Auroral Lyman α and H₂ bands from the giant planets: 1. Excitation by proton precipitation in the Jovian atmosphere

Abstract

This paper is part of a work aimed at modeling the ratio of the observed Jovian auroral intensity at H Lyman α and in the H₂ Lyman and Werner bands and interpreting them as diagnostic of the incident magnetospheric particle species and energy. The work is planned in three steps: (1) modeling of the volume excitation rate, (2) modeling of the radiative transfer at Lyman α, (3) application to existing observational data and new data obtained from the Hubble Space Telescope. The present paper deals with the first step. Models of the volume excitation rate have previously been developed for low energy electrons and oxygen ions. However, the energy range of the study has to be extended towards higher energy in view of recent results on the penetration depth of the primary particles. Protons have not been modeled so far. We have used an existing electron code of degradation of energy [Gérard and Singh, 1982] which has been improved, updated and adapted to the case of precipitating protons. The issues of nonequilibrium beam H/H⁺ fractions and of getting reliable cross sections over a wide energy range have been considered with particular care. The altitude distribution of the volume excitation rate is compared for electrons and protons, for various initial energies in the range 10-50 keV and 50 keV to 1 MeV, respectively.