Determining the beaming of Io decametric emissions : a remote diagnostic to probe the Io-Jupiter interaction

TL;DR

This study uses multi-instrument observations and models to precisely locate Io-Jupiter decametric radio sources, analyze their beaming angles, and infer electron energies, enhancing understanding of the Io-Jupiter interaction.

Contribution

It introduces new methods combining UV imaging and multi-point radio data to accurately locate Io flux tube sources and analyze their emission angles and electron energies.

Findings

Beaming angle varies with frequency and hemisphere.

Multiple UV spots are linked to Io-DAM arcs.

Electron energies range from 3 to 16 keV.

Abstract

We investigate the beaming of 11 Io-Jupiter decametric (Io-DAM) emissions observed by Juno/Waves, the Nan\c cay 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 $\theta$ from the local magnetic field vector. These methods use (i) updated models of the IFT equatorial lead angle, (ii) ultraviolet (UV) images of Jupiter's aurorae and (iii) multi-point radio measurements. The kinetic energy $E_{e-}$ of source electrons is then inferred from $\theta$ in the framework of the Cyclotron Maser Instability. The precise position of the active IFT achieved from methods (ii,iii) can be used to test the effective torus plasma density. 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, $\theta$ varies a function of frequency (altitude), by decreasing from $75^\circ-80^\circ$ to $70^\circ-75^\circ$ 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 $E_{e-}$ also vary as a function of altitude and time. For the 11 investigated cases, they range from 3 to 16 keV, with a $6.6\pm2.7$ keV average.