V. S. Semenov

Magnetic field reconnection theory and the solar wind magnetosphere interaction: a review

This review considers the theory of the magnetic field line reconnection and its application to the problem of the interaction between the solar wind and the Earths magnetosphere. In particular, we discuss the reconnection models by Sonnerup and by Petschek (for both incompressible and compressible plasmas, for the asymmetric and nonsteady-state cases), the magnetic field annihilation model by Parker; Syrovatskys model of the current sheet; and Birns and Schindlers solution for the plasma sheet structure. A review of laboratory and numerical modelling experiments is given.Results concerning the field line reconnection, combined with the peculiarities of the MHD flow, were used in investigating the solar wind flow around the magnetosphere. We found that in the presence of a frozen-in magnetic field, the flow differs significantly from that in a pure gas dynamic case; in particular, at the subsolar. part of the magnetopause a ‘stagnation’ line appears (i.e., a line along which the stream lines are branching) instead of a stagnation point. The length and location of the stagnation line determine the character of the interaction of the solar wind with the Earths magnetosphere. We have developed the theory of that interaction for a steady-state case, and compare the results of the calculations with the experimental data.In the last section of the review, we propose a qualitative model of the solar wind — the Earths magnetosphere interaction in the nonsteady-state case on the basis of the solution of the problem of the spontaneous magnetic field line reconnection.

A reconnection layer associated with a magnetic cloud

Abstract We examine a 3-hour long interval on December 24, 1996, containing a magnetic hole associated with an interplanetary magnetic cloud. Two sets of perturbations are observed by the Wind spacecraft at 1 AU. In the first, the field and flow rotate at constant field strength, and the plasma is accelerated to the local Alfven speed. We show this to be a rotational discontinuity. In the second, observed 25 min later, the plasma is heated and the field decreases. We show this to be a slow shock. The whole structure is in pressure balance. We interpret the observations as MHD discontinuities arriving with varying delays from a reconnection site closer to the Sun. Energetic particle observations suggest further that ejecta material is present for many hours prior to the magnetic cloud observation and separated from it by the layer. This suggests that reconnection took place between field lines of a CME of which the magnetic cloud formed a part.

Rapid reconnection in compressible plasma

A study of set‐up, propagation, and interaction of non‐linear and linear magnetohydrodynamic waves driven by magnetic reconnection is presented. The source term of the waves generated by magnetic reconnection is obtained explicitly in terms of the initial background conditions and the local reconnection electric field. The non‐linear solution of the problem found earlier, serves as a basis for formulation and extensive investigation of the corresponding linear initial‐boundary value problem of compressible magnetohydrodynamics. In plane geometry, the Green’s function of the problem is obtained and its properties are discussed. For the numerical evaluation it turns out that a specific choice of the integration contour in the complex plane of phase velocities is much more effective than the convolution with the real Green’s function. Many complex effects like intrinsic wave coupling, anisotropic propagation characteristics, generation of surface and side wave modes in a finite beta plasma are retained in th...

A comparison and review of steady-state and time-varying reconnection

Abstract Reconnection is a ubiquitous energy conversion process operating in current sheets. It appears in a variety of applications and is, for example, the dominant coupling process at the Earths magnetopause, the current sheet which separates the solar wind and the terrestrial magnetosphere. Reconnection at the magnetopause is investigated theoretically and experimentally, and in both areas a division exists between so-called steady-state and time-dependent reconnection. In theoretical research the former is associated with the time-invariant analysis of Petschek and coworkers, and this is often put into contrast with an intrinsically time-varying process such as tearing. In experimental research, manifestations of reconnection have been classified either as large scale and (quasi) steady-state, or time dependent, with the former corresponding to accelerated plasma flows along the magnetopause, and the latter to the flux transfer event signature. This division is a source of confusion, in particular since it is unlikely that a true steady-state is ever achieved in nature. To clarify the relationship between steady-state and time-dependent reconnection we discuss here an extension of Petscheks analysis to include time variations in the reconnection rate. In this generalized analysis the reconnection electric field is imposed as an initial-boundary condition which can be specified as an arbitrary function of space and time. Different types of reconnection behaviour can therefore be investigated and we take advantage of this to compare steady-state and time-varying reconnection. We show that the former is just a special case of the latter and that there are no jumps in conceptual understanding required from one to the other. Furthermore, the time-dependent analysis is easily understood and gives a framework which unifies the interpretation of reconnection phenomena observed at the magnetopause. In particular, the theoretical results indicate that the same reconnection rate can give rise to both accelerated plasma flows and the flux transfer event signature; thus there is no physical reason to make a distinction in the underlying process giving rise to different reconnection phenomena.

Time-dependent localized reconnection of skewed magnetic fields

We describe and analyze a model for time-varying, localized reconnection in a current sheet with skewed magnetic field orientations on opposite sides. As in Petscheks description, disruption is initiated in a localized part of the current sheet known as the diffusion region, and the disturbances are subsequently propagated into the system at large through magnetohydrodynamic (MHD) waves. The MHD waves therefore play the dominant role in energy conversion, and collectively they form an outflow for plasma streaming toward the current sheet and a field reversal region joining magnetic field lines from opposite sides. We restrict the analysis to an incompressible plasma, in which case the Alfven wave and the slow shock merge to form shocks bounding the field reversal or outflow region, and to the case of weak reconnection, which implies that the reconnection electric field is much smaller than the product of the characteristic values of the external field strength and Alfven speed. It is then possible to perform a perturbation analysis of the MHD equations which govern the plasma and field behavior. The analysis can be formulated as a mixture of three well-known problems. The problem of determining the appropriate combination of MHD waves corresponds to the Riemann problem, which also specifies the tangential field and flow components in the field reversal region. These results, it is important to note, are not sensitive to variations in the reconnection rate. Reconnection also acts as a source of surface waves, and their analysis determines the behavior of the perpendicular field and flow components and the shape of the shocks. Lastly, the field reversal region can be considered as a thin boundary layer in our treatment, and the external disturbances can therefore be solved in a way similar to the flow around a thin aerofoil. The model presented here can be applied to the Earths magnetopause, where reconnection is considered to be the dominant process coupling the solar wind and the magnetosphere. In particular, the results can be used to interpret different manifestations of reconnection such as accelerated plasma flows along the magnetopause and flux transfer events.

Model of electron pressure anisotropy in the electron diffusion region of collisionless magnetic reconnection

A new model of the electron pressure anisotropy in the electron diffusion region in collisionless magnetic reconnection is presented for the case of antiparallel configuration of magnetic fields. The plasma anisotropy is investigated as source of collisionless dissipation. By separating electrons in the vicinity of the neutral line into two broad classes of inflowing and accelerating populations, it is possible to derive a simple closure for the off-diagonal electron pressure component. The appearance of these two electron populations near the neutral line is responsible for the anisotropy and collisionless dissipation in the magnetic reconnection. Particle-in-cell simulations verify the proposed model, confirming first the presence of two particle populations and second the analytical results for the off-diagonal electron pressure component. Furthermore, test-particle calculations are performed to compare our approach with the model of electron pressure anisotropy in the inner electron diffusion region by Fujimoto and Sydora [Phys. Plasmas 16, 112309 (2009)].

Magnetic double-gradient instability and flapping waves in a current sheet.

A new kind of magnetohydrodynamic instability and waves are analyzed for a current sheet in the presence of a small normal magnetic field component varying along the sheet. These waves and instability are related to the existence of two gradients of the tangential (B_{tau}) and normal (B_{n}) magnetic field components along the normal (nabla_{n}B_{tau}) and tangential (nabla_{tau}B_{n}) directions with respect to the current sheet. The current sheet can be stable or unstable if the multiplication of two magnetic gradients is positive or negative. In the stable region, the kinklike wave mode is interpreted as so-called flapping waves observed in Earths magnetotail current sheet. The kink wave group velocity estimated for the Earths current sheet is of the order of a few tens of kilometers per second. This is in good agreement with the observations of the flapping motions of the magnetotail current sheet.

Impulsive reconnection in the magnetotail during substorm expansion

Abstract Magnetic and electric field variations measured by the ISEE-1 spacecraft within the plasma sheet (PS) have been studied during the initial stage of a strong substorm event. A plasmoid structure was generated at the substorm onset and then passed tailwards removing most of the previously large northward magnetic flux closed across the equatorial plane. During the following 20 min, a few individual bursts of positive Ey and southward Bz field components were detected within the PS tailwards of the reconnection site simultaneously with Pil bursts in the conjugate sector of the auroral zone. No signatures of ejected or growing magnetic islands were found in association with these bursts. The details of these burst-like variations agree with those predicted by the transient reconnection model, which describes the propagation of the disturbance launched from the local reconnection region. We conclude that the impulsive development of the expansion phase related phenomena shown by recent studies originates from impulsive modulation of the reconnection rate in the magnetotail.

Magnetic reconnection with space and time varying reconnection rates in a compressible plasma

Fast magnetic reconnection of Petschek-type including moving shock waves and discontinuities in a compressible plasma is studied. Magnetic flux tubes of finite size are reconnected by a localized dissipative electric field pulse. This process generates nonlinear perturbations propagating along the initial current surface. The linear wave problem in the outer regions is solved analytically in terms of the reconnection induced sources which move in different directions and with different speeds along the surface. The time-coordinate representation of the solution is given in form of convolution integrals over the reconnection initializing electric field. As an example, reconnection of flux tubes in a sheared magnetic field geometry is analyzed.

Time-dependent reconnection in a current sheet with velocity shear

Abstract Magnetic field reconnection is a macroscopic energy-conversion and transport process which operates in current sheets. Here we investigate analytically a physico-mathematical model of reconnection in a 2-D configuration consisting of a current sheet separating two different plasma regions with antiparallel magnetic field orientations. Reconnection is initiated in a localized region of the current sheet known as the diffusion region. The disruption of the current sheet generates MHD waves, which propagate the local disturbances into the system at large. We apply the MHD equations to describe and analyse the macroscopic response of the plasma and magnetic field to an imposed reconnection rate, which is generally a function of time. The dissipative process leading to disruption is not specified, and instead an arbitrary reconnection rate is used as an initial-boundary condition for solving the MHD equations. Analysis is restricted to an incompressible plasma, and to the case of weak reconnection, which implies that the magnetic field component perpendicular to the current sheet remains small relative to the field strength specified in the initial configuration. Time dependency and the inclusion of different plasma parameter values and field strengths on opposite sides of the current sheet lead to new features which are not evident in Petscheks analysis of steady-state reconnection in a symmetric current sheet configuration. These features include an asymmetric evolution of the outflow region on opposite sides of the current sheet, and a shift of the reconnection line into the region of higher field strength. The outflow region is partially bounded by a tangential discontinuity as a result of the different propagation speeds of the shocks and the surface waves. Once the reconnection rate drops to zero, i.e. reconnection is no longer active, the outflow region splits at the reconnection line and propagates like two solitary waves moving in opposite directions along the current sheet. The previous shift of the reconnection line now corresponds to a displacement of the current sheet downstream of the outflow region. Although no more flux is reconnected at this stage, the outflow region continues to grow in size and gathers up an increasingly large volume of reconnected plasma. The growth is eventually confined to a stretching of the outflow region along the current sheet. With velocity shear there is an additional asymmetry in the evolution on opposite sides of the reconnection line, and the conditions for the appearance and outward propagation of MHD waves are shown to be related to the criterion for Kelvin-Helmholtz stability of the current sheet.