Statistical visualization of the Earth's magnetotail and the implied mechanism of substorm triggering based on superposed-epoch analysis of THEMIS data

摘要

To investigate the physical mechanism responsible forsubstorm triggering, we performed a superposed-epoch analysis using plasmaand magnetic-field data from THEMIS probes. Substorm onset timing wasdetermined based on auroral breakups detected by all-sky imagers at theTHEMIS ground-based observatories. We found earthward flows associated withnorth–south auroral streamers during the substorm growth phase. At aroundX = −12 Earth radii (R_E), the northward magnetic field and itselevation angle decreased markedly approximately 4 min before substormonset. Moreover, a northward magnetic-field increase associated withpre-onset earthward flows was found at around X = −17 R_E. Thisvariation indicates that local dipolarization occurs. Interestingly, in theregion earthwards of X = −18 R_E, earthward flows in the central plasmasheet (CPS) reduced significantly approximately 3 min before substorm onset,which was followed by a weakening of dawn-/duskward plasma-sheetboundary-layer flows (subject to a 1 min time lag). Subsequently,approximately 1 min before substorm onset, earthward flows in the CPS wereenhanced again and at the onset, tailward flows started at around X = −20R_E. Following substorm onset, an increase in the northward magneticfield caused by dipolarization was found in the near-Earth region.Synthesizing these results, we confirm our previous results based on GEOTAILdata, which implied that significant variations start earlier than bothcurrent disruption and magnetic reconnection, at approximately 4 min beforesubstorm onset roughly halfway between the two regions of interest; i.e. inthe catapult current sheet.