D. Hubert

ELECTRON PROPERTIES AND COULOMB COLLISIONS IN THE SOLAR WIND AT 1 AU: WIND OBSERVATIONS

The question of what controls the electron properties in the solar wind has been the subject of several extensive analyses over the past 20 years. We analyze here the electron properties of the solar wind observed by the Wind satellite at 1 AU in the ecliptic plane, during 50 days close to the last minimum of solar activity. The electron temperature anisotropy Te∥/Te⊥, which seems to depend on the wind speed Vsw, the density Np, the heliomagnetic latitude λm, or the time, actually depends mainly on the Coulomb collisions. The collisional age Ae is the number of transverse collisions suffered by a thermal electron during the expansion of the wind over the scale of the density gradient. The Ae depends on Vsw, on Np, and thus on λm; it also depends on the time because it changes strongly at the crossing of a stream interface. We show that Te∥/Te⊥ is strongly correlated with 1/Ae. The effect of Coulomb collisions on the electron heat flux are also investigated. We find that the total electron heat flux Qe displays an upper bound that is inversely proportional to the collisional age, in favor of a regulation of the heat flux by Coulomb collisions. The observed heat flux is then compared to the collisional heat flux of the classical Spitzer-Harm (SH) theory. Although earlier observations have shown that the electron heat flux in the solar wind at 1 AU is well below the values given by the SH theory, we find that the observed heat flux reaches the SH limit for the lowest values of the electron mean free paths. The Coulomb collisions thus seem to play a part in the regulation of the electron heat flux in the solar wind.

Interplanetary rotational discontinuities: From the solar wind to the magnetosphere through the magnetosheath

For the first time the evolution of interplanetary rotational discontinuities (RDs) are observed in the solar wind, in the magnetosheath and in the dayside magnetosphere by the ISEE 1-2-3 spacecraft. Interaction of a RD with the bow shock builds up a transient density event (TDE) in the magnetosheath, displaying two density peaks anticorrelated to the magnetic field modulus. TDEs are convected from the bow shock to the dayside magnetopause without spreading. The duration of a TDE is around 15 s and two close TDEs can merge in a single structure. The density in a TDE exceeds the surrounding level by a factor of about 35%, while the field modulus can decrease by a large amount. The interaction of a TDE with the magnetopause is analyzed in terms of impulsive penetration process. The element of plasma injected in the magnetosphere is filamentary, its signature is different from the signature of a flux transfer event or a surface wave.

A Generalized Model for the Proton Expansion in Astrophysical Winds. I. The Velocity Distribution Function Representation

We construct a new approach to model the velocity distribution function (VDF) for the protons in stellar atmosphere expansions or planetary polar winds. The generalized Grad method of construction is used, and comparisons with the bi-Maxwellian polynomial expansion model are made in applications to the solar wind in the context of the measurements made by the Helios probes between 0.3 and 1 AU. A fitting procedure based on a sum of two Maxwellian functions is used to check the convergence property of both polynomial expansions and to calculate the predicted polynomial expansion profiles along the magnetic field orientation for typical proton VDFs in the solar wind. The generalized model is better adapted than the bi-Maxwellian polynomial expansion function to reproduce the long-tail features of a majority of the observed proton VDFs; moreover, our model does not display negative values of the VDF, contrary to the bi-Maxwellian expansion for normalized heat flux larger than unity. A 16 moment approximation, which corresponds to a third order of development, allows us to provide an associated set of generalized transport equations better closed than the equivalent system associated with a bi-Maxwellian polynomial expansion.

Evidence of anisotropic temperatures of molecular ions in the auroral ionsphere

Using incoherent scatter measurements obtained from Kiruna and Sodankyla EISCAT remote stations, and corresponding to different aspect angles, the authors show, for the first time, anisotropic temperatures of molecular ions. The electric field obtained for the 5 hours period of the 2nd February 1990 experiment presented here range from 20 to 50 mV/m. They show that the line of sight ion temperature deduced from Kiruna incoherent scatter spectra is larger than the one deduced from Sodankyla spectra which correspond to a smaller aspect angle. A statistical approach has been chosen to evaluate a {beta} parameter for Sodankyla measurements which describes the anisotropy. They found a value of 0.60 {plus minus} 0.02 which compares very well with values inferred from theoretical studies.

Interplanetary magnetic field variations and slow mode transitions in the Earth's magnetosheath

The event observed on September 17, 1978 on ISEE 1–2, which led to the concept of a stationary slow mode transition region (SMT) in the magnetosheath in front of the magnetopause, is revisited. We establish that the two edges of this SMT have an exogenous origin induced by two discontinuities of the interplanetary magnetic field. The key of our analysis is that the outer edge of the SMT is built up by a tangential interplanetary discontinuity which is observed on ISEE-3 at a large distance from the Sun-Earth line and which has an unusual direction. In this SMT the subsolar magnetosheath is entirely downstream of a quasi-parallel bow shock, while upstream this SMT the subsolar magnetosheath is downstream of a quasi-perpendicular shock. We identify three effects at the origin of the density enhancement in this SMT. We extend this approach to the original statistical study and we find that any SMT is connected to interplanetary magnetic field variations. This corroborates our hypothesis that SMTs have an exogeneous origin driven by interplanetary magnetic field variations.

A Generalized Model for the Proton Expansion in Astrophysical Winds. II. The Associated Set of Transport Equations

In Paper I, we presented a new model of the velocity distribution function for protons composing a stellar atmosphere expanding in interstellar space, valid from collisional to collisionless regions. In this paper, the set of generalized transport equations associated with this model and the closure assumptions for higher order velocity moments are provided for 9 and 16 moment approximations. The study of the properties of such a set of transport equations in the collisionless limit is presented and discussed. A comparison with the similar bi-Maxwellian approximation is made using two kinds of analysis, in the context of an application to solar wind expansion. Our model is better adapted to high values of the heat flux and thus is able to provide a macroscopic parameter evolution for stellar atmosphere expansion in a state far from local equilibrium, as well as for the expansion of planetary polar winds.

Density fluctuations measured by ISEE 1-2 in the Earth's magnetosheath and the resultant scattering of radio waves

Radio waves undergo angular scattering when they propagate through a plasma with fluctuating density. We show how the angular scattering coefficient can be calculated as a function of the frequency spectrum of the local density fluctuations. In the Earth’s magnetosheath, the ISEE 1–2 propagation experiment measured the spectral power of the density fluctuations for periods in the range 300 to 1 s, which produce most of the scattering. The resultant local angular scattering coefficient can then be calculated for the first time with realistic density fluctuation spectra, which are neither Gaussian nor power laws. We present results on the variation of the local angular scattering coefficient during two crossings of the dayside magnetosheath, from the quasi-perpendicular bow shock to the magnetopause. For a radio wave at twice the local electron plasma frequency, the scattering coefficient in the major part of the magnetosheath is b(2fp) ≃ 0.5–4 × 10−9 rad2/m. The scattering coefficient is about ten times stronger in a thin sheet (0.1 to IRE) just downstream of the shock ramp, and close to the magnetopause.

Comparison of the generalized and bi‐Maxwellian multimoment multispecies approaches of the terrestrial polar wind

A comparison between two multimoment approaches is provided in the context of an application to the terrestrial polar wind. We compare the bi-Maxwellian 16-moment approach with the 16-moment generalized model, which has been built in order to account for the suprathermal part of the velocity distribution function better than the previous multimoment approaches. This comparison has shown a general similarity between the two approaches and has shown that the better adapted closure assumption of the set of transport equations of the generalized model generates a higher acceleration. Moreover, the better determination of the collisional energy transfers generates a smaller increase of the temperature of the supersonic species that generally agrees with the adiabatic cooling assumption predicted by Monte Carlo or direct resolution of the Fokker Planck equation. The generalized model also provides the typical profiles of the velocity distribution function from the original collision-dominated region to the collisionless region. These profiles are in good agreement with the Monte Carlo and collision kinetic resolution in the collision-dominated region and in the lower part of the transition region.

A GENERALIZED MODEL FOR THE PROTON EXPANSION IN ASTROPHYSICAL WINDS. III. THE COLLISIONAL TRANSFERS AND THEIR PROPERTIES

This is the third and last of a series of papers that present a new theoretical approach to modeling the expansion of the solar and terrestrial polar winds by solving the Fokker-Planck equation. The Coulomb collisional transfers between the different species that compose these winds are presented after the velocity distribution function and the set of transport equations associated with the generalized model. The method and the assumptions used to calculate these terms are described. They are derived from generic expressions, which must be numerically estimated for specific applications. Their new properties are analyzed in the context of the terrestrial polar wind, and their potential importance in the processes of heating and acceleration of the solar wind is discussed. Because the generalized model is adapted to reproduce the high suprathermal part of the velocity distribution function currently observed in the solar wind, we also emphasize the role of this contribution in the collisional transfers. In the terrestrial polar wind, the region of energy transfer between H+ and O+ ions is thinner than previously predicted. In the solar wind, the region of energy transfer between electrons and protons in the inner corona is larger than previously thought. The role of the thermal and mirror forces and the mechanisms of isotropization of the electrons are underlined.

Density and field structure of a FTE observed in the magnetosphere

ISEE1 and 2 high resolution electron density and magnetic field measurements have been used to study characteristic structures, identified as FTEs, observed inside the magnetosphere close to the magnetopause. We investigate new properties: a volcano-like signature in the magnetic field strength, a thin sheet structure at the leading edge, and a wake. The plasma compressibility is analysed through the structure. Auto and cross spectral analysis of the data reveal the signature of compressive modes in the wake of the FTE.