The source location of certain Jovian decametric radio emissions

Abstract

New evidence supporting the widely accepted concept that certain of the Jovian decametric radio waves originate as northern hemisphere extraordinary mode cyclotron emissions (possibly from the Io flux tube) has been found in the Voyager radio observations. Shortly after the closest approach to Jupiter, the wave signals received by Voyager 1 near 10 MHz exhibited cusps in the fringe pattern which is attributed to Faraday rotation in the Io plasma torus. The diminished Faraday rotation indicated by these cusps implies that the wave path had become perpendicular to the magnetic field in the torus. At nearly the same time, the wave polarization near 1 MHz (where Faraday rotation is absent) exhibited a sudden reversal of its rotation sense, indicating that the wave path for those frequencies had also become perpendicular to the magnetic field at the spacecraft. For these two events it was possible to project the wave paths back toward Jupiter and determine to some extent where the waves originated. It was found that the waves came from the northern hemisphere, at progressively lower altitudes with increasing frequency, and if the source is assumed to be associated with an L = 6 field line, the emission seems to have occurred near the source cyclotron frequency somewhere in the local midnight sector. This suggests that the source could be at the Io flux tube, since Io was then also near local midnight. Since the polarization rotation sense was apparently right handed with respect to the source magnetic field, the emitted wave mode must have been extraordinary. Furthermore, in order for the torus to produce a detectable Faraday rotation, the emitted wave polarization must have been substantially noncircular, and that would require a low plasma density near the source, much like that which occurs with auroral kilometric radiation at the earth.