Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771

In this paper, we study gradients of the energetic ion intensity observed at the edge of the plasma sheet boundary layer (PSBL) by the energetic ion anisotropy spectrometer (EPAS) on ISEE 3. In particular, we have determined the velocity of the boundary relative to the spacecraft in the direction perpendicular to the tail axis and the angle which the boundary normal makes to the spacecraft spin axis for 1160 PSBL encounters 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 able 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 motion of the entire tail induced by solar wind direction variations, although 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 of ~50° ). (4) The width of the distribution decreases with downtail distance. (5) The variation of the distributions with cross-tail position reveals that this decrease in width is most likely due to the edge of the PSBL being concave, or significantly flared at the tail flanks, in the near-Earth region. This flaring is absent further 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 reduced towards the flanks. (6) During days on which the IMF has "away" sector structure, the north lobe of the tail is twisted on average towards dawn by 7.0 ± 2.4° . (7) During days on which the IMF has "toward" sector structure, the north lobe is tilted towards dusk by 3.8 ± 2.3° . (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 mean twist of -12.3 ± 5.0° for IMF B_y > 0 and 5.5 ± 3.8° for IMF B_y < 0 (positive twist angles correspond to a tilt of the northern lobe towards dusk.). (9) For northward IMF B_z, the tail has a twist of -23.9 ± 5.0° for IMF B_y > 0 and 13.4 ± 6.0° for IMF B_y < 0. Hence the tail appears more twisted on average for the IMF B_z northward case. (10) The distribution of tail twists is wider for lower levels of geomagnetic activity, indicating that the tail is able to twist more at lower levels of activity. (11) The data set reveals no evident effect of the Earth's dipole wobble; tail orientation appears to be controlled by the solar wind and IMF, such that the GSE coordinate system may be appropriate for the study of field and plasma structures in the distant tail region.

J. Geophys. Res., 100, No. A1, 185-204, Jan. 1995