Source of radio emissions induced by the Galilean moons Io, Europa and
Ganymede: in situ measurements by Juno
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 Alfvén 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 Alfvén perturbations and downward field aligned
currents are necessary for the radio emissions to be amplified.