Statistical visualization of the Earth's magnetotail based on Geotail data and the implied substorm model

• Published in: Annales geophysicae (1988) Vol. 27; no. 3; pp. 1035 - 1046
• Main Authors: MACHIDA, S, MIYASHITA, Y, IEDA, A, NOSE, M, NAGATA, D, LIOU, K, OBARA, T, NISHIDA, A, SAITO, Y, MUKAI, T
• Format: Journal Article
• Language: English
• Published: Göttingen Copernicus 02-03-2009; Copernicus GmbH
• Subjects: Analysis; Earth, ocean, space; Exact sciences and technology; External geophysics; Magnetic fields; Physics of the ionosphere; Physics of the magnetosphere; Visualization (Computer); Ballooning instability; Plasma; Polar satellite; magnetic field; Plasma physics; Poynting flux; Plasmoid; Convection flow; Parameter variation; Non linear phenomenon; Superposed epoch analysis; Relaxation; Proxy; Coastal current; fluctuations; IMAGE satellite; Magnetospheric physics (Magnetotail; Magnetic line; Magnetospheric tail; Storms and substorms; models; Magnetic reconnection; efficiency; Current sheet; Observable; electrical field; Plasma layer; Neutral line; storms; Space plasma physics (Nonlinear phenomena)
• ISSN: 0992-7689; 1432-0576; 1432-0576
• DOI: 10.5194/angeo-27-1035-2009

We investigated the temporal and spatial development of the near-Earth magnetotail during substorms based on multi-dimensional superposed-epoch analysis of Geotail data. The start time of the auroral break-up (t=0) of each substorm was determined from auroral data obtained by the Polar and IMAGE spacecraft. The key parameters derived from the plasma, magnetic-field, and electric-field data from Geotail were sorted by their meridional X(GSM)–Z(proxy) coordinates. The results show that the Poynting flux toward the plasma-sheet center starts at least 10 min before the substorm onset, and is further enhanced at X~−12 RE (Earth radii) around 4 min before the onset. Simultaneously, large-amplitude fluctuations occurred, and earthward flows in the central plasma sheet between X~−11 RE and X~−19 RE and a duskward flow around X=−10 RE were enhanced. The total pressure starts to decrease around X=−16 RE about 4 min before the onset of the substorm. After the substorm onset, a notable dipolarization is observed and tailward flows commence, characterised by southward magnetic fields in the form of a plasmoid. We confirm various observable-parameter variations based on or predicted by the relevant substorm models; however, none of these can explain our results perfectly. Therefore, we propose a catapult (slingshot) current-sheet relaxation model, in which an earthward convective flow produced by catapult current-sheet relaxation and a converted duskward flow near the Earth are enhanced through flow braking around 4 min before the substorm onset. These flows induce a ballooning instability or other instabilities, causing the observed current disruption. The formation of the magnetic neutral line is a natural consequence of the present model, because the relaxation of a highly stretched catapult current-sheet produces a very thin current at its tailward edge being surrounded by intense earthward and tailward magnetic fields which were formerly the off-equatorial lobe magnetic fields. This location is the boundary between a highly stressed catapult current sheet and a Harris-type current sheet characterized by little stress. In addition, the flows induced around the boundary toward the current-sheet center may enhance the formation of the magnetic neutral line and the efficiency of magnetic reconnection. After magnetic reconnection is induced, it plays a significant role in driving the substorm.