Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109

Observations from the JPL vector helium magnetometer and the EPAS energetic ion spectrometer on ISEE 3 are used to investigate the magnetic configuration of the central plasma sheet as a function of distance down the tail, the magnitude and orientation of the IMF, and substorm activity. The ISEE 3 magnetotail measurements are supplemented with IMP 8 GSFC interplanetary magnetic field observations and University of Kyoto/NOAA AE indices. Southward B_Z in the plasma sheet is observed to be correlated with high-speed antisolar bulk flows as predicted by the Dungey model of the magnetosphere. It is also found that southward plasma-sheet magnetic fields are correlated with southward IMF upstream of the magnetosphere. An effective dayside reconnection efficiency of 25 ± 4% is inferred, in good agreement with theory and near-earth studies of magnetic flux transfer. This unique data set has also allowed an examination of the variation in plasma-sheet B_Z and V_X with distance down the tail for both substorm and quiet conditions. During substorms, |AL| > 100 nT, negative magnetic fields, and high-speed antisolar flows are observed in the central plasma sheet at |X| > 80-100 R_e. Earthward of that distance positive B_Z and solar-directed V_X are measured. At nearly all distances, á V_XB_Z ñ is positive (i.e., dawn-to-dusk electric field) under substorm conditions indicating that magnetic field energy is being converted to plasma kinetic energy as required in the open model of the magnetosphere. During quiet conditions, |AL| < 100 nT, the magnetic field in the plasma sheet is positive out to the most distant ISEE 3 apogee, |X| = 240 R_e, despite the presence of tailward flow in the plasma sheet beyond |X| = 80 R_e. This result suggests that the quiet time central plasma sheet at |X| > 80-100 R_e is threaded by closed field lines that are being carried slowly tailward. The energy required to do work against these closed field lines must be supplied by the kinetic energy in the mantle and low latitude boundary layer flows as described in the quasi-viscous models of the solar wind interaction with the geomagnetic field. These quiet-time results stand out in contrast with the ISEE 3 observations during substorms, which show very good agreement with the original merging models by Dungey.

in Magnetotail Physics, 59-63, A. T. Y. Lui, ed., Johns Hopkins, University Press, Baltimore, 1987