Statistical study of thin current sheet evolution around substorm onset

The spatial distribution of current sheet thinning in the substorm growth phase is examined statistically using Geotail data. The current density is calculated from the ion and electron velocity moments. It is found that the current sheet becomes thin before the onset mainly on the Earthward side of the initial near-Earth neutral line (NENL) position and becomes less than 0.5 R-E on average. However, on the tailward side of the NENL position, the current sheet becomes thin only after the onset. This result implies that the NENL forms at or near the tailward edge of the thin current sheet. An intense current sheet is formed in the center of the plasma sheet on average, but the largest current density is temporarily observed in the off-neutral sheet region, indicating the formation of a double-peaked current sheet. The current carriers becomes mainly electrons in the thin current sheet under the existence of the ambipolar electric field directed toward the neutral sheet.