Photometric Auroral Particle Measurements

Abstract

Within certain limitations, ground based or airborne measurements of auroral emissions can be used to determine the type, energy, and flux of precipitating particles. The technique uses absolute intensity measurements of total height integrated column emissions in 4861 Å Hβ, 4278 Å N₂ ⁺, and 6300 Å OI. The data reduction procedure is illustrated schematically in Figure 1. The measured intensities are corrected for atmospheric extinction. The corrected Hβ intensity gives an estimate of the proton flux and the proton contribution to the 4278 Å N₂ ⁺ and 6300 Å OI excitation. The residual 4278 ÅN₂ ⁺ intensity then gives a value of the total energy influx of the precipitating electrons. At times when this residue is zero, airglow corrections can be made to the 6300 ÅOI intensity. The ratio r of the corrected 6300 ÅOI emission and the N₂ ⁺4278 Å emission can be used as a parameter related to the mean energy of electron precipitation because soft auroras are generated at higher altitudes where the 6300 ÅOI quenching is inefficient. The quantity r therefore, can be used to interpret the mean height and mean energy of the precipitating electrons. The mean height provides a correction for the total electron energy flux as obtained from the 4278 Å emission. The mean energy is obtained by applying a one parameter model of the particle spectrum of the type:

$$ N\left( E \right){\text{d}}E = N_0 E{\text{e}}^{ - E/\alpha } {\text{d}}E $$

, where the spectral parameter α is half the mean energy and N(E) is the number flux per energy interval. The adopted theoretical dependence of r on 4278 Å intensity and Å is shown (Rees, 1971) in Figure 2.