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 × 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 ) ∼ (−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 ∼ −10 R E and X ∼ −28 R E have the same magnitudes and develop considerably about 1 min after onset, while that around ( X, Y ) ∼ (−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 ) ∼ (−19, 6) R E .