A state-of-the-art picture of substorm-associated evolution of the near-Earth magnetotail obtained from superposed epoch analysis

We have obtained a state-of-the-art picture of substorm-associated evolution of the near-Earth magnetotail and the inner magnetosphere for understanding the substorm triggering mechanism. We performed superposed epoch analysis of Geotail, Polar, and GOES data with 2-min resolution, utilizing a total of 3787 substorms for each of which auroral breakup was determined from Polar UVI or IMAGE FUV auroral imager data. The decrease of the north-south magnetic field associated with plasmoids and the initial total pressure decrease suggest that the magnetic reconnection first occurs in the premidnight tail, on average, at X similar to -16 to -20 R-E at least 2 min before auroral onset. The magnetic reconnection site is located near the tailward edge of a region of considerably taillike magnetic field lines and intense cross-tail current, which extends from X similar to -5 to -20 R-E in the premidnight sector. Then the plasmoid substantially evolves tailward of X similar to -20 R-E immediately after onset. Almost simultaneously with the magnetic reconnection, the dipolarization begins first at X similar to -7 to -10 R-E 2 min before onset. The dipolarization region then expands tailward as well as in the dawn-dusk directions and earthward. We find that the total pressure generally enhances in association with the dipolarization, with the contribution of high-energy particles. Also, energy release is more significant between the regions of the magnetic reconnection and the initial dipolarization. The present results will be helpful as a reference guide to developing the overall picture of magnetotail evolution and studying the causal relationship between the magnetic reconnection and the dipolarization as well as detailed mechanisms of each of the two processes on the basis of multispacecraft observations.