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.

Click dot to see picture.

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