The Altitude and Colors of the Aurora

The altitude at which auroral emissions are excited depends upon the energy of the precipitating particles and the density of the atmosphere. In the nightside oval, where more energetic particle precipitation occurs than on the day side, the bulk of the auroral emissions are produced between 100 and 150 km, in the ionospheric E region. On the day side, most of the emissions occur at higher altitudes.

The principal visible emissions are those from atomic oxygen at 630 nm (blood red) and 557.7 nm (yellow-green) and from singly ionized molecular nitrogen at 427.8 nm (blue). The red line emissions, which are excited by less energetic particles, dominate at altitudes above 200 km, while green line emissions dominate between 100 and 200 km. Below 100 km, the aurora occasionally takes on a magneta color due to a mix of blueish N2+ emissions and of reddish emissions from N2 and O2+. Auroral emissions also occur at ultraviolet, x-ray, and infrared wavelengths. In addition to the optical emissions, extremely intense radio emissions--auroral kilometric radiation (AKR)--are generated in the field-aligned potential structures that accelerate the particles responsible for the discrete aurora.

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