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X. Blanco-Cano (Instituto de Geofisica, UNAM), C. T. Russell (IGPP, UCLA), R. J. Strangeway (IGPP,UCLA), M. G. Kivelson (IGPP, UCLA)
Previous Io flyby passes occurred at solar phase angles near noon (I0), near 1012 LT (I24, I25), and near 0900 LT (I27). These passes showed systematic changes with solar phase angle. On I0 the ion cyclotron waves had been peaked very strongly near the SO2+ ion cyclotron frequency inbound when the spacecraft was above the daylit hemisphere of Io but were very broad in frequency extent. Above the dark side of Io the wave frequency switched to that of SO+. On subsequent passes (at earlier local times) the wave amplitude was less than on I0 and ion cyclotron waves of multiple species were seen simultaneously in narrow frequency bands. Thus on the I31 pass, which occurred at about 0420 LT, we expected to observe weak narrow-banded signals. Instead the signals are as strong as on the passes near 1030 LT and they are quite broad in frequency extent. The strength of the waves may be explained by the trajectory of Galileo that remained close to the edge of the co-rotation wake. The broad spectrum of waves suggests that ions from about the mass of sulfuric acid (98mp) to that of sulfur (32mp) were being picked up. Over this whole band the waves are left-hand elliptically polarized and propagate at a small angle to the field. We perform kinetic dispersion analysis to estimate growth rates of the observed waves.