Auroral Lyman alpha and H2 bands from the giant planets. 2: Effect of the anisotropy of the precipitating particles on the interpretation of the 'color ratio'

Previous spectral analyses have given evidence of collisionally excited Jovian and (at times) Saturnian H2 Werner bands being absorbed by hydrocarbons at the shortest wavelengths along the auroral ovals, and of a longitudinal dependence of this absorption in the Jovian aurorae. This 'color ratio' has been used to estimate the energy of the primary particles. In such estimates, particles are generally assumed to penetrate vertically into the atmosphere. However, the precipitating particle angular distribution is unknown, and a model developed for a diffuse aurora by Prange and Elkhamsi (1991), for instance, predicts quite different possible distributions. We consider here the influence of the angular distribution used in the model, and show that distributions peaking far from vertical may increase the energy derived from a given color ratio by as much as a factor of 3. We discuss previous interpretations of the color ratio longitudinal modulation (variation of the auroral atmosphere structure, or of the incident particle energy) in view of the subsequent increase in energy input. We argue that an interpretation in terms of energy variations only is not consistent with the energy available in the magnetosphere if the aurorae are diffuse, and we discuss this finding in the context of recent Hubble Space Telescope (HST) images.