Substorm dipolarization and recovery

On the basis of ∼2 years of Geotail data, we use a superposed epoch approach to study the average behavior of plasma and magnetic fields at different radial distances, between 11 and 31 RE, during 66 substorms in the premidnight sector. Magnetic field dipolarization is first seen in the innermost region (11–16 RE) around substorm onset and subsequently moves tailward at a rate of 35 km/s. Fast earthward and tailward ion bulk flows in the central plasma sheet indicate that during substorm expansion the near-Earth neutral line is located between 21 and 26 RE, with a tendency to be closer to 21 RE 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 RE. This is the classical signature of the start of the recovery phase. We conclude that substorm recovery sets in when the tailward moving dipolarization front reaches the near-Earth neutral line, because the near-Earth neutral line cannot operate in a dipolar field geometry.