Io's hot plasma torus—A synoptic view from Voyager

A study of the morphology of Io's hot plasma torus has encompassed hundreds of Voyager UVS measurements of torus intensity. The long‐term average state of the torus can be characterized by an axial asymmetry in the brightness of the prominent S III 685‐Å feature manifested as an enhancement in brightness whose peak is fixed near 1900 local time. No long‐term correlation of brightness with magnetic longitude is present. On time scales of a few Jupiter rotations, the torus can differ markedly from its average axial asymmetry, and its brightness is correlated with magnetic longitude for short times.