The vertical profile of the northern Io footprint auroral emission in the far-ultraviolet from Juno

Variability and energy spectrum of the precipitating electrons

¹ Laboratory for Planetary and Atmospheric Physics, Space Sciences, Technologies and Astrophysical Research Institute, University of Liège, Liège, Belgium
² Institute for Space Astrophysics and Planetology, National Institute for Astrophysics (INAF-IAPS), Rome, Italy
³ Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA
⁴ Aix-Marseille Université, CNRS, CNES, Institut Origines, LAM, Marseille, France
⁵ Space Science Division, Southwest Research Institute, San Antonio, TX, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 29 September 2025
Accepted: 10 December 2025

Abstract

Context. The interaction between Io and the Jovian magnetosphere produces the auroral “Io footprint”, which can be observed in the ultraviolet thanks to the deexcitation of atmospheric H and H₂ under electron precipitation. Since 2016, Jupiter has been explored with Juno’s Ultraviolet Spectrograph, which monitors the emission from 68 to 210 nm.

Aims. We used Juno observations of the Io footprint near the planetary limb to determine its vertical ultraviolet emission profile in the northern hemisphere. We simulated emission as a function of the altitude and the mean energy of the precipitating electrons, and we used the results to determine the energy spectrum associated with the footprint profile.

Methods. We estimated the source location of the Io footprint UV emission, and we extracted its vertical profile. We analyzed the variability of the emission altitude, and we derived the corresponding energy spectrum of precipitating electrons using TransPlanet. The results were compared with the in situ measurements from Juno’s particle detectors.

Results. The main spot emission peaks around ~500 km, with a variability of ~200 km correlated with longitude. The emission of the footprint tail within 20° from the main spot peaks around ~500±300 km, and it is moderately correlated with the magnitude of the magnetic field near Jupiter. The trans-hemispheric electron beam spot is located at ~300 km, and the associated precipitation appears depleted at low energy. The retrieved energy spectrum of the precipitation shows remarkable agreement with the particle measurements.

Conclusions. The longitudinal modulation of the Io footprint altitude suggests that the morphology of the torus affects the transmission of the Alfvén waves responsible for the acceleration of auroral electrons. The dependency of the footprint tail altitude on the magnetic field strength indicates that the magnetic field also plays a role in the acceleration mechanism.