Location and spatial shape of electron beams in Io's wake

Abstract

The Galileo spacecraft observed energetic field‐aligned electron beams very close to Io during several flybys. We apply a three‐dimensional magnetohydrodynamic (MHD) model of the far‐field Io‐Jupiter interaction to simulate for the first time the location and spatial shape of field‐aligned electron beams. Io continuously generates MHD waves by disturbing the Jovian magnetoplasm. Currents carried by Alfvén waves propagate predominantly along the magnetic field lines. As the number of charge carriers decreases along the travel path, electrons are accelerated toward Jupiter. These energetic electrons precipitate into the Jovian ionosphere, visible as prominent Io footprint emission. Electrons are also accelerated toward Io and form the equatorial beams observed by the Galileo spacecraft. Unlike the beam formation, the position and spatial structure of these beams have not been addressed in detail before. We use a 3‐D MHD model with initial conditions corresponding to the individual Galileo flyby and determine the spatial morphology of the beams in Io's orbital plane. Our results for the beam locations are in good agreement with the Galileo Energetic Particle Detector observations. We find that the ratio of the one‐way travel time of the Alfvén wave from Io to Jupiter and the convection time of the plasma past the obstacle controls the location of the beam. This leads to the conclusion that at other satellites with other plasma environments, the electrons might not be close to the satellite but can be shifted significantly downstream along its plasma wake. Thus, the future search for electron beams near a satellite should be further extended to the wake region.