Thick Bifurcated Current Sheet in the Near-Earth Tail Plasma Sheet

Abstract:

The bifurcated structure of the current sheet in the mid tail (X~-20 R_E) has been reported in several in-situ observational studies. The presented study examines a spatial distribution of current densities that statistically infer a thick bifurcated current sheet as a typical structure in the near-Earth tail (X=-8 to -12 R_E). The current density is evaluated by using any two of THEMIS spacecraft measurements when certain conditions, such as spacecraft separation and orientation, are strictly met. A survey from 2007 to 2013 results in approximately 3000 current densities, which made it possible to study north-south profile of the current sheet. The peak of the current density is often found 0.5 R_E to 1 R_E off the magnetic equator, while the median half-thickness of the current sheet is approximately 3 R_E. These indicate that the current sheet is thick and bifurcated on average. Presumably, owing to this non-uniform profile, local current densities sometimes become very intense. The intense current density preferentially occurs during growth phase and expansion phase of substorms, but also occur in quiet time. The intense current densities are found to be independent from the solar wind dynamic pressure. It is concluded that the intense current density is not caused by the compression of the plasma sheet. Other mechanisms need to be suggested to fully understand the structure and the evolution of the current sheet in the near-Earth tail.