Evolution of the magnetotail associated with substorm auroral breakups

We have statistically studied the three‐dimensional structure of evolution of the magnetotail associated with substorm expansion onsets. We selected 397 auroral breakups from Polar UVI data with a timing uncertainty of less than 2 min and then performed the two‐dimensional superposed epoch analysis using GEOTAIL data with 2 min resolution. We also performed the regional superposed epoch analysis with 1 min resolution using only the events for which we could determine the onset time with an uncertainty of less than 1 min. In particular, we found the simultaneous variations in the north‐south magnetic field, the duskward electric field, and the thermal flux immediately after onset around X ∼ −10 R E and X ∼ −30 R E , as well as the first total pressure decrease slightly (0–2 min) before onset around X ∼ −20 R E . These results suggest that the merging of magnetic field lines or the magnetic reconnection first occurs around X ∼ −20 R E in the premidnight tail at least 0–2 min before onset. Then, immediately after onset, the dipolarization occurs around X ∼ −10 R E , which is simultaneous with substantial evolution of the plasmoid around X ∼ −30 R E .