Ion-cyclotron waves at Io Original Research Article

The massloading of the near Io torus by sulfur dioxide leads to the generation of intense ion cyclotron waves, near the singly ionized sulfur dioxide ion gyro frequency, that have been detected by the Galileo magnetometer. While other ions are produced near Io, the Io torus plasma contains nearly isotropic Maxwellian distributions of these ions that damp any waves near their ion cyclotron frequencies. Since SO2+ dissociates rapidly in the Io torus environment, there is no background of damping ions at the SO2+ ion gyro frequency. To zeroth order these waves grow at the ion cyclotron frequency, are left-hand circularly polarized and propagate along the magnetic field. In actuality the waves grow over a finite bandwidth, propagate at an angle to the field and are elliptically polarized. At the upper edge of the band of wave growth the waves propagate at their greatest angles to the magnetic field, and are more elliptically polarized. In fact in some regions the waves become slightly right-handed. These polarizations may arise due to the propagation of the waves in an increasing field strength in a multi ion environment. Finally, we examine the data obtained by the Voyager spacecraft along the “same” field line as Galileo but 16.5 years earlier and 12 RIo southward of Io. No ion cyclotron waves are seen here or anywhere else along the Voyager Io passage. If we assume that ion cyclotron waves were generated at the same strength in 1979, then the waves must have been significantly attenuated by propagation or absorption.