Valentina V. Lebedeva

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.

Magnetosheath model in the Chew–Goldberger–Low approximation

Parameters of the solar wind plasma and magnetic field in the magnetosheath are calculated for an anisotropic plasma model in the Chew–Goldberger–Low approximation. It is shown that in the case when the energy transfer between the perpendicular and parallel (with respect to the magnetic field) degrees of freedom is absent, the resulting temperature anisotropy may significantly affect the plasma density and magnetic field intensity profiles across the magnetosheath. However, in this case, the value of the temperature anisotropy (the ratio of the perpendicular to the parallel component of the temperature with respect to the magnetic field, T⊥/T‖) becomes unrealistic high. To bring agreement between the model values of the temperature anisotropy and experimental data, the existence of an intensive proton pitch-angle diffusion is assumed. In the case when the temperature anisotropy relaxation time is much smaller than the time taken by the solar wind plasma to move from the bow shock to the magnetopause, one ...

Time‐varying reconnection: Implications for magnetopause observations

We discuss the implications of results arising from an analysis of a Petschek-type reconnection model for the interpretation of data obtained at the terrestrial magnetopause. In this model, reconnection is initiated through the introduction of a reconnection electric field in the diffusion region. The magnitude of the electric field is considered to be small compared to the product of characteristic values of the magnetic field strength and Alfven speed in the system; that is, we study the case of weak reconnection only. Outside the diffusion region, the behavior of the plasma is governed by the ideal MHD equations. Petscheks original analysis is generalized through the introduction of a spatially and temporally varying reconnection rate, that is, the reconnection line has a finite length and the reconnection electric field along it varies in time. Additionally, the magnetic fields on either side of the current sheet (although uniform initially) may have arbitrary strength and are skewed relative to each other. New features are that (1) the plasma velocity may have a shear across the current layer, and (2) the densities on either side of the current sheet may be different in general. The reconnection electric field initiates a localized disruption of the current sheet, and the associated disturbances are propagated into the system by MHD waves. With this model we are able to explain and interpret various features observed at the terrestrial magnetopause, such as accelerated plasma flows and flux transfer events. We describe magnetic field signatures predicted by our model. We also show that reconnection is capable of generating surface waves. A property of our model is that it predicts a displacement of the magnetopause when time-dependent reconnection is occurring.

MHD-modelling of the magnetosheath

Abstract A short discussion of some problems of magnetosheath physics is presented. In particular, anisotropic MHD models of the magnetosheath are discussed. A method to estimate the value of the characteristic relaxation time (τ) of the proton temperature anisotropy from experimental data is proposed. Another problem considered in the review concerns the conditions of formation of a magnetic barrier within the magnetosheath. The existing controversy in this question is explained in the authors’ opinion by different definitions of the term “magnetic barrier” used in papers by Pudovkin et al. J. Geophys. Res., 87 (1982) 8131; Ann. Geophys. 13 (1995) 828) and Phan et al. J. Geophys. Res. 99 (1994) 121). Experimental data on the magnetic barrier dependence on the IMF orientation are discussed.

Magnetosheath structure in an anisotropic plasma model

Variations of the magnetic field and plasma parameters across the Earths magnetosheath are calculated for an anisotropic plasma in the Chew-Goldberger-Low approximation. Additionally, proton pitch-angle diffusion is taken into account as the energy transfer mechanism between the direction perpendicular and parallel to the magnetic field. We discuss the main characteristics of the magnetic barrier for different temperature relaxation times and their dependence on the interplanetary magnetic field orientation.