FLUX ROPE STRUCTURES IN THE MAGNETOTAIL - COMPARISON BETWEEN WIND GEOTAIL OBSERVATIONS AND GLOBAL SIMULATIONS/

On December 11, 1994, Geotail observed several reconnection (negative Bz) 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 we re 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 Geotai l, but at high latitudes. Global fluid simulations that incorporate hi gher-order corrections to Ohm's law are used to produce a three dimens ional picture of the reconnection site in conjunction with the Geotail data. It is shown that many of the negative B-z events have appreciab le core magnetic fields, which in the modeling appear to form around 2 0 R-E about 10 min earlier than observed by Geotail. The predicted tim e 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 appea r 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 m agnetic field, but the probability that a spacecraft observes a unidir ectional or bipolar signature is highly dependent on the position of t he center of the current sheet relative to the spacecraft. Bipolar sig natures are predicted to be most easily seen when the spacecraft is ne ar the center of the current sheet, while unidirectional signatures ar e most likely to be observed at more distant encounters.