GALILEO DIRECTION FINDING OF JOVIAN RADIO EMISSIONS

The Galileo spacecraft, in orbit about Jupiter, has observed distinct spin modulation of plasma wave emissions near the Ganymede (G1 and G2) encounters in the frequency range from about 100 kHz to approximately 6 MHz. Assuming circularly polarized, transverse electromagnetic radi ation, we have used the spin modulation of the sweep-frequency receive rs of the electric dipole antenna over many spins to estimate the sour ce location in the spin plane of the spacecraft. Hectometric (HOM) and decametric (DAM) emission is observed by Galileo as a general and con tinuous background with frequent bursts that last tens of minutes and can be separated by minutes or hours. We have analyzed HOM and DAM emi ssions observed near Jupiter just after the G1 and G2 encounters, incl uding two HOM/DAM ''arc'' signatures observed after the G2 encounter. These latter appear to be low-frequency extensions of DAM arcs, with s ource regions along either the lo or the Ganymede flux tube. While the uncertainties associated with the data analysis do not allow a precis e source location, the HOM/DAM emission observed near the G1 and G2 en counters is consistent with a gyroresonant source region, but it is ne cessary to require refraction due to the Io torus to understand the re sults. To explain emission from apparent source regions above a gyrore sonant source region, wave refraction from asymmetries in the Io plasm a torus that extend along magnetic field lines is postulated. Alternat ively, if such torus density asymmetries do not exist, emission with s ources above a gyroresonant source region would require another free-e nergy source such as energetic plasma beams in the presence of density gradients or temperature anisotropies.