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

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 x 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.