BOUNDARY-LAYER FORMATION IN THE MAGNETOTAIL - GEOTAIL OBSERVATIONS AND COMPARISONS WITH A GLOBAL MHD SIMULATION

We present Geotail plasma and field observations from the middle magne totail near X-GSE = -46 R-E for the time period 1400 to 1800 UT on Dec ember 14, 1994. During that period, the Wind satellite monitored the s olar wind plasma and interplanetary magnetic field (IMF) up stream of the bow shock. The IMF was northward and the plasma parameters near av erage. Geotail observed slow tailward flows and a northward field. The plasma and field parameters indicate that Geotail is either in the pl asma sheet or in a boundary layer. We used the Wind solar wind plasma and IMF data as input for a global simulation of that time interval. C omparison of the simulation results with the observational data show v ery good overall agreement of the magnitudes of the plasma and field p arameters. In particular, the simulation reproduces the slow tailward flows and northward field found at Geotail. Small scale temporal varia tions are less well reproduced. The simulation shows the formation of a broad boundary layer (which we call tail flank boundary layer, TFBL) that consists of closed flux which formed by magnetic reconnection of IMF and lobe field lines. The simulation results indicate that Geotai l is located very close to the TFBL and may have entered the TFBL prop er. We show that the TFBL plays an important role in energy transport from the solar wind into the magnetosphere during northward IMF condit ions.