Magnetic Reconnection in the Near-Earth Magnetotail

Abstract

The Geotail mission has added significantly to our understanding of magnetic reconnection in the near-Earth plasma sheet for substorm onset. Tailward fast convection flows in association with onsets are found to occur mostly beyond 22 R E in the magnetotail, whereas earthward fast convection flows in association with onsets are found to occur only inside 30 R E These convection flows are seen mainly in the premidnight plasma sheet. The observations locate the site of typical observation of magnetic reconnection for substorm onsets in the premidnight plasma sheet between X GSM = -22 R E and X GSM = -30 R E . Furthermore, the flows are found to precede the ground onset time in a number of cases near this site, which implies that magnetic reconnection is the cause of the substorm. In the vicinity of the magnetic reconnection site, ion and electron distribution functions show characteristics that depend on distance from the neutral sheet. Ions convect away from the X-type neutral line near the neutral sheet, whereas ions form field-aligned outflows off the neutral sheet. Heating and acceleration processes are observed for ions being convected toward the neutral sheet. High-energy (10-keV) electrons appear in this region. Off the neutral sheet, the highest-energy component of electrons escapes from the X-type neutral line region. Farther off the neutral sheet, high-energy (10-keV) electrons form outward stream, while medium-energy (3-keV) electrons form stream toward the X-type neutral line region. Thus, the micro structure and distribution functions within the magnetic reconnection site has been emerging with Geotail observations.