MODE CONVERSION AT THE JOVIAN PLASMA SHEET BOUNDARY

The plasma wave data obtained by Galileo in Jupiter's magnetosphere of ten exhibit three distinct frequency bands in the frequency range betw een a few hertz and a few kilohertz. It is shown that these emissions are generally electromagnetic. They are identified by relating their c haracteristic frequencies to the solutions of the cold plasma dispersi on relation. Four modes are possible: X, Z, O, and whistler. Knowing t he electron gyrofrequency f(ce),, measured by the fluxgate magnetomete r, we have considered two different hypotheses for the observed lower- frequency cutoff of the intermediate frequency emissions which occur b elow f(ce),,. Under these assumptions, characteristic frequencies have been computed from the cold plasma theory and compared with the set o f cutoff frequencies derived from the observations. Consistency checks lead to the identification of the intermediate frequency band as bein g on O mode with a low-frequency cutoff at the electron plasma frequen cy f(p),. Below the O mode, Galileo detects whistler mode emissions (b elow f(p),). Above f(ce),, the observed emission is consistent with be ing X mode. An attempt is made to identify the source of the O mode ra diation. Quasi-electrostatic waves are sometimes identified below the upper hybrid frequency when the plasma sheet boundary is crossed. We s uggest that these electrostatic waves, which are presumably generated by field-aligned electron beams flowing along plasma sheet boundary, a re successively mode converted into Z and later O mode. Thus the O mod e observed mostly outside the plasma sheet is generated by mode conver sion of primary electrostatic waves.