AURORAL LYMAN-ALPHA AND H-2 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 excite d Jovian and (at times) Saturnian H-2 Werner bands being absorbed by h ydrocarbons at the shortest wavelengths along the auroral ovals, and o f a longitudinal dependence of this absorption in the Jovian aurorae. This ''color ratio'' has been used to estimate the energy of the prima ry particles. In such estimates, particles are generally assumed to pe netrate vertically into the atmosphere. However, the precipitating par ticle angular distribution is unknown, and a model developed for a dif fuse aurora by Prange and Elkhamsi (1991), for instance, predicts quit e different possible distributions. We consider here the influence of the angular distribution used in the model, and show that distribution s peaking far from vertical may increase the energy derived from a giv en color ratio by as much as a factor of 3. We discuss previous interp retations 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 inte rpretation in terms of energy variations only is not consistent with t he energy available in the magnetosphere if the aurorae are diffuse, a nd we discuss this finding in the context of recent Hubble Space Teles cope images.