Identification of sulfur compounds in IOʹs exosphere Original Research Article

Ion cyclotron waves have been identified in the vicinity of Io on each of the four Galileo passes, and on each pass there have been differences in the ion cyclotron waves observed suggestive of variations in the composition of Ioʹs exosphere. On the first flyby, IO, waves were seen centered on the SO2+ gyrofrequency and additionally at the SO+ gyrofrequency outbound. On the next flyby, I24, first only waves associated with the SO+ gyrofrequency were seen but later in the pass SO2+ waves appeared, and still later S+ waves. On I25 the spectrum resembled that of I24 but the SO2+ waves were stronger, not the SO+ waves. Finally on I27, SO2+ and SO+ waves were seen as on the previous two passes but later in the pass a band of waves centered around the H2S+ gyrofrequency was seen, including lines that could be due to Cl+. Because each of these flybys occurred at a different orbital phase of Io, some of these differences may reflect the Io solar-zenith-angle variation of the atmosphere, but other changes may reflect variations in volcanic activity.