Evolution of the thin current sheet in a substorm observed by Geotail

[1] Time evolution of a thin current sheet during a substorm is studied in the near-Earth magnetotail of X = -15 R-E. Specifically, we study the thickness of the current sheet and its dynamical structure. Using the Geotail plasma data, we obtain the electric current density directly from both ion and electron velocity moments, and the thickness of the current sheet is evaluated from the Ampere's law with the pressure balance of the plasma sheet. It is found that the averaged thickness of the highly fluctuating current sheet becomes as thin as the ion inertial scale during the late growth phase and the early expansion phase. The current sheet temporally forms a double-peaked current sheet in which the current density becomes the largest away from the neutral sheet. We also found that the intense current is mainly carried by electrons, contrary to the ion-dominated diamagnetic current caused by the higher ion temperature than the electron one. We discuss that the Hall electric field plays an important role on the formation of the double-peaked current sheet. The Hall electric field toward the neutral sheet causes the electron dominated current sheet by enhancing the dawnward drift for both ion and electron.