# What exactly is interplanetary scintillation; what was the Interplanetary Scintillation Array looking for? Did it successfully observe any?

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The Interplanetary Scintillation Array is the radioastronomy observatory (i.e. big antenna) where the first pulsar was discovered by then graduate student Jocelyn Bell Burnell through careful and painstaking review of chart recorder data.

Question: What exactly is interplanetary scintillation; what was the Interplanetary Scintillation Array looking for? Did it successfully observe any?

For observing ionizing radiation such as cosmic rays and photons, a scintillator is used to convert energy to lower energy photons (usually visible light) but I don't know if the term scintillation in radio astronomy is related to that process in any way or not.

update: I've just asked in Aviation SE: What is “scintillation” and are “qualified pilots” aware of it? which was quickly answered and points out that in that context scintillation can also be called "twinkling". If a radio source can twinkle as well, is this an atmospheric effect? If so is it neutrals or ions that generate it. If not, is it turbulence in the ionized interstellar medium?

To fully answer your questions, let me introduce scintillation before interplanetary scintillation.

Atmospheric scintillation

The imaging of an astronomical source is affected by a collection of effects that goes under the name of astronomical seeing, the main ones being smearing, motion and scintillation of the image. All these effects are caused by the deformation of the light wavefront due to random inhomogeneities in the refractive index of the atmosphere.

Let's visualize the basic idea behind the seeing. Suppose to have a light source located at infinite distance, such that it's ideally a point source and its light reaches Earth in plane waves. When entering the atmosphere these waves face changes in the refractive index, and in geometrical optics approximation the wavefronts are deformed according to Snell's law. In the following image you have two simplified cases: on the left, it is shown that a vertical gradient in the refractive index produces a simple tilt of the wavefront; on the right, it is shown that a horizontal gradient produces a deformation of the wavefront. Here $$extrm{WF}_1$$ is the incoming unperturbed wavefront, $$extrm{WF}_2$$ is the same wavefront after entering the atmosphere and $$n_i$$ are the refractive indices.

$$hskip2in$$

Let's now approach the real case. It is known that variations in the refractive index are related to variations in density (e.g. via Gladstone-Dale relation). Since our atmosphere has an overall more-or-less stationary distribution of temperatures, densities, and pressures, the most dramatic variations of the refractive index are found only in the turbulent layers. Here each turbulent eddy can have different density, temperature and so on, causing local variations in the refractive index. These layers are located in the first km of atmosphere (the planetary boundary layer) and at $$sim$$10 km (near the tropopause). In the following image, it's depicted how a plane wavefront is deformed after crossing a turbulent layer and all of its eddies with different $$n_i$$. The lenght $$r_0$$ is the Fried parameter, which can be interpreted as the characteristic lenght of the turbulent eddies.

$$hskip2in$$

What is atmospheric scintillation then? Well, as you see from the previous image, the curvature of the wavefront leads to a convergence (or a divergence) of the light rays (i.e. the lines perpendicular to the wavefront). This means that when you take an image by collecting photons with your sensor (i.e. when you collect a portion of the wavefront) the image brightness can rise or decrease depending on whether light rays are converging or diverging. Furthermore, this brightness variation is time-dependent, since the eddies evolve with time and the turbulent layer has an horizontal drift velocity. This is scintillation.

To see scintillation you need that (i) the source has small angular dimension, (ii) the turbulent layer is far from the telescope, (iii) the characteristic length of the collected wavefront's portion (e.g. the diameter of the telescope) is comparable to $$r_0$$, (iv) the exposure time is less than the lifetime of the deformation. If (i) and (ii) are not satisfied you might see the motion of the source instead of scintillation, while if (iii) and (iv) are not satisfied you will see speckles or a smeared image.

In the radio domain, scintillation is not limited to atmospheric scintillation.

For wavelengths between millimeters and centimeters we still have atmospheric effects, mainly due to the water vapor vertical gradient near the ground. Indeed, for this wavelengths the refractive index of air deviates from unity less than few houndreds of ppm. Since the turbulent layers are closer to the ground and usually the radiotelescope apertures are bigger than the characteristic length of the turbulence, seeing is not dominated by scintillation.

For wavelengths beyonds centimeters, the frequency of the waves gets closer to the plasma frequency in the ionosphere, hence the wavefront deformation provoked by the passage through the electron clouds in the ionosphere becomes relevant. Since their great distance from the ground, this will result in scintillation.

The timescale of ionospheric scintillation is between minutes and tens of minutes, but Hewish (1955) started to notice that there was another scintillation with a timescale of few seconds, and that its intensity was greater for sources near the Sun. Following this hint Hewish et al. (1964) have shown that in fact this faster scintillation was produced by plasma clouds in the interplanetary medium, delivered by the solar wind.

Going further, Sieber (1982) have shown that there's also a scintillation with timescales between days and months due to plasma clouds in the interstellar medium.

Conclusions

What exactly is interplanetary scintillation?

Scintillation is a time-depending change in the intensity of a light signal, and it is caused by deformations in the wavefront due to random variations of the refractive index. In the case of interplanetary scintillation, the refractive index variations are caused by plasma clouds traveling in the interplanetary medium and delivered by the solar wind.

What was the Interplanetary Scintillation Array looking for?

Precisely interplanetary scintillation. From it you can study the properties of both the interplanetary plasma and the scintillating radio sources.

Did it successfully observe any?

I guess yes!

## Could a large radio telescope survive interstellar spaceflight?

I recently came across Could pulsars really act as "lighthouses" to help in interstellar travel?, asked a week ago. The author was trying to figure out if pulsars could be useful for interstellar FTL travel, because they could be used to find a craft's position the answer was, of course, yes. I had some reservations, though. My main issue was that it seems, from an engineering standpoint, not very feasible. Here's my thought process:

• To be certain of successfully triangulating your position to a high degree of accuracy, you'd need to have probably half a dozen or more candidates to observe from any one location.
• Observing a pulsar isn't easy. The issue is that if you make a random jump out of hyperspace, you won't know where any pulsar should appear unless you can quickly determine a general location. This means that you'd need to do a lot of guessing, and essentially discover pulsars all over again.
• This in turn means that you'd need a large radio telescope, and that's not really easy to attach to a typical spacecraft. Optimally, the dish is over 100 meters in length.

Let's say that we overcome various other technical hurdles, and need to attach a 100-meter parabolic radio telescope (although I'm open to other designs, if you can make a very convincing argument otherwise) to an interstellar spacecraft, for the purposes of finding and monitoring pulsars for navigation. I'm concerned as to whether or not the telescope could survive sub-lightspeed travel for any extended period of time. By this I mean acceleration for perhaps two weeks to a speed of maybe

## X-ray pulsars

Apparenly x-ray pulsars are easier to see

Or, you could look for pulsars that emit X-rays, a much brighter signal. X-ray antennas are also smaller and lighter, says physicist Richard Matzner at the University of Texas at Austin. Their drawback is oversensitivity to electrons surrounding the Earth.

But an X-ray–based positioning system could pinpoint an object to within 10 meters, an improvement on the 100-meter or so accuracy of the radio pulsar system.

The physical stress of motion won't be a problem.

Every telescope on earth is built to sustain 9.8 m/s 2 acceleration indefinitely.

Accelerating up to 0.01 c (299,792,4.58 m/s) over the course of 2 weeks (1209600s) can be accomplished with a constant acceleration of 2.48 m/s 2 .

Since this is much lower than what we engineer telescopes on earth to it should be easy to engineer a telescope to withstand that level of acceleration.

.01c\$, staying at constant speed for three months, then decelerating for two weeks. Propulsion would likely be from chemical rockets.

• Will the telescope survive the harsh conditions of space, including micrometeoroid impacts?
• Will there be any physical stresses from the motion of the ship that could damage it?
• Are there any other potential dangers to the telescope itself, and can they be overcome?

So far, nobody's actually addressed the original scenario in as much detail as I'd like. I'd love answers that do that it's why I asked the question. However, I wouldn't be totally opposed to answers that suggest different but related options, such as using a different type of telescope, or using pulsars a different way. But you'd have to make a really good case for doing so, and you'd still have to justify that this option would survive the spaceflight.

My motivation for asking this is that I've considered using pulsars for this purpose in several stories, but I've always gotten hung up on how to solve this sort of problem.

## An Interview with Manuel F. Varela and Ann F. Varela: Who was Jocelyn Bell Burnell, and what did she have to do with Pulsars?

1) In this interview—we delve into astronomy and another famous female scientist—Jocelyn Bell Burnell. Where was Bell born, and where did she attend school in her youth?

Bell’s full name was Susan Jocelyn Bell. Her birthdate was July 15, 1943, in the town of Belfast, Northern Ireland. Her parents were well-educated Quakers who supported their daughter’s early interest in science with books and trips to the nearby Armagh Observatory, of which her architect father helped design. The staff at the observatory would also encourage her interest in astronomy during her visitations.

Bell attended the Preparatory Department of Lurgan College, a coeducational selective grammar school for students aged 14-19. Female students were not permitted to register for science classes at this institution until Bell’s parents, along with other parents, protested the school’s policy. Up to this point, the girl’s curriculum included cooking and needlepoint, but not any science-related courses. Despite her enthusiasm for learning, however, Bell struggled in grade school and failed an exam meant to measure her preparedness for higher education.

Undiscouraged, her parents sent her to England to study at a Quaker boarding school, The Mount School, where she promptly gained recognition for herself in her science classes. Having established her ability and talent for higher learning, Bell attended the University of Glasgow, where, in 1965, she earned her B. Sc. degree in physics with honors. She later earned a Ph.D. in radio astronomy from Cambridge University in 1969.

2) Her supervisor at Cambridge—Antony Hewish and radio galaxies—seemed to pique her interest. What was Hewish doing, and how did Bell fit into the picture?

In 1965, Bell started graduate studies in astronomy at Cambridge, working under her graduate advisor Anthony Hewish. At the time, Hewish was a radio astronomer designing and building a radio telescope to detect quasars in outer space. Quasars are incredibly bright centers of galaxies with supermassive black holes. See Figure 1.

These galactic centers are highly active from an electromagnetic perspective. Such highly active galactic centers of quasars are star-like objects with a circular accretion disk of hot gas. As the gas in the spinning disk is drawn into the supermassive engine of the black hole, the center becomes a compact source of radio wave emissions characterized by electromagnetic radiation with a wide-ranging spectrum. The matter that plunges into the depths of a black hole is heated by intense gravity, generating massive blasts of radiation beams. The edges of the spinning hot disks form a donut-shaped ring of stellar dust. So-called radio jets of material composed of charged particles shoot outward from the magnetic pole of the black hole engine, creating long plumes that are thousands of light-years in their distance.

Figure 1. The first direct visual image of a black hole in Messier 87, a supergiant elliptical galaxy in the constellation Virgo.

During the time in which Bell had started her graduate studies at Cambridge under Hewish, the new radio telescopes were meant to detect the scintillating behavior of quasars. As the light of a quasar hurls through ionized solar wind, the twinkling property manifests itself and could be spotted by their new radio antenna telescope. Bell spent her first two years as a Cambridge graduate student building the giant instrument by hammering and connecting wires. The new radio telescope instrument consisted of over four acres of land, 120 miles of cable wires suspended on about 1,000 wooden beams, and 200 hand-made transformers. See Figure 2. When the quasar-detecting machine was complete, Bell was the only person who operated the new instrument. She collected the data, which consisted of ink tracings on reams of paper, and she analyzed the machine’s output, a tremendous undertaking. The amount of data was immense, and Bell had to sort out any confounding artificial interference from the twinkling activities of the naturally occurring objects from outer space.

Figure 2. Remains of the Interplanetary Scintillation Array at the Mullard Radio Astronomy Observatory, Cambridgeshire, in June 2014.

Shortly after turning on the new radio telescope in July of 1967, Bell noticed a strange signal in her reams of printed data from outer space. She called new tracings “scruff.” This scruffy data did not appear to align with either human-made interference or scintillating pulsars. Instead, Bell observed that the scruff appeared periodically, about every 1.3 seconds, from the exact location in the night sky of outer space. It was an ordered signal coming from the same patch of the night sky. Such regularly repeating scruff signals did not seem to belong to any previously known natural phenomena from space. See Figure 3 for Bell’s scruff pulsar data.

Bell and Hewish began systematically ruling out various artificial, manufactured sources, such as fellow radio astronomers, radio or TV broadcast signals, Earth-orbiting satellites, radar signals bouncing off the moon and entering their instrument, and even aberrant signals reflecting off nearby buildings with corrugated metal roofs.

Figure 3. Jocelyn Burnell examined the chart in August 1967, showing the trace of the first identified pulsar, subsequently designated PST B1919+21.

The space signals appeared as intense pulses, regularly repeating every 1.3 seconds, too quickly to have originated from any known star at the time. Bell and Hewish called their new source, LGM-1 (for Little Green Men). She also nicknamed the signal “Belisha Beacon,” after the orange flashing lights meant to warn motorists of pedestrian street-crossings. While they felt that the space signal, though seemingly artificial, was likely not from aliens in outer space, they did rule it out, nonetheless. If the LGM-1 was an actual signal from a planet of beings revolving around another sun-like star, then the call should move about like a world in its orbit. The orbiting exoplanet should exhibit Doppler shifts during its “transmissions.” The LGM-1 pulse, however, showed no such Doppler effect, indicating that the signal could not have come from alien beings on an exoplanet orbiting their sun.

Instead, Bell and Hewish learned that their novel signal came from a star. This star source appeared to be distant from our solar system but well within the Milky Way galaxy.

The shortness of the pulsing transmission, only 1.3 seconds, suggested that the star must be relatively tiny, like a white dwarf star. Shortly after this historical pulsar discovery, Bell discovered three additional pulsars. Bell and Hewish, along with co-authors John Pilkington, Paul Scott, and R.A. Collins, published their novel findings in the prestigious journal Nature in February of 1968. Bell was the second author in the now-famous paper. The Nature article was the first published evidence for the existence of radio pulsars. Bell, a young graduate student, had played a significant role in the historic discovery of the legendary radio pulsars. This first pulsar, “Bell’s Star,” was first known as CP 1919, for Cambridge Pulsar with celestial coordinates 19 h 19 m . Later, Bell’s pulsar was updated with the designation PSR B1919+21. We now know that the object is 978.5 light-years away from Earth.

3) Now, for the layperson—what exactly is a pulsar?

A pulsar can be described as a rapidly spinning remnant of a dead star, called a neutron star. As the star rotates about its axis in precisely timed intervals, astronomers observe short pulses of radiation. Hence, they were pulsating radio stars, or pulsars, a term coined by Bell and Hewish. Neutron stars have strong magnetic fields and rapid spinning rates. In general, a neutron star is a compact and highly dense star consisting almost entirely of neutrons. These neutrons are tightly packed within the star’s diameter, the mass of which can resemble our solar system’s sun. However, a typical neutron star has a diameter of only about six miles (10 kilometers), whereas our sun is about 864,900 miles (1.4 million kilometers). A neutron star rapidly spins between 1.4 milliseconds to about 30 seconds per rotation, while our sun rotates once every 25 days.

We know that a neutron star forms when the core of a highly dense star collapses upon itself and undergoes an explosion of supernova proportions. What remains in the aftermath of a so-called type II supernova explosion is the spinning neutron star. When such stars go supernova, the material in the outer crust of the exploding star is sent away, leaving behind its neutrons that are tightly compacted into a spinning radiation-pulsing star.

The pulsars operate when charged particles are spiraling along the magnetic field lines of the neutron star, producing the radiation beam. As pulsars rotate, they emit a beam of radiation, sort of like a lighthouse with its rotating light shaft. When observers on Earth detect the radiation beam from a neutron star, we see the energy as a pulse, and when the pulsar is “off,” the radiation beam does not face Earth observers. The energy level of the radiation beams can vary, ranging between the radio, X-ray, ultraviolet, and gamma-ray intensities of the electromagnetic spectrum.

Pulsars are known to emit visible light. After the gases of the exploding supernova cool down, the visible light seems to fade but will shine with infrared radiation and pulse with perfectly timed radio waves. As such, pulsars are thought to be extremely precise keepers of time in our current universe, a so-called cosmic metronome.

When Bell and colleagues published their discovery of the first observed pulsar, it garnered a tremendous amount of attention. Soon additional pulsars were discovered, starting a new field of study in the astronomic sciences.

For her role in being the first person in the world to find the first pulsar signal ever, Bell would become one of the most famous graduate students in the history of stellar evolutionary science. See Figure 4.

Figure 4. Susan Jocelyn Bell (Burnell), June 15, 1967.

4) Radio galaxies—what exactly are these?

Radio galaxies are a type of so-called active galactic nuclei, also called active galaxies, and they represent natural sources of radio waves from objects in outer space. See Figure 5. In general, there are several types of active galaxies. These active galactic nuclei differ by their intensity and orientations of dust rings and radio jets.

The radio galaxies are compact centers of galaxies with emissions of extremely wide radio wavelengths. The extreme luminosities, characteristic of radio galaxies, strongly suggest that non-star objects are responsible, such as supermassive black holes. Frequently, the emissions arise from two giant plumes, or radio jets, of a radio galaxy. A perpendicular accretion disc with a massive circular dust ring at the edge accompanies these jetting radio lobes. The accretion discs of dust are known to rotate.

The quasars represent the second type of active galaxy. These objects appear in the night sky as star-like points of light but are tremendously distant galaxies, as indicated by their red-shift characteristics. Quasars are thus galaxies with exceptionally bright cores, which emerge from giant dust rings. Quasars are thought to be supermassive versions of pulsars, with millions of densely packed stars forming galactic nuclei. It is believed such quasars are powered by giant matter-antimatter explosions and occur when gas and dust get sucked into the cores of black holes in the centers of galaxies.

The third type of radio galaxy is known as the blazar, also called a BL Lacertae object. The blazars are also star-like points in the sky but do not have significant spectral lines as the quasars do. Blazars are oriented towards Earth observers such that the radio-emitting plumes or lobes face us directly.

Lastly, the so-called Seyfert galaxy type has regular-looking spirals but is compact with light-emitting cores. The Seyfert galaxies are typically oriented in which the dust ring and accretion disc are visible but are less powerful than the quasars.

5) At first, Bell thought that space aliens, or “little green men,” as she called them—were sending signals to her—What was going on in reality?

Bell never seriously considered that her radio signals were from “little green men.” However, Bell and her colleagues did have to rule out such extraterrestrial activity to understand their discovery. It was a historical encounter with objects from outer space. Bell has described the pulsar discovery as sort of an accident. They were seeking quasars, which are enormously distant objects in outer space. Quasars were already known to astronomers. Instead, Bell had obtained contaminating noise signals that were closer than the sought-after quasars, and the annoying pulses got in the way of their attempts to study these quasars—at first.

As Bell examined the interrupting signals from outer space, she noticed that amongst the data, there appeared on her printed readouts various intense pulses, occurring periodically every few seconds. The timeframe of the pulsing radio waves was too short, suggesting that they did not come from any recognized stellar or planetary object. The pulses lasted only briefly, about 0.3 seconds per pulse, but they occurred precisely every 1.3 seconds. Furthermore, the energy beatings had no relation to the movement of the Earth.

Instead, the pulsations adhered to “star time,” a phenomenon known as sidereal time, as the pulse occurrences related to the activity of the stars and not the Earth.

As we mentioned above, Bell and Hewish called their new radio signal, LGM-1 (for Little Green Men). The idea, mused about only briefly and not to any serious-minded degree, was that perhaps the pulses were from aliens in space—little green men! The possibility that they were receiving messages from an alien civilization was intriguing. Bell and colleagues, however, immediately reasoned that their new radio signals were not from extraterrestrial beings from outer space. The astronomers understood that the signs originated from a neutron star and not necessarily from an exoplanet.

Further, the neutron star had likely gone supernova previously and, thus, could not harbor planets that might support living alien beings. As mentioned above, the LGM-1 signal did not move in a pattern like an orbiting planet around any star, as backed by the accompanying Doppler shift data. Bell and colleagues also ruled out the possibility that their novel radio signals were of Earth origin, as the emissions were mapped to locations far outside the confines of our solar system. They had systematically ruled out contaminating signals from Earth-derived sources and orbiting human-manufactured satellites.

When Earth- and alien-based civilizations were ruled out as sources of the energy pulses, the attention turned to the stars way beyond our solar system as a naturally occurring radio source. Bell and her astronomer colleagues would learn that the pulsating signal came from a neutron star. No one in the world had seen such a phenomenon, whether in space or on Earth. No one had ever imagined such an energy-radiating object could be possible.

As for the little green men, none have been found as of this writing. There are, however, active projects with the prime aim of finding bona fide alien signals from civilizations in outer space. The program is called SETI for Search for Extra-Terrestrial Intelligence, and scientists are still seeking evidence for such aliens. Bell Burnell had never been a part of SETI.

6) Her supervisor, Antony Hewish, won the Nobel Prize (for physics), and many felt that Bell was somewhat ignored. This episode was back in 1974.

British astronomer Antony Hewish took the physics Nobel Prize that year for the discovery of the pulsars. He shared the prize that year with Martin Ryle, Hewish’s mentor, noted for his (Ryle’s) contributions to the invention of the so-called aperture-synthesis method. Hewish had designed the radio telescope instrument for detecting quasars. Together, Hewish and Ryle were recognized for their pioneering studies in radio astrophysics.

Jocelyn Bell Burnell did not ever share in the Nobel Prize. She was not even invited to the royal ceremonies. During that era in the 1970s and before, it would have been widely considered unprecedented for a student who did the work to share the Nobel Prize with the project’s principal investigator. However, soon after the Nobel nod to Hewish, critics pointed out a degree of unfairness in leaving Bell Burnell out.

She had, after all, built the machine, operated it, collected much of the data, been the first to pay attention to the pulsar’s signal, and was the first to suggest the neutron star as the radio source of the pulses. Yet, the Nobel went to Bell’s advisor, Hewish, who oversaw the lab and had designed the antenna device.

Bell had heard rumors that she would share the Nobel with Hewish. Solid evidence about the Nobel emerged when Bell and Hewish had been jointly awarded the 1973 Albert A. Michaelson prize. Then she learned that the prestigious accolade went to Hewish and Ryle.

Then or later, not even Bell Burnell seemed to be surprised about her omission. Bestowing the Nobel to lab assistants or students was not done in the day. In 1923, Frederick Banting took the Nobel for discovering insulin. Banting’s student, Charles Best, was not named in the official accolade. However, Banting had given Best a share of the Nobel award money. One might argue that it is not the fault of the Laureates for leaving out lab assistants or students when the choice of who gets the Nobel is determined. The Nobel Laureates are not, after all, routinely involved in the Prize nominations or the commission’s decisions.

Hewish, in his Nobel lecture, did name Bell. He gave her credit for connecting the cable network of dipoles in the antenna, keeping up with the paper flow from the machine’s recorders, and even bringing the new pulsing signals to his attention in the first place, in August of 1967. He even attributed Bell as providing a list of additional pulsars. Hewish further acknowledged that the discovery was a team effort, consisting of many Cambridge personnel.

Thus, while he seemed to have given due credit as he saw it during the Nobel festivities, Hewish nevertheless appeared to have mishandled the affair with news reporters afterward. When questioned by science reporters, Hewish had taken to naming only himself during his discussions, leaving out any mention of Bell or others. Hewish had to be prodded to provide additional details of the discovery if his team’s contributions were concerned.

Hewish’s dealings with the science reporters took a downturn. When specifically asked whether he or a graduate student of his who made the initial readings of the pulsar data, he seemed to have implied that he had taken the recordings himself, saying, “Oh, yes, I did.” He further stated that his graduate student was “doing observations, which I had designed,” keeping himself in the picture when conferring any credit to others.

Rather than blame the Nobel authorities for Bell’s omission, Hewing chose to defend their decision. He was not, after all, responsible for any of the selections made by the Nobel commission. At some point, Hewish became “fed up” with the “stupid business that Jocelyn did all the work, and I got all the credit.” Hewish went on record to elaborate that “If she’s [Bell Burnell] disgruntled about the Nobel, well that’s too bad quite honestly,” declaring that her work was not creative enough for a Nobel consideration. Nevertheless, in many circles, it is recognized that bestowing credit or recognition to additional contributors does not necessarily diminish those of the original recipient of the accolades.

Eventually, the controversy regarding who deserved credit for the pulsars then and the question of fairness (or lack thereof) faded away—until 1993, that is. Physics professor Joseph Taylor was given the Nobel Prize in physics because he discovered the so-called double pulsars, and his student, Russell Hulse, was named a co-Laureate! In the 1990s, both the professor and the student were given an equal share of the Nobel credit. Naturally, the old wounds from 20 years earlier regarding the discovery of pulsars and lack of equal credit were reopened. The issue was widely debated again. Taylor, who felt that Bell Burnell had been unduly overlooked concerning her contributions to the pulsar discovery, had generously invited her to his Nobel ceremonies. Anders Bárány, who was chair of the physics Nobel committee, gave Bell Burnell a replica of the Nobel medal as sort of a compensatory gesture.

Throughout this era, Jocelyn Bell Burnell had been benevolent about her exclusion by the Nobel commission. She had even been pleased in 1974 that an astronomer had taken the physics Nobel. Bell Burnell was proud to have been involved in such a historical, scientific discovery. To have been so closely associated with the pulsar discovery, however directly or indirectly, Bell Burnell related in the ensuing years that it had nevertheless provided her with “enormous enjoyment, [and] some undeserved fame.”

7) Bell Burnell held several positions—one at the Royal Observatory in England! Where else did she teach during her long career?

After receiving her doctorate from Cambridge, Bell Burnell taught and researched gamma-ray astronomy at Southampton from 1968 to 1973. Then, Bell Burnell spent eight years as a professor at University College London, concentrating on x-ray astronomy until 1982.

Bell was a tutor, adviser, examiner, and senior lecturer for the Open University From 1973 to 1987. Later, she worked at the Open University as a professor of physics while simultaneously studying neurons and binary stars and conducted research focusing on infrared astronomy at the Royal Observatory located in Edinburgh, Scotland.

Bell was the Dean of Science at the University of Bath from 2001 to 2004 and has been a visiting professor at such respected institutions as Princeton University and Oxford University.

While at Edinburgh’s Royal Observatory, she was head of the James Clerk Maxwell Telescope section, responsible for the British end of the telescope project based in Hawaii. Presently, Bell Burnell is a Professor of Physics and Department Chair, Open University, England.

8) Bell Burnell has also been involved in gamma rays, X-rays, infrared rays, and millimeter-wave astronomy. What do all these waves and rays have in common?

Bell Burnell used her expertise with the antenna to study each of these forms of electromagnetic waves. At Cambridge, radio astronomy was the area of investigation held by the newly minted Dr. Bell Burnell. But at Southampton University, she studied gamma rays. At Edinburgh, Bell Burnell had become interested in learning about infrared and millimeter-wave detection.

Each of these entities is a form of radiative energy and has specific electromagnetic radiation field characteristics. Thus, these electromagnetic components are all detectable on various antennas, each geared for the specialized detection of their specific wave energies. Another thing in common with these various electromagnetic fields is that technical devices can transmit their particular rays. For example, magnetrons were invented during World War II to be used as radar transmitters. Another example is the microwave oven, in which each range has its dedicated microwave magnetron. The substances that are heated in a microwave can serve as a sort of antenna. The third commonality to these energy waves is that they can be stored in gadgets colloquially called pillboxes. These apparatuses reflect the waves throughout the insides of the pillbox containers to keep the energy for later use. Lastly, these waves can be interrupted in their pathways through space, increasing their heat temperatures.

9) Although Bell Burnell never won the Nobel Prize, she did receive many other awards. Can you tell us about some of them?

Bell Burnell’s accomplishments have been recognized with numerous honors and awards. Among these awards include Commander and Dame of the Order of the British Empire in 1999 and 2007, respectively the Oppenheimer prize in 1978 the 1989 Herschel Medal and the 2015 Royal Medal from the Royal Astronomical Society. Bell served as president of several institutions, including the Royal Astronomical Society from 2002 to 2004, the Institute of Physics, headquartered in London, from 2008 to 2010, and the Royal Society of Edinburgh since 2014. In 2018, she received the Special Breakthrough Prize in Fundamental Physics, which included a £2.3 million prize money that she entirely donated towards scholarships for women, minorities, and refugee students pursuing degrees in physics-related research. After her gracious gesture, the Institute of Physics later renamed this prize the “Bell Burnell Graduate Scholarship Fund.” In addition, Bell Burnell has honorary degrees from a wide assortment of universities.

## 1 Introduction

As our modern society relies more and more on Global Navigation Satellite Systems (GNSS) technology in positioning and precise timing, any extended disruption of the GNSS service will affect our daily life. However, the performance of GNSS services is highly dependent on processes in the ionosphere. When the GNSS signal passes through regions of ionospheric irregularities, the ionospheric scintillation occurs due to the interference of the radio waves. Consequently, the signal that has traversed the ionosphere exhibits rapid fluctuations in amplitude and phase, which are referred to as amplitude and phase scintillations, respectively (see, e.g., Yeh & Liu, 1982 Kintner et al., 2007 , and references therein). The signal quality is degraded during intense scintillations, and the GNSS receiver may not be able to maintain lock of the signal. In this case, the GNSS service may become unavailable.

At high latitudes, the scintillation effect on Global Positioning System (GPS) signals has been associated with phenomena like storm-enhanced density, polar cap patches, and auroral precipitation (Alfonsi et al., 2011 De Franceschi et al., 2008 Jin et al., 2014 , 2015 Jin, Moen, et al., 2016 Li et al., 2010 Mitchell et al., 2005 Moen et al., 2013 Oksavik et al., 2015 Prikryl et al., 2010 , 2011 , 2013 Smith et al., 2008 Spogli et al., 2009 van der Meeren et al., 2014 , 2015 , 2016 ). The strongest GPS phase scintillations are associated with auroral blobs that are formed when polar cap patches enter the nightside auroral region (Jin et al., 2014 Jin, Moen, et al., 2016 van der Meeren et al., 2015 ). Similar results have been reported in the dayside cusp ionosphere, where the polar cap patches combined with the cusp auroral dynamics are associated with the strongest GPS phase scintillation (Jin et al., 2015 , 2017 Oksavik et al., 2015 ).

The GNSS service can be particularly disturbed during severe geomagnetic storms which are driven by the interplanetary coronal mass ejections (ICMEs) or corotating interaction regions (e.g., Prikryl et al., 2014 ). In this paper we now investigate one of the largest geomagnetic storms during the last solar maximum, the St. Patrick's Day storm on 17 March 2015. Most studies have focused on geomagnetic conditions that are far less active and mostly report phase scintillations of GPS signals at high latitudes. However, extreme events like the St. Patrick's Day storm are likely candidates for even stronger disruptions. In the current study we report significant amplitude scintillation of GPS signals at high latitudes as well, in addition to phase scintillation. We also quantify the signal power drop and loss of signal lock during the St. Patrick's Day storm. Finally, we relate the disruptions to large-scale phenomena like total electron content (TEC) blobs.

## 3. System Specifications

[7] The specifications of the SWIFT are shown in Table 2, while the overall structure and dipole elements of the phased array are shown in Figure 1. A schematic illustration of the SWIFT configuration is presented in Figure 2. The observation frequency of the SWIFT is 327 MHz, the same as that of the existing STEL IPS system. The SWIFT is composed of a pair of asymmetric cylindrical parabolic reflectors with a physical dimension of 108 m (north-south) by 19 m (east-west), and a low-noise phased array receiver with 192 elements. The parabolic reflectors are fixed on the ground, and its cylinder axes are oriented north-south. The antenna directivity of the SWIFT is formed in the meridian plane by the phased array. Thus, the SWIFT is dedicated to meridian transit observations of radio sources.

## What is a living individual and is it naturally universally mobile?

Earth is gone. Complements of some natural occurrence, you name it. Perhaps a primordial black hole or giant rogue planet that happens to be passing through this solar system which sends the Earth into direct collision with Jupiter. Or perhaps there is an immense solar flare that perturbs Earths' orbit, sending our magnificent crucible for life careening into the sun. Result? All that you, and I, and your pet otter were, every cell and every DNA molecule, every atom that was on, or in the Earth, is now ionized nuclear fuel within the sun. The Darwinian evolved chemistry and biology that many fall back upon to describe life on Earth, particularly human life, has ceased to exist in this solar system. Along with its thermodynamically described chemistry and biological processes once used to describe the entirety of Earths' ecosystem.

Additionally, imagine if you will that there is life elsewhere in this universe. Let us imagine there exists at least one other evolved ecosystem (ECO-2) capable of hosting Darwinian life. Different from Earth but governed by the same laws of physics and biology and thermodynamic processes that manifested Earths' ecology. This planet orbiting a viable star may be located anywhere in this universe since the laws of physics are expected to be consistently applied throughout. Also for this anecdote, let us say that this other bastion of life is some 10 billion light-years from Earths' sun. A distance so vast it would take much longer than the age of the big-bang to relativistically travel that distance, assuming, of course, there were any classically defined remnants of ones' biology left to make the journey.

The question becomes could you or I or any individual formerly hosted by Earth's ecology ever find oneself a part of ECO-2s' ecology? Is the nature of life in this universe such that one could at some point find oneself naturally born to ECO-2 in the form of a species indigenous (present or future) to ECO-2, just as we were born on Earth to species indigenous to Earths' ecology? If one adheres solely to the classically understood, thermodynamically described, relativistically constrained mechanisms to explain life writ large then you are forced to say no, and in so doing you would necessarily be Earth and human-centric as one discounts the rest of the cosmos. Because in nature, what is possible here is necessarily possible elsewhere, ergo if you can live here, you can live anywhere. And yet, clearly, some aspect of what biologically, thermodynamically, chemically, defined ones' singular existence on Earth, must relativistically (Below the speed of light) travel to bridge the unbridgeable distance between your last physical location, Earths' solar system, and ECO-2s'.

#### Tonylang

Your response is quite appreciated. Here is the active bit:

The question becomes could you or I or any individual formerly hosted by Earth's ecology ever find oneself a part of ECO-2s' ecology? Is the nature of life in this universe such that one could at some point find oneself naturally born to ECO-2 in the form of a species indigenous (present or future) to ECO-2, just as we were born on Earth to species indigenous to Earths' ecology?

#### Tonylang

The proposal being made is that if you can live in one viable habitat, i.e. Earth, then the very laws of physics that guide our scientific method demand that you can also live in any other viable habitat i.e. ECO-2 in this universe. Ergo Earth is not special, at least not that special. The distance factor (10B LY) is the interesting bit. How can one be naturally reinstantiated (born) elsewhere regardless of distance and with no physical travel (no comets or spacecraft or photons from Earth can reach ECO-2)?

The trouble most will have with this realization is one's individuality has always been misperceived to be instantiated by one's host form, one's species. However, the atoms and molecules that compose your body, is a part of the current indigenous ecosystem, Earth or ECO-2. The demand this realization makes upon all cognizant living beings is the acceptance of the abstraction of one's current host form (body) from your universally mobile position of view (POV), one's individuality. This implies the universal mobility of individuality and demands a natural, scientifically describable mechanism for its implementation.

#### Tonylang

Naturally invasive scenarios such as this don't reveal questions posed by individuals, but questions posed by nature. Such scenarios essentially ask how could it be otherwise? Such questions reveal their own answers to any species sufficiently developed to comprehend and honestly confront them. The point of this scenario is the inescapable conclusion that each individualized instance of life must involve a non-classical, non-local, relativistically unconstrained, scientifically describable, naturally recurring component. This individualizing phenomenon must exist separately and distinctly from any local physical form and must be definable by some discretely quantifiable property of nature with degrees-of-freedom much greater than that of matter. Such a mechanism may also not be indigenous to this universe but instead is native to the underlying Hilbert-space, or 'Metaverse' if you will. This need for non-locality is necessary to instantiate individuality not just on Earth while it exists and is viable, but also within the systems and galaxies of this vast Higgs constrained universe, and throughout nature.

The only life that has ever existed on Earth is the living cell, in all of its forms. The aspect of being and individuality had by a single living cell is that which defines all life, no more and no less is required. This aspect, which instantiates the first person being of a single cell as a living individual every bit as alive as any multi-cellular creature, is the position of view (POV). All of the skills and talents that tend to distract from this fact are only emergent features of the host form. Beneath it all is ones' POV. In this universe, there isn't one implementation of life for mammalian forms and another for insects, and yet another for vegetation or microbial forms of life. Nature is an efficient system of cause and effect, and life is one holistic effect. It isn't my intention to change anyone's' mind on this topic. Rather, to expose open-minded readers to a new and practical way of thinking about a very old, perhaps the most personal of all ideas known to humankind. The recognition of a unique and scientifically plausible description of how nature governs not only species but the individual, you. There is a very good chance, as is often the case with such invasive ideas about nature that I and everyone who reads this volume would be long gone before either the capability or the courage to prove or disprove the LINE hypothesis is achieved. However, every first step is worth taking.

The natural processes that implement life are the same for the cell as it is for the bacteria as it is for a fruit fly as for a human being. It is folly for us to think we could only experience life in this very temporary, randomly emerged bipedal primate form. Further, your cells and molecules come and go continuously over the course of your lifetime. Nonetheless, you remain you. Then there are the other trillions of living individuals in millions of different forms all around us coming into being and going out of life continuously. I realized that the only form we need to consider in this regard is the single living cell. The answers that are true for the cell are the answers that apply to all life.

Furthermore, you and I and your pet octopus and every living cell are instances of life, each a temporary instantiation of some natural, empirically definable phenomena of nature. This instantiating phenomenon must have the relativistically unconstrained reach to establish individual life (you), biological or perhaps otherwise, on any planet orbiting any star or indeed in any viable environment in the cosmos or in existence where viable hosts may emerge. It is a tragic mistake to feel that this describes something that could not possibly be natural, but must be supernatural. While, as usual, natures' genius is a practical and ubiquitous, even if a bit unfamiliar implementation. There is a phenomenon known to science for some time that meets all of these requirements: Quantum Entanglement (QE). Einstein called it spooky action at a distance. Today we play with it in the lab as a mere tech curiosity. It is the most plausible mechanism by which individuality is universally instantiated.

#### Tonylang

The LINE "Life Instantiated By Natural Entanglement" hypothesis presents perhaps for the first time, a practical scientifically plausible hypothesis for the natural implementation that governs the instantiation of the living individual as a being distinct from the evolution of that beings current species. It will introduce you to

• The Instantiation Of Individuality: The natural process which establishes each instance of individual life, you.
• The Entanglement Molecule (EM) A primordial molecule, is hypothesized to naturally interact with the QE spectrum to entangle metamatter. It is the Alice in the process of natural entanglement and is utilized by the living cell to establish individualized life.
• The Position Of View (POV): That component of the instantiation process which defines your presence in your current host form within this space-time.
• The Metaverse: Hilbert-Space, the only real verse, and that from which this universe emerges.
• The Quantum Entanglement Spectrum (QE): The degrees of freedom which define the phenomenon of natural quantum coherent interaction. Einsteins' 'spooky action'.
• The Quantum Entanglement Frequency (QEF): Ones' immutable unique value of the QE degrees of freedom which instantiates your POV.
• The Cell and (Proto-Cell): The only life on Earth, natures’ entanglement circuit. The original instantiated living individual which implement all other biological hosts on Earth.
• The LifeID: A calculated value that defines ones' current unique QE connection, your LINE.
• The Entanglement Cells Individual cells responsible for heterodyning their unique LINES in complex hosts to establish your LifeID.
• Metamatter: A non-local Weakly Interacting Cosmic Background Bose Condensate (CBBC) is hypothesized to be as necessary to life as dark-matter is to galaxy formation. Where the EM is the Alice, then metamatter is the Bob of natural entanglement.
• The Fidelity of Teleportation (FT): A calculated value that describes the individuals’ current reinstantiation prospects for your next life.
• The Monogamy of Entanglement: The property of the QE connection that enforces a singleton instance of individuality and the role of death.

The hypothesis in summary:

The most fundamental element of life is a molecule called the Entanglement Molecule (EM). This molecule composed of normal baryonic matter manifests the unique property of prolifically establishing a natural teleportation channel, which is a shared quantum coherent state, a quantum entanglement connection (QE), with a hypothesized form of matter called metamatter. Metamatter is composed of an undiscovered type of particle that necessarily resides entirely beyond this space-time, in Hilbert-space or the metaverse if you will. Metamatter is as essential to life as dark matter is to galaxy formation. Entanglement molecules in this universe are at all times entangled to particles of metamatter in Hilbert-space. It is their natural state to do so. Metamatter, as is possible with any natural entity having only subtle degrees-of-freedom within this space-time, is not subject to locality or relativistic constraints and so, via this QE connection, is non-classically, instantaneously accessible to entanglement molecules (EM) everywhere in this universe.

These entanglement molecules and metamatter are the Alice and Bob endpoints of each isolated, naturally occurring, QE connection established within every living cell that has ever existed. An entanglement molecule once arranged from its constituent atoms, not unlike the molecules in the ferrite magnet in a transistor radio, is instantly sensitive to available, uninstantiated QE degrees of freedom (DOF) of the QE spectrum, or quantum entanglement frequencies (QEF). It is the QEF that define the unique natural teleportation channel upon which to entangle available metamatter. Such isolated pairings existed on Earth for eons, and in this universe, for even longer before the naturally occurring circumstances arose, on Earth, and perhaps elsewhere, to provide a sphere of molecules that could be described as an early cell wall. Not all entanglement molecules were likely to encounter a cell wall, but those that did, enclosed by this barrier, obtained the benefit of an extra level of protection. This enclosure allowed them to develop beyond the typical. This basic entanglement relationship is the most fundamental manifestation of life. It establishes the position of view (POV). Over time other types of molecules joined with these proto-cells sometimes to their mutual benefit, sometimes not. Those that added no benefit or diminished the proto-cells survival prospects would not survive.

The QE connection gave surviving proto-cells something very special. It gave the otherwise inanimate molecular components on the inside of this early cell a form of intra-cellular communication. That is, the ability to interact at a distance, but more critically at that point, the QE connection gave the proto-cell the capacity to share or imprint internal cellular state information upon its entangled metamatter. Metamatter because of its extra-dimensional, non-locality and relativistically unconstrained nature essentially acts as a kind of cloud-storage for information accessible instantaneously from any location in this universe, and in any other as well. This universal cloud storage repository of information is the critical factor required to get evolution started. This natural cosmic background Bose condensate (CBBC) is what makes being possible anywhere in this universe. At that point, evolution existed only via random environmental contact between proto-cells with other structures in the primordial environment of early Earth.

Thus, the cell became natures' biological entanglement circuit. Each such entanglement pairing constitutes an instantiation of life, whether on Earth, elsewhere in this universe, or anywhere in existence. Consequently, life could now be hosted by any viable formation of cell(s) that may emerge anywhere in existence. Ones' instantiation is established at one specific QEF, a unique value of the degrees of freedom among the infinity of possible values on the quantum entanglement spectrum. A QEF that is unique in all existence to each individual and to no other, but only while that QE connection, ones' natural teleportation (LINE) channel, persists. These yet to be determined DOF's, perhaps frequency and others, on the QE spectrum, is the singular property in nature that defines each living individual. All other components of the instantiation process may change or be exchanged, but it is your QEF that positions you as the central and only target of your instantiation, of your life, and not someone else's. Change or retune ones' QEF enough, and you change the being, the individual. You are your QEF you are not your cells or your metamatter.

It is very likely that the QE spectrum predated even the big bang. Your QEF is the immutable, the classically indestructible you. When entanglement molecules, contained within viable hosts such as the cell, located on any viable planet, orbiting any viable star, anywhere in existence, entangles metamatter at your QEF, that is where you will instantiate. That is where you will be. A place such as that is where you are right now. A place such as that is where you are likely to have been many times before your current instantiation. Places such as that are where you will inevitably reinstantiate many more times in your future. This is instantiation this is life. You and I, and your pet otter, every insect, every cell and every organization of cells, all life anywhere in existence instantiates by this mechanism. While each cell entangles at a unique QEF, a few specialized cells in complex organisms, called entanglement cells (EC), have evolved to heterodyne, or combine their own unique QEF's. This combination of distinct LINE channels entangle metamatter at yet a different unique QEF, called a composite or emerged QEF, thus instantiating the emerged individual, you.

This composite degree of freedom called the QEF together with the metamatter it entangles is called the lifeID. No memories or behavior of the host body is carried or transferred by the lifeID. In nature, such properties are electromagnetic manifestations of the host species or vessel only. The closest cultural meme to the lifeID come via religions throughout human history having referred to this, using one word or another, as the soul. Once any QE connection is terminated, by sufficiently disrupting the cellular component (inducing death of the host vessel), the previously entangled metamatter becomes available for entanglement by other cells. However, this particular metamatter has been imprinted to some extent by its previous entanglement. Each generation of entanglement, each instantiation, each life, imprints information from both the host and QEF, to its entangled metamatter. The degree of this imprinting is yet to be determined.

This time-dependent, perishable imprinting of cellular state in metamatter becomes available to future cells that entangle this metamatter while simultaneously limiting its entanglement opportunities to cells of matching state. The passage of time decays the imprint left on metamatter causing a return to a state best described as stem-metamatter (to be discussed later in this volume). This transfer of cellular state information may impact cellular behavior and development and to the extent that this imprinted information manifests an advantage for the cell, may provide a survival benefit. This is the evolutionary mechanism used by early life that predated the development of the DNA and RNA molecules. With QE communication, ergo life, the proto-cell became the laboratory of evolutionary innovation we see today from which emerged a great many useful cellular structures and processes, but most pivotally, a clear benefit to augment the cloud storage mechanism of metamatter with a more local, more expandable and flexible information storage mechanism which became RNA and eventually DNA. This was the birth of the modern living cell. Much is yet to be learned but the implications of this process are vast and pervasive.

The degree to which metamatter is imprinted by its entangled host and unique QEF will determine, after deinstantiation (death), the likelihood that your imprinted metamatter will, for a time, reject entanglement opportunities from dissimilar host cells (of even your same or similar species), in favor of entanglement with cells that contain your familial DNA. These are cells that are more compatible with its imprinting. Thereby increasing the probability of reinstantiating you into your former family line, or if less finely imprinted, to any random line in your previous species or if less finely tuned still, to another species entirely. Longevity may be a factor in this regard. Also when we discover the entanglement molecule in nature or within the cell, just as we eventually discovered the DNA molecule in the cell decades after Darwin presented his theory of evolution by natural selection, likewise this may allow us to develop technologies capable of detecting and tracking each individuals unique QEF in this life or across multiple instantiations. This will change the world, at the very least it will change the way we write our wills. As for practical implementations, discovering and using metamatter could change everything. Metamatter satellites would be very different yet similar to regular orbital satellites, even though they will reside outside of this space-time they'll permit instantaneous communication with any point in the cosmos. This will forever alter the human relationship not just to each other, but to all living creatures biological or otherwise. Also for the first time in human history, we could begin to take practical actions in life that would affect the individuals' reinstantiation prospects into ones' next life, thereby tailoring your next instantiation ahead of time, minus the mysticism and ideology.

## Contents

In a 1959 paper, Cornell University physicists Philip Morrison and Giuseppe Cocconi had speculated that any extraterrestrial civilization attempting to communicate via radio signals might do so using a frequency of 1420 megahertz ( 21 centimeters), which is naturally emitted by hydrogen, the most common element in the universe and therefore likely familiar to all technologically advanced civilizations. [5]

In 1973, after completing an extensive survey of extragalactic radio sources, Ohio State University assigned the now-defunct Ohio State University Radio Observatory (nicknamed "Big Ear") to the scientific search for extraterrestrial intelligence (SETI), in the longest-running program of this kind in history. [6] The radio telescope was located near the Perkins Observatory on the campus of Ohio Wesleyan University in Delaware, Ohio. [7] [8]

By 1977, Ehman was working at the SETI project as a volunteer his job involved analyzing by hand large amounts of data processed by an IBM 1130 computer and recorded on line printer paper. While perusing data collected on August 15 at 22:16 EDT (02:16 UTC), he spotted a series of values of signal intensity and frequency that left him and his colleagues astonished. [5] The event was later documented in technical detail by the observatory's director. [9]

The string 6EQUJ5, commonly misinterpreted as a message encoded in the radio signal, represents in fact the signal's intensity variation over time, expressed in the particular measuring system adopted for the experiment. The signal itself appeared to be an unmodulated continuous wave, although any modulation with a period of less than 10 seconds or longer than 72 seconds would not have been detectable. [10] [11]

### Intensity Edit

The signal intensity was measured as signal-to-noise ratio, with the noise (or baseline) averaged over the previous few minutes. The signal was sampled for 10 seconds and then processed by the computer, which took 2 seconds. Therefore, every 12 seconds the result for each frequency channel was output on the printout as a single alphanumeric character, representing the 10-second average intensity, minus the baseline, expressed as a dimensionless multiple of the signal's standard deviation. [12]

In this particular intensity scale, a space character denoted an intensity between 0 and 1, that is between baseline and one standard deviation above it. The numbers 1 to 9 denoted the correspondingly numbered intensities (from 1 to 9) intensities of 10 and above were indicated by a letter: "A" corresponded to intensities between 10 and 11, "B" to 11 to 12, and so on. The Wow! signal's highest measured value was "U" (an intensity between 30 and 31), that is thirty standard deviations above background noise. [2] [12]

### Frequency Edit

John Kraus, the director of the observatory, gave a value of 1420.3556 MHz in a 1994 summary written for Carl Sagan. [9] However, Ehman in 1998 gave a value of 1420.4556 ± 0.005 MHz . [13] This is ( 50 ± 5 kHz ) above the hydrogen line value (with no red- or blue-shift) of 1420.4058 MHz . If due to blue-shift, it would correspond to the source moving about 10 km/s (6.2 mi/s) towards Earth.

An explanation of the difference between Ehman's value and Kraus's can be found in Ehman's paper. An oscillator, which became the first local oscillator, was ordered for the frequency of 1450.4056 MHz . However, the university's purchasing department made a typographical error in the order and wrote 1450.5056 MHz (i.e., 0.1 MHz higher than desired). The software used in the experiment was then written to adjust for this error. When Ehman computed the frequency of the Wow! signal, he took this error into account. [ citation needed ]

### Bandwidth Edit

The Wow! signal was a narrowband emission: its bandwidth was less than 10 kHz . The Big Ear telescope was equipped with a receiver capable of measuring fifty 10 kHz -wide channels. The output from each channel was represented in the computer printout as a column of alphanumeric intensity values. The Wow! signal is essentially confined to one column. [13]

### Time variation Edit

At the time of the observation, the Big Ear radio telescope was only adjustable for altitude (or height above the horizon), and relied on the rotation of the Earth to scan across the sky. Given the speed of Earth's rotation and the spatial width of the telescope's observation window, the Big Ear could observe any given point for just 72 seconds. [3] A continuous extraterrestrial signal, therefore, would be expected to register for exactly 72 seconds, and the recorded intensity of such signal would display a gradual increase for the first 36 seconds—peaking at the center of the observation window—and then a gradual decrease as the telescope moved away from it. All these characteristics are present in the Wow! signal. [14] [15]

The precise location in the sky where the signal apparently originated is uncertain due to the design of the Big Ear telescope, which featured two feed horns, each receiving a beam from slightly different directions, while following Earth's rotation. The Wow! signal was detected in one beam but not in the other, and the data was processed in such a way that it is impossible to determine which of the two horns received the signal. [16] There are, therefore, two possible right ascension (RA) values for the location of the signal (expressed below in terms of the two main reference systems): [17]

B1950 equinox J2000 equinox
RA (positive horn) 19 h 22 m 24.64 s ± 5 s 19 h 25 m 31 s ± 10 s
RA (negative horn) 19 h 25 m 17.01 s ± 5 s 19 h 28 m 22 s ± 10 s

In contrast, the declination was unambiguously determined to be as follows:

B1950 equinox J2000 equinox
Declination −27°03′ ± 20′ −26°57′ ± 20′

The galactic coordinates for the positive horn are l =11.7°, b =−18.9°, and for the negative horn l =11.9°, b =−19.5°, both being therefore about 19° toward the southeast of the galactic plane, and about 24° or 25° east of the galactic centre. The region of the sky in question lies northwest of the globular cluster M55, in the constellation Sagittarius, roughly 2.5 degrees south of the fifth-magnitude star group Chi Sagittarii, and about 3.5 degrees south of the plane of the ecliptic. The closest easily visible star is Tau Sagittarii. [18]

No nearby sun-like stars were within the antenna coordinates, although in any direction the antenna pattern would encompass about six distant stars. [10]

A number of hypotheses have been advanced as to the source and nature of the Wow! signal. None of them have achieved widespread acceptance. Interstellar scintillation of a weaker continuous signal—similar in effect to atmospheric twinkling—could be an explanation, but that would not exclude the possibility of the signal being artificial in origin. The significantly more sensitive Very Large Array did not detect the signal, and the probability that a signal below the detection threshold of the Very Large Array could be detected by the Big Ear due to interstellar scintillation is low. [19] Other hypotheses include a rotating lighthouse-like source, a signal sweeping in frequency, or a one-time burst. [17]

Ehman has said: "We should have seen it again when we looked for it 50 times. Something suggests it was an Earth-sourced signal that simply got reflected off a piece of space debris." [20] He later recanted his skepticism somewhat, after further research showed an Earth-borne signal to be very unlikely, given the requirements of a space-borne reflector being bound to certain unrealistic requirements to sufficiently explain the signal. [13] Also, it is problematic to propose that the 1420 MHz signal originated from Earth since this is within a protected spectrum: a bandwidth reserved for astronomical purposes in which terrestrial transmitters are forbidden to transmit. [21] [22] In a 1997 paper, Ehman resists "drawing vast conclusions from half-vast data"—acknowledging the possibility that the source may have been military or otherwise a product of Earth-bound humans. [23]

METI president Douglas Vakoch told Die Welt that any putative SETI signal detections must be replicated for confirmation, and the lack of such replication for the Wow! signal means it has little credibility. [24]

### Discredited hypotheses Edit

In 2017, Antonio Paris, a teacher from Florida, proposed that the hydrogen cloud surrounding two comets, 266P/Christensen and 335P/Gibbs, now known to have been in the same region of the sky, could have been the source of the Wow! signal. [25] [26] [27] This hypothesis was dismissed by astronomers, including members of the original Big Ear research team, as the cited comets were not in the beam at the correct time. Furthermore, comets do not emit strongly at the frequencies involved, and there is no explanation for why a comet would be observed in one beam but not in the other. [28] [29] [30]

Several attempts were made by Ehman and other astronomers to recover and identify the signal. The signal was expected to occur three minutes apart in each of the telescope's feed horns, but that did not happen. [15] Ehman unsuccessfully searched for recurrences using Big Ear in the months after the detection. [19]

In 1987 and 1989, Robert H. Gray searched for the event using the META array at Oak Ridge Observatory, but did not detect it. [19] [31] [ page needed ] In a July 1995 test of signal detection software to be used in its upcoming Project Argus, SETI League executive director H. Paul Shuch made several drift-scan observations of the Wow! signal's coordinates with a 12-meter radio telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia, also achieving a null result.

In 1995 and 1996, Gray again searched for the signal using the Very Large Array, which is significantly more sensitive than Big Ear. [19] [31] [ page needed ] Gray and Simon Ellingsen later searched for recurrences of the event in 1999 using the 26-meter radio telescope at the University of Tasmania's Mount Pleasant Radio Observatory. [32] Six 14-hour observations were made at positions in the vicinity, but nothing like the Wow! signal was detected. [15] [31] [ page needed ]

In 2012, on the 35th anniversary of the Wow! signal, Arecibo Observatory beamed a digital stream towards Hipparcos 34511, 33277, and 43587. [33] The transmission consisted of approximately 10,000 Twitter messages solicited for the purpose by the National Geographic Channel, bearing the hashtag "#ChasingUFOs" (a promotion for one of the channel's TV series). [34] The sponsor also included a series of video vignettes featuring verbal messages from various celebrities. [35]

To increase the probability that any extraterrestrial recipients would recognize the signal as an intentional communication from another intelligent life form, Arecibo scientists attached a repeating-sequence header to each individual message, and beamed the transmission at roughly 20 times the power of the most powerful commercial radio transmitter. [34]

ISEE is a NASA/ESA cooperative program consisting of three satellites intended to study the dynamic properties of Earth's magnetosphere and the solar wind in front of the magnetosphere (study the interaction of the interplanetary medium with the earth's immediate environment and to study the magnetosphere bow shock and magnetosheath in order to derive a better model of the interaction). Specific objectives of the mission were: 1) 2) 3) 4)

&bull to investigate the solar-terrestrial relationships at the outermost boundaries of the Earth's magnetosphere

&bull to examine in detail the structure of the solar wind near the Earth and the shock wave that forms the interface between the solar wind and Earth's magnetosphere

&bull to investigate motions of and mechanisms operating in the plasma sheets

&bull to continue the investigation of cosmic rays and solar flare emissions in the interplanetary region near 1 AU.

The ISEE-1 and ISEE-3 spacecraft were the principal contributions of NASA, while ISEE-2 was built and managed by ESA. More than 100 investigators, representing most of the magnetospheric community, from 33 institutes were involved in the ISEE mission and its 32 instruments.

The three spacecraft carried a number of complementary instruments for making measurements of plasmas, energetic particles, waves, and fields. The mission thus extended the investigations of previous IMP (interplanetary Monitoring Platform) spacecraft.

Figure 1: ISEE mission poster (image credit: UCLA) 5)

The overall objectives were the observation of the near-Earth magnetosphere and its boundaries, better understanding of many phenomena, such as the Earth's bow shock, the magnetosheath and magnetopause, interactions between the tail and aurorae, and particle populations and flows in the tail.

ISEE-1 and ISEE-3 spacecraft are based on the IMP design pattern and were built by NASA as their main contribution to the IMS. The ISEE-1 spacecraft was spin-stabilized, had a mass of 340 kg (minisatellite) and a nominal power of 175 W. 6) 7) 8) 9)

The ISEE-1 mission has also the designations of ISEE-A and Explorer 56.

Figure 2: Artist's view of the ISEE-1 spacecraft in orbit (image credit: NASA)

Figure 3: Line drawing of the ISEE-1 spacecraft configuration (image credit: NASA)

The Explorer-class daughter spacecraft, ISEE-2, was part of the mother/daughter/heliocentric mission (ISEE-1, ISEE-2, ISEE-3). The mother/daughter portion of the mission consisted of two spacecraft (ISEE-1 and ISEE-2) with station-keeping capability in the same highly eccentric geocentric orbit.

The ISEE-2 minisatellite featured a spin-stabilized cylindrical bus with three deployed instrument booms. Strict measures were followed to eliminate interference from the spacecraft to some of the experiments: the entire exterior was made conductive to reduce potential difference to 1 V, the use of non-magnetic materials restricted ISEE&rsquos DC field to < 0.25 gamma at the magnetometer, and stringent limits were imposed on the electromagnetic radiation emitted by ISEE&rsquos interior. 10)

Attitude/orbit control: 20 rpm spin-stabilized about longitudinal axis, perpendicular to ecliptic plane 4 spin nozzles, 2 precession nozzles, also used for separation maneuvers from ISEE-1. Cold gas propellant: 10.7 kg, Freon-14. The attitude was determined by two Earth albedo and solar aspect sensors.

The EPS (Electric Power Subsystem) used silicon cells on cylindrical panels providing a power of > 100 W (65 W after 10 years) 27 W were required by the science payload. The EPS was supported by a NiCd battery which failed after 2 years (as predicted).

Figure 4: Photo of the ISEE-2 spacecraft in the dynamic rest chamber at ESTEC (image credit: ESA)

The ISEE-2 S/C was built by Dornier-System GmbH (prime contractor), heading the STAR consortium. The ISEE-2 spacecraft had launch mass mass of 166 kg (27.7 kg science payload) with a design life of 3 years.

RF communications: S-band data was returned at data rates of 8192 bit/s (high) or 2048 bit/s (low). The spacecraft was controlled from NASA/GSFC (Goddard Space Flight Center).

Figure 5: Mating of ESA's ISEE-2 (top) with NASA&rsquos ISEE-1 S/C at Cape Canaveral (image credit: ESA, NASA)

Launch: ISEE-1 and ISEE-2 were launched in tandem (Delta-2914 launch vehicle, joint launch provided by NASA) on October 22, 1977 from Cape Canaveral into highly elliptical geocentric orbits. The satellites passed through the magnetosphere and into the magnetosheath during each orbit providing good coverage of all the magnetosphere features over the period of a year.

Orbit: HEO (Highly Elliptical Orbit) with an apogee of 23 RE (137,806 km) and a perigee of 1.04 RE (6,600 km), inclination = 28.76º. Both spacecraft penetrated into the interplanetary medium for up to 3/4 of an orbital period depending upon the time of year.

ISEE-1 and ISEE-2 were in almost coincident orbits around the Earth with periods of approximately 57 hours (3441 minutes), and their time separation in this orbit could be altered by maneuvering ISEE-2. These two spacecraft, separated by a variable distance (50 -5000 km) and with similar instrument complements, were able to break the space-time ambiguity inevitably associated with measurements by a single spacecraft on thin boundaries which may be in motion, such as the bow shock and the magnetopause. 11)

Status of the missions ISEE-1 and ISEE-2:

&bull Both spacecraft reentered the Earth's atmosphere on September 26, 1987 - completing 1517 orbits of the Earth (nearly 10 years of operational life was provided).

&bull ISEE-1 operated in a somewhat degraded mode due to the loss of one experiment and partial loss of four others of the total complement of 13 experiments. The battery failed due to normal wear-out after 4 1/2 years of service however, this did not curtail operations due to the spacecraft being in a full-sunlight orbit.

&bull ISEE-2: No units failed, apart from the expected loss of its battery.

&bull Following the re-entry of ISEE-1 and -2 in 1987, a special effort was undertaken to archive at the NSSDC high-quality, high time resolution data about particles, fields and waves for specific time periods deemed to be of interest to the scientific community. For ISEE-1, these special archival periods are:

1) the early years of the mission (Aug. 12, 1978 - Feb. 17, 1980) 12)

2) the period when ISEE-3 was in Earth's magnetotail (Oct. 15, 1982 - Dec. 25, 1983)

3) the "PROMIS" campaign period (March 29, 1986 - June 16, 1986).

Sensor complement of ISEE-1 and ISEE-2:

A special issue on instrumentation for the International Sun-Earth Explorer Spacecraft was provided in IEEE Transactions on Geoscience Electronics, Vol. GE-16, July 1978.

ANM/AND (Electrons & Protons)

PI: K. A. Anderson, UCB, the instrument is flown on ISEE-1 and -2. Objectives: Study of the varies energetic particle phenomena found in the Earth's magnetosphere, magnetopause, magnetosheath, bow shock, and upstream medium. Measurement over a wide range of energies, from

1.5 to 300 keV for both electrons and protons.

The instrument was developed at UCB and consists of a pair of surface barrier semiconductor detector telescopes (one with a foil and one without a foil) and four fixed energy electric field particle analyzers. The analyzers are used to measure electrons and protons separately at 2 and 6 thousand electron volts.

LEPEDEA (Low-Energy Proton and Electron Differential Energy Analyzer):

PI: Louis A. Frank, University of Iowa. The instrument is also known by the designation FRM/FRD and is flown on ISEE-1 and ISEE-2. Objective: study of directional intensities of positive ions and electrons over a large solid angle. Energy range: 1 eV&le E/Q &le 50 keV in 63 bands with 17% resolution. 13)

The instrument is a quadrispherical low-energy proton and electron differential energy analyzer (LEPEDEA), employing seven continuous channel electron multipliers in each of its two (one for protons and one for electrons). All but 2% of the 4&pi sr solid angle was covered for particle velocity vectors. A GM tube was also included, with a conical field of view of 40° full-angle, perpendicular to the spin axis. This detector was sensitive to electrons with E > 45 keV, and to protons with E > 600 keV. Instrument mass = 5 kg, power = 5 W.

RUM/RUD (Fluxgate Magnetometer Experiment):

PI: C. T. Russell, UCLA. The RUM/RUD fluxgate magnetometers were flown on ISEE-1 and ISEE-2. The overall objective was to obtain a quantitative understanding of the dynamic plasma and field environment of the Earth. 14)

Three NOL (Naval Ordnance Laboratory) ring core sensors in anorthogonal triad are enclosed in a flipper mechanism at the end of the magnetometer boom, 3 m from the skin of the spacecraft on ISEE l, and 2 m on ISEE 2. The flipper mechanism is actuated by heating a bimetallic strip which rotates the sensor from one stable spring-held position through 90º to a second position. During a "flip left" operation sensor which is initially anti parallel to the spin axis in the flip position, is rotated into the spin plane to look in the direction opposite spacecraft rotation. Sensor 3 is rotated from the spin plane looking in the direction of spacecraft rotation to a direction anti parallel to the spacecraft spin axis. A flip takes about 4 min at room temperature in vacuum, and requires about 5 W.

Mass of sensor assembly, electronics

3.9 W (normal operations)
7.8 W (during flip operations)

21 cm x 12 cm x 15 cm (electronics)
11 cm x 9 cm (diameter) of sensors

Table 1: Instrument characteristics

Figure 6: RUM/RUD sensor configuration (image credit: UCLA)

Both the ISEE l and 2 magnetometers were turned on a few hours after launch and have operated continuously since that time except for brief periods during interference testing. The only operational anomalies have been a couple of status changes of the ISEE 2 instrument which were not commanded from the ground. These both occurred during the first two weeks and have not reoccurred. The flippers have been exercised every five days on both spacecraft for a total of over 50 flips to date with no evidence of aging. 15)

The instrument has two commandable ranges of ±256 &gamma and±8192 &gamma with an accuracy of 0.025%.

FPE (Fast Plasma Experiment):

PIs: S. J. Bame, Los Alamos Scientific Lab, G. Paschmann, MPI Garching. Identical fast plasma experiment (FPE) systems were placed on the ISEE-1 and ISEE-2 spacecraft. Three electrostatic analyzers (with 90º spherical section) provide electron and proton measurements. Each instrument uses a divided secondary emitter system to intercept the analyzed particles. ISEE-1 carries also a solar wind experiment (SWE) to measure solar wind ions with high resolution. The SWE is composed of two 150Â spherical section analyzers using the same set of plates. The two acceptance fans are tilted with respect to each other so that 3D characteristics of the ion distributions can be derived. 16)

WIM/KED (Medium Energy Particles Experiment):

PI: D. J. Williams, JHU/APL, Laurel, MD. Objective: Study and identify the physical mechanisms of medium energy particles associated with acceleration, source and loss processes, and boundary and interface phenomena throughout the orbits of ISEE-1 and -2.The instrument has also the designation MEPE (Medium Energy Particles Experiment) 17)

The experiment consists of the WIM instrument (Wide Angle Particle Spectrometer and a Heavy Ion Telescope) on ISEE-1 and the KED instrument (five sensor systems mounted at various angular positions with respect to the S/C spin axis) on ISEE-2.

- Protons: 20 keV - 2 MeV in 8 channels, in 16 channels on ISEE-1

- Electrons: 20 keV - 1.2 MeV in 8 channels, in 16 channels on ISEE-1

- Protons: 20 keV - 2 MeV in 12 channels on ISEE-2

- Electrons: 20 keV - 300 keV (to 1.2 MeV for 90º unit) on ISEE-2

GUM/GUD (Plasma Wave Investigation):

PI: D. A. Gurnett, University of Iowa. GUM/GUD is flown on ISEE-1 and ISEE-2. Objective: Study of wave/particle interaction in the Earth's magnetosphere and in the solar wind. The instrument on ISEE-1 uses three electric dipole antennas with lengths of 215 m, 73.5 m, and 0.6 m for the electric field measurements, and a triaxial search coil antenna for magnetic field measurements.

The ISEE-2 instrument uses two electric dipoles with lengths of 30 m and 0.6 m, and a single-axis search coil antenna for magnetic field measurements. The ISEE-2 plasma wave instrumentation consists of a 16 channel spectrum analyzer covering the frequency range from 5.62 Hz to 31.1 kHz and a wide-band waveform receiver with the capability of making waveform measurements in selected frequency ranges up to 2.0 MHz. 18) 19)

10 - 100 kHz (3 axis, 16 channels)

10 Hz - 10 kHz (3 axis, 12 channels)

10 kHz - 200 kHz (128 steps), analysis of the electric field signals

Table 2: GUM/GUD parameters

HEM (VLF Wave Propagation Experiment)

PI: R. A. Helliwell, Stanford University the instrument is flown on ISEE-1. Objective: Study of VLF-wave-particle interactions in the magnetosphere (note: VLF = Very Low Frequency in the 10 - 30 kHz range). A second goal is the determination of the effects upon energetic particles in the magnetosphere of electrical power transmission line radiation. 20)

The instrument setup consists of three separate elements:

- a broadband VLF transmitter located at Siple station in the Antarctic

- ground stations in the Antarctic and Canada

During the IMS (International Magnetospheric Study), the ISEE-1 spacecraft has been an important component of the VLF wave-injection experiments for studying interactions between coherent VLF waves and energetic particles. The coherent waves are injected into the magnetosphere by ground-based transmitters such as the Siple Station, Antarctica, and those of the Omega navigation network.

EGD (Solar Wind Ion Experiment):

PIs: E. Egidi, G. Moreno, CNR Frascati, Italy the instrument is flown on ISEE-2, it has also the designation of SWE (Solar Wind Ion Experiment). Objective: Study of the transient phenomena in the solar wind to obtain a spatial gradients of the interplanetary plasma. The instrument measures the flow directions and energy spectra of the positive ions in the solar wind. Two modes of operation are provided, one concentrates on high angular resolution and the other on high energy resolution. The main region of interest for this instrument is outward from and including the magnetopause.

The instrument is based on two identical hemispherical electrostatic energy selectors for the measurement of positive ions in two different energy windows.

HPM (DC Electric Field Experiment):

PI: J. P. Heppner, GSFC the instrument is flown on ISEE-1. Objective: Study of the transfer mechanisms (mass, momentum, and energy at the magnetopause), in particular the spatial extent and variability of the zone of strong electric fields, or fast convection in adjacent magnetospheric regions.
Instrument: 8 channel spectrum analyzer. Measurement ranges: 0.1 Hz - 3200 Hz in 9 steps.

The double probe, floating potential instrumentation on ISEE-1 is producing reliable direct measurements of the ambient DC electric field at the bow shock, at the magnetopause, and throughout the magnetosheath, tail plasma sheet and plasmasphere. In the solar wind and in middle latitude regions of the magnetosphere spacecraft sheath fields obscure the ambient field under low plasma flux conditions such that valid measurements are confined to periods of moderately intense flux. Initial results show: 21)

&bull a) that the DC electric field is enhanced by roughly a factor of two in a narrow region at the front, increasing B, edge of the bow shock

&bull b) that scale lengths for large changes in E at the sub-solar magnetopause are considerably shorter than scale lengths associated with the magnetic structure of the magnetopause

&bull c) that the transverse distribution of B-aligned E-fields between the outer magnetosphere and ionospheric levels must be highly complex to account for the random turbulent appearance of the magnetospheric fields and the lack of corresponding time-space variations at ionospheric levels.

HOM (Low Energy Cosmic Ray Experiment):

PI: Dieter K. Hovestadt, MPI Garching, Germany. The instrument is flown on ISEE-1 and ISEE-3. Objective: Measurement of elemental abundances, charge state composition, energy spectra, and angular distributions of energetic ions in the energy range of 2 keV/charge to 80 MeV/nucleon, and of electrons between 75 - 1300 keV. The instrument consists of three sensor systems: 22)

- ULECA is an electrostatic deflection analyzer, its energy range from about 3 to 560 keV/charge

- ULEWAT is a double dE/dX versus E thin-window flow-through proportional counter/solid-state detector telescope covering the energy range from 0.2 to 80 MeV/nucleon (Fe).

- the ULEZEQ sensor consists of a combination of an electrostatic deflection analyzer and a thin-window proportional counter. The energy range is 0.4 MeV/nucleon to 6 MeV/nucleon. Objective: collection of composition data in the trapped radiation zone.

MOM (Quasi-Static Electric Field Experiment):

PI: F. S. Mozer, UCB. The instrument is flown on ISEE-1. Objectives: 23)

- study of the quasi-static electric field over a dynamic range of 0.1 - 200mV/m

- study of wave electric fields at frequencies <1000 Hz with a sensitivity < 1 µV/m (Hz) 1/2 at all frequencies

- study of plasma density and temperature

Measurements are made of the potential difference between a pair of 8 cm diameter vitereous carbon spheres which are mounted on the ends of wire booms and are separated by 73.5 m in the spin plane of the satellite.

OGM (Fast Electron Spectrometer Experiment):

PI: K. W. Ogilvie, GSFC. The instrument is flown on ISEE-1. Objective: Study of three-dimensional plasma distribution in the solar wind, magnetosheath, outer magnetosphere, and near tail regions. Instrument provides three energy ranges: 7.5-512 eV, 11-2062 eV, and 109-7285 eV. Two channel electron multipliers are used at the output of each of six cylindrical electrostatic analyzers. The total mass of two sensors and a data processing unit is 4.9 kg and the power consumption is 3.5 W. Two hundred information bits/s telemetry rate is required. 24) 25)

SHM (Ion Composition Experiment):

PI: R. D. Sharp, Lockheed, Palo Alto, CA. The instrument is flown on ISEE-1. Objective: Study of the composition of the hot magnetospheric plasma. Ion composition of the ring current, the plasma sheet, the plasmasphere, the magnetosheath, and the solar wind in order to establish the origin of the plasmas in the various regimes of the magnetosphere and to identify mass and charge dependent acceleration, transport, and loss processes. 26)

The instrument consists of two ion mass spectrometers which can be operated independently. The spectrometers point 5º above and 5º below the ISEE¿1 spin plane. Measurement ranges: 1 AMU to > 150 AMU in 64 channels at each of 32 energy channels covering the energy per charge range from 0 to

ISEE-3 / ICE (International Cometary Explorer) mission

The ISEE-3 spacecraft had two 3 m booms for the magnetometer and plasma wave sensors, and four 49 m wire antennas for radio and plasma wave studies. The drum-shaped spacecraft was spin-stabilized with a nominal spin rate of 20 rpm. A pair of sun sensors provided an attitude knowledge of

0.1º. A hydrazine propulsion system was used for attitude and &DeltaV maneuvers. There are 12 thrusters, four radial, four spin-change, two upper-axial, and two lower-axial. Eight conospherical tanks held 89 kg of hydrazine at launch, providing a total &DeltaV capacity of about 430 m/s. Since a libration-point mission had never been flown before, this large capacity provided a margin in case the actual station-keeping costs were higher than theoretical models predicted. 27) 28) 29) 30)

Spacecraft size: 1.77 m diameter, height = 1.58 m. The launch mass of the ISEE-3 spacecraft was 479 kg (including 89 kg of hydrazine), and power of 173 W.

RF communications: Communications are provided in S-band.

Figure 7: Artist's view of the ISEE-3 spacecraft in orbit (image credit: NASA)

Figure 8: Photo of the ISEE-3 spacecraft during test and integration at GSFC (image credit: NASA)

Figure 9: The ISEE-3 spacecraft in flight configuration (image credit: JHU/APL)

Launch: ISEE-3 was launched on August 12, 1978 from Cape Canaveral and subsequently inserted into a "halo orbit" about the the libration point situated about 240 earth radii (Re) upstream between the Earth and the Sun.

Orbit: ISEE-3 was first placed into a halo orbit around the Lagrangian Point L1, located

240 Earth radii, Re) sunward from the Earth. At L1 the spacecraft co-rotated with the Earth around the sun during the course of each year.

ISEE-3 used the tight control technique in an attempt to maintain its trajectory as close to a nominal halo orbit as possible. This mission, being the first to orbit a Sun-Earth libration point, had the luxury of a large supply of fuel to allow for uncertainties in the insertion to and maintenance of the new orbit. The relatively small errors encountered during insertion into the halo orbit left a large amount of fuel that could be used specifically for stationkeeping. Over the four years that ISEE-3 was established at the L1 point, 15 SK (Station Keeping) maneuvers were performed totaling 30.06 m/s at an average of 2.00 m/s per maneuver. The time between the maneuvers averaged 82 days.

The Earth-Moon-Sun system was used as a catapult to maneuver the spacecraft into its various mission phases (Figures 11 and 12).

Figure 10: Isometric view of the ISEE-3 halo orbit around the Sun-Earth L1 point (image credit: JHU/APL, Ref. 29)

ISEE-3 / ICE mission chronology and status:

&bull The original mission of ISEE-3: ISEE-3 was the first artificial object placed in a halo orbit about the Sun-Earth L1 point, proving that such a suspension between gravitational fields was possible. - Plasma passing this point arrives at the Earth approximately 1 h later where it may cause changes which can be observed by instruments on ISEE-1 and ISEE-2 (Ref. 29).

&bull In June 1982, after completing its original mission, ISEE-3 began the magnetotail and comet encounter phases of its mission. At this time, the spacecraft was renamed to ICE (International Cometary Explorer) for its 2.d mission period.

- A maneuver was conducted on June 10, 1982, to remove the spacecraft from the halo orbit around the L1 point and place it in a transfer orbit involving a series of passages between Earth and the L2 (magnetotail) Lagrangian libration point. - After several passes through the Earth's magnetotail, with gravity assists from lunar flybys in March, April, September and October of 1983, a final close lunar flyby (119.4 km above the moon's surface) on December 22, 1983, ejected the spacecraft out of the Earth-Moon system and into a heliocentric orbit ahead of the Earth, on a trajectory intercepting that of Comet Giacobini-Zinner.

- A total of fifteen propulsive maneuvers (four of which were planned) and five lunar flybys were needed to carry out the transfer from the halo orbit to an escape trajectory from the Earth-Moon system into a heliocentric orbit.

Figure 11: ISEE-3 spacecraft trajectory overview from halo orbit to geomagnetic tail

&bull The primary scientific objective of the ICE (International Cometary Explorer) mission was to study the interaction between the solar wind and a cometary atmosphere. As planned, the spacecraft traversed the plasma tail of Comet Giacobini-Zinner on September 11, 1985, and made in situ measurements of particles, fields, and waves. The represented the first ever comet encounter by a spacecraft. 31) 32)

&bull ICE also transited between the Sun and Comet Halley in late March 1986, when other spacecraft (Giotto, Planet-A, MS-T5, VEGA) were also in the vicinity of Comet Halley on their early March comet rendezvous missions. ICE became the first spacecraft to directly investigate two comets.

&bull As of January 1990, ICE was in a 355-day heliocentric orbit with an aphelion of 1.03 AU, a perihelion of 0.93 AU and an inclination of 0.1º. This will bring it back to the vicinity of the Earth-Moon system in August 2014.

&bull An extended ICE mission was approved by NASA in 1991 for the continued investigation of coronal mass ejections, continued cosmic ray studies, and coordinated observations with Ulysses.

&bull On May 5, 1997, NASA ended the ICE mission, and commanded a deactivation of the probe, with only a carrier signal left operating.

&bull In 1999, NASA made a brief contact to verify its carrier signal.

&bull On Sept. 18, 2008, NASA successfully located and reactivated ICE using the Deep Space Network. A status check revealed that all but one of its 13 experiments were still functioning, and it still has enough propellant for 150 m/s of &DeltaV. NASA scientists are considering reusing the probe to observe additional comets in 2017 or 2018. 33)

The ISEE-3 mission proved the utility of an orbit about the Sun-Earth L1 point for space physics (especially upstream solar wind) measurements. Orbits about the Sun-Earth L2 point could be used to measure the geomagnetic tail, but already ISEE-3 showed that double-lunar swingby orbits were better for that purpose. However, in the late 1980&rsquos, many mission planners learned the value of orbits near the Sun-Earth L2 point for astronomical observations. A satellite there would have an unobstructed view of well over half of the sky with no interference from either the Sun, the Earth, or the Moon, all of which would remain within about 15º of the direction to the Sun. Especially observations in the infrared would benefit since the geometry and construction of the spacecraft would allow passive cooling to very low temperatures the solar cell panels pointing towards the Sun could shade the scientific instruments. A small-amplitude Lissajous orbit about L2 would be better than the large-amplitude one that would be required by a periodic halo orbit (Ref. 29).

&bull NASA scientists, including a team lead by Robert Farquhar, are considering several options for the future of ICE, including redirecting it towards additional comet encounters in 2017 or 2018. Still other missions are possible for this robust, reused spacecraft before it once again drifts back into interplanetary space and subsequently returns to the vicinity of the Earth sometime in the 2040s (Ref. 30).

Figure 12: Artist's view of the various trajectory phases of the ISEE-3 (yellow, red) and ICE missions (green, blue), image credit: NASA 34)

Sensor complement of ISEE-3:

The ISEE-3 payload consisted of 13 instruments provided by both US and European groups.

ANH (X-Rays and Electrons Instrument):

PI: K. A. Anderson, UCB (University of California, Berkeley). This instrument represented the first successful flight of a high purity germanium detector on a satellite. It provided an order of magnitude improvement in the measurement of spectral properties of gamma-ray bursts than any previously flown detector. 35)

- Measurement of solar flare X-ray bursts and transient cosmic gamma-ray bursts. A proportional counter and scintillation detector cover the energy range from 5 - 228 keV.

- Measurement of electrons from

1MeV with high energy and angular resolution. (Study of interplanetary and solar electrons in the energy range between the solar wind and galactic cosmic rays).

BAH (Solar Wind Plasma Experiment):

PI: S. J. Bame, Los Alamos Scientific Lab. Two electrostatic analyzers ( with 135º spherical section) provide electron and ion measurements. Each instrument uses a divided secondary emitter system to intercept the analyzed particles.

HKH (High Energy Cosmic Ray Experiment):

PI: H. H. Heckman, UCB. Multidetector cosmic ray experiment to identify the charge and mass of incident cosmic ray nuclei from H through Fe species (over energy ranges from 20 to 500 MeV/nucleon).

HOH (Low Energy Cosmic Ray Experiment):

PI: D. Hovestadt, MPI Garching, Germany. Objective: Study of nuclear and ionic composition of solar, interplanetary, and magnetospheric accelerated and trapped particles. Measurement of elemental abundances, charge state composition, energy spectra, and angular distributions of energetic ions in the energy range of 2 keV/charge to 80 MeV/nucleon, and of electrons between 75 - 1300 keV.

DFH (Low Energy Proton Experiment):

PI: R. J. Hynds, Imperial College, London. Objective: Study of low energy protons from a solar flare to relate particle fluxes measured near the Earth to fluxes in the upper corona (investigation of the gross scale of coronal control). DFH experiment to measure low energy protons in the energy range from 35-1600 keV. The instrument was designed and built by Imperial College, the Space Science Department of ESA and the Space Research Institute of Utrecht. 36) 37) 38)

Note: The DFH is also known under the designation of EPAS (Energetic Particle Anisotropy Spectrometer). EPAS consists of a system of three identical semi-conductor particle telescopes mounted on the body of the spacecraft and inclined at 30º (Telescope 1), 60º (Telescope 2) and 135º (Telescope 3) to the spacecraft spin axis which is maintained perpendicular to the ecliptic plane (to within 1º). The spacecraft spin period is 3.04 s. Each telescope has a conical field of view of 16º semi-cone angle and a geometrical factor of 0.05 cm 2 sr. 39)

The telescopes detect ions (electrons being excluded by "broom" magnets) and measure their total kinetic energy (but not their mass) by each using a stack of two silicon surface barrier detectors. The front detector (A) is 33 µm thick while the second (B) is 150 µm thick. Particle counts are defined by anticoincidence (A not B), i.e. the ions deposit all their energy in the A detector and do not intercept and trigger the B detector. The amplitude of the signal produced in the A detector is dependent on the energy deposited in the silicon, and hence on the incident ion energy. This signal is fed to pulse height discriminators which define 8 primary energy channels, E1 to E8.

In addition, a further channel, E0, monitors the instrument thermal noise but can register ions above the background if the ion flux is sufficiently high. No background noise counting-rate correction is required in any of the primary energy channels, i.e. the counts recorded are actual particle counts. The channel energy ranges depend slightly on ion mass. This is due principally to mass-dependent energy losses when the ions pass through a thin gold electrode on the front surface of the A detector.

Figure 13: The Low Energy Particle Telescope System on ISEE-3 (image credit: Imperial College, London)

MEH (Cosmic Ray Electrons and Nuclei):

PI: P. Meyer, University of Chicago). Objective: Study of the long and short-term variability of cosmic ray electrons and nuclei. Measurement of the energy spectrum of cosmic electrons in the range of 5-400 MeV. In addition, determination of the energy spectra and relative abundances of nuclei from protons in the iron group (energies from 30 MeV/n to 15 GeV/n). 40)

OGH (Plasma Composition Experiment):

PI: Keith W. Ogilvie, NASA/GSFC. Objective: Study of the dynamics and energetics of the solar wind acceleration region. Ion mass spectrometer for the measurement of ionic composition of the solar wind.

SCH (Plasma Wave Instrument):

PI: F. L. Scarf, TRW, Los Angeles. Objective: Study of interplanetary wave-particle interactions in the spectral range from 1 Hz to 100 kHz. Measurements of magnetic field and electric field components on long booms (90 m tip to tip). Magnetic field levels: 8 channels, 60 dB range, 20 Hz - 1 kHz. Electric field levels: 16 channels, 80 dB range, 20 Hz - 100 kHz.

PI: J. L. Steinberg, Meudon Observatory, Paris. Objectives: a) monitoring the solar wind flow and perturbations of the magnetic field in conjunction with simultaneous measurements on ISEE-1 and -2 (bow shock, magnetopause, neutral sheet), and b) propagation studies of particle fluxes and shock waves in the solar wind (large scale structure of the magnetic field).
Measurement of the interplanetary scintillation of natural radio sources using two dipole antennas, one in the spin plane (90 m tip to tip) and one along the spin axis (15 m tip-to-tip). Each of these antennas drives two radiometers (10 kHz bandwidth and 3 kHz bandwidth).

SMH (Helium Vector Magnetometer):

PI: E. J. Smith, JPL. Objective: Continuous observation of the interplanetary magnetic field near 1 AU (structure, direction, polarity north-south component, magnitude, dynamic phenomena). Boom-mounted magnetometer sensor (3 m) with the following characteristics: 41)

- frequency response: 0 - 3 Hz within three bands (0.1 - 1, 1 - 3, and 3 - 10 Hz) to measure fluctuations parallel to the S/C spin axis.

STH (Heavy Isotope Spectrometer Telescope, HIST):

PI: E. C. Stone, CIT (California Institute of Technology). Objective: measurement of the isotopic composition and energy of solar, galactic, and interplanetary cosmic ray nuclei for the elements Li through Ni in the energy range from

5 to 250 MeV/nucleon. Charge, isotope, and energy range: Z 3 - 28 (Li to Ni) A 6 - 64 ( 6 Li to 64 Ni). Mass resolution: Li 0.065 - 0.83 proton masses Fe 0.18 - -0.22 proton masses. 42) 43)

The HIST instrument consists of a telescope of solid-state detectors and associated signal-processing electronics. The telescope consists of 11 silicon solid-state detectors of graduated thicknesses. The front two detectors (M1 and M2) are two-dimensional position-sensitive detectors which measure the trajectories of individual particles entering the telescope. Use of this trajectory information results in a significant improvement in the mass resolution as compared with telescopes with similar opening angles that do not have trajectory-measuring capability.

Figure 14: Photo of the solar isotope spectrometer (image credit: NASA/JPL)

TYH (Medium Energy Cosmic Ray Experiment):

PI: Tycho T. von Rosenvinge, NASA/GSFC. Objective: measurement of the charge composition of nuclear energetic particles over the energy ranges from

1 - 500 MeV/ nucleon, and charges from Z=1 to Z=28.

The experiment consists of two telescopes. The combined charge, mass, and energy intervals covered by these two telescopes are as follows: 44)

- Nuclei charge of energy spectra: Z = 1-30, energy range 1-500 MeV/nucleon

- Isotopes: Z=1, &DeltaM=1, from 4-70 MeV/n Z=2, &DeltaM=1 from 1-70 MeV/n Z=3-7, &DeltaM=1 from 30-140 MeV/n

- Anisotropies: Z=1-26 (1-150 MeV/n for Z=1,2) Electrons: 2-10 MeV.

3) K. W. Ogilvie, T. von Rosenvinge, A. C. Durney, &ldquoInternational Sun Earth Explorer - A three spacecraft program,&rdquo Science, 198, No. 4313, pp. 131-138, Oct. 1977, DOI: 10.1126/science.198.4313.131

4) K. W. Ogilvie, et al., &ldquoInternational Sun-Earth Explorer: A Three-Spacecraft Program,&rdquo Science, Vol. 198, No. 4313, October 14, 1977, pp. 131-138

8) A. C. Durney, K. W. Ogilvie, &ldquoIntroduction to the ISEE Mission (article published in the special issues: Advances in Magnetospheric Physics with GEOS-1 and ISEE - 1 and 2.),&rdquo Space Science Reviews, Volume 22, Issue 6, Dec. 1978, p. 679, DOI: 10.1007/BF00212618

12) B. M. Walsh, T. A. Fritz, N. M. Lender, J. Chen, K. E. Whitaker, &ldquoEnergetic particles observed by ISEE-1 and ISEE-2 in a cusp diamagnetic cavity on 29 September 1978,&rdquo Annales Geophysicae, Vol. 25, 2007, pp.2633-2640, URL: http://www.ann-geophys.net/25/2633/2007/angeo-25-2633-2007.pdf

13) &ldquoInternational Sun-Earth Explorer (ISEE) 1 and 2 LEPEDEA Observations,&rdquo URL: http://www-pi.physics.uiowa.edu/www/lep/

14) C. T. Russell, &ldquoThe ISEE 1 and 2 Fluxgate Magnetometers,&rdquo Transactions on Geoscience Electronics, Vol. GE-16, No 3, July 1978, also in URL: http://www-ssc.igpp.ucla.edu/personnel/russell/papers/ISEE_fluxgate/

15) X. M. Zhu, M. G. Kivelson, R. J. Walker, C. T. Russell, M. F. Thomsen, D. J. McComas, &ldquoAn ISEE-1/2 spacecraft study of an unusual flux transfer event,&rdquo Advances in Space Research, Vol. 8, No 9-10, pp. (9)259-(9)262, 1988, URL: http://www-ssc.igpp.ucla.edu/personnel/russell/papers/isee1-2_event.pdf

16) S. J. Bame, J. R. Asbridge, H. E. Felthauser, J. P. Glore, G.. Paschmann, P. Hemmerich, K. Lehmann, H. Rosenbauer, &ldquoSEE-1 and ISEE-2 Fast Plasma Experiment and the ISEE-1 Solar Wind Experiment,&rdquo Transactions on Geoscience Electronics, Vol. 16, Issue 3, July 1978, pp. 216-220

17) D. J. Williams, E.. Keppler, T. A. Fritz, B. Wilken, G. Wibberenz, &ldquoThe ISEE 1 and 2 Medium Energy Particles Experiment,&rdquo IEEE Transactions on Geoscience Electronics, Vol. GE-16, No 3, pp 270-280, July 1978.

18) D. A. Gurnett, F. L. Scarf, R. W. Fredricks, E. J. Smith, IEEE Transactions on Geoscience Electronics, Vol. GE-16, Issue 3, July 1978 pp.:225 - 230

19) D. A. Gurnett, R. R. Anderson, F. L. Scarf, R. W. Fredricks, E. J. Smith, &ldquoInitial results from the ISEE-1 and -2 plasma wave investigation,&rdquo Space Science Reviews, Volume 23, Number 1, March 1979, pp. 103-122

20) T. F. Bell, U. S. Inan, R. A. Helliwell, &ldquoISEE-1 Satellite Observations of VLF Signals and associated triggered emissions from the Siple Station Transmitter,&rdquo NIPR (National Institute of Polar Research), 1980, URL: http://vlf.stanford.edu/sites/default/files/publications/236.pdf

21) J. P. Heppner, N. C. Maynard, T. L. Aggson, &ldquoEarly results from ISEE-1 electric field measurements,&rdquo Space Science Reviews, Volume 22, No 6 , Dec. 1978, pp.777-789

22) D. Hovestadt, G. Gloeckler, C. Y. Fan, L. A. Fisk, F. M. Ipavich, B. Klecker, Oapos, J. J. Gallagher, M. Scholer, H. Arbinger, J. Cain, H. Hofner, E. Kunneth, P. Laeverenz, E. Tums, &ldquoThe Nuclear and Ionic Charge Distribution Particle Experiments on the ISEE-1 and ISEE-C Spacecraft,&rdquo IEEE Transactions on Geoscience Electronics, Vol. 16, Issue 3, July 1978, pp. 166-175

23) F. S. Mozer, R. B. Torbert, U. V. Fahleson, C. G. Falthammar, A. Gonfalone,A. Pedersen, &ldquoMeasurements of Quasi-Static and Low-Frequency Electric Fields with Spherical Double Probes on the ISEE-1 Spacecraft,&rdquo IEEE Transactions on Geoscience Electronics, Vol. 16, Issue 3, July 1978, pp. 258-261

24) K. W. Ogilvie, J. D. Scudder, H. Doong, &ldquoThe Electron Spectrometer Experiment on ISEE-1,&rdquo IEEE Transaction on Geoscience Electronics, Vol. 16, Issue 3, July 1978, pp. 261-265

25) K. W. Ogilvie, J. D. Scudder, &ldquoFirst results from the six-axis electron spectrometer on ISEE-1,&rdquo Space Science Reviews, Vol. 23, No 1, March 1979, pp. 123-133

26) M. A. Coplan, K. W. Ogilvie, P. A. Bochsler, J. Geiss, &ldquoIon Composition Experiment,&rdquo IEEE Transaction on Geoscience Electronics, Vol. 16, Issue 3, July 1978, pp. 185-191

28) Robert W. Farquhar, &ldquoThe Flight of ISEE-3/ICE: Origins, Mission History, and a Legacy,&rdquo The Journal of the Astronautical Sciences, ISSN 0021-9142, Vol. 49, No 1, January-March 2001, pp. 23-73 and previously presented at the AIAA/AAS Astrodynamics Conference, Boston, Massachusetts, August 11, 1998 (AIAA paper 98-4464).

30) Andrew J. LePage, &ldquoThe ICE mission: the first cometary encounter,&rdquo The Space Review, Sept. 20, 2010, URL: http://www.thespacereview.com/article/1697/1

31) Robert Farquhar, Daniel Muhonen, Leonard C. Church, &ldquoTrajectories and orbital maneuvers for the ISEE-3/ICE comet mission ,&rdquo American Institute of Aeronautics and Astronautics and American Astronautical Society, Astrodynamics Conference, Seattle, WA, Aug 20-22,1984., paper: AIAA-1984-1976

35) K. A. Anderson, S. R. Kane, J. H. Primbsch, R. H. Weitzmann, W. D. Evans, R. W. Klebesadel, W. P. Aiello, &ldquoX-ray spectrometer experiment aboard the ISEE-C (heliocentric) spacecraft,&rdquo IEEE Transactions on Geoscience Electronics, Vol. GE-16, Issue 3, July 1978, p. 157

36) A. Balogh, R. J. Hynds, J. J. van Rooijen, G. A. Stevens, T. R. Sanderson, K. P. Wenzel, &ldquoEnergetic Particles in the Heliosphere - Results from the ISEE-3 Spacecraft,&rdquo ESA Bulletin 27, 1981, pp. 4-12

37) A. Balogh, G. Van Dijen, J. Van Genechten, J. Henrion, R. Hynds, G. Korfmann, T. Iversen, J. Van Rooijen, T. Sanderson, G. Stevens, K. P. Wenzel, &ldquoThe Low Energy Proton Experiment on ISEE-C,&rdquo IEEE Transactions on Geoscience Electronics, Vol. GE-16, Issue 3, July 1978, pp. 176-180

40) P. Meyer, P. Evenson, &ldquoUniversity of Chicago cosmic ray electrons and nuclei experiment on the H spacecraft,&rdquo IEEE Transactions on Geoscience Electronics, GE-16, No. 3, July 1978., pp.180-185

41) A.M.A. Frandsen, B. V. Connor, J. Van Amersfoort, E. J. Smith, &ldquoThe ISEE-C Vector Helium Magnetometer,&rdquo IEEE Transactions on Geoscience Electronics, GE-16, No. 3, July 1978., pp. 195-198

42) Edward C. Stone, Richard A. Mewaldt, &ldquoResearch relative to the heavy isotope spectrometer telescope experiment,&rdquo Final Report, 1 Dec. 1985 - 30 Nov. 1992, California Institute of Technology, Pasadena, Division of Physics, Mathematics, and Astronomy

43) W. E. Althouse, A. C. Cummings, T. L. Garrard, R. A. Mewaldt, E. C. Stone, R. E. Vogt, &ldquoA cosmic ray isotope spectrometer,&rdquo IEEE Transactions on Geoscience Electronics, Vol. 16, Issue 3, July 1978, p.204

44) T. T. von Rosenvinge, F. B. McDonald, J. H. Trainor, M. A. I. Van Hollebeke, I. A. Fisk, &ldquo The Medium Energy Cosmic Ray Experiment for ISEE-C,&rdquo IEEE Transactions on Geoscience Electronics, Vol. GE-16, No. 3, July 1978, pp. 208-212

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: &rdquoObservation of the Earth and Its Environment: Survey of Missions and Sensors&rdquo (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.

## 2 Methods and Instrumentation

Phase scintillation can be modeled as the interference between different wavefront phases exiting the phase screen planes (Chartier et al., 2016 Rino, 1979a , 1979b ), which, in the auroral region, are oriented perpendicular to the magnetic field (Chartier et al., 2016 ). Plasma density perturbations there have a length scale greater than the first Fresnel radius , thus producing refractive exiting wavefront patterns (e.g., Forte et al., 2017 Kintner et al., 2007 ). Fluctuations in the signal phase ΔΦ are a direct measurement of the receiver and are proportional to variations in the integrated electron density in a plasma. Phase variations induced by plasma are defined as (1) where Q is the charge of an electron, f is the radio frequency, me is the mass of an electron, c is the speed of light, and is the integrated electron density along the radio link, referred to as total electron content (TEC). The phase change is either negative (advance) or positive (delay), corresponding to the velocity (phase and group) of propagation. (2) measured in TEC units (TECU), where 1 TECU = . Slant TEC is a function of elevation angle, which, in this study, is corrected by an obliquity factor (Klobuchar, 1987 ) to obtain vertical TEC (vTEC). We use the relative vTEC, ignoring system biases (Rideout & Coster, 2006 ) as a measure of relative density perturbation caused by precipitating energetic particles from the magnetosphere. In addition, we use the first-order difference of the vTEC, ΔTEC (TECU/s), as a measure of temporal variability of the TEC.

Phase scintillation activity is usually expressed in terms of the phase scintillation index σΦ. Van Dierendonck ( 1999 ) suggested a minimum receiver phase locked loop bandwidth of 15 Hz and recommended an output sample rate of 50 Hz for a phase scintillation receiver monitor. The receivers used in this study do not satisfy the latter criterion, and therefore, we use the detrended carrier phase on L1 channel to represent the phase scintillation instead. Phase scintillation data are sensitive to the detrending process, as pointed out by Forte and Radicella ( 2002 ) and Strangeways ( 2009 ). We use standard polynomial detrending and high-pass filtering with a cutoff frequency of 0.1 Hz. Amplitude scintillation is not presented in our results since there was no amplitude scintillation noted during the presented event.

In addition to GNSS and optical observables, we present a LOS comparison with coaligned PFISR observations. For the presented event there was a favorable geometry of all remote sensing nodes. In particular, PFISR was running the Themis36 mode with a pattern of 23 beams. Six of its beams were directed toward the PRN23 LOS, as indicated in Figure 1. We use two levels of ISR measurements. First, we use received power information, averaged over 64 long pulses with a pulse width 330 μs and interpulse period 5 ms, resulting in a time resolution of ∼3 Hz. Second, we use derived vector parameters, E region electric field and F region drift, derived from long pulse mode (Heinselman & Nicolls, 2008 ).

Global maps of ionospheric vTEC are commonly produced using LOS TEC data from hundreds of receivers, each viewing multiple satellites simultaneously (Rideout & Coster, 2006 Vierinen et al., 2016 ). In contrast to this conventional approach, our study demands extremely high spatiotemporal resolution with a uniform probing angle. We therefore use data recorded between a single satellite and multiple receivers. This approach has previously been used in the Japanese GEONET project (Tsugawa et al., 2011 ) to discover coherent traveling ionospheric disturbances propagating concentrically away from the epicenter of the magnitude 9 Tōhoku earthquake in 2011.

The Mahali GPS array consisted of nine Trimble NetR9 GNSS receivers, deployed in the vicinity of PFRR. Figure 1 shows the locations of the Mahali array and associated sensors. The NetR9 receivers are multifrequency GPS receivers, capable of monitoring PRN phase, carrier phase, and signal-to-noise (SNR) at L1, L2, and L5 (1176 MHz) bands simultaneously. The receiver output data rate is 1 Hz, stored using the standard receiver-independent exchange format. For the purposes of this study, L1 and L2 carrier phase data are used, since the observational objective is focused on phase scintillation only and satellite vehicle 23 does not operate in the L5 band.

Along with GPS receivers and ISR, we use optical emissions and magnetic field measurements to complement the study. In particular, we use green line (558 nm) emissions to spatially correlate the GPS phase scintillation with precipitation patterns. Red line (630 nm) and blue line (428 nm) emissions are used to infer the characteristic energy of precipitating electrons along the flux tube and to estimate the altitude of contributing plasma irregularities. The ASI is colocated with Mahali receiver #8 (MAH8), PFISR, and a magnetometer at PFRR. The ASI records images at a cadence of 12.5 s per wavelength, with a mutual delay between different wavelengths of ∼4 s. The ASI is operated by the University of Alaska, which provided data and calibration files. We also use the perturbed northward magnetic field component Bx in our study, monitored with a colocated three-axes fluxgate magnetometer, operated by the Alaska Satellite Facility.

## What exactly is interplanetary scintillation what was the Interplanetary Scintillation Array looking for? Did it successfully observe any? - Astronomy

by Luisa Bonolis

Anthony Hewish

Nobel Prize for Physics 1974 tgether with Martin Ryle "for their pioneering research in radio astrophysics: Ryle for his observations and inventions, in particular of the aperture synthesis technique, and Hewish for his decisive role in the discovery of pulsars".

Twinkle, twinkle little star: becoming a radio astronomer

Anthony Hewish was born in Fowey, Cornwall, in 1924. After attending King's College in Taunton, he entered Cambridge University in 1942, but the following year he left college for wartime work. He first worked on radio receivers, but after a few months he was transferred to the Telecommunications Research Establishment in Malvern (TRE), which was the top-secret centre for the development of airborne radar devices. The leader of the Malvern team was Martin Ryle, an Oxford scientist who was at that time working on the design of antennas for airborne radar equipment. Hewish’s main job was to visit the Bomber Command airfield where this equipment was first installed and to instruct the RAF technicians how to use it. He found the teaching experience fulfilling and during three years of war service he began to develop a real understanding of physics.
Hewish's first task as a new student was sawing a large pile of brass tubing into pieces and then soldering them onto copper wires to make a dipole array. Ryle's speciality was the use of two such arrays separated by a considerable distance and connected together to form an interferometer. Radio interferometry was used at that time to perform the first high-resolution radio astronomy observations. In this technique, the data from each antenna are combined (or “interfered”) and joined to the same receiver. Coincident signals reinforce one another, while conflicting signals cancel each other out in a manner analogous to the way fringes are produced in the optical interferometer. The principle of the radio interferometer's operation is the same as for an optical interferometer, but, because radio waves are much longer than light waves, the scale of the instrument is generally correspondingly greater. The effect is to achieve the collecting power of a single large instrument encompassing the individual collecting sub-components. Today, the technique has evolved into powerful radio interferometers such as the Very Large Array located on the Plains of San Augustin in central New Mexico, consisting of 27 parabolic antennas, each measuring 25 metres in diameter, with a total collecting area equivalent to a single 130-metre antenna.

Hewish was lucky to participate in these pioneering efforts and to share in the team spirit, which became a notable characteristic of Ryle's group for many years. Following a suggestion from Ryle, he started by setting up a simple antenna to measure the polarisation of intense bursts of solar radiation emitted when there were large sunspots present, a radiation which jammed the radars in 1942. But he soon got bored with waiting for solar activity to occur and got involved with a much more exciting problem. The first few radio stars had been observed to vary in intensity - they sometimes scintillated on a timescale of seconds to minutes, twinkling rather like ordinary stars. Was this an intrinsic effect due to physical changes within the sources or was it caused by propagation through the atmosphere?
Ratcliffe was highly impressed by Hewish’s theory and encouraged him to write two papers, which were published in the Proceedings of the Royal Society of London. So this research became the main topic for his doctoral dissertation and resulted in further publications.

After earning his Ph.D. for his radio studies of the upper atmosphere in 1952, ionospheric research could have continued as a major interest, but Hewish was diverted by another development. By the early 1950s, it had been found that most of the so-called radio stars were actually a new kind of galaxy, rare, powerful, and at huge distances. Others were

In 1964 came another breakthrough, when Ryle's group noticed that some radio galaxies showed scintillation during daytime, which could not be related to the ionosphere. He wondered if the solar atmosphere might be causing this twinkling to occur. A few of these sources coincided with quasars. Quasars had shown unexpected variations of intensity, and Hewish realised that these sources might, indeed, have a small enough angular size for the solar atmosphere to cause the scintillation. They immediately checked this by special observations and found that the rate of variation of intensity fitted his theory rather well, provided that the speed of the solar wind was roughly the same near the sun as they are farther out. The speed of distant solar winds had been detected by spacecrafts between the end of the 1950s and the beginning of the 1960s. Spacecrafts, however, were limited to observations in the plane of the Earth's orbit due to constraints set by energy requirements at launch. By repeating the techniques that he had used earlier for the ionosphere, Hewish was able to overcome this limitation and measure the wind emitted from the Sun's polar regions. Since the solar wind was so fast, it was necessary to set up radio telescopes at sites one hundred kilometres apart, and to use larger antennas. By choosing suitably located radio galaxies so that the line of sight passed over the solar pole, they found an enhanced wind speed in this region. Thirty years were to elapse before the space probe Ulysses, a joint venture of NASA and the European Space Agency, could orbit the Sun and study it at all latitudes, confirming his finding.
Another application of interplanetary scintillation that Hewish wanted to exploit was its use in finding more quasars, as these turned out to be sources of prodigious energy. In the early 1960s, radio telescopes were unable to obtain sharp enough images to distinguish between quasars and normal radio galaxies, whereas scintillation gave a direct indication of their tiny angular size. A highly sensitive radio telescope was needed to detect a large number of faint radio galaxies. His experience with setting up simple but effective antennas for studying the solar wind showed how a large array of similar structures could be combined to produce the necessary sensitivity. So in 1965 Hewish drew up plans for a radio telescope with which he intended to carry out a large-scale survey of more than 1000 radio galaxies using interplanetary scintillation to provide high angular resolution. To achieve the required sensitivity it was necessary to cover an area of 18 000 square meters. The final design was an array containing 2048 dipole antennas arranged in 16 rows of 128 elements. Each row was 470 m long and the north-south extent of the array was 45 m. Later that year he was joined by a new graduate student, Jocelyn Bell, who became responsible for the network of cables connecting the dipoles. The entire array was connected by 120 miles of wire and cable.

“Little Green Men”. or a new kind of astrophysical object?

The commissioning of the 4.5-acre array proceeded through the summer of 1967. Hewish suggested that Bell create sky charts for each strip of the sky each day, noting all the scintillating sources. If the scintillating sources were present on successive weeks at the same astronomical coordinates, they were likely to be real sources, whereas if the scintillation were simply the result of interference with other sources, they would not recur at the same astronomical coordinates. This was a very demanding task requiring great persistence, patience, and attention to detail on Bell’s part since she had to keep up with the high rate at which the charts were being produced by the telescope. The radio telescope was completed and tested by July 1967, and they immediately began a survey of the sky, making repeated observations so that interplanetary scintillation could be observed over a wide range of angular distance from the Sun for any radio galaxy. They surveyed the entire range of accessible sky at intervals of 1 week. To maintain a continuous assessment of the survey, they arranged to plot the positions of scintillating radio sources on a sky chart, as each record was analysed, and to add points as the observations were repeated at weekly intervals. In this way genuine sources could be distinguished from electrical interference since the latter would be unlikely to recur with the same celestial coordinates. This is a feature of all radio astronomy. Radio telescopes are very sensitive instruments, and it takes little radio interference from nearby on earth to swamp the cosmic signals.
One day around the middle of August 1967, Bell became aware that on occasions there was a bit of “scruff” on the records which did not look exactly like a scintillating source, and yet did not look exactly like man-made interference either. Furthermore, in examining previous recordings, she realised that this scruff had been seen before on the same part of the records, from the same patch of sky. The source was transiting during the night, a time when interplanetary scintillation should be at a minimum.
They first thought that the signals might be electrical interference. However, by the end of September, routing survey records showed that the source sometimes appeared on the sky map in the same position, but occasionally it was not present. More detailed examination revealed that it was emitting pulses of radio waves of a very stable frequency. Another odd fact was that it scintillated too strongly. Whatever it was they decided that it deserved closer inspection, and that this would involve making faster chart recordings as it transited. Towards the end of October, Bell started going out to the observatory each day to make the fast recordings. They were useless. For weeks she recorded nothing but receiver noise. The “source” was apparently gone. Then one day she skipped the observations to go to a lecture, and next day she saw the scruff had been there. A few days after that, at the end of November 1967, she got it on the fast recordings and with this improved time-resolution the pulses were detected separately for the first time. She immediately saw that the signal was a succession of short pulses repeating at regular intervals. Hewish's first reaction was that they must be man-made. The pulses appeared again the next day and Hewish checked the recording establishing that this signal, whatever it was, kept accurately to sidereal time. But a sequence of pulses with repetition period 1.33 seconds apart seemed suspiciously man-made. Besides it was far too fast a pulsation rate for anything as large as a star. It could not be anything earth-bound because it kept sidereal time. Hewish could not believe that any natural source would radiate in this fashion and he immediately consulted astronomical colleagues at other observatories to inquire whether they had any equipment in operation that might possibly generate electrical interference at a fixed sidereal time near 19 h 19 m. They also considered and eliminated radar reflected off the moon into their telescope, satellites in peculiar orbits, and other anomalous effects caused by a large, corrugated metal building just to the south of the telescope area.

They spent a whole month trying to find out what was wrong, so unexpected was the signal, whose period was found to be stable to better than 1 part in 106. They speculated that the signal might possibly be from a distant civilisation, and nicknamed it “Little Green Men”. Then, John Pilkington in their group measured the bandwidth of the signal from which he estimated that the source was well outside the solar system but inside the galaxy, the distance being around 100 light years, while Paul Scott and his student R. A. Collins made observations with a separate telescope, with its own receivers, which eliminated instrumental effects. Having found no satisfactory terrestrial explanation for the pulses, they now began to believe that they could only be generated by some source far beyond the solar system the short duration of each pulse suggested that the radiator could not be larger than a small planet. In his 1975 Nobel lecture, Hewish says, “We had to face the possibility that the signals were, indeed, generated on a planet circling some distant star, and that they were artificial. I knew that timing measurements, if continued for a few weeks, would reveal any orbital motion of the source as a Doppler shift, and I felt compelled to maintain a curtain of silence until this result was known with some certainty. Without doubt, those weeks in December 1967 were the most exciting in my life.”
As Bell recalled: “We did not really believe that we had picked up signals from another civilisation, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission.” She was very worried: “. here was I trying to get a Ph.D. out of a new technique, and some silly lot of little green men had to choose my aerial and my frequency to communicate with us.” During the following days, in analysing a recording of a completely different part of the sky, amongst a strong, heavily modulated signal from Cassiopea A, she thought she saw some scruff. She rapidly checked through previous recordings of that part of the sky, and on occasions there was scruff there. Knowing that the scruff would transit in the early hours of the morning, she went early in the morning to the laboratory, and she found that this scruff too was a series of pulses, this time 1.2 seconds apart. She left the recordings on Hewish's desk and went off, much happier, for Christmas: “It was very unlikely that two lots of little green men would both choose the same, improbable frequency, and the same time, to try signalling to the same planet Earth.” Over Christmas, Hewish kept the survey running, put fresh paper in the chart recorders, ink in the ink wells, and piled the charts, unanalysed, on Bell's desk. When she returned after the holiday, she immediately settled down to do some chart analysis. Soon, on the one piece of chart, an hour or so apart in right ascension, she saw two more lots of scruff. It was another fortnight or so before another one was confirmed, and soon after that the third and fourth were also.

At the beginning of February 1968, the paper “Observation of a Rapidly Pulsating Radio Source”, announcing the first pulsar, was submitted to Nature. It was signed by Hewish, Bell, Pilkington, Scott, and Collins. By that time they were confident that three additional pulsars existed although their parameters were then only crudely known. They also mentioned that at one stage they had thought the signals might be from another civilisation. They suggested that the pulses might be generated by a white dwarf star, or, more likely, a hypothetical neutron star. The existence of neutron stars was first proposed by Walter Baade and Fritz Zwicky in 1934, when they argued that a small, dense star consisting primarily of neutrons would result from a supernova explosion. In the late 1930s, Robert Oppenheimer and his collaborators predicted that when a massive star died, it would collapse into an incredibly dense, spinning body, a neutron star. In 1967, shortly before the discovery of pulsars, Franco Pacini suggested that a rotating neutron star with a magnetic field would emit radiation. After the discovery of the first pulsar, Thomas Gold independently suggested a rotating neutron star model similar to that of Pacini and explicitly argued that a pulsar is accompanied by an enormously powerful magnetic field surrounded by a plasma cloud, emitting a rotating beam. This model could explain the pulsed radiation observed by Bell and Hewish. When Stephen Hawking heard their news he was delighted and phoned Hewish to say that if neutron stars existed then black holes were almost certain to occur also. The detection of pulsed light from the star that had previously been identified as the remnant of the original explosion, observed in 1054 A. D., provided further impressive support for the neutron star hypothesis. This, according to theories of stellar evolution, was precisely where a young neutron star should be created.
By 1968 most opinion settled on neutron stars as the best solution for pulsars. These extremely dense stars, which form from the collapsed remnants of massive stars after a supernova, have strong magnetic fields that are not aligned with the star’s rotation axis. They spin very rapidly, up to nearly 1000 times per second. The strong field and rapid rotation produces a beam of radiation that sweeps around as the star spins. Jets of particles moving almost at the speed of light are streaming out above their magnetic poles. These jets produce very powerful beams of light and their radio emission is concentrated along a narrow cone. For a similar reason that “true north” and “magnetic north” are different on Earth, the magnetic and rotational axes of a pulsar are also misaligned. Therefore, the beams of light from the jets sweep around as the pulsar rotates and on Earth, we see this as a series of pulses, much like the beacon from a rotating lighthouse lamp. Pulsars are formed with a certain amount of angular momentum. As gravity causes them to shrink (and thus have a smaller radius) they must spin faster in order to conserve angular momentum.

In 1968, Bell earned her PhD - pulsars appeared in the appendix of her dissertation. It became soon clear that this discovery must rank as one of the great events in astronomy. Their unique properties made them nearly ideal probes for a wide range of physical studies, opening up new horizons in fields as diverse as quantum-degenerate fluids, relativistic gravity and interstellar magnetic fields. The outstanding observational characteristic of pulsars is the pulsed emission and its precise periodicity. This great stability provides the basis for many of the applications of pulsars to studies of physical phenomena. The neutron stars represent matter in bulk at nuclear densities and offer many challenges for physicists and astrophysicists.
In 1974, Antony Hewish and Martin Ryle became the first astronomers to be awarded the Nobel Prize in physics, with the Royal Swedish Academy of Sciences noting that Hewish had played a “decisive role in the discovery of pulsars”. By that time, more than 130 pulsars charted in the heavens, there was overwhelming evidence that the neutron star “lighthouse” model was correct. No other star could spin fast enough, without fragmenting. However, considerable controversy was associated with the fact that Hewish was awarded the prize while Bell, who played a pivotal role in the initial discovery while she was his Ph.D student, was not. Bell claimed no bitterness upon this point, supporting the decision of the Nobel committee.
That same year, using the Arecibo Observatory, Joseph Taylor, Jr. and Russell Hulse discovered for the first time a pulsar in a binary system, PSR B1913+16. This pulsar orbits another neutron star with an orbital period of just eight hours. Einstein's theory of general relativity predicts that this system should emit strong gravitational radiation, causing the orbit to continually contract as it loses orbital energy. Observations of the pulsar soon confirmed this prediction, providing the first ever evidence of the existence of gravitational waves. In 1993, the Nobel Prize in Physics was awarded to Taylor and Hulse for the discovery of this pulsar.
In PSR 1913+16 only one of the pair of neutron stars is observed as a pulsar, but in the binary system PSR J0737-3039, both neutron stars are observed as pulsars. This remarkable system discovered in 2003 at Australia's Parkes Observatory by an international team led by the radio astronomer Marta Burgay, makes it possible to determine the parameters of their binary orbits very precisely. The binary period about the centre of momentum of the system is only 2.4 hours, the shortest yet known for such an object (one-third that of the Taylor-Hulse object). The effects of general relativity are even stronger than in PSR 1913+16, thus this binary neutron star system can provide some of the most stringent tests of general relativity. Radio observations from Australia, Germany, England, and the United States show that as a result of energy loss due to gravitational waves the orbit shrinks by 7 millimetres per day, exactly in accordance with what Einstein's theory predicts. The two components will coalesce in about 85 million years.

Bibliography

Bell Burnell J. Little Green Men, White Dwarfs or Pulsars? Cosmic Search 1 (1), http://www.bigear.org/vol1no1/burnell.htm

Interview With Antony Hewish, 1974 Nobel Prize Laureate In physics, By Kourosh Ziabari. 17 October, 2012, http://www.countercurrents.org/ziabari171012.htm

Hewish A. (2001) Prelude to Discovery. The Kenyon Review 23 (2): 147-157

Longair M. (2011) The Discovery of Pulsars and the Aftermath. Proceedings of the American Philosophical Society 155: 147-157