EVIDENCE FOR SHORT COOLING TIME IN THE IO PLASMA TORUS

We present empirical evidence for a radiative cooling time for the Io plasma torus that is about a factor of ten less than presently accepte d values. We show that brightness fluctuations of the torus in the ext reme ultraviolet (EUV) at one ansa are uncorrelated with the brightnes s at the other ansa displaced in time by five hours, either later or e arlier. Because the time for a volume of plasma to move from one ansa to the other is only five hours, the cooling time must be less than th is transport time in order to wipe out memory of the temperatures betw een ansae. Most (similar to 80 - 85%) of the EUV emission comes from a narrow (presumably ribbon-like) feature within the torus. The short c ooling time we observe is compatible with theoretical estimates if the electron density in the ribbon is similar to 10(4)/cm(3). The cooling time for the rest of the torus (which radiates the remaining 15-20% o f the power) is presumably consistent with the previously derived 20-h our values. A nearly-continuous heating in both longitude and time is needed to maintain the EUV visibility of the torus ribbon - a requirem ent not satisfied by presently available theories.