Temperature inversion in the Io plasma torus.

Abstract

We present in situ measurements of electron parameters as a function of latitude in the Io plasma torus, deduced from an extended analysis of the radio and plasma wave data acquired on board Ulysses. We find that the density and temperature are anti-correlated and obey an approximate polytrope law T_e ∝ √{1/n_e}. We interpret this result with a simple model based on velocity filtration by the potential which confines the particles to the equator. In the absence of local thermal equilibrium, suprathermal particles overcome more easily the potential; thus the temperature increases with latitude in anticorrelation with the density, which decreases less steeply than the Gaussian corresponding to equilibrium. This explains the observed polytrope relation, and the calculated densities fit quite well the measured density profile. These results illustrate in the Io torus a general behavior of low-density inhomogeneous plasmas, anticipated by J. D. Scudder (1992, Astrophys. J. 398, 299-319).