Thin current sheet in the substorm late growth phase: THEMIS observations

The formation of a thin current sheet prior to the expansion onset of a substorm event, which occurred on Feb 26, 2008, is studied in the near-Earth magnetotail based on THEMIS observations. During this time interval, the ion distribution showed mushroom-shaped structures with clear non-gyrotropic features, indicating that the warmer component of the ions were unmagnetized, which becomes possible only if the gyroradii of these ions are comparable with the current sheet thickness. By comparing the observations with the Sitnov (2006) model, which is a generalization of the Harris (1962) model by considering the effect of the meandering ions in the thin current sheet, we reconstruct the current sheet structure in the late growth phase of the substorm, and discuss the role of the ion kinetics in the thin current sheet.