The variability of the Io plasma torus

A review of observations of Io's plasma torus with particular reference to time variable phenomena is provided. The observed variations can be separated into six different catagories ranging from short timescale (of order 15 min) oscillations of the entire torus to long‐term, possibly aperiodic, changes in the strength of optical and UV emissions. Much of the evidence for variability can be interpreted in several different ways and is, in some cases, contradictory. Similarly, some qualitative descriptions of the processes leading to observed variations have been discussed in the literature but often alternative explanations are also available and without a more quantitative analysis it is impossible to differentiate between the proposed models. In short, there is a clear need for a more quantitative approach to the modelling of Jovian magnetospheric variability in support of dedicated long‐term observational programs.