Using data from 41 substorm events in the near‐Earth magnetotail, we have combined plasma, energetic ion, and magnetic field data from the AMPTE/IRM spacecraft to perform a superposed epoch analysis of changes in the total pressure and in the magnetic field configuration as a function of time relative to substorm onset. We find that unloading is evident in the total pressure profile, with the pressure beginning to decrease at onset time and reaching a minimum about 45 min later. The pressure decreases by ∼20%. The pressure increase associated with the growth phase is not evident in this analysis. This indicates that the pressure changes during the growth phase are not as uniform for the different substorms as the pressure changes during the expansion phase. To study the changes in the magnetic field configuration, we have determined the development of the plasma pressure profiles in z for an average of data from 15 to 19 RE. We combined this information with the known radial gradient in the total pressure to determine the field configuration changes during a substorm. The stretching of the field during the growth phase is clearly evident in this analysis. At substorm onset, the field line dipolarization begins on the innermost 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 substorm.
