Current sheet structure around the near‐Earth neutral line observed by Geotail

Abstract

The current sheet structure around the near‐Earth neutral line during substorms is investigated using ion and electron velocity moments directly obtained by the Geotail satellite. The intense cross‐tail current sheet is found to be thinner closer to the X line and becomes as thin as 500 km. This is less than the ion inertial length (720 km), where ions are decoupled from electrons. The current sheet is found to be bifurcated from the ion diffusion region to the off‐neutral sheet and forms a double‐peaked current sheet away from the X line. Fast dawnward moving electrons with the strong ambipolar electric field toward the neutral sheet (Ens) carry the intense electron‐dominated current, and such Ens is larger than Ey in the bursty bulk flows. So‐called Hall current system into/out of the X line caused by the large separation between ion and electron motions is fully identified. The Hall current system is observed around the neutral sheet in the vicinity of the X line and bifurcates to the north/south boundary regions as the distance from the X line increases. There, the inward and outward currents form antiparallel current layers, whose boundaries are identified as 0.8 ∼ 0.85 of lobe magnetic field intensity.