Mode conversion at the Jovian plasma sheet boundary

Abstract

The plasma wave data obtained by Galileo in Jupiter's magnetosphere often exhibit three distinct frequency bands in the frequency range between a few hertz and a few kilohertz. It is shown that these emissions are generally electromagnetic. They are identified by relating their characteristic frequencies to the solutions of the cold plasma dispersion relation. Four modes are possible: X, Z, O, and whistler. Knowing the electron gyrofrequency ƒce measured by the fluxgate magnetometer, we have considered two different hypotheses for the observed lower‐frequency cutoff of the intermediate frequency emissions which occur below ƒce. Under these assumptions, characteristic frequencies have been computed from the cold plasma theory and compared with the set of cutoff frequencies derived from the observations. Consistency checks lead to the identification of the intermediate frequency band as being on O mode with a low‐frequency cutoff at the electron plasma frequency ƒp. Below the O mode, Galileo detects whistler mode emissions (below ƒp). Above ƒce the observed emission is consistent with being X mode. An attempt is made to identify the source of the O mode radiation. Quasi‐electrostatic waves are sometimes identified below the upper hybrid frequency when the plasma sheet boundary is crossed. We suggest that these electrostatic waves, which are presumably generated by field‐aligned electron beams flowing along plasma sheet boundary, are successively mode converted into Z and later O mode. Thus the O mode observed mostly outside the plasma sheet is generated by mode conversion of primary electrostatic waves.