A simple transport model for the Io plasma torus “ribbon”

Abstract

A model for the generation of the Io plasma torus “ribbon” density feature is proposed. In this model a self‐regulation mechanism for plasma pressure is hypothesized, operating through ring current inhibition of flux‐tube interchange transport. Torus plasma outflow drives hot plasma from the middle magnetosphere inwards, raising its confining pressure and impeding further outflow of torus plasma. As the hot plasma cools by collisions with torus ribbon electrons its pressure diminishes, allowing more torus outflow. This negative feedback modulates torus ion flux‐tube content NL² in such a way as to hold it relatively constant. In order to simulate this process, we have constructed numerical models which approximate plasma transport by a non‐linear diffusion process, with the feedback mechanisms built into the diffusivity. For certain reasonable parameter values, these models produce a density enhancement near L = 5.6 to 5.8 which somewhat resembles the “ribbon” feature that is actually observed near that position.