Abstract :
[en] The general introduction of this work presents the main characteristics of the planet Jupiter and a detailed description of its magnetosphere. The latter reminds the basic motion of a charged particle in a magnetic field. It describes the interplanetary magnetic field and its reconnection with the planetary magnetosphere. The notions of corotating field and magnetospheric convection are used to differentiate the Earth and Jovian magnetospheres. Among these differences the interaction with the satellite Io is stressed out as Io provides most of the magnetospheric plasma material. These notions allow to discuss the potential origin of the particles responsible for the Earth aurorae and, by extension, the Jovian aurorae. It is postulated that the auroral particles are related to the presence of field-aligned currents and fields. The main characteristics of the Hubble Space Telescope and of the WFPC2 and GHRS instruments, which were used for this work, are briefly described at the end of the introduction.
The first part describes a model simulating Earth views of UV auroral arcs and diffuse emissions in the Jovian north polar region. Simple geometric cases are described to illustrate the dependence of the altitude, atmospheric scale height and central meridian longitude of an idealized auroral morphology seen from Earth orbit. As an application of the simulation model, four images obtained with the WFPC2 camera on board the Hubble Space Telescope are used to determine the characteristics of their auroral (discrete and diffuse) structures. A composite average auroral distribution is built by mapping 10 WFPC2 images from the same dataset. It illustrates the dichotomy frequently observed between a narrow single structure confined to the morning sector, and the multiple arc and broad diffuse emission in the afternoon sector. Location of these structures are given and constrained in a reference frame linked to the GSFC-O6 magnetic field model.
This model is then applied to assess the role of the viewing geometry on the auroral far UV color ratio. This value gives the ratio between the intensity measured in an unabsorbed spectral band and the intensity in a methane-absorbed band. The simulated color ratios, obtained from a geometry deduced from images taken with HST, are compared to the color ratio measurements obtained with the IUE spectrograph. We attempt to reproduce the IUE observations by imposing an intrinsic longitudinal dependence of the column of methane above the level of the auroral emission.
The second part is devoted to the energy degradation model of the auroral electrons in the jovian atmosphere. It then describes the coupling of this model with a thermal conduction model. The theoretical section includes an introduction on the jovian atmosphere and its confinement in regions characterized by different dynamical and thermal regimes. The notion of hydrostatic equilibrium is reminded and used to establish a pressure-altitude relationship. We describe the electron transport model of Banks and Nagy and the numerical resolution method that we applied to it. The set of cross sections used to quantify the energy loss processes is described along with the numerical treatment of the energy "reapportionment" of the auroral electrons.
The vertical thermal profile is calculated from the heat conduction equation. Among the different heat sources we consider H2 dissociation, thermal electron heating, and chemical heating. Other sources, such as the breaking gravity waves, are indirectly accounted for. The heat sinks account for the IR radiative cooling from H3+, CH4 and C2H2. A correction regarding the departure from local thermodynamic equilibrium is applied for these species.
In order to calculate the response of the atmospheric structure to the auroral precipitation, the model iteratively solves the diffusion equation for the major constituents. The vertical profiles of the eddy and molecular diffusion coefficients and their connection are addressed. The adopted method for the approximation of the H3+ density in the ionosphere as a function of the auroral activity is presented. The thermostatic role of H3+ in the thermosphere is then discussed.
The heat conduction equation, the diffusion equation and the electron transport equations are tightly coupled. The resolution of this set of equations therefore requires an iterative approch for which we describe a strategy meant to limit the convergence speed.
The energy degradation model is then applied with different energy distributions to assess the importance of the energy spectrum of the incident electrons for the thermal balance of Jupiter's auroral thermosphere. The values of observable quantities such as the altitude of the H2 emission peak, the IR and UV emissions, the FUV color ratio and temperatures associated with various optical signatures are used to constrain the parameters of these energy distributions. A series of sensitivity tests are carried out to analyse the role of critical parameters such as the value of the eddy diffusion coefficient at the homopause.
The third part describes the H2 UV high-resolution spectral generator and the global coupling of the different models.
We begin with an overview of H2 far-UV spectroscopy notions that are used in the spectral generator, especially regarding the Lyman and Werner band systems. For the Werner bands, the cascade effect from the E,F state is considered. The coupling of the energy degradation model with the spectral generator is described. In a first stage an unconverged thermal profile is adopted. Three examples are used to illustrate the effect of the electron energy distribution on the spectra. The temperature effect is also highlighted. The H2 temperature is determined from two GHRS spectra. It gives a best fit temperature of 600 K, in disagreement with the temperature predicted by the energy degradation model. The latter predicts an average temperature, weighted by the H2 UV emission profile, of the order of 200 K. It is shown that the use of converged thermal profiles, obtained with the heat conduction equation, does not remove the contradiction.
The coupling of the three models (energy degradation, spectral generator, and morphology) is performed in the last section. This coupling reveals a wavelength and a viewing geometry effect on the temperature deduced from the observed spectra. It is shown how these effects impinge on the thermal, density and emission vertical profiles to produce an effective H2 temperature of 600 K in agreement with the temperature deduced from the observed spectra.
We finally discuss a possible application of the coupled models that would allow a spectroscopic probing of the jovian thermal profile.
