Variability of [SII] emission from the Io plasma torus

Io, the innnermost Galilean satellite of Jupiter, is a main source of heavy ions in the Jovian magnetosphere. Therefore variability of the plasma in the vicinity of Io produces large effects to the magnetospheric environment. We have started optical observations of the Io plasma torus in order to study the plasma environment in the Jovian magnetosphere. Observed emissions are forbidden line emissions of S ⁺ ions (λλ 671.6 and 673.1 nm) in the plasma torus. In 1997, the observations were carried out at Alice Springs, Northern Territory of Australia. We mainly report the results of the observations at [SII] 673.1 nm.