Geophysics Program, University of Washington, Seattle

On December 11, 1994, Geotail observed several reconnection (negative B_z) events at a position 30-40 R_E down the tail over a 24 hour period. This period is also interesting because the solar wind conditions were well monitored by Wind, which was only about 20 R_E in front of the bow shock, and because IMP 8 was at nearly the same x value as Geotail, but at higher latitudes. Global fluid simulations that incorporate higher-order corrections to Ohm's law are used to produce a three dimensional picture of the reconnection site in conjunction with the Geotail data. It is shown that many of the negative B_z events have appreciable core magnetic fields, which in the modeling appear to form around 20 R_E about 20 min earlier than observed by Geotail. The predicted time of the formation coincides approximately with increased ionospheric activity observed by the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometer chain. The flux ropes also appear to be highly localized (2-4 R_E), particularly in x and z when they are first created. In addition, they have a distinctive bipolar core magnetic field, but the probability that a spacecraft observes a unidirectional or bipolar signature is highly dependent on the position of the center of the current sheet relative to the spacecraft. Bipolar signatures are predicted to be most easily seen when the spacecraft is near the center of the current sheet, while unidirectional signatures are most likely to be observed at more distant encounters.

J. Geophys. Res., 103, No. A1, 135-150, Jan. 1998