Ion cyclotron waves observed at Galileo's Io encounter: Implications for neutral cloud distribution and plasma composition

We present observations of ion cyclotron waves measured by the magnetic field experiment onboard the Galileo spacecraft at its encounter with Jupiter's satellite Io on December 7, 1995. The waves exhibit highly coherent oscillations, with a period between 2 and 3 seconds. They are left-handed, nearly circularly polarized, and their plane of polarization is nearly perpendicular to the background field. Their power spectra reveal peaks close to the gyrofrequencies of SO₂⁺ and, sometimes, SO⁺. The waves are interpreted as L-mode waves generated by a gyroresonance of pickup ions, whose source is neutral particles originating from Io. On the inbound leg of the flyby, in regions where the electron density is approximately constant, the observed decay of the wave power with distance from Io roughly follows the expected distribution of neutral clouds. The presence of wave power just above the SO₂⁺ gyrofrequency suggests that there is only a small amount of SO₂⁺ in the background plasma.