The polar night sky glows with shimmering colors. Hot particles spewing from the sun bend in the Earth's magnetice field and irradiate the air beyond where the sun shines. Excited fragments of the atmosphere dispose of their excess energy by emittinbg light--both visible and invisible. The human eye sees bands of red and green and blue.
Near the poles, the night sky glows with shimmering curtains of light. These glorious light shows are powered by nuclear radiation from the sun. The aurora borealis and aurora australis are the lights at the ends of the world.
Storms on the sun eject showers of protons and electrons into space at speeds up to a million miles per hour. This so-called "solar wind" floods the solar system with hazardous radiation. Earth's magnetic field deflects the solar wind around the planet and toward the magnetic poles. The ricocheting particles are bent into areas where the sun doesn't shine. There, they impact the outer reaches of the atmosphere. Molecules are torn apart; electrons are stripped away. Repair requires the collision fragments dissipate their excess energies somehow. The mechanisms to do that vary with atmospheric pressure. The radiochemistry of the atmosphere is complex. All roads ultimately lead to light emission—visible or otherwise.
Atmospheric recovery modes that involve emission of visible light create the auroras we see in the night sky. At the highest altitudes, free oxygen atoms in a long-lived electronically excited state emit red light. In the less diffuse atmosphere below that, collisions chemically remove those atoms before they can emit. There, only oxygen atoms with shorter-lived electronic excitations survive to give auroras a green glow. Still farther down, air gets even denser and free oxygen atoms can't survive long enough to emit at all. At these lowest altitudes, excited nitrogen molecules glow in the blue. Bands of one, two, or all three colors are seen at arctic and antarctic latitudes.
Earth's auroras are not unique. The solar wind blows out past Pluto. The wind's radiation may light up the atmosphere of any planet it hits along the way. The gas giants Jupiter and Saturn are surrounded by strong magnetic fields and exhibit auroras. The Juno probe is exploring Jupiter's polar regions. Its auroras will give us a peek deep into the planet's atmosphere. The Hubble telescope has also detected auroras on Uranus and Neptune. Visiting probes have seen auroras on Venus and Mars as well. Will auroral glows someday tell us about the atmospheres of exo-planets?
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