Model of field aligned potential drops near Io

Abstract

The Galileo spacecraft passed through an unexpectedly dense plasma and mass loading region near Io closest approach. This paper shows how the prodigious mass loading observed in this region can produce field‐aligned potential drops. For parameters appropriate to Io and consistent with Galileo measurements, this mechanism is shown to support up to two hundred volt, field‐aligned potential drops near Io and field‐aligned electron beams. The total power in these electron beams is less than ∼4×108 W, a small fraction of the total energy requirement for maintaining torus ultraviolet emissions. This mechanism may nevertheless be important for the Jupiter‐Io interaction, because it injects a seed population of energized electrons and ions into the Io flux tube with hundred volt parallel energies. A two step acceleration process may operate in the Io flux tube, in which a seed population of electrons with parallel energies above an eV is created, followed by field‐aligned acceleration in the Alfven wings.