Most auroras are caused by the excited electrons of the solar wind, directed to the Earth's magnetic field. These electrons collide with the air molecules, releasing other electrons in the shock, which emit light when they are recaptured.
However, sometimes, the auroras are mainly caused by solar wind protons (heavier than electrons), which impact the Earth causing a more intense aurora, with a strong ultraviolet emission.
A proton aurora captured in ultraviolet by the IMAGE satellite is seen in the photograph surrounding the magnetic north pole of the Earth.
Most of the electrons and protons never reach the Earth to give rise to the auroras, since they are completely reflected into space by the Earth's magnetic field. The bright spot on the auroral ring indicates a particularly deep fracture in the Earth's magnetic field where protons could flow, almost without being reflected, along a temporary passage between the Sun and Earth, until impacting with the ionosphere land.
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