Structure of Magnetic Reconnection in the Near-Earth Magnetotail

Abstract:

Ion and electron dynamics in magnetic reconnection in the near-Earth magnetotail has been investigated on the basis of the Geotail observations for the period of 1994-2014. Magnetic reconnection forms in association with substorm onset mostly at radial distances of 20-30 Re in the premidnight sector of the magnetotail. The ion-electron decoupling region where electron outflow speed exceeds significantly ion outflow speed is formed in the magnetic reconnection site. The ion-electron decoupling region has a spatial extent of approximately 11 λi (ion inertial length) along the GSM x direction, and the dawn-dusk current sheet with main current carriers of electrons exists over this region. An intense electron current layer with a spatial extent of 0.5–1 λi occupies in its center around the X line. The dawn-dusk length of the X line is estimated to be 6-8 Re in the GSM y direction, and its duskside edge is located around YGSM = 10 Re. High-speed electron outflow jets are formed just outside the central intense electron current layer. They are decelerated and become non-jet outflows with speed slightly higher than ion outflow speed. Electrons have flat-top distribution functions indicating heating and acceleration in both the outflow jets and the non-jet outflows; however, heating and acceleration are weak in the central intense current layer surrounding the X line. Inflowing ions enter the central intense electron current layer, and these ions are accelerated up to 10 keV inside the electron outflow jet regions. Ion acceleration beyond10 keV and thermalization operate mostly in the non-jet electron outflow regions. Electrons show thermal distributions without any heating/acceleration signatures immediately beyond the edge of the ion-electron decoupling region, while higher-energy ions pervade even beyond the edge and hot MHD plasma flows are produced.