Jovian auroral spectroscopy with FUSE: analysis of self-absorption and implications for electron precipitation

High-resolution (similar to 0.22 Angstrom) spectra of the north jovian aurora were obtained in the 905-1180 Angstrom window with the Far Ultraviolet Spectroscopic Explorer (FUSE) on October 28, 2000. The FUSE instrument resolves the rotational structure of the H-2 spectra and the spectral range allows the study of self-absorption. Below 1100 Angstrom, transitions connecting to the upsilon" less than or equal to 2 levels of the H-2 ground state are partially or totally absorbed by the overlying H2 molecules. The FUSE spectra provide information on the overlying H2 column and on the vibrational distribution of H-2. Transitions from high-energy H-2 Rydberg states and treatment of self-absorption are considered in our synthetic spectral generator. We show comparisons between synthetic and observed spectra in the 920-970, 1030-1080, and 1090-1180 Angstrom spectral windows. In a first approach (single-layer model), the synthetic spectra are venerated in a thin emitting layer and the emerging photons are absorbed by a layer located above the source. It is found that the parameters of the single-layer model best fitting the three spectral windows are 850, 800, and 800 K respectively for the H-2 gas temperature and 1.3 x 10(18), 1.5 x 10(20), and 1.3 x 10(20) cm(-2) for the H-2 self-absorbing vertical column respectively. Comparison between the H-2 column and a 1-D atmospheric model indicates that the short-wavelength FUV auroral emission originates from just above the homopause. This is confirmed by the high H-2 rovibrational temperatures, close to those deduced from spectral analyses of H-3(+) auroral emission. In a second approach, the synthetic spectral generator is coupled with a vertically distributed 3 energy degradation model, where the only input is the energy distribution of incoming electrons (multi-layer model). The model that best fits globally the three FUSE spectra is a sum of Maxwellian functions, with characteristic energies ranging from 1 to 100 keV, giving rise to an emission peak located at 5 mubar, that is similar to 100 km below the methane homopause. This multi-layer model is also applied to a re-analysis of the Hopkins Ultraviolet Telescope (HUT) auroral spectrum and accounts for the H2 self-absorption as well as the methane absorption. It is found that no additional discrete soft electron precipitation is necessary to fit either the FUSE or the HUT observations. (C) 2004 Elsevier Inc. All rights reserved.