Karine Issautier

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.

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.

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.

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.

Solar wind plasma parameters on Ulysses: Detailed comparison between the URAP and SWOOPS experiments

The presence of several instruments measuring plasma parameters aboard Ulysses made possible an extensive comparison between them. In this paper, we focus on solar wind electron parameters measured by the Unified Radio and Plasma (URAP) wave receiver, using the thermal noise spectroscopy method, and by the Solar Wind Observations Over the Poles of the Sun (SWOOPS) electron and ion analyzers. We compare the data sets acquired by both experiments during the ecliptic path in 1990/1991 and the first pole-to-pole fast transit of Ulysses in 1994/1995, near the maximum and minimum of solar activity, respectively. From the comparison, we find very good agreement between the total density derived by both experiments; the average offset between the two data sets differs by less than 5%. We find a systematic offset of 5% to 10% in the core electron temperature values depending on the solar wind flow speed. We present detailed observations obtained during the crossing of the stream interface on March 28, 1995, by Ulysses, when it returned to the high-speed wind region in the northern hemisphere. We find that the core electron temperature increases around 40% at the interface, which is in agreement with the statistical result obtained using IMP plasma measurements at 1 AU. We point out that SWOOPS provides reliable ion diagnostics in addition to suprathermal (halo) electron parameters and yields fundamental information on particle velocity distributions, whereas the improved quasi-thermal noise method provides total electron densities and core electron temperatures accurately.

Wind-Ulysses in-situ thermal noise measurements of solar wind electron density and core temperature at solar maximum and minimum

The radio receivers on the Wind and Ulysses spacecraft in the solar wind continuously record spectra of the quasi-thermal plasma noise near the electron plasma frequency, from which the electron density and core temperature can be determined using the method of quasi-thermal noise spectroscopy. Such in-situ thermal noise measurements were obtained by Wind in the in-ecliptic solar wind upstream of the Earth, and by Ulysses during its two pole-to-pole fast latitude scans, in 2000–2001 at solar maximum and in 1994–1995 at solar minimum. We present histograms of these measurements performed over common time spans of several months by the two spacecraft. We compare and discuss these histograms, together with those provided by Ulysses in 1990–1991 at solar maximum, thus extending our study to over a full solar activity cycle. From the resulting distributions, we classify the solar wind flow into distinct populations both in density and temperature. Their variations in number and importance with the solar activity and heliolatitude are also investigated.

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.

Quasi-Thermal Noise Diagnostics in Space Plasmas

We present the method of plasma diagnostics by quasi-thermal noise spectroscopy and show examples of application in the solar wind and the Earths plasmasphere. Using only an electric antenna and a radio receiver, diagnostics of various space environments (magnetized or not) can be obtained in situ. Because of its accuracy, this technique can be used to cross-check other plasma sensors.

Electron Temperature in the Solar Wind from a Kinetic Collisionless Model: Application to High-Latitude Ulysses Observations

We use a kinetic collisionless model of the solar wind to calculate the radial variation of the electron temperature and obtain analytical expressions at large radial distances. In order to be compared with Ulysses observations, the model, which initially assumed a radial magnetic field, has been generalized to a spiral magnetic field. We present a preliminary comparison with Ulysses observations in the fast solar wind at high heliospheric latitudes.

High-speed solar wind from Ulysses measurements and comparison with exospheric models

We outline the main results obtained in situ by thermal noise spectroscopy on Ulysses at high latitudes. The Ulysses trajectory allows for the first time to obtain a data sample of the steady-state fast solar wind, which is free of bias due to the presence of interaction regions and to data selection between different types of winds. This enables us to derive in particular the core electron temperature radial profile. Besides, we present a kinetic collisionless model of the solar wind which predicts the electron temperature profile and the terminal velocity. We give analytical expressions at large distances, which can explain the trends of the temperature profiles obtained by previous studies.