Modeling the interaction of electrons and dispersive Alfvén waves in the Io plasma torus

Observations of the Galileo satellite illustrated small-scale magnetic field fluctuations near Io suggesting the presence of kinetic Alfvén waves (KAWs) in the Io plasma torus. In the terrestrial magnetosphere, both in observations and simulations, the presence of KAWs has also been linked to the formation of electron distribution functions elongated in the direction parallel to the ambient magnetic field. This plateau is commonly associated with the trapping of electrons within the KAW potential. In this paper, we present simulations of KAW propagation in the Io plasma torus using a 2D gyrofluid-kinetic electron model in a dipolar geometry. We consider realistic heavy ion mass and temperature and illustrate the formation of plateaued electron distributions in upward current regions that are qualitatively consistent with both the observations seen in the terrestrial and Jovian magnetospheres. We illustrate that in downward current regions, electron Landau damping leads to significant dissipation of wave energy over time, which increases as the perpendicular scale length of the wave narrows. This dissipation acts to restrict the transfer of wave energy through the torus boundary for smaller perpendicular scales. Although much of the KAW energy is reflected at the torus boundary, the reflection also acts to energize electrons up to several hundred electron volts, for the parameters considered, and thus could be a significant source of superthermal electrons that are also relevant to the chemistry of the Io–Jupiter interaction.