Ganymede-Induced Decametric Radio Emission: In Situ Observations and Measurements by Juno

Abstract

At Jupiter, part of the auroral radio emissions are induced by the Galilean moons Io, Europa, and Ganymede. Until now, they have been 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 magnetic flux tubes connected to these moons, or their tail, and gives a direct measure of the characteristics of these decametric moon-induced radio emissions. In this study, we focus on the detection of a radio emission during the crossing of magnetic field lines connected to Ganymede's tail. Using electromagnetic waves (Juno/Waves) and in situ electron measurements (Juno/JADE-E), we estimate the radio source size of ∼250 km, a radio emission growth rate >3 × 10−4, a resonant electron population of energy (Formula presented.) –15 keV and an emission beaming angle of θ = 76–83°, at a frequency ∼1.005–1.021 × fce. We also confirmed that radio emission is associated with Ganymede's downtail far ultraviolet emission.