SUPERPOSED EPOCH ANALYSIS OF PRESSURE AND MAGNETIC-FIELD CONFIGURATION CHANGES IN THE PLASMA SHEET

Using data from 41 substorm events in the near-Earth magnetotail, we h ave combined plasma, energetic ion, and magnetic field data from the A MPTE/IRM spacecraft to perform a superposed epoch analysis of changes in the total pressure and in the magnetic field configuration as a fun ction of time relative to substorm onset. We find that unloading is ev ident in the total pressure profile, with the pressure beginning to de crease at onset time and reaching a minimum about 45 min later. The pr essure decreases by approximately 20%. The pressure increase associate d with the growth phase is not evident in this analysis. This indicate s that the pressure changes during the growth phase are not as uniform for the different substorms as the pressure changes during the expans ion phase. To study the changes in the magnetic field configuration, w e have determined the development of the plasma pressure profiles in z for an average of data from 15 to 19 R(E). We combined this informati on with the known radial gradient in the total pressure to determine t he field configuration changes during a substorm. The stretching of th e field during the growth phase is clearly evident in this analysis. A t substorm onset, the field line dipolarization begins on the innermos t field lines and then progresses to the outer field lines. The field lines map the closest to the Earth about 45 min after substorm onset, and then begin to stretch out again during the recovery phase of the s ubstorm.