Characterizing Auroral Acceleration Mechanisms at Jupiter: Statistical Analyses of Juno-UVS-Derived Electron Energies.

Resulting from the precipitation of magnetospheric particles into the upper atmosphere, planetary aurorae form an image of the dynamics of the magnetosphere and its coupling with the ionosphere and atmosphere. Hence, exploring the upper atmospheres of the Solar System's giant planets allows us to test our theories of the magnetosphere-ionosphere-thermosphere-atmosphere (MITA) interactions under very different conditions. At Jupiter, the coupling between the magnetosphere and the ionosphere has been the subject of numerous studies since the discovery of the Jovian ultraviolet (UV) aurorae by Voyager 1 in 1979 (Broadfoot et al., 1979). Each auroral feature arises from the precipitation of charged particles (mostly electrons) accelerated by a different magnetospheric process. The penetration depth of precipitating electrons in the atmosphere depends directly on their energy, and it is thus possible to use the absorption of UV light by the various hydrocarbons in the deepest layers to estimate their mean energy.

In this study, we compute the energy of the electrons precipitating in the auroral regions of Jupiter using observations from Juno’s UltraViolet Spectrograph (Juno-UVS), with the electron transport model TransPlanet (Lilensten et al., 1989; Benmahi et al., 2024). We compare the correlation between the H₂ brightness and the electron energy to those predicted by several theoretical models to constrain the acceleration mechanisms that produce each auroral feature. Indeed, observational studies have established that such a correlation exists for some features but not others (Gérard et al., 2016), thus indicating that several acceleration processes are at play at Jupiter.

We find that most of the data recorded by Juno-UVS is well calibrated, but in some unique circumstances (especially times of very localized intense emissions) the calibration for some wavelength ranges can be unreliable due to higher order instrumental effects. To circumvent this problem, we define a new color ratio between unabsorbed and absorbed wavelengths. Utilizing these results, we can locate the specific Juno orbits and auroral regions most affected by the instrument effects which make the nominal color ratio values less accurate. Using the new ratio and the corresponding relationship between color ratio and energy of the precipitating electrons, we map the brightness and the electron energy (see fig 1) for each Juno orbit for both hemispheres. We will show results of how this new color ratio and the energies inferred from the modeling compare to the previous method. We then compute the correlation between H₂ brightness and electron energy and determine if they follow a relation resembling the one predicted by Knight (1973), or not. These results thus constitute a test for the various explanations that were put forward to explain the complex morphology of the Jovian aurorae.

Figure 1 Electron energy map obtained from the CR(E) relation for an electron population with a kappa distribution of energies for the northern auroral region during PJ6. The subsolar longitude is marked by the yellow cross.

Bibliography :

Broadfoot et al., Science (1979), doi:10.1126/science.204.4396.979.

Lilensten et al., Annales Geophysicae. 7, 83–90 (1989).

Benmahi et al., A&A. 685, A26 (2024).

J.-C. Gérard et al., Planetary and Space Science. 131, 14–23 (2016).

Knight, Planetary and Space Science. 21, 741–750 (1973).