Variability of the Auroral Footprint of Io Detected by Juno‐JIRAM and Modeling of the Io Plasma Torus

Abstract

One of the auroral features of Jupiter is the emission associated with the orbital motion of its moon Io. The relative velocity between Io and the surrounding plasma trigger perturbations that travels as Alfvén waves along the magnetic field lines towards the Jovian ionosphere. These waves can accelerate electrons into the atmosphere and ultimately produce an auroral emission, called the Io footprint. The speed of the Alfvén waves ‐ and hence the position of the footprint ‐ depends on the magnetic field and on the plasma distribution along the field line passing through Io, whose SO2‐rich atmosphere is the source of a dense plasma torus around Jupiter. Since 2016, the Jovian InfraRed Auroral Mapper (JIRAM) onboard Juno has been observing the Io footprint with a spatial resolution of ∼ few tens of km/pixel. JIRAM detected evidences of variability in the Io footprint position that are not dependent on the System III longitude of Io. The position of the Io footprint in the JIRAM images is compared with the position predicted by a model of the Io Plasma Torus and of the magnetic field. This is the first attempt to retrieve quantitative information on the variability of the torus by looking at the Io footprint. The results are consistent with previous observations of the density and temperature of the Io Plasma Torus. However, we found that the plasma density and temperature exhibit considerable non‐System III variability that can be due either to local time asymmetry of the torus or to its temporal variability.This article is protected by copyright. All rights reserved.

The Infrared Auroral Footprint Tracks of Io, Europa and Ganymede at Jupiter Observed by Juno‐JIRAM

“…Therefore, we suggest using the reference track reported in the present manuscript to calibrate the localization of ground‐based observations, when the MAW spot of the footprint can be identified. Due to the variability of the Ganymede footprint (and of the Europa footprint, to a lesser extent), we recommend using the Io footprint whenever possible. The footprint position can be used to investigate the plasma environment near the moons (Moirano et al., 2023) by studying the lead angle associated with the footprints themselves (Hue et al., 2023). For this reason, the footprints represent (a) a source of information that complements the particle measurements carried by Juno during the satellite flybys in its extended mission, and (b) a supporting data set for the next missions Juice and Europa Clipper, which are dedicated to the Jovian major moons.…”

“…Due to the variability of the Ganymede footprint (and of the Europa footprint, to a lesser extent), we recommend using the Io footprint whenever possible. • The footprint position can be used to investigate the plasma environment near the moons (Moirano et al, 2023) by studying the lead angle associated with the footprints themselves (Hue et al, 2023). For this reason, the footprints represent (a) a source of information that complements the particle measurements carried by Juno during the satellite flybys in its extended mission, and (b) a supporting data set for the next missions Juice and Europa Clipper, which are dedicated to the Jovian major moons.…”

“…Moreover, the lead angle associated with Io, Europa and Ganymede shows variability by comparing observation with the satellites at the same System III longitude at different epochs. This implies a variation in the Alfvén travel time, which determines the mapping between the moons and their emission (e.g.,: Hinton et al., 2019; Moirano et al., 2023). This might be caused by either a change in the plasma conditions or a transient longitudinal stretching (or contraction) of the magnetic field, which in turn can be due to variations in the radial plasmadisk currents.…”

“…Case a shows the IFP during PJ11 and PJ32, with lead angles 4.7 ± 0.2°and 6.0 ± 0.3°, respectively. This pair of observations was analyzed in Moirano et al (2023), where the difference was attributed either to changes in the state of the Io Plasma Torus (Bagenal & Dols, 2020) between the two orbits or to the local-time asymmetry of the plasma torus. Case b shows the longitudinal displacement of the EFP by comparing PJ7 and PJ37, when the lead angle was 4.3 ± 0.8°and 7.5 ± 0.5°r espectively, while Europa was at the same longitude within less than 1°.…”

“…Furthermore, the MAW spot can be a reference for calibrating ground‐based observations: this will help to better determine the position of the aurorae observed from near Earth, hence improving the quality of the analysis based on those data. Lastly, the JIRAM survey can be used to study the variability of the plasma environment at the orbit of the Galilean satellites (Moirano et al., 2023), as the position of the footprints depends on both the magnetic field and the plasma distribution along the magnetic field lines. The possibility of investigating the plasma parameters near the moons by looking at the footprint is an important tool for complementing the in‐situ particle measurements that Juno is performing during the flybys of the satellites.…”