Nicole Meyer-Vernet

Plasma diagnosis from thermal noise and limits on dust flux or mass in comet Giacobini-Zinner

Thermal noise spectroscopy was used to measure the density and temperature of the main (cold) electron plasma population during 2 hours (1.5x105 kilometers perpendicular to the tail axis) around the point of closest approach of the International Cometary Explorer (ICE) to Comet Giacobini-Zinner. The time resolution was 18 seconds (370 kilometers) in the plasma tail and 54 seconds (1100 kilometers) elsewhere. Near the tail axis, the maximum plasma density was 670 per cubic centimeter and the temperature slightly above 1 electron volt. Away from the axis, the plasma density dropped to 100 per cubic centimeter (temperature, 2x 104 K) over 2000 kilometers, then decreased to 10 (1.5x 105K) over 15,000 kilometers; outside that region (plasma tail), the density fluctuated between 10 and 30 per cubic centimeter and the temperature between 1x 105 and 4 x105 K. The relative density of the hot population rarely exceeded a few percent. The tail was highly asymmetrical and showed much structure. On the other antenna, shot noise was recorded from the plasma particle impacts on the spacecraft body. No evidence was found of grain impacts on the antennas or spacecraft in the plasma tail. This yields an upper limit for the dust flux or particle mass, indicating either fluxes or masses in the tail smaller than implied by the models or an anomalous grain structure. This seems to support earlier suggestions that these grains are featherlike. Outside the tail, and particularly near 105 kilometers from its axis, impulsive noises indicating plasma turbulence were observed.

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

Electrostatic noise in non-Maxwellian plasmas: Generic properties and Kappa distributions

Although the conventional use of electric antennae is for remote sensing by detection of electromagnetic waves, they can also be used for in situ measurements, by detecting electrostatic waves produced by the random motion of the ambient plasma particles. The spectroscopy of this quasithermal noise near the plasma frequency is currently used for measuring electron parameters in space plasmas [see Meyer-Vernet and Perche, 1989 and references therein]. It is also planned to be used on future missions such as Ulysses and Wind in the solar wind, and CRAF and Cassini in a cometary and the Saturnian environment, respectively. This method is complementary to cofiventional electron

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.

Large scale structure of planetary environments: the importance of not being Maxwellian

Abstract The velocity distributions observed in space have too many fast particles, by Maxwells standards. This ubiquitous property raises doubts about the validity of models based on a set of fluid equations whose closure requires the distributions to be nearly Maxwellian. I discuss here two generic cases: bound structures and winds. Near rapidly rotating magnetised planets, particles channelled along co-rotating magnetic field lines are acted on by the field-aligned component of the centrifugal force, which exceeds the gravitational attraction beyond a few planetary radii. With dipolar magnetic fields, this tends to trap particles near the equator and produce torus-shaped structures, whereas gravitational confinement occurs closer to the planet. These confining forces act as high-pass filters for particle speeds, so that the temperatures are rising with distance from the potential wells, if the velocity distributions are not Maxwellian — in sharp contrast to classical isothermal equilibrium; and the density profiles fall off less steeply than a Gaussian — just as the velocity distributions fall off less steeply than a Maxwellian. While these bound structures are shaped along closed magnetic field lines, winds can blow along open field lines. A suprathermal tail in the electron velocity distribution increases the electric field which ensures the balance of ion and electron fluxes, and should thus increase the wind speed above the value predicted by classical hydrodynamic escape.

Electron temperature in the solar wind: Generic radial variation from kinetic collisionless models

We calculate analytically the radial profile of the average electron temperature in the solar wind with a kinetic collisionless model. The electron temperature profile at large distances r is the sum of a term r -4/3 plus a constant, with both terms of the same order of magnitude near r ∼ 1 AU. This result is generic as it is weakly dependent on the particle velocity distributions in the corona. It provides a natural explanation for the observed electron temperature profile near 1 AU, which is in the low or middle part of the range between isothermal and adiabatic behaviors. The r -4/3 term comes from the isotropically distributed electrons confined by the heliospheric electric potential, which is found to have a similar radial variation. The constant term comes from the parallel temperature of the electrons energetic enough to escape. The calculated profile flattens as r increases and tends to be flatter in the high-speed wind. We also give simple explicit expressions for the electron temperature and density at large distances and for the terminal wind velocity as a function of coronal parameters when the electron velocity distribution is a Kappa function, which is close to a Maxwellian with a suprathermal tail.

Dust distribution around Neptune: Grain impacts near the ring plane measured by the Voyager Planetary Radio Astronomy Experiment

During the Voyager 2 flyby of Neptune the planetary radio astronomy (PRA) experiment recorded an intense noise near the equatorial plane around 3.4 and 4.2 RN, as already observed during previous Voyager ring plane crossings at Saturn and Uranus. This noise is interpreted as being due to impact ionization of dust grains striking the spacecraft. We deduce a power law index of the grain mass distribution of about 2. The PRA system is sensitive to particles with radii larger than ∼1.6 μm, and the largest particles, detected near the ring plane, are evaluated to have a radius of ∼10 μm. The spatial dust distribution along the spacecraft trajectory around the two equatorial crossings is found not to be symmetrical with respect to the ring plane and spread over wide regions: over ∼2 RN perpendicularly to the equatorial plane with the densest part concentrated within ∼700 km. The vertical optical depth τ of this dense region is found to be 10−6 – 10−8.

Collisionless model of the solar wind in a spiral magnetic field

We present a kinetic collisionless model of the solar wind generalized to take into account the spiral structure of the interplanetary magnetic field. This model, which also includes Kappa velocity distributions, calculates self-consistently the electric potential profile and derives the solar wind speed and the temperatures of the medium. We study how the inclusion of the spiral geometry changes the plasma parameters compared to the case of a radial magnetic field. Whereas the interplanetary electric potential, the wind density and bulk speed are not significantly changed, we show that the electron and proton temperatures are modified; in particular, we find a decrease of the proton temperature and of its anisotropy, and an increase of the electron temperature. We discuss these results and the validity of the model.