Substorm dipolarization and recovery

On the basis of similar to 2 years of Geotail data, we use a superposed epo ch approach to study the average behavior of plasma and magnetic fields at different radial distances, between 11 and 31 R-E, during 66 substorms in t he premidnight sector. Magnetic held dipolarization is first seen in the in nermost region (11-16 R-E) around substorm onset and subsequently moves tai lward at a rate of 35 km/s. Fast earthward and tailward ion bulk flows in t he central plasma sheet indicate that during substorm expansion the near-Ea rth neutral line is located between 21 and 26 R-E, With a tendency to be cl oser to 21 R-E near substorm onset. About 45 min after onset, the tailward moving dipolarization front reaches the distance range where the near-Earth neutral line is located. Thereafter the near-Earth neutral line disappears beyond 31 R-E. This is the classical signature of the start of the recover y phase. We conclude that substorm recovery sets in when the tailward movin g dipolarization front reaches the near-Earth neutral line, because the nea r-Earth neutral line cannot operate in a dipolar field geometry.