Excitation of O(1D) atoms in aurorae and emission of the [OI] 6300-Å line

Abstract

The electron aurora leads to six processes capable of exciting the O(1D2) metastable state of the atomic-oxygen ground-state configuration, the parent state of the 6300-Å red line. Altitude profiles of the volume emission rate resulting from each process are computed for Maxwellian electron spectra with characteristic energies between 0.1 and 2.0 keV. Since each process peaks at a different altitude, the sum or total volume emission rate extends over a wide altitude range. Measurements of 6300-Å emission obtained by rocket and satellite-borne instruments are summarized, and it is shown that the chemical reaction of N(2D) with O2 is the major source of O(1D) atoms in the electron aurora. New calculations of the 6300-Å:4728-Å column emission-rate ratio are presented for a range of characteristic energies in an assumed Maxwellian electron spectrum. An approximate equation for the red-line emission per unit energy input is given as a function of electron-spectrum characteristic energy.