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

The structure of a thin current sheet prior to the expansion onset of a substorm event that occurred on 26 February 2008 is studied in the near-Earth magnetotail on the basis of Time History of Events and Macroscale Interactions During Substorms (THEMIS) observations. During this time interval, the ion distribution showed mushroom-shaped structures with clear nongyrotropic features, indicating that the warmer component of the ions was unmagnetized, which becomes possible only if the gyroradii of these ions are comparable with the current sheet thickness. By comparing the observations with the model proposed by Sitnov et al. (2003), which is a modification of the Harris (1962) model by considering the effect of the meandering ions in thin current sheets, we reconstruct the current sheet structure in the late growth phase of the substorm. Warmer ions, mostly following meandering orbits across the neutral sheet, are found to remarkably alter the current sheet profiles and therefore play an important role in the formation of the thin current sheet.