On the [formula omitted] emission rate ratio in aurora

Abstract

We examine the sources of variability in the vertically integrated I(557.7 nm) I(427.8 nm) ratio of auroral green line emission to N 2 + 1 NG emission, in terms of its response to changes both in the electron energy spectrum and the atomic oxygen concentration. Assuming that N 2(A) + O and O + e are the only mechanisms producing the O( 1S) 557.7 nm emission, the emission rate ratio was modelled for varying atomic oxygen concentration and a fixed electron energy spectrum. The range of ratios obtained corresponds well with the range of observed ratios, from 1 to 10. The ratios were then computed for an MSIS atomic oxygen profile, but for electron fluxes having Maxwellian energy distributions with E 0 varying from 0.125 keV to 8 keV. The resulting ratios all lie between 5 and 6, a range that is much smaller than the observed ratio range. The curve obtained is in good agreement with coordinated ground-based and satellite observations. Through the introduction of an energy-independent ratio, it is shown that the insensitivity of the emission rate ratio to electron characteristic energy results from a balancing of the N 2(A) contribution at low altitude against the O + e contribution at high altitude, making the overall contribution relatively constant with altitude. On this basis, it is concluded that atomic oxygen variability is dominantly responsible for changes in the emission rate ratio. It is also shown that additional mechanisms are unlikely to influence this conclusion.