STRUCTURE AND DYNAMICS OF MAGNETIC RECONNECTION FOR SUBSTORM ONSETS WITH GEOTAIL OBSERVATIONS

Fast tailward ion flows with strongly southward magnetic fields are fr equently observed near the neutral sheet in the premidnight sector of the magnetotail at 20-30 R-E for substorm onsets in Geotail observatio ns. These fast tailward flows are occasionally accompanied by a few ke V electrons. With these events, we study the structure and dynamics of magnetic reconnection. The plasma sheet near the magnetic reconnectio n site can be divided into three regions: the neutral sheet region (ne ar the neutral sheet with the absolute magnitude of B-x of < 5 nT), th e boundary region (near the plasma sheet/tail lobe boundary with the a bsolute magnitude of B-x is near or > 10 nT), and the off-equatorial p lasma sheet (the rest). In the neutral sheet region, plasmas are trans ported with strong convection, and accelerated electrons show nearly i sotropic distributions. In the off-equatorial plasma sheet, two ion co mponents coexist: ions being accelerated and heated during convection toward the neutral sheet and ions flowing at a high speed almost along the magnetic field. In this region, highly accelerated electrons are observed. Although electron distributions are basically isotropic, hig h-energy (higher than 10 keV) electrons show streaming away from the r econnection site along the magnetic field line. In the boundary region , ions also show two components: ions with convection toward the neutr al sheet and field-aligned ions flowing out of the reconnection region , although acceleration and heating during convection are weak. In the boundary region, high-energy (10 keV) electrons stream away, while me dium-energy (3 keV) electrons stream into the reconnection site. Magne tic reconnection usually starts in the premidnight sector of the magne totail between X-GSM = -20 R-E and X-GSM = -30 R-E prior to an onset s ignature identified with Pi 2 pulsation on the ground. Magnetic reconn ection proceeds on a timescale of 10 min. After magnetic reconnection ends, adjacent plasmas are transported into the postreconnection site, and plasmas can become stationary even in the expansion phase.