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Auroras are the result of the emissions of photons in the Earth's upper atmosphere, above 80 km (50 miles), from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state. They are ionized or excited by the collision of solar wind particles being funneled down and accelerated along the Earth's magnetic field lines; excitation energy is lost by the emission of a photon of light, or by collision with another atom or molecule:
Green or brownish-red, depending on the amount of energy absorbed. nitrogen emissions
Blue or red.
Blue if the atom regains an electron after it has been ionized. Red if returning to ground state from an excited state.
Gassendi's interests in unusual celestial phenomena dates back as early as 1621, when he observed the colorful illumination of the sky and dubbed these lights ‘aurora borealis’. Based on his correspondence with observers as far away as the Levant, he located the source of the illumination at very high altitude, above the Northern Polar region.