GEOTAIL observations of total pressure and electric field variations in the near and mid-distant tail associated with substorm onsets

Selecting 263 substorm events with Pi 2 pulsation, we have statistically investigated temporal variations in the total pressure and the electric field (E -V x B) in the near-Earth magnetotail with GEOTAIL data. It was found that a total pressure decrease occurs first in the premidnight region around (X,Y) similar to (-18,7) R-E in the GSM coordinates about 1 or 2 min before onset, and propagates to the surrounding regions successively. The dawn-to-dusk electric fields around X similar to -10R(E) and X similar to -28 R-E have the same magnitudes and develop considerably about 1 min after onset, while that around (X, Y) similar to (-20, 5) R-E is smaller. These results strongly suggest that the magnetic reconnection takes place at least 2 min earlier than the dipolarization, and that, on the average, the center of the energy release is initially located around the premidnight region of (X,Y) similar to (-19,6) R-E.