No document available.
Abstract :
[en] Abstract: Auroral emissions provide key insights into interactions between planetary magnetic fields, atmospheres, and surrounding plasma. While Jupiter’s aurorae are well-studied, those of Ganymede, the only moon with an intrinsic magnetic field in the Solar System, remain poorly understood. Recent observations by the Hubbe Space Telescope (HST) and Juno/UVS revealed UV auroral ovals on Ganymede, shaped by its magnetospheric interaction with Jupiter’s plasma and linked to atomic oxygen emissions at 130 and 135 nm produced by oxygenated species like H₂O, O and O₂ that are excited by electron impacts. However, the mechanisms behind these UV emissions are not yet fully characterized.Building on our previous work on Jupiter’s aurorae (Benmahi et al. 2024a,b), we apply a similar approach to Ganymede, focusing on oxygen emission in the 125–135 nm range. Using our electron transport model TransPlanet and a custom non-LTE radiative transfer model, we reproduced the 130 and 135 nm emissions observed by Juno/UVS during the Ganymede flyby of PJ34, across sunlit regions 0 to 16 (see figure below).Accounting for the UV solar flux reflection by Ganymede’s surface, we used the intensity ratio (I135/I130) and total brightness of both oxygen emission lines to constrain the mean energy and flux of precipitating electrons. These quantities depend on the electron distribution functions, solar zenith angle, surface albedo, and atmospheric composition.The theoretical I135/I130 ratio is approximately 0.2 for H₂O dissociation, around 2.2 for O₂ dissociation and around 0.02 for O excitation. Observations yield a median ratio of 2.22, indicating a strong predominance of O₂ over H₂O and O in these regions. Since the H₂O abundance is linked to surface ice sublimation, it varies strongly with solar zenith angle. For this reason, we adopted an atmospheric profile corresponding to a region not exposed to sunlight, as the auroral zones analyzed lie far from the subsolar point.To constrain the precipitating electron flux distributions, we modeled both the total brightness and line ratio as functions of the mean energy E0 and energy flux ϕ, for two initial types of distributions: a broadband kappa distribution and a monoenergetic distribution.Our simulations indicate that, in the regions studied, a kappa distribution does not allow for a satisfactory simultaneous reproduction of the observed brightness and line ratio. In contrast, in a large portion of the auroral regions analyzed, assuming a monoenergetic distribution yields results that are reasonably consistent with the observations, with characteristic energies around 20 eV in quiescent regions and up to 200 eV in the brightest areas, and with corresponding precipitating energy fluxes ranging from 0.5 to 5.0 mW/m².These results imply that the precipitating electrons are relatively low in energy, yet carry energy fluxes comparable to those driving Jupiter’s aurorae. Most regions analyzed seems to be consistent with either monoenergetic distributions or broadband spectra with energy cutoffs of a few tens of eV. The absence of very energetic electrons is attributed to Ganymede’s extremely tenuous atmosphere, where excitation cross sections for the 3S and 5S states of atomic oxygen drop significantly at high electron energies. Consequently, these energetic electrons tend to reach the surface without meaningful interaction with atmospheric particles, producing only a minimal number of secondary electrons and having little impact on the emission intensities at 130 and 135 nm.In conclusion, while the best fits are obtained with monoenergetic distributions, some regions exhibit significant discrepancies, indicating that neither the monoenergetic nor the broadband model can fully reproduce the observations. These limitations are likely due to uncertainties in the atmospheric model, particularly in the H₂O abundance profile, whose influence becomes critical when its concentration is comparable to or exceeds that of O₂. They may also arise from alternative types of energy flux distributions among the precipitating electrons in these auroral regions. References : Benmahi et al. 2024. « Energy Mapping of Jupiter’s Auroral Electrons from Juno/UVS Data Using a New H2 UV Emission Model ». Astronomy & Astrophysics 685 (mai):A26. https://doi.org/10.1051/0004-6361/202348634.
Benmahi et al. 2024. « Auroral 3D Structure Retrieval from the Juno/UVS Data ». Astronomy & Astrophysics 691 (novembre):A91. https://doi.org/10.1051/0004-6361/202451439.
Figure : Legend : Total brightness map around the 130 and 135 nm lines observed with the Juno/UVS instrument during the Ganymede flyby of PJ34. The polygons from 0 to 16 are the auroral regions of interest.
