Evidence of Fresh Injections Related to the Interchange Instability in the Io Torus

The Juno Waves instrument often detects brief, band-limited emissions when the spacecraft crosses magnetic fields threading Io’s M-shell, or vicinity thereof, up to about 30 degrees magnetic latitude. The disturbances have durations of order one minute and are observed below the electron cyclotron frequency. While plasma densities are often not available, it is thought that the frequency of the events are below the plasma frequency of the surrounding medium. Often, the first electron cyclotron half-harmonic is weakened or disrupted at the time of the events. While the events can be seen in isolation, there are typically a few of them with temporal spacing between about 15 to 40 minutes. Similar features were commonly seen by the Cassini radio and plasma wave instrument at Saturn and were identified as ’fresh’ injections or evidence of inward-moving flux tubes due to the centrifugal interchange instability. As such, they were characterized as having depleted thermal plasma and enhanced energetic plasma with electron distributions unstable to wave modes such as the upper hybrid band and chorus. Such events were also observed by Galileo. As fresh injections, the energetic particles associated with them have not had time to drift in longitude due to gradient and curvature drift forces.