aurora

A faint visual phenomenon associated with geomagnetic activity that is visible mainly in the high-latitude night sky. Aurorae occur within a band of latitudes known as the auroral oval, the location of which is dependent on geomagnetic activity. Aurorae are a result of collisions between atmospheric gases and precipitating charged particles (mostly electrons) guided by the geomagnetic field from the magnetotail. Each gas (oxygen, nitrogen molecules, and atoms) emits a particular color depending on the energy of the precipitating particles, and atmospheric composition varies with altitude. Since the faster precipitating particles penetrate deeper, certain auroral colors, originate preferentially from certain heights in the sky. The auroral altitude range is 80 to 1000 km, but typical aurorae are 100 to 250 km above the ground; the color of the typical aurora is yellow-green, from a specific transition of atomic oxygen. Auroral light from lower levels in the atmosphere is dominated by blue and red bands from molecular nitrogen and oxygen. Above 250 km, auroral light is characterized by a red spectral line of atomic oxygen.  Aurorae in the Northern Hemisphere are called the aurora borealis or ”northern lights.” Aurorae in the Southern Hemisphere are called aurora australis. The patterns and forms of the aurora include quiescent arcs, rapidly moving rays, curtains, patches, and veils.