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

Abstract

[1] 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 $ À16 to À20 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 $ À5 to À20 R E in the premidnight sector. Then the plasmoid substantially evolves tailward of X $ À20 R E immediately after onset. Almost simultaneously with the magnetic reconnection, the dipolarization begins first at X $ À7 to À10 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.

Spatial distributions of electromagnetic field variations and injection regions during the 20 November 2007 sawtooth event

“…It is likely that the midnight dipolarisation (0 h MLT) started slightly tailward of L89. This interpretation is consistent with the fact that a typical location of the dipolarisation initiation region is X∼−7 R E -−10 R E (Miyashita et al, 2009). Almost the same discussion can be found in Ohtani et al (1993), in which a significant dipolarisation was absent at geosynchronous orbit inside the SCW.…”

Azimuthal auroral expansion associated with fast flows in the near-Earth plasma sheet: Coordinated observations of the THEMIS all-sky imagers and multiple spacecraft

“…Intensifications of the electric fields correspond to local depletions of the plasma pressure. While a net increase in plasma pressure has often been observed in the inner plasma sheet [e.g., Miyashita et al, 2009;Xing et al, 2010], the local depletion of the plasma pressure that we observed is similar to the one observed in the midtail plasma sheet (∼20 R E ), where earthward flows enhance with entropy reduction [e.g., Sergeev et al, 1996].…”

X lines in the magnetotail for southward and northward IMF conditions

“…The X lines in the tail are frequently to be observed simultaneously with the auroral brightening Kan, 2007;Ieda et al, 2008;Miyashita et al, 2009]. Recently, depending on a newly developed method to categorize the different phases of a substorm, Zhang et al [2015] statistically analyzed the occurrence of the X lines at different activity levels in the Earth's tail.…”

“…According to Zhang et al [2010], both locally occurred X lines and the X lines from somewhere else contribute to the occurrences of the TFF. Such TFFs are almost simultaneously with the auroral breakup within a few minutes [Ieda et al, 2008;Miyashita et al, 2009].…”