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

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 H 2 spectra and the spectral range allows the study of self-absorption. Below 1100 Å, transitions connecting to the v 2 levels of the H 2 ground state are partially or totally absorbed by the overlying H 2 molecules. The FUSE spectra provide information on the overlying H 2 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 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 H 2 gas temperature and 1.3 × 10 18 , 1.5 × 10 20 , and 1.3 × 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 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 H 2 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.

Parameterization of ionization rate by auroral electron precipitation in Jupiter

“…It is shown that the conductance at this current is P ≈4 mho with small variation on the spectra, whereas the conductance in the other two cases is smaller by a factor of 2. This may be due to the effect of the low energy component of spectra in Grodent et al (2001) and Gustin et al (2004) to reproduce observed high atmospheric temperature. Assuming that the low energy component cannot reach the peak altitude of ion mobility, we try to eliminate the component from the definition of current j .…”

“…11a, b). Furthermore, we adopt two flux functions derived on the basis of observation results (Grodent et al, 2001;Gustin et al, 2004), with a simple normalization of these magnitudes to adjust j . Variations in the conductance through spectral configurations are shown in Fig.…”

Characteristics of north jovian aurora from STIS FUV spectral images

“…observations with the Far Ultraviolet Spectroscopic Explorer by Gustin et al 2004b). Such former spectroscopic observations thus do not allow us to draw a global picture of the processes responsible for polar auroras.…”

“…These images were obtained by integrating the spectrum corresponding to each spatial pixel in the 1550-1620 Å bandwidth, unaffected by hydrocarbon absorption, and multiplied by 8.1 to extend the result over the whole UV. The latter factor has been determined from a synthetic H 2 spectrum as 235 described by Gustin et al (2004b). All brightnesses mentioned hereafter correspond to unabsorbed emission for the whole UV bandwidth.…”

The H Lyman-α emission line from the upper atmosphere of Jupiter: Parametric radiative transfer study and comparison with data

“…Far from the Galileo probe descent region and the Voyager occultation latitudes, the temperature profile is uncertain apart from scattered values obtained indirectly through H + 3 or H 2 vibrational emissions, etc. (Grodent et al, 2001;Gustin et al, 2004;Raynaud et al, 2004;etc. ).…”