Thin current sheets in the presence of a guiding magnetic field in Earth's magnetosphere

摘要

A self-consistent theory of relatively thin anisotropic current sheets (TCS) in collisionless plasma is developed, taking into account the presence of a guiding field By (all notations are used in the GSM coordinate system). TCS configurations with a finite value of guiding field B_y are often observed in Earth's magnetotail and are typical for Earth's magnetopause. A characteristic signature of such configurations is the existence of a magnetic field component along the direction of TCS current. A general case is considered in this paper with global sheared magnetic field B_y = const. Analytical and numerical (particle-in-cell) models for such plasma equilibria are analyzed and compared with each other as well as with Cluster observations. It is shown that, in contrast to the case with B_y = 0, the character of "particle-current sheet" interaction is drastically changed in the case of a global magnetic shear. Specifically, serpentine-like parts of ion trajectories in the neutral plane become more tortuous, leading to a thicker current sheet. The reflection coefficient of particles coming from northern and southern sources also becomes asymmetric and depends upon the value of the By component. As a result, the degree of asymmetry of magnetic field, plasma, and current density profiles appears characteristic of current sheets with a constant By. In addition, in the presence of nonzero guiding field, the curvature current of electrons in the center of the current sheet decreases, yielding an effective thickening of the sheet. Implications of these results for current sheets in Earth's magnetosphere are discussed.