Formation of thin bifurcated current sheets by quasisteady compression

Abstract

Thin current sheets with half-widths in the range of about 10 or less ion inertial lengths (or ion gyroradii) have been identified as the sites of important dynamical phenomena in space plasmas. Recent space observations established that thin current sheets often have a bifurcated (double-peaked) current density. Earlier suggestions of possible bifurcation mechanisms are based on the presence of microfluctuations, magnetic reconnection, or a normal magnetic field component or assumed simplified models of adiabatic dynamics. Despite these efforts, the cause (or causes) of the formation of thin bifurcated current sheets and the conditions under which they form have remained unclear. In this paper, we identify quasisteady compression of a plane, initially wide collisionless current sheet as an effective physical mechanism for the formation of thin bifurcated current sheets. Our main tool is electromagnetic particle simulation. The initial sheet has a singly peaked current and a half-width that is five times larger than the ion inertial length. This sheet is quasisteadily compressed by external forces. A three-scale structure develops and the current bifurcates during compression. It is shown that the bifurcation, pressure anisotropy, and other major properties of the embedded current sheet can be understood in terms of basic physical principles, such as electric field shielding and momentum conservation. Sufficient conditions for bifurcation of symmetric current sheets are presented. They suggest that bifurcation must generally occur by quasisteady compression if the (singly peaked) initial current sheet is sufficiently wide.

Conditions for the formation of nongyrotropic current sheets in slowly evolving plasmas

“…Adopting the widely held view that reconnection requires a thin current sheet to form [1][2][3][4][5][6][7][8][9][10][11][12] and realizing that, in the present geometry, reconnection needs a reversal of the magnetic field, we see that, within the frame of models considered in this paper, sheets of nongyrotopic current form when thin current sheets are required for reconnection. In the plasma of case 4 where nongyrotropic currents do not form, reconnection is also not expected to occur as the magnetic field is unidirectional.…”

“…4 Our criterion predicts nongyrotropic currents for any slow evolution starting from a Harris sheet. This includes expansion, which motivated our case 2, showing explicitly that a nongyrotropic current sheet indeed forms in a slowly expanding initially smooth field.…”

“…[1][2][3] Numerous theoretical investigations, simulation studies, and observations have explored detailed properties of thin current sheets and their formation. For an outline of earlier results, we refer to a previous paper 4 ͑Paper I͒. More recent confirmation of the importance of thin current sheets as the sites of magnetic reconnection resulted from observations in the Earth's magnetotail [5][6][7][8] as well as in the solar wind.…”

The structure of the electron outflow jet in collisionless magnetic reconnection

“…Equation ͑8͒ is the standard equation for a diamagnetic current sheet, where electrons carry the majority of the current due to momentum conservation. 13 The electron motion is thus a composite of E ϫ B drift and this current carrying velocity. This result is consistent with a nondissipative current sheet.…”

Ion motion in a polarized current sheet

“…And indeed, there is some observational 5,53,55 and modeling 22,28,29,42 evidence of such Hall electric fields in thin current sheets. In the coordinate system shown in Fig.…”