Source of radio emissions induced by the Galilean moons Io, Europa and Ganymede: in situ measurements by Juno

TL;DR

This paper reports in situ measurements by Juno spacecraft that identify the sources and mechanisms of radio emissions induced by Jupiter's Galilean moons Io, Europa, and Ganymede, revealing new insights into their magnetic interactions.

Contribution

It provides the first in situ characterization of radio sources associated with Europa and Ganymede, confirming the role of Cyclotron Maser Instability driven by loss-cone electrons.

Findings

Radio emissions linked to Main and Reflected Alfvén Wings.

Europa and Ganymede emissions associated with Transhemispheric Electron Beam.

Presence of Alfvén perturbations necessary for amplification.

Abstract

At Jupiter, part of the auroral radio emissions are induced by the Galilean moons Io, Europa and Ganymede. Until now, except for Ganymede, they have been only remotely detected, using ground-based radio-telescopes or electric antennas aboard spacecraft. The polar trajectory of the Juno orbiter allows the spacecraft to cross the range of magnetic flux tubes which sustain the various Jupiter-satellite interactions, and in turn to sample in situ the associated radio emission regions. In this study, we focus on the detection and the characterization of radio sources associated with Io, Europa and Ganymede. Using electric wave measurements or radio observations (Juno/Waves), in situ electron measurements (Juno/JADE-E), and magnetic field measurements (Juno/MAG) we demonstrate that the Cyclotron Maser Instability (CMI) driven by a loss-cone electron distribution function is responsible for the encountered radio sources. We confirmed that radio emissions are associated with Main (MAW) or Reflected Alfv\'en Wing (RAW), but also show that for Europa and Ganymede, induced radio emissions are associated with Transhemispheric Electron Beam (TEB). For each traversed radio source, we determine the latitudinal extension, the CMI-resonant electron energy, and the bandwidth of the emission. We show that the presence of Alfv\'en perturbations and downward field aligned currents are necessary for the radio emissions to be amplified.