M. Roth

Vlasov theory of the equilibrium structure of tangential discontinuities in space plasmas

Extensive theoretical work has been performed on the equilibrium structure of tangential discontinuities (TDs) in collisionless plasmas. This paper reviews kinetic models based on steady-state solutions of the Vlasov equation. It is shown that most of the existing models are special cases of a generalized multi-species model. In this generalized model all particle populations -from both outer regions and from inside the layer — are described using a unique formalism for the velocity distribution functions. Because of their historical importance, the Harris and Sestero models are reviewed and deduced from the generalized model. The Lee and Kan model is also a special case of the generalized model. The generalized model, however, is also able to describe TDs with velocity shear and large angles of magnetic field rotation. Such a multi-species model with a large number of free parameters and different gradient scales illustrates many observable features of TDs, including their multiscale fine structure. Particular attention is paid to the magnetopause. Observed magnetopause crossings are simulated. The effects of the relative flow velocity and asymmetrical magnetic field profiles on the structure of the magnetopause and on its stability with respect to tearing perturbations are discussed. We also present calculations that demonstrate the potential of the generalized model in explaining the origin of discrete auroral arcs. Numerical simulations of solar wind TDs with heavy ions and a large spectrum of thicknesses are also feasible. This indicates that such a model is of fundamental importance for understanding the detailed structure of solar wind TDs, like those observed by the interplanetary spacecraft ULYSSES. The problems associated with the one-dimensional, time-independent Vlasov approach are discussed and a variational principle is suggested to reduce the arbitrariness resulting from the large number of free parameters.

Equilibrium conditions for the tangential discontinuity magnetopause

Early satellite observations of the dayside magnetopause have suggested that the magnetic field typically rotates clockwise above the solar-magnetospheric equatorial plane and counterclockwise below it, in agreement with the predictions of first-order orbit theory for magnetopause crossings of the rotational discontinuity type. The present paper treats the tangential discontinuity (TD) case. The influence of magnetosheath magnetic field and plasma flow on the magnetopause equilibrium structure is analyzed by means of a Vlasov model. The nature of the current layer plays a major role; the analysis is carried out for ion-dominated, electron-dominated, and mixed layers. Necessary and sufficient conditions for the existence of an equilibrium magnetopause are derived. It is found that (1) the magnetopause is best modeled as a transition layer of mixed type; (2) for high magnetic shear the magnetic field at the dayside magnetopause preferentially rotates clockwise above the equatorial plane and counterclockwise below it, while it rotates counterclockwise above and clockwise below the equatorial plane at the tail flanks; this effect becomes more manifest as the magnetosheath flow is faster and as the difference in proton and electron transition lengths is more pronounced; (3) for low magnetic shear, TD equilibrium is expected to be lost more easily at the dawnside than at the duskside; and (4) the model provides a magnetopause thickness estimate; in particular, the low magnetic shear dawn magnetopause is predicted to be thinner than the dusk magnetopause.

Equilibrium conditions and magnetic field rotation at the tangential discontinuity magnetopause

De Keyser and Roth recently have developed a kinetic model of the tangential discontinuity magnetopause. This model predicts (1) that not all configurations of magnetic field vectors and magnetosheath velocity allow an equilibrium to exist and (2) that there is a preference for a particular magnetic field rotation sense across the magnetopause due to the different response of ions and electrons to the electric field in the current layer. In the present paper we extend the original model to allow for different magnetospheric and magnetosheath densities and temperatures, and we show that the conclusions remain essentially unchanged. Given the large-scale magnetosheath flow pattern around the magnetosphere, we also compute which regions of the dayside magnetopause may be in tangential discontinuity equilibrium for a given magnetosheath field orientation.

Effect of the relative flow velocity on the structure and stability of the magnetopause current layer

The Vlasov kinetic approach is used to study the stability of the magnetopause current layer (MCL) when a sheared flow velocity and a sheared magnetic field both exist simultaneously within it. A modified Harris-Sestero equilibrium where the magnetic field and bulk velocity are changing direction on the same spatial scale is suggested to illustrate the generation of a y component of the magnetic field in the center of the MCL. With this equilibrium it is shown that By(0) can be of the order of Bz(∞) when the value of the shear flow (U) tends to the ion drift velocity (Ud). The modifications of the initial symmetrical Harris configuration, introduced by the presence of a shear flow, strongly influence the adiabatic interaction of the plasma with low-frequency tearing-type electromagnetic perturbations as well as the nonadiabatic response of the particles near the center of the MCL. This results in a reduction of the growth rate of the tearing mode.

Magnetic field rotation at the dayside magnetopause: AMPTE/IRM observations

Given the large-scale magnetosheath flow pattern around the magnetosphere, the tangential discontinuity magnetopause model of De Keyser and Roth predicts, for a prescribed magnetic field rotation angle and rotation sense, where equilibrium is possible on the dayside magnetopause surface and where it is not. In this paper we verify these predictions using 5 s time resolution magnetic field and plasma observations of the low-latitude dayside magnetospheric boundary acquired by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module satellite. The model is confirmed by (1) the dominant presence of large positive magnetic field rotations among the dawnside crossings north of the equator, (2) the observation of positive and negative rotations near the stagnation point and at the duskside, and (3) the rare occurrence and questionable tangential discontinuity nature of low magnetic shear dawnside crossings. The absence of tangential discontinuity equilibrium in dawnside low shear crossings is consistent with the observation of increased dawnside low-latitude boundary layer thickness for northward magnetosheath field reported in the literature.

The plasmapause as a plasma sheath: a minimum thickness

We consider the plasmapause as a stationary boundary layer (a sheath) separating two types of plasmas characterized by different temperatures and densities: on one side, the hot trapped particles imbedded in the cold exospheric plasma of ionospheric origin, and on the other side the plasmatrough including the hot ring current particles. The structure of this layer is described by a kinetic model using the Vlasov-Maxwells equations for the charged particles and fields. In absence of any collisional effects or wave-particle interactions a minimum value for the thickness of the plasmapause is deduced which is of the order of 5 times the Larmor radius of the cold ions.

A Survey of Field-Aligned Mach Number and Plasma Beta in the Solar Wind

We have surveyed solar wind plasma beta and field-aligned Alfvenic Mach number using Ulysses and Wind data. We show the characteristic timescale and occurrence frequency of ‘magnetically dominated’ solar wind, whose interaction with a planetary magnetosphere may produce a bow shock with multiple shock fronts. We discuss radial, latitudinal, and solar cycle effects.