Auroras are now known to be caused by the collision of charged particles (ions (+) (-) ) 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 and 15
keV and, as they collide with atoms of gases in the atmosphere, the atoms become excited. Shortly afterwards, 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 at higher altitudes - the dark-red glow (at 630.0 nm of wavelength). Both of these represent
forbidden transitions of electrons of atomic oxygen that, in absence of newer collisions, persist for a long time and account for the slow brightening and fading (0.5-1 s) of auroral rays. Many other colors - especially those emitted by atomic and molecular nitrogen (blue and purple, respectively)
[1] - can also be observed. These, however, vary much faster and reveal the true dynamic nature of auroras.
