Magnetic fluctuations close to Io: ion cyclotron and mirror mode wave properties

As Galileo approached Io on December 7, 1995 and passed through the Io wake it passed from a region of ion cyclotron waves at the SO+2 gyrofrequency into a region of strong compressional waves at the edge of the wake region and through a region of waves linearly polarized transverse to the magnetic field in the center of the wake region. As the Io wake was approached the properties of the ion cyclotron waves changed as if there was a change in the wave generating region. The cyclotron waves were also significantly weaker outbound than inbound. We interpret the compressional waves to be mirror mode waves generated by the anisotropic pick-up distribution of SO+2. The appearance of the mirror mode waves at the edges of the wake suggests that the magnetic field or plasma gradient plays a role in limiting the growth rate of the ion cyclotron instability in this region, in addition to the disappearance of the isotropic warm torus plasma that stabilizes the mirror mode outside of the wake region.