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Jovian auroral spectroscopy with FUSE: analysis of self-absorption and implications for electron precipitation |
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| Authors: | J Gustin PD Feldman D Grodent L Ben Jaffel HW Moos HA Weaver JM Ajello E Roueff |
| Institution: | a Laboratoire de Physique Atmosphérique et Planétaire, allée du 6 aout, 17, 4000 Liège, Belgium b Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA c Institut d'Astrophysique de Paris, 75014 Paris, France d Applied Physics Laboratory, Space Department, The Johns Hopkins University, Laurel, MD 20723, USA e Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA f Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109, USA g Observatoire de Paris, Section de Meudon, DAEC and CNRS UMR 8631, 92195 Meudon cedex, France |
| Abstract: | High-resolution (∼0.22 Å) spectra of the north jovian aurora were obtained in the 905-1180 Å window with the Far Ultraviolet Spectroscopic Explorer (FUSE) on October 28, 2000. The FUSE instrument resolves the rotational structure of the H2 spectra and the spectral range allows the study of self-absorption. Below 1100 Å, transitions connecting to the v″?2 levels of the H2 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 H2. Transitions from high-energy H2 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 Å spectral windows. In a first approach (single-layer model ), the synthetic spectra are generated 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 H2 gas temperature and 1.3×1018, 1.5×1020, and 1.3×1020 cm−2 for the H2 self-absorbing vertical column respectively. Comparison between the H2 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 H2 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 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 μbar, that is ∼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. |
| Keywords: | Aurora Jupiter Spectroscopy Ultraviolet observations |
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