Determining the Beaming of Io Decametric Emissions: A Remote Diagnostic to Probe the Io‐Jupiter Interaction

Abstract

We investigate the beaming of 11 Io‐Jupiter decametric (Io‐DAM) emissions observed by Juno/Waves, the Nançay Decameter Array, and NenuFAR. Using an up‐to‐date magnetic field model and three methods to position the active Io Flux Tube (IFT), we accurately locate the radiosources and determine their emission angle θ from the local magnetic field vector. These methods use (a) updated models of the IFT equatorial lead angle, (b) ultraviolet (UV) images of Jupiter's aurorae, and (c) multi‐point radio measurements. The kinetic energy Ee− of source electrons is then inferred from θ in the framework of the Cyclotron Maser Instability. The precise position of the active IFT achieved from methods (b and c) can be used to test the effective plasma density of the Io torus. Simultaneous radio/UV observations reveal that multiple Io‐DAM arcs are associated with multiple UV spots and provide the first direct evidence of an Io‐DAM arc associated with a trans‐hemispheric beam UV spot. Multi‐point radio observations probe the Io‐DAM sources at various altitudes, times and hemispheres. Overall, θ varies a function of frequency (altitude), by decreasing from 75°−80° to 70°−75° over 10−40 MHz with slightly larger values in the northern hemisphere, and independently varies as a function of time (or longitude of Io). Its uncertainty of a few degrees is dominated by the error on the longitude of the active IFT. The inferred values of Ee− also vary as a function of altitude and time. For the 11 investigated cases, they range from 3 to 16 keV, with a 6.6 ± 2.7 keV average.

The Io, Europa, and Ganymede Auroral Footprints at Jupiter in the Ultraviolet: Positions and Equatorial Lead Angles

“…For instance, comparing in more details the simulations that account for the lead angle and refractions effects (Figure 8c) with the circularly polarized radio emission from RPWS indicates that the early part of the radio emission arc (from 22:00:00 to 22:15:00) are better reproduced with electrons with lower energies, that is, 5 keV, while the center and end of the arc are better reproduced with electrons with energies ranging from 10 to 15 keV. It is worth noting here that Lamy et al (2022) showed that, in the case of Io, the electrons energy derived from radio emission analysis is variable as a function of the moon longitude. It is not surprising here to observe that in order to reproduce the arc in the time-frequency plane, the energy of the electrons must vary with time (i.e., the longitude of the moon).…”

“…It is worth noting here that Lamy et al. (2022) showed that, in the case of Io, the electrons energy derived from radio emission analysis is variable as a function of the moon longitude. It is not surprising here to observe that in order to reproduce the arc in the time‐frequency plane, the energy of the electrons must vary with time ( i.e., the longitude of the moon).…”

“…Because the directionality of these radio emissions depend both on the electron energy and the lead angle, this may lead to situations where non‐unique solutions exist, in order to reproduce the observed decametric radio arcs. The knowledge of the active field line (where the radio emission sources are located) and thus the lead angle, constitutes one of the main sources of uncertainty in determining the electron energy responsible for the radio emission (Hess, Pétin, et al., 2010; Lamy et al., 2022). Previous estimate of the absolute lead angle values were not accurate enough mostly because of the uncertainty in the magnetic field models as well as the uncertainties in the Europa and Ganymede measured footprint positions from HST (Hess, Pétin, et al., 2010), leading to non‐physical cases with negative lead‐angle.…”

“…The knowledge of the lead angle is also important for the interpretation of the moon‐induced decametric emissions (e.g., Hess, Pétin, et al., 2010; Louis et al., 2019; Lamy et al., 2022; Marques et al., 2017). This section illustrates the added benefit from the equatorial lead angle knowledge in a case related to the interpretion of the Ganymede‐induced decametric radio emission.…”

Radio-loud Exoplanet-exomoon Survey: GMRT Search for Electron Cyclotron Maser Emission

“…The radio emission from the exomoon interaction could also be a time variable similar to the variability seen in the Jupiter-Io emission. Furthermore, the emission due to the interaction between Io and Jupiter is also highly beamed (e.g., Queinnec & Zarka 1998;Zarka et al 2004;Ray & Hess 2008;Lamy et al 2022). Similar beaming is expected from exomoon-exoplanet interactions.…”