Ion cyclotron waves in Ioʹs wake region

When Galileo flew almost directly through the center of Ioʹs wake in December 1995 it encountered a region of depressed field strength, devoid of the strong ion cyclotron waves that were present inbound and outbound on either side of the wake. In August 2001 Galileo made an almost orthogonal pass, parallel to the wake axis, passing within 200 km of the Io surface, and staying close to 1RIo above the center of the wake. Downstream from Io (but not upstream) strong ion cyclotron waves were seen. The very low frequency of these waves, well below the local SO2+ gyro frequency, suggests that they were created in the low field of Ioʹs wake. Thus in contrast to implications of the December 1995, near-Io, wake passage, ion cyclotron waves are produced in the Io wake. Furthermore, these waves persisted beyond 6.7RIo downstream, with little sign of recovery in frequency, indicating that Ioʹs depressed field wake extends well downstream of Io.