Volcanic activity on Io at the time of the Ulysses encounter

The population of heavy ions in Io's torus is ultimately derived from Io volcanism. Ground-based infrared observations of Io between October 1991 and March 1992, contemporaneous with the 8 February 1992 Ulysses observations of the Io torus, show that volcanic thermal emission was at the low end of the normal range at all Io longitudes during this period. In particular, the dominant hot spot Loki was quiescent. Resolved images show that there were at least four hot spots on Io's Jupiter-facing hemisphere, including Loki and a long-lived spot on the leading hemisphere (Kanehekili), of comparable 3.5-micrometer brightness but higher temperature.