Colors of the Auroras
The following color types are as classified by the International
Auroral Atlas (International Union of geodesy and Geophysics, 1963)
- Type-a (Type-A red): Theoretically, one might expect
all sufficiently long and bright rays to show a red upper region.
- Type-b (Type-B red): Aurora with red lower borders are
well-known. Approximate height of the lower border is 80 km which implies
electrons with energies of more than 100 keV.
- Type-d (Type-A red): Height range of 250-450 km;
overall red type of display seems to be more observed at low latitudes.
- Type-e: This form appears to be a natural consequence
of the .7s lifetime of the upper state for the 5577 Angstroms green line
and occurs when the lateral motion of the rays in a RA is sufficiently
large and the intensity bright enough for color vision.
- Type-f: Such auroras take the form of long rays of
which the upper sunlit parts may appear blue or violet as a result of the
enhancement of the first N2 ion bands by fluorescence in sunlight.
These color variations are largely height effects arising from
variations in the energy spectra of the incoming particles.
dot to see picture.