How low to the ground do auroras get?

Because the phenomenon depends on the fluorescence of oxygen and nitrogen atoms at low densities, aurora rarely, if ever, extend below 50 kilometers from the Earth's surface. Most of the nitrogen and oxygen lines we see as red, green or purple colors are produced over 90 kilometers above the ground. The light from nitrogen atoms between 6610 and 6861 Angstroms, also a red color, are produced as low as perhaps 65 kilometers.