Theory of electron acceleration by lower hybrid waves in the Io plasma torus of Jupiter

Abstract

A theoretical model of superthermal electron acceleration in the Io plasma torus is developed. The model assumes that sulfur and oxygen ions are created out of Io's extended neutral cloud forming a ring distribution in velocity space which is unstable to the growth of lower hybrid waves traveling perpendicular to the magnetic field. The waves dissipate through Landau damping by plasma electrons accelerating them to kiloelectron volt energies. The present model computes the electron distribution function evolving under the action of the lower hybrid waves with the simultaneous occurrence of Coulomb collisions. The results show that for moderate electric field amplitudes characteristic of the lower hybrid waves observed there, a power law velocity dependence is generated along with a quasi‐isotropic pitch angle distribution due to Coulomb collisions. The power law extends up to a characteristic speed where the electron energy equals the pickup ion energy. At higher energies, an exponential distribution is prevalent with an effective temperature proportional to the wave acceleration efficiency. The model has a wide applicability to space and astrophysical plasmas where the presence of hot electrons gives rise to anomalous ionization effects.