ICARUS 116, 202-213 (1995)

Temperature inversion in the Io plasma torus

Nicole Meyer-Vernet, Michel Moncuquet, and Sang Hoang

Département de Recherche Spatiale, CNRS URA 264, Observatoire de Paris, 92195 Meudon Cedex, France

Received September 27, 1994; revised January 19, 1995

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 anticorrelated and obey an approximate polytrope law . 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 behaviour of low-density inhomogeneous plasmas, anticipated by Scudder (1992, Astrophys. J. 398, 299-319).

• INTRODUCTION
• ELECTRON MEASUREMENTS FROM BERNSTEIN WAVES
  • Electron density and temperature
  • Significance of the temperature
• POLYTROPE RELATION
  • Relation between density and temperature
  • Discussion
• VELOCITY FILTRATION IN THE IO TORUS
  • Position of the problem
  • Velocity distribution and generalized temperatures
  • Generic anticorrelation between density and generalized temperatures
  • Kappa distribution
  • Relation between Kappa and the polytrope exponent
• PLASMA DENSITY PROFILE
  • Density profile with a Kappa velocity distribution
  • Density profile in a fluid approximation
  • Comparison with the measured density profile
• SUMMARY AND FINAL REMARKS
• APPENDIX
• References