AVERAGE MOTION, STRUCTURE AND ORIENTATION OF THE DISTANT MAGNETOTAIL DETERMINED FROM REMOTE-SENSING OF THE EDGE OF THE PLASMA SHEET BOUNDARY-LAYER WITH E-GREATER-THAN-35 KEV IONS

In this paper, we study gradients of the energetic ion intensity obser ved at the edge of the plasma sheet boundary layer (PSBL) by the energ etic ion anisotropy spectrometer (EPAS) on ISEE 3. In particular, we h ave determined the velocity of the boundary relative to the spacecraft in the direction Perpendicular to the tail axis and the angle which t he boundary normal makes to the spacecraft spin axis for 1160 PSBL enc ounters at X(GSM) > -240 R(E). By assuming that, on average, the edge of the PSBL is parallel to the cross-tail current sheet, we are then a ble to determine a number of properties of the structure, orientation and motion of the deep geomagnetic tail. We conclude the following: (1 ) Most crossings of the edge of the PSBL are caused by transverse moti on of the entire tail induced by solar wind direction variations, alth ough some are caused by reconfiguration of the tail due to geomagnetic activity. (2) The typical velocity of the PSBL (and hence of the tail ) in a direction perpendicular to the tail axis is 50-85 km s(-1). (3) The average twist of the tail is near zero, with the edge of the PSBL (and by inference the cross-tail current sheet) lying parallel to the ecliptic plane (however, large twists are found in individual events and the distribution of twists is broad, with one standard deviation o f similar to 50 degrees. (4) The width of the distribution decreases w ith downtail distance. (5) The variation of the distributions with cro ss-tail position reveals that this decrease in width is most likely du e to the edge of the PSBL being concave, or significantly flared at th e tail flanks, in the near-Earth region. This flaring is absent furthe r downtail. In fact, at X(GSM) < -200 R(E), the combined thickness of the plasma sheet and PSBL may be greatest at the tail centre and reduc ed towards the flanks. (6) During days on which the IMF has ''away'' s ector structure, the north lobe of the tail is twisted on average towa rds dawn by 7.0 +/- 2.4 degrees. (7) During days on which the IMF has ''toward'' sector structure, the north lobe is tilted towards dusk by 3.8 +/- 2.3 degrees. (8) A subset of events for which IMP 8 solar wind data are available show that, for southward IMF B-Z, the tail has a m ean twist of -12.3 +/- 5.0 degrees for IMF B-y > 0 and 5.5 +/- 3.8 deg rees for IMF B-Y < 0 (positive twist angles correspond to a tilt of th e northern lobe towards dusk). (9) For northward IMF B-Z, the tail has a twist of -23.9 +/- 5.0 degrees for IMF B-y > 0 and 13.4 +/- 6.0 deg rees for IMF B-y < 0. Hence the tail appears more twisted on average f or the IMF B-Z northward case. (10) The distribution of tail twists is wider for lower levels of geomagnetic activity, indicating that the t ail is able to twist more at lower levels of activity. (11) The data s et reveals no evident effect of the Earth's dipole wobble; tail orient ation appears to be controlled by the solar wind and IMF, such that th e GSE coordinate system may be appropriate for the study of field and plasma structures in the distant tail region.