THEMIS two‐point measurements of the cross‐tail current density: A thick bifurcated current sheet in the near‐Earth plasma sheet

Abstract

The basic properties of the near‐Earth current sheet from 8 RE to 12 RE were determined based on Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations from 2007 to 2013. Ampere's law was used to estimate the current density when the locations of two spacecraft were suitable for the calculation. A total of 3838 current density observations were obtained to study the vertical profile. For typical solar wind conditions, the current density near (off) the central plane of the current sheet ranged from 1 to 2 nA/m2 (1 to 8 nA/m2). All the high current densities appeared off the central plane of the current sheet, indicating the formation of a bifurcated current sheet structure when the current density increased above 2 nA/m2. The median profile also showed a bifurcated structure, in which the half thickness was about 3 RE. The distance between the peak of the current density and the central plane of the current sheet was 0.5 to 1 RE. High current densities above 4 nA/m2 were observed in some cases that occurred preferentially during substorms, but they also occurred in quiet times. In contrast to the commonly accepted picture, these high current densities can form without a high solar wind dynamic pressure. In addition, these high current densities can appear in two magnetic configurations: tail‐like and dipolar structures. At least two mechanisms, magnetic flux depletion and new current system formation during the expansion phase, other than plasma sheet compression are responsible for the formation of the bifurcated current sheets.

Generalized magnetotail equilibria: Effects of the dipole field, thin current sheets, and magnetic flux accumulation

“…Resolving their structure to describe the magnetotail on the scales comparable to the thermal ion gyroradius requires a generalization of the isotropic model assuming plasma anisotropy and nongyrotropy as well as an explicit description of the details of quasi‐adiabatic particle motion in sharply curved magnetic fields [ Speiser , ; Sonnerup , ]. We consider one such model and show that the accompanying perturbation of the vector potential outside the current sheet similar to the dipole field effect corresponds to a bifurcation of the central current region, consistent with near‐Earth plasma sheet observations [ Saito , ] and global MHD simulations [ Meng et al , ], and is similar to other TCS models [ Birn et al , ].…”

“…Since A d <0 and in the adopted coordinated system the unperturbed equilibrium current is negative, the singular current is directed oppositely relative to the equilibrium current, creating a bifurcated current sheet [ Sitnov et al , ]. Such current sheets have indeed been observed in the magnetotail [ Runov et al , ; Sergeev et al , ; Asano et al , ] and, in particular, in the near‐Earth plasma sheet [ Saito , ]. Their singular nature indicates either the loss of equilibrium [ Birn and Schindler , ] or a deviation from the isotropic model .…”

“…10.1002/2016JA023001 Such advances are necessary to meet the challenges of more recent observations of the preonset equilibria and new effects found therein [Wang et al, 2004;Machida et al, 2009Machida et al, , 2014Petrukovich et al, 2013;Artemyev et al, , 2015Saito, 2015].…”

“…At the same time, the equilibrium theory of the magnetotail, including a finite B z component with isotropic plasmas, was developed several decades ago [ Schindler , ; Birn et al , ; Zwingmann , ] and has not been advanced substantially since then. Such advances are necessary to meet the challenges of more recent observations of the preonset equilibria and new effects found therein [ Wang et al , ; Machida et al , , ; Petrukovich et al , ; Artemyev et al , , ; Saito , ].…”

Vertical Current Density Structure of Saturn's Equatorial Current Sheet

“…Phenomenologically, at Earth bifurcated current sheets are more often found during fast flow events and are associated with substorms (Saito, 2015), thus related to magnetic reconnection. Theory and simulations have explored the role of instabilities or plasma anisotropy that can give rise to bifurcations.…”