Structure, force balance, and evolution of incompressible cross-tail current sheet thinning

Abstract

[1] THEMIS five-point observations on April 8, 2009 were used to study thinning of the current sheet in the near-Earth tail that led to the onset of a small substorm. Taking advantage of a fortuitous alignment of the five spacecraft near 2300 LT and 11 R E and within 1.5 R E of the current sheet center, latitudinal gradients are analyzed. A significant latitudinal pressure gradient is present indicating the necessity of a (J × B) z force to maintain the pre-onset equilibrium state. During thinning the total pressure remained approximately constant at all spacecraft rather than increasing. Within the plasma sheet, magnetic field strength increased while plasma pressure decreased due to decreasing temperature. We present a comprehensive explanation for the relationship between the thinning, the stretched structure, and development of intense current density. Our analysis of this event suggests that (1) the thinning in this event is an MHD force-balanced self-evolving process and is not a forced process due to an increased lobe field; (2) the thinning changes flux tube structure in length and curvature but not significantly in volume; (3) the thinning evolves with a change of the radial plasma pressure profile in the near-Earth tail, which is associated with a locally intensified current sheet. The conclusion is that the increased lobe field strength is not the necessary and the primary cause for cross tail current sheet thinning but rather thinning can occur within the plasma sheet as a result of unknown internal processes.

Ballooning instability‐induced plasmoid formation in near‐Earth plasma sheet

“…Our work has demonstrated for the first time that as a macroscopic coherent process, the ideal MHD ballooning instability is capable of inducing the formation of plasmoids in the magnetotail configuration without relying on any microscopic, kinetic, or turbulent processes. In light of recent evidence found in ground and in situ observations for the presence of ballooning instability in the pre‐onset auroral and plasma sheet structures [ Saito et al , , ; Panov et al , ; Motoba et al , , ], our findings on the ballooning instability‐induced plasmoid formation may indeed provide a solid and practical scheme for ballooning instability in the near‐Earth magnetotail to play a critical role in triggering the substorm onset process.…”

Tailward leap of multiple expansions of the plasma sheet during a moderately intense substorm: THEMIS observations

“…The data gaps in Ey GSM are caused by this processing. For the calculation of the plasma pressure, ion density, ion velocity and ion temperature, we use the method discussed in the Appendix A of Saito et al [2011].…”

Properties of current sheet thinning at x ∼− 10 to −12 RE

“…Both patterns of B L behavior have been found in THEMIS and Cluster data. Using a case study Saito et al [] suggested that the j y increase may not necessarily be accompanied by growth of the lobe magnetic field B L , and thus, CS thinning can be (at least at sometimes) driven by internal magnetotail processes rather than by the solar wind. From statistics of Cluster observations of significant j y growth, Thompson et al [] and Snekvik et al [] showed that j y and B L both increase during CS thinning.…”