Construction of magnetic reconnection in the near‐Earth magnetotail with Geotail

[1] The Geotail spacecraft made in situ observations of magnetic reconnection on 15 May 2003 in the near-Earth magnetotail at a radial distance of 28 RE when a moderate substorm started on the ground. For this event, the intense cross-tail electron current layer was detected in association with the simultaneous plasma flow and magnetic field reversals, and the scale length along the GSM x axis was obtained. This observation enables us to deduce scales for the basic structure of magnetic reconnection in the near-Earth magnetotail. In the center of the electron current layer (a possible X line), the speed of the dawnward electron flow carrying cross-tail current exceeds the maximum of the electron outflow speed (earthward/tailward), and it is highly super Alfvenic. The full extent of this central intense cross-tail electron current layer is approximately 1 λi (ion inertial length) in the x direction, which corresponds to 0.2 RE in the magnetotail. Electron outflow speed reaches its maximum, which is also super Alfvenic, at distances of less than 1 λi from the X line, and ion outflow speed reaches its maximum farther away from the center. Electron outflow speed decreases in the downstream region, and it becomes the same as the ion speed at distances of 4 λi. The full extent of the ion-electron decoupling region is 8 λi in the x direction, which corresponds to 1.5 RE in the magnetotail, and the outer region belongs to the MHD regime. Inside the ion-electron decoupling region, ions are accelerated up to 10 keV during inflow processes and further accelerated beyond 40 keV toward the duskward direction near the center and along the x axis slightly away from the center. These observations of the ion and electron dynamics in the close vicinity of the X line are fairly consistent with results from the two-dimensional particle-in-cell simulation described here and others. The present Geotail results provide the observational basis for the structure of magnetic reconnection in the near-Earth magnetotail.

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