CONSTRAINTS ON JOVIAN PLASMA PROPERTIES FROM A DISPERSION ANALYSIS OFUNDUCTED WHISTLERS IN THE WARM IO TORUS

The dispersion of unducted lightning-generated whistlers observed by V oyager I in the warm torus around density peaks at L = 5.7 and L = 5.9 are analyzed using the HOTRAY code, which incorporates a newly develo ped diffusive equilibrium density model for the Io torus. Since the wa ve propagation characteristics are primarily controlled by electron de nsity, a simplified two-ion (H+ and O+) model has been used to simulat e wave dispersion. The properties of O+ are adjusted to simulate the e lectron density variation at low latitudes (less than or equal to 20 d egrees), where heavy ions dominate, and a variable Hf component is add ed to model the electron density at higher latitudes. Both the offset and tilt of the Jovian magnetic dipole are taken into account to deter mine the electron distribution as a function of System III longitude. The results confirm earlier suggestions that modest therml anisotropie s (T-perpendicular to > T-parallel to) Of heavy ions are required to m atch the observed whistler dispersion. Proton concentrations typically lie in the range 5-10%, with larger values in the outer torus. On the basis of these optimum plasma parameters, the observed upper cutoff f requencies (similar to 6 kHz) imply a minimum electron density of abou t 8 cm(-3) at high latitudes along field lines that map into the warm torus. This analysis of unducted whistlers indicates that all observed waves originate in the northern hemisphere rather than the southern h emisphere, as assumed in earlier studies of ducted waves. This new res ult is consistent with optical lightning events, which were only obser ved in the northern hemisphere by Voyager 1.