Michel Moncuquet

Solar wind radial and latitudinal structure: Electron density and core temperature from Ulysses thermal noise spectroscopy

We present new in situ solar wind plasma measurements obtained during Ulysses fast transit from the south solar pole to the north one, which took place 1 year before the 1996 sunspot minimum. The data were obtained with the radio receiver of the Unified Radio and Plasma Wave Experiment, using the method of quasi-thermal noise spectroscopy, which is relatively immune to spacecraft potential perturbations and whose density measurements are independent on gain calibrations. We analyze the electron density and the core electron temperature. We deduce their radial profiles in the steady state fast solar wind; southward of 40° latitude, between 1.52 and 2.31 AU, the total electron density varies as n e ∞ r (-2.003±0.015) , while the core temperature varies as T c ∞ r (-064±0.03) . This allows to estimate the interplanetary electrostatic field using a simplified fluid equation. We also study, poleward of 40° (where the variance of both parameters are very low), the histograms of the electron density and core temperature scaled to 1 AU, assuming the above determined radial variation. Each histogram shows a single class of flow with a roughly normal distribution. We find a mean electron density of 2.65 cm -3 in the southern hemisphere which is about 8% larger than in the northern one. The core temperature histogram is centered at a mean of 7.5x10 4 K in the south, and of 7x10 4 K in the north. This small asymmetry may be due to a genuine solar asymmetry between the two hemispheres and/or to a temporal variation since solar activity slightly decreased during the Ulysses exploration.

Latitudinal structure of outer Io plasma torus

[1] We present a model of the latitudinal structure of the Io plasma torus (IPT), which is able to explain Ulysses results and to reconcile several in situ data sets. Basically, the observed temperature inversion and the polytropic law are due to ‘‘velocity filtration’’ of particles having non-Maxwellian velocity distributions. This mechanism acts as a highpass filter for particle energies if the particles are confined in an attractive monotonic potential well. These conditions are met in the IPT, where the attractive potential is due to the centrifugal force that confines plasma ions since the plasma is corotating with Jupiter, whereas electrons are confined by an ambipolar electric field preserving electric neutrality, and the electron velocity distribution is known to have a suprathermal tail. The suprathermal electron population has a velocity distribution that decreases with increasing energy as a power law, as is frequently observed in space plasmas, and the velocity distribution can be conveniently modeled with a ‘‘kappa’’ function [Meyer-Vernet et al., 1995]. Adopting such a kappa distribution for the electrons and for all ion species detected in the torus and including temperature anisotropy, we construct a collisionless kinetic model based on the so-called ‘‘bi-kappa distributions’’ to calculate the latitudinal structure. Following Bagenal [1994], we adopt the nearly equatorial data set from Voyager 1 to represent empirically the radial structure. The model reconciles the Voyager 1 and 2 and Ulysses observations and demonstrates that these data sets possess similar latitudinal and radial variations of the IPT densities and temperatures. This model also generates a radial ion temperature profile past � 7.5 Jovian radii, which is compatible with a quasiadiabatic radial temperature decrease at the torus equator. INDEX TERMS: 5780 Planetology: Fluid Planets: Tori and exospheres; 6218 Planetology: Solar System Objects: Jovian satellites; 2756 Magnetospheric Physics: Planetary magnetospheres (5443, 5737, 6030); KEYWORDS: Io, plasma, torus, kinetic, model, confinement

Bernstein waves in the Io plasma torus: A novel kind of electron temperature sensor

During Ulysses passage through the Io plasma torus, along a basically north-to-south trajectory crossing the magnetic equator at R ∼ 7.8 RJ from Jupiter, the Unified Radio and Plasma Wave experiment observed weakly banded emissions with well-defined minima at gyroharmonics. These noise bands are interpreted as stable electrostatic fluctuations in Bernstein modes. The finite size of the antenna is shown to produce an apparent polarization depending on the wavelength, so that measuring the spin modulation as a function of frequency yields the gyroradius and thus the local cold electron temperature. This determination is not affected by a very small concentration of suprathermal electrons, is independent of any gain calibration, and does not require an independent magnetic field measurement. We find that the temperature increases with latitude, from ∼1.3 × 105 K near the magnetic (or centrifugal) equator, to approximately twice this value at ±10° latitude (i.e., a distance of ∼1.3 RJ from the magnetic equatorial plane). As a by-product, we also deduce the magnetic field strength with a few percent error.

Quasi-thermal noise in a drifting plasma: Theory and application to solar wind diagnostic on Ulysses

The present paper provides the basic principles and analytic expressions of the quasi-thermal noise spectroscopy extended to measure the plasma bulk speed, as a tool for in situ space plasma diagnostics. This method is based on the analysis of the electrostatic field spectrum produced by the quasi-thermal fluctuations of the electrons and by the Doppler- shifted thermal fluctuations of the ions; it requires a sensitive radio receiver connected to an electric wire dipole antenna. Neglecting the plasma bulk speed, the technique has been routinely used in the low-speed solar wind, and it gives accurate measurements of the electron density and core temperature, in addition to estimates of parameters of the hot electron component. The present generalization of the method takes into account the plasma speed and thereby improves the thermal electron temperature diagnostic. The technique, which is relatively immune to spacecraft potential and photoelectron perturbations, is complementary to standard electrostatic analysers. Application to the radio receiver data from the Ulysses spacecraft yields an accurate plasma diagnostic. Comparisons of these results with those deduced from the particle analyser experiment on board Ulysses are presented and discussed.

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

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 ∼8 R j ). 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 B, 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 (N. Meyer-Vernet, M. Moncuquet, and S. Hoang, 1995).

Solar wind electron parameters from quasi‐thermal noise spectroscopy and comparison with other measurements on Ulysses

Plasma thermal noise spectroscopy was used for the first time on a large scale on the Ulysses radio receiver data to measure the solar wind electron density and temperature in the ecliptic plane. The validity and limitations of the results obtained with this method are discussed. Nearly simultaneous measurements of the electron density and temperature from the radio receiver, the sounder, and the electron analyzer on Ulysses are intercompared. The thermal noise measurements are found to compare quite well with the other measurements, apart from some discrepancies, which are discussed. The uncertainties on the core temperature, derived from a least squares model fitting of the radio data, are shown to be statistically consistent and significant.

Solar wind thermal electrons in the ecliptic plane between 1 and 4 AU - Preliminary results from the Ulysses radio receiver

The radio receiver of the Unified Radio and Plasma (URAP) experiment aboard the Ulysses spacecraft records spectra of the quasi-thermal plasma noise. The interpretation of these spectra allows the determination of the total electron density Ne and of the cold (core) electron temperature Tc in the solar wind. A single power law does not fit the variations of Ne which result from the contribution from different solar wind structures. The distribution of the values of Tc suggests that, on the average, the solar wind is nearly isothermal.

Detection of Bernstein wave forbidden bands in the Jovian magnetosphere : A new way to measure the electron density

We analyze the power spectra measured by the radio receiver of the Unified Radio and Plasma Wave experiment on Ulysses during its passage through the Jovian inner magnetosphere from ∼ 9 RJ in the outskirts of the Io plasma torus to ∼ 13 RJ near the plasma sheet. Below the plasma frequency ƒp, these spectra are weakly banded between gyroharmonics. These observations were interpreted by Meyer-Vernet et al. [1993] as quasi-thermal fluctuations in Bernstein waves. We show that above ƒp each observed gyroharmonic band falls off very abruptly on its high-frequency side. We interpret it as the “forbidden band” predicted by the Bernstein wave dispersion equation between the so-called ƒQ frequency and the consecutive gyroharmonic, that is, a region where no Bernstein wave can propagate. This allows a determination of the local cold plasma frequency and thus of the core electron density with a ∼ 16% uncertainty. As a consistency check, we show that the ƒQ thus determined are very close to the frequencies of the resonances excited by the relaxation sounder on Ulysses.

Electron density and temperature in the Io plasma torus from Ulysses thermal noise measurements

Abstract During the Ulysses flyby of Jupiter, the spacecraft crossed the outer part of the Io plasma torus along a basically North-to-South trajectory at a Jovicentric distance of about 8 R J . The quasi-thermal noise measured by the Unified Radio and Plasma Wave (URAP) experiment is used to deduce the electron density and temperature along the trajectory. The density is deduced from the upper hybrid frequency line and the temperature from the spin modulation of Bernstein waves. These results are used to build a simplified Gaussian model of the torus. The density profile is roughly symmetric with respect to the centrifugal equator, with a scale height of about 0.9 R J . The density at equator crossing is twice as large as that expected from the Divine-Garrett Voyager -based model at the same radial distance. The density scale height is lower than that found by Voyager 1; it is consistent with an ion temperature of about 5 × 10 5 K, assuming an effective mass of about 20 proton masses. The fitting of the pressure distribution, symmetric with respect to the centrifugal equator, yields a cold electron temperature of about 1.4 × 10 5 K at the equator, which is of the same order of magnitude as found by Voyager 1.

A novel method to measure the solar wind speed

We propose a novel method to measure in situ the bulk speed of a space plasma. It is based on the analysis of the electrostatic field spectrum produced by the Doppler-shifted thermal fluctuations of the plasma ions which can be measured with a sensitive receiver at the terminals of a passive electric antenna. We present a preliminary application in the solar wind using the data acquired in the ecliptic plane by the Unified Radio and Plasma experiment (URAP) on the Ulysses spacecraft. This should allow us to extend to the bulk speed the method of thermal noise spectroscopy which already gives an accurate in situ diagnosis of the electron density and bulk temperature. This method can be complementary to classical electrostatic analyzers for both interplanetary and magnetospheric studies.