Io's interaction with the plasma torus

TLDR

Pickup is the process by which ions newly created near Io are accelerated by the corotational electric field. The study presents a new model of Io’s interaction with the dense corotating plasma of the Io torus, involving pickup and Alfvén waves carrying a field‑aligned current. The model self‑consistently determines the electric field near Io by combining pickup and field‑aligned currents, and uses a simplified electric‑field solution to calculate pickup rate, field‑aligned current, plasma density enhancement, thermal energy injection, UV radiation power, and inertial drag on the magnetosphere. The pickup current is linked to a field‑aligned current carried by an Alfvén wave, and Voyager observations are consistent with electron‑impact ionization and a neutral SO₂ Io atmosphere of density 10⁹ cm⁻³.

Abstract

A new model for the interaction of Io with the dense corotating plasma of the Io torus is described which involves pickup and Alfvén waves carrying a field‐aligned current. Pickup refers to the process whereby ions freshly created near Io are accelerated by the corotational electric field. It is shown that the pickup current is connected to a field‐aligned current carried by an Alfvén wave. The combination of currents allows for a self‐consistent determination of the electric field in Io's vicinity. Using a simplified solution for the electric field, we calculate the total pickup rate, field‐aligned current, enhancement of plasma density in Io's vicinity, thermal energy injected into the torus, UV radiation power, and inertial drag on the magnetosphere. All observations made by Voyager are compatible with an electron impact ionization mechanism and a neutral SO 2 Io atmosphere with a density of 10 9 cm −3 .