Auroral Mechanism

Auroras can also be understood as being caused by the collision of charged particles (such as electrons) found in the magnetosphere with atoms in the Earth's upper atmosphere (at altitudes above 80 km). These charged particles are typically energized to levels between 1,000 and 15,000 electronvolts and, as they collide with atoms of gases in the atmosphere, the atoms become energized. Shortly afterward, the atoms emit their gained energy as light (see Fluorescence). Light emitted by the aurora tends to be dominated by emissions from atomic oxygen, resulting in a greenish glow (at a wavelength of 557.7 nm) and—especially at lower energy levels and higher altitudes—a dark-red glow (at 630.0 nm wavelength). Both these represent forbidden transitions of electrons of atomic oxygen that (in the absence of newer collisions) persist for a long time and account for the relatively slow (0.5-1 s) brightening and fading of auroral rays. Many other colors—especially those emitted by atomic and molecular nitrogen (blue and purple, respectively)—can also be observed. These, however, vary much faster and reveal the truly dynamic nature of auroras.

The Aurora Borealis as viewed from the International Space Station Expedition 6 team.