THEORY OF ELECTRON ACCELERATION BY LOWER-HYBRID WAVES IN THE IO PLASMA TORUS OF JUPITER

A theoretical model of superthermal electron acceleration in the lo pl asma torus is developed. The model assumes that sulfur and oxygen ions are created out of lo's extended neutral cloud forming a ring distrib ution in velocity space which is unstable to the growth of lower hybri d waves traveling perpendicular to the magnetic field. The waves dissi pate through Landau damping by plasma electrons accelerating them to k iloelectron volt energies. The present model computes the electron dis tribution function evolving under the action of the lower hybrid waves with the simultaneous occurrence of Coulomb collisions. The results s how 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 spee d where the electron energy equals the pickup ion energy, At higher en ergies, an exponential distribution is prevalent with an effective tem perature proportional to the wave acceleration efficiency. The model h as a wide applicability to space and astrophysical plasmas where the p resence of hot electrons gives rise to anomalous ionization effects.