JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. A11, PAGES 21,697-21,708, NOVEMBER 1, 1995

Dispersion of electrostatic waves in the Io plasma torus
and derived electron temperature

M. Moncuquet, N. Meyer-Vernet, and S. Hoang

Département de Recherche Spatiale, Centre National de la Recherche Scientifique,
Observatoire de Paris, France

Abstract:

We present a detailed analysis of the set of radio spectra acquired during the Ulysses spacecraft passage through the outer part of the Io plasma torus (at ). Since Ulysses is the first spacecraft to explore the torus far from the equator and the onboard plasma analyzers were shut off, these wave data are the only ones providing an in situ plasma diagnostics outside the equatorial region. We present here two main results. First, by comparing the observed spin modulation of the signal measured between electron gyroharmonics to the theoretical modulation for electrostatic waves propagating roughly normal to , we deduce experimental dispersion curves for these waves. These curves are very similar to Bernstein dispersion curves in a quasi-thermal plasma. Second, by fitting these theoretical dispersion relations to the experimental ones, we are able to deduce the core electron temperature with about 20% uncertainty when the density is measured independently. Otherwise, we can get a rough evaluation of both the density and the temperature. The corresponding latitudinal variation of these parameters is analyzed in a related paper [Meyer-Vernet, Moncuquet and Hoang, 1995].

S. Hoang, N. Meyer-Vernet, and M. Moncuquet, Département de Recherche Spatiale, Observatoire de Paris, 5 Place Jules Janssen, F-92195 Meudon Cedex, France. (e-mail: hoang@megasx.obspm.fr; meyer@obspm.fr; moncuquet@obspm.fr)

December 28, 1994; revised July 24, 1995; accepted July 25, 1995

• 1. Introduction
• 2. From Measured Spectra to Experimental Dispersion Curves
  • 2.1. Preliminary Remarks About the Observations
  • 2.2. Computing the Antenna Response
  • 2.3. The Experimental Dispersion Curves
  • 2.4. Some Limitations of the Method
• 3. From Dispersion Characteristics to Electron Temperature
  • 3.1. Fitting the Temperature When the Plasma Frequency is Known
  • 3.2. Fitting with both Adjustable Temperature and Density
• 4. Temperature Measurement in a Non-Maxwellian Plasma
• 5. Summary and Final Remarks
• Appendix A: Doppler Effect on the Antenna Response to Bernstein Waves
• Appendix B: Dispersion Relations in a Maxwellian Plasma
• References