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

• Main Authors: S. Machida, Y. Miyashita, A. Ieda, M. Nosé, V. Angelopoulos, J. P. McFadden
• Format: Article
• Language: English
• Published: Copernicus Publications 2014-02-01
• Series: Annales Geophysicae
• Online Access: https://www.ann-geophys.net/32/99/2014/angeo-32-99-2014.pdf

To investigate the physical mechanism responsible for substorm triggering, we performed a superposed-epoch analysis using plasma and magnetic-field data from THEMIS probes. Substorm onset timing was determined based on auroral breakups detected by all-sky imagers at the THEMIS ground-based observatories. We found earthward flows associated with north–south auroral streamers during the substorm growth phase. At around <i>X</i> = &minus;12 Earth radii (<i>R</i>_E), the northward magnetic field and its elevation angle decreased markedly approximately 4 min before substorm onset. Moreover, a northward magnetic-field increase associated with pre-onset earthward flows was found at around <i>X</i> = &minus;17 <i>R</i>_E. This variation indicates that local dipolarization occurs. Interestingly, in the region earthwards of <i>X</i> = &minus;18 <i>R</i>_E, earthward flows in the central plasma sheet (CPS) reduced significantly approximately 3 min before substorm onset, which was followed by a weakening of dawn-/duskward plasma-sheet boundary-layer flows (subject to a 1 min time lag). Subsequently, approximately 1 min before substorm onset, earthward flows in the CPS were enhanced again and at the onset, tailward flows started at around <i>X</i> = &minus;20 <i>R</i>_E. Following substorm onset, an increase in the northward magnetic field caused by dipolarization was found in the near-Earth region. Synthesizing these results, we confirm our previous results based on GEOTAIL data, which implied that significant variations start earlier than both current disruption and magnetic reconnection, at approximately 4 min before substorm onset roughly halfway between the two regions of interest; i.e. in the catapult current sheet.