Properties of electrons accelerated by the Ganymede-magnetosphere interaction: survey of Juno high-latitude observations

TL;DR

This study uses Juno spacecraft data to analyze electron acceleration mechanisms in Ganymede's magnetosphere, revealing energy decay patterns, electron distribution types, and the extent of the acceleration region, enhancing understanding of moon-magnetosphere interactions.

Contribution

It provides the first detailed characterization of electron acceleration properties in Ganymede's magnetosphere using Juno data, including energy distributions and spatial extent of the acceleration region.

Findings

Electron energy flux decays exponentially with distance from Ganymede.

Two distinct electron energy distribution types are identified.

The acceleration region extends between 0.5 and 1.3 Jupiter radii above the surface.

Abstract

The encounter between the Jovian co-rotating plasma and Ganymede gives rise to electromagnetic waves that propagate along the magnetic field lines and accelerate particles by resonant or non-resonant wave-particle interaction. They ultimately precipitate into Jupiter's atmosphere and trigger auroral emissions. In this study, we use Juno/JADE, Juno/UVS data, and magnetic field line tracing to characterize the properties of electrons accelerated by the Ganymede-magnetosphere interaction in the far-field region. We show that the precipitating energy flux exhibits an exponential decay as a function of downtail distance from the moon, with an e-folding value of 29{\deg}, consistent with previous UV observations from the Hubble Space Telescope (HST). We characterize the electron energy distributions and show that two distributions exist. Electrons creating the Main Alfv\'en Wing (MAW) spot and the auroral tail always have broadband distribution and a mean characteristic energy of 2.2 keV while in the region connected to the Transhemispheric Electron Beam (TEB) spot the electrons are distributed non-monotonically, with a higher characteristic energy above 10 keV. Based on the observation of bidirectional electron beams, we suggest that Juno was located within the acceleration region during the 11 observations reported. We thus estimate that the acceleration region is extended, at least, between an altitude of 0.5 and 1.3 Jupiter radius above the 1-bar surface. Finally, we estimate the size of the interaction region in the Ganymede orbital plane using far-field measurements. These observations provide important insights for the study of particle acceleration processes involved in moon-magnetosphere interactions.