Collisionless Space Research Articles

Theory of a thin one‐dimensional current sheet in collisionless space plasma

It is widely believed that evolution of a current sheet in collisionless space plasma often results in fast magnetic field merging, for example, during substorm onsets. The current sheet structure on the merging sites should exhibit a considerable change, in order for the field energy transformation into the energy of particles to become possible. The specific current sheet structure is the quasi‐one‐dimensional kinetic forced current sheet. A full analytical theory of such a sheet has been constructed for the typical case when the plasma parameter in the background plasma is small, β ≪ 1, and bulk motions of the plasma are sub‐alfvenic, MA ≪ 1. Theoretical consideration of that equilibrium state is based on the existence of a specific adiabatic invariant, corresponding to ion oscillations about the sheet central plane in their “Speiser” orbits. The theory describes the sheet structure self‐consistently, identifying its dependence on the features of the ion distribution function. Detailed structure of the emerging equilibrium sheet has been numerically studied. For a certain current sheet profile, while outside the sheet the calculated ion distribution function is close to a pair of interpenetrating shifted Maxwellians, inside the sheet it is also highly anisotropic but quite different, involving all velocities from 0 up to 2vA. Dependence of the structure scale length on the parameters of the problem corresponds to estimates obtained earlier. This scale length also agrees with observational estimates of the current sheet thickness in the magnetotail during its extreme thinning near substorm onset.

Electromagnetic properties of hot collisionless plasma in magnetic reconnection regions

Investigation of magnetic reconnection processes in a hot collisionless space plasma requires development of new methods for a self-consistent description of plasma dynamics in the region of an X-type magnetic fieldB(r),where kinetic effects of resonant and nonresonant interaction of particles and fields are important and the conventional MHD description is inadequate. Inside the reconnection regions, there is a very strong electric fieldE(r),which accelerates particles and produces a plasma flow with a self-consistent plasma sheet, which is known as Sweet-Parker flow. The fine structure of fields, currents, and flows in such a hot collisionless plasma remains unclear and needs special examination. In the analytical investigation, we confine ourselves to the internal part of the reconnection region, where the electric field is much stronger than the magnetic field, ¦E¦≫¦B¦.We observe a regular behavior of particles there and develop a perturbation method for Maxwell's and Vlasov's equations. The electric field is often related to inductive fields, but we focus mainly on time-invariant reconnection processes and consider a stationary electric field imposed externally and defined by external boundary conditions. In a zeroth order approximation, the magnetic field is absent, and the particles are stationary and have equilibrium distribution between acceleration and deceleration in the homogeneous electric field E0.This field is a free parameter of the problem and produces a new scale of length τ Eα =1/(-ik) Eα where k Eα =iqαE0/Tα, and a scale of frequency ω Eα =-ik Eα vαfor reconnection scaling. The linear response of the particles under fieldsE1(r)andB1(r)and the distributions of flow velocityu1(r),charge ρ1(r),and currentj1(r)are found by integrating along the particle trajectory in the homogeneous electric field. The external electric fieldE0redefines the linear electromagnetic properties of the plasma and connects, in particular, the electrostatic and electromagnetic modes via drift terms. As a result, we obtain a system of linear integral equations with external sources for a self-consistent description of the plasma behavior in regions with a strong accelerating magnetic field. We explore these regions in a small-scale field limit (kτ Eα ≫1),using a local approximation.

Ion distribution functions behind an RF sheath

Measurements of ion energy distribution functions were made at the grounded electrode of a parallel plate radiofrequency discharge by means of a gridded energy analyser in discharges in Ar, He, and O2 at pressures of the order of 0.001-0.1 mbar. In this regime the transition from the collisionless space charge sheath (in front of the electrode) to a sheath dominated by collisions, mainly charge exchange collisions, appears. Although the Debye length is small compared to the mean free path for charge exchange, for high sheath voltages the sheath thickness increases and may exceed the mean free path. Measured distributions in Ar are compared to energy distribution functions calculated in a self-consistent sheath model, including charge exchange collisions. The agreement of theory and experiment is very good.

Kelvin–Helmholtz instability in collisionless space plasmas

The drift kinetic approximation (DKA) is developed by combining the macro- and microdynamics of guiding center plasmas. It is shown that the DKA provides a better description for collisionless processes in plasmas than the Chew–Goldberger–Low approximation. A new theory of the Kelvin–Helmholtz instability (KHI) in collisionless space plasmas is established. Calculations show that the general pattern of the growth rate of the KHI versus flow velocity in the DKA is similar to that in compressional magnetohydrodynamics. However, the critical velocity decreases if pressure is anisotropic, and the growth rate and energy flux are reduced in the presence of wave–particle interactions.

The bowshock and magnetopause

The explosive growth of magnetopause and bowshock research reported by Russell and Greenstadt [1983] has continued unabated through the 1983 to 1986 quadrennium. Shock research has gone far beyond the tentative exploratory work seen in the last two quadrennia: a sizable volume of collisionless shock parameter space has been explored both in careful data analysis and in theory and sophisticated simulations. Research on the magnetopause has progressed somewhat more slowly than shock research, but new results and exciting ideas concerning reconnection and energy transfer through the magnetopause have not been lacking in the past four years. Much of this shock and magnetopause research has been due, directly or indirectly, to knowledge gained from the multi‐spacecraft ISEE mission. Exciting results from AMPTE and the recent comet missions have opened whole new areas of shock research.

Nonlinear wave interactions in a collisionless space and time dependent warm fluid plasma

Expressions for the current response operators of first and second order are calculated for a fluid plasma with isotropic pressure terms. The unperturbed state is an arbitrary space and time dependent solution of the basic equations. The response operators are given in a form which shows explicitly the mathematical structure behind the approximate conservation of wave energy in resonant wave interactions. The coefficients in the coupled mode equations are expressed in terms of these response operators.