Precipitating electron properties of Ganymede’s aurora retrieved from Juno/UVS observations during PJ34

Context. Ganymede’s UV aurorae, observed by HST and Juno/UVS, trace interactions between its atmosphere and Jupiter’s magnetosphere. These emissions, dominated by O I lines at 130.4 and 135.6 nm, are driven by electron impact on species such as H2O, O, and O2, and yet the properties of the precipitating electrons remain poorly constrained. Aims. Our aim was to retrieve the energy and flux of precipitating electrons using UV observations from Juno/UVS during PJ34 and to assess the dominant atmospheric species producing the observed emissions. Methods. Using the TransPlanet electron transport model and a non-local thermodynamic equilibrium (non-LTE) radiative transfer module, we simulated O I emissions for 17 auroral subregions, testing both monoenergetic and kappa-type electron distributions. The I(135.6 nm)/I(130.4 nm) line ratio was used as a diagnostic, with values varying by target species. Results. Monoenergetic distributions fit most regions better, with mean energies of 17–300 eV and fluxes up to 2 mW m-2. Kappa and Maxwellian distributions yielded higher fluxes, but poorer spectral fits. Poor fits in some regions reflect low S/N or non-ideal electron populations. Conclusions. Our results suggest that Ganymede’s UV aurorae are mainly driven by low- to intermediate-energy electrons. Upcoming high-resolution observations and in situ data from Juice and Europa Clipper will be key to refining these diagnostics.