Jupiter's Moon-Induced Infrared Aurorae: Spectral Insights from Juno's JIRAM

Since the discovery of Jupiter’s auroral footprints linked to the Galilean moons - Io, Europa, Ganymede, and Callisto - extensive efforts have been made to unravel the mechanisms behind these unique features, which have no direct analog on Earth. These auroral emissions arise from interactions between Jupiter’s magnetic field, co-rotating iogenic plasma, and the moons themselves, generating disturbances that propagate as Alfvén waves along magnetic field lines. Further insights into these aurorae have been made possible by NASA’s Juno mission, which has provided unprecedented access to Jupiter’s polar regions and significantly advanced our understanding of auroral processes and the coupling between the planet’s ionosphere and its moons. Central to this progress is the Jovian InfraRed Auroral Mapper (JIRAM), which combines an L-band imager (3.3–3.6 μm) with a spectrometer spanning the 2–5 μm range. Throughout the mission, JIRAM has provided numerous high-spatial-resolution images of H₃⁺ infrared emissions associated with the footprints of Io, Europa, and Ganymede, revealing fine-scale structures and enhancing our understanding of their morphology and the electrodynamic processes that shape them. Nonetheless, the spectral properties of these features remain poorly characterized.

This study investigates the infrared signatures observed by JIRAM’s spectrometer at the footprint locations. We analyze L-band images and spectra acquired during perijove (PJ) passes 1 through 40. The images provide the spatial context necessary to identify the spectra corresponding to the auroral footprints driven by Io, Europa, and Ganymede, which are the primary focus of this work. From these spectra, we derive key parameters such as the temperature and column density of H₃⁺ across the distinct spots that make up the footprints: the Main Alfvén Wing (MAW) spot, formed at the magnetic foot of Alfvén waves directly connected to the moon; the Reflected Alfvén Wing (RAW) spot, generated by wave reflection on the density gradient at the plasma sheet boundary; and the Transhemispheric Electron Beam (TEB) spot, produced by electrons accelerated away from Jupiter and precipitating into the opposite hemisphere (Figures 1 and 2).

  

Figure 1. JIRAM image-slit composite maps of the southern Io footprint, created by combining L-band images from (a) PJ 14 on July 7, 2018 (scanning session from 07:01:44 to 07:11:46) and (b) PJ 27 on June 2, 2020 (scanning session from 12:07:16 to 12:08:19), with simultaneously measured spectral slits (green lines). Red lines indicate the trajectories of the footprints, while red squares mark the positions of the footprints in the first and last images of the sequence, as predicted by the Con2020 magnetic field model.

 

  

Figure 2. JIRAM image-slit composite maps of the southern Ganymede footprint obtained from (a) PJ 33 on April 15, 2021, in the scanning session from 00:42:55 to 00:45:55, (b) PJ 8 on September 2, 2017, in the scanning session from 00:06:06 to 00:22:43, and (c) PJ 15 on November 8, 2020, in the scanning session from 03:25:19 to 03:34:51. Red lines indicate the trajectories of the footprints, while red squares mark the positions of the footprints in the first and last images of the sequence, as predicted by the Con2020 magnetic field model.

 

Here, we present the results of the analysis of the auroral footprints of Io and Ganymede (Figure 3), where JIRAM spectra sampled all three considered spots. We also compare the derived parameters with those from previous JIRAM spectral studies of the main aurora, providing a better understanding of how the footprints relate to the main emissions. The MAW spots of the Io and Ganymede footprints exhibit H₃⁺temperatures and abundances comparable to those previously observed along the main auroral ovals. The Io footprint's MAW shows H₃⁺ temperatures ranging from 750 K to 1070 K and column densities between 0.47 and 1.6 x 10¹³ cm^-2, while the Ganymede footprint's MAW displays H₃⁺temperatures ranging from 860 K to 1230 K and column densities between 4.3 and 9.2 x 10¹² cm^-2. Similarly, the RAWs of both footprints reveal H₃⁺ temperatures and abundances consistent with those found in the main aurora. In contrast, the TEB structures show divergent behaviors: the Io footprint’s TEB is approximately 75 K hotter than its MAW, whereas the TEB in Ganymede’s footprint is about 70–90 K cooler. These thermal profiles indicate that the TEBs associated with Io and Ganymede form at different altitudes in Jupiter’s upper atmosphere - above the MAW and RAW for Io, but below them for Ganymede - pointing to possible differences in magnetosphere-ionosphere coupling processes between the different moon-aurora systems.

Figure 3. Retrieved values of H₃⁺ temperature and column density from the inversion of the JIRAM mean spectra of the MAW and TEB of the footprints of Io (a,b) and Ganymede (c,d).