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

¹ Aix-Marseille Université, CNRS, LAM, Marseille, France
² Aix-Marseille Université, CNES, Institut Origines, Marseille, France
³ Laboratory of Atmospheric and Planetary Physics, STAR Institute, University of Liège, Belgium
⁴ University of Bern, Faculty of Science, Physics Institute, Space Research & Planetary Sciences (WP), Switzerland
⁵ The University of Edinburgh, School of GeoSciences, Edinburgh, UK
⁶ Centre for Exoplanet Science, University of Edinburgh, Edinburgh, UK
⁷ LATMOS/CNRS, Sorbonne Université, UVSQ, Paris, France
⁸ Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA
⁹ Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX, USA
¹⁰ Physique des Interactions Ioniques et Moléculaires, CNRS et Aix-Marseille Université, France
¹¹ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France
¹² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹³ Univ. Grenoble Alpes, CSUG, 38000 Grenoble, France
¹⁴ Institute for Space Astrophysics and Planetology, National Institute for Astrophysics (INAF-IAPS), Rome, Italy
¹⁵ IRAP, CNRS-Université Paul Sabatier, Toulouse Cedex 4, France
¹⁶ Department of Physics and Astronomy, The University of Alabama, Tuscaloosa, AL 35487, USA
¹⁷ NASA Langley Research Center, Hampton, Va, USA
¹⁸ Science Systems and Applications Inc., Hampton, VA, USA
¹⁹ Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX, USA

^★ Corresponding author: bilal.benmahi@lam.fr

Received: 26 August 2025
Accepted: 30 September 2025

Abstract

Context. Ganymede’s UV aurorae, observed by HST and Juno/UVS, trace interactions between its atmosphere and Jupiter’s magneto-sphere. 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.