Sounder stimulated D_n resonances in Jupiter's Io plasma torus

Abstract

On February 8, 1992, the Ulysses spacecraft passed through Jupiter's Io plasma torus, where rich spectra of narrow-band resonances were stimulated by the relaxation sounder of the Ulysses unified radio and plasma wave (URAP) instrument. Since the gyrofrequency f_g is comparable to the plasma frequency f_p in the Io torus, it was predicted that the general classification of stimulated ionospheric D_n resonances, developed for 1<=f_p/f_g<=8 in the Earth's topside ionosphere, should apply in the Io torus as well as the Earth's magnetosphere (Osherovich, 1989). The URAP plasmagrams (sounder spectra) in the portions of the Io torus satisfying these plasma conditions are dominated by the D_n resonances for frequencies below f_p. On most of these plasmagrams the f_p resonance is also present, but it is seldom the dominant resonance. Neither upper hybrid nor nf_g resonances have been found on these plasmagrams. The identification of D_n resonances has allowed both the electron density and the magnetic field amplitude to be calculated. The derived densities on the outbound pass agree well with a Voyager model of Bagenal (1992). The derived magnetic field values are close to the Goddard Space Flight Center O₆ model magnetic field.