Constraints on Jovian plasma properties from a dispersion analysis of unducted whistlers in the warm Io torus

The dispersion of unducted lightning-generated whistlers observed by Voyager 1 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 developed diffusive equilibrium density model for the Io torus. Since the wave propagation characteristics are primarily controlled by electron density, a simplified two-ion (H+ and O+) model has been used to simulate wave dispersion. The properties of O+ are adjusted to simulate the electron density variation at low latitudes (≤20°), where heavy ions dominate, and a variable H+ component is added to model the electron density at higher latitudes. Both the offset and tilt of the Jovian magnetic dipole are taken into account to determine the electron distribution as a function of System III longitude. The results confirm earlier suggestions that modest therml anisotropies (T⊥ > T‖) of heavy ions are required to match 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 frequencies (∼ 6 kHz) imply a minimum electron density of about 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 hemisphere, as assumed in earlier studies of ducted waves. This new result is consistent with optical lightning events, which were only observed in the northern hemisphere by Voyager 1.