The Substorm Current Wedge (SCW) occurrence in the late growth and onset phases of substorms was proposed as the current system which disrupts cross-tail current by diverting it to the ionosphere. The closure current for the SCW originally was suggested to be the strong westward auroral electrojet (WEJ). However, the SCW-WEJ system has no viable generator current. Similarly, the asymmetric or Partial Ring Current (PRC) increases in strength during the growth phase, and is sometimes associated with an enhanced Region 2 field-aligned current (FAC) closing to the ionosphere, but specifics of that closure have been lacking. Here we present a unifying picture which includes the SCW post- and pre-midnight (AM and PM, respectively) currents and a generator current in the midnight portion of the PRC system, with these currents based upon a model of the nightside magnetotail magnetic geometry. That geometry consists of open north and south lobe regions surrounding a plasmasheet with two types of closed field line regions-stretched lines in the central part of the plasmasheet (SPS) and dipolar lines (DPS) between the low latitude boundary layer (LLBL) regions and the SPS. There is also an important plasmasheet transition region (TPS) in which the dipolar field near the plasmapause gradually transforms to stretched lines near the earthward edge of the SPS, and in which the midnight part of the PRC flows. We propose that our proposed near-onset current system consists of a central current which becomes part of the midnight sector PRC and which is the generator, to which are linked two three-part current systems, one on the dawnside and one on the duskside. The three-part systems consist of up and down FACs closing as Pedersen currents in the ionosphere. These 3-part systems are not activated until near-onset is reached, because of a lack of ionospheric conductivity in the appropriate locations where the Pedersen current closure occurs. The initial downward FAC of the 3-part dawnside system and the final upward FAC of the 3-part duskside system correspond to the AM and PM current segments, respectively, of the originally proposed SCW.
RESEARCH-ARTICLE
The substorm current wedge and midnight sector partial ring current near substorm onset: A synthesis based on a magnetotail magnetic field geometry model

Vol. 24, Issue 1, pp. 32-41 (2013) • DOI
Abstract
Basic Infomations
References
Attachments
Cited By
Abstract
Author Address:
1. Institute of Space & Atmospheric Studies, University of Saskatchewan, Saskatoon S7N 5E2, Canada
1 McPherron R L, Russell C T, Aubry M P. Satellite studies of magnetospheric substorms on August 15, 1968: 9. Phenomenological model for substorms. J Geophys Res, 1973, 78(16): 3131–3149 DOI:10.1029/JA078i016p03131.
2 Dungey J W. Interplanetary magnetic field and the auroral zones. Phys Rev Lett, 1961, 6(2): 47–48 DOI:10.1103/PhysRevLett.6.47.
3 Bristow W A, Sofko G J, Stenbaek-Nielsen H C, et al. Detailed analysis of substorm observations using SuperDARN, UVI, ground-based magnetometers, and all-sky imagers. J Geophys Res, 2003, 108(A3): 1124 DOI:10.1029/2002JA009242.
4 Haerendel G. Equatorward moving arcs and substorm onset. J Geophys Res, 2010, 115(A7): A07212 DOI:10.1029/2009JA015117.
5 Shen C, Li X, Dunlop M, et al. Analyses on the geometrical structure of magnetic field in the current sheet based on Cluster measurements. J Geophys Res, 2003, 108(A5): 1168 DOI:10.1029/2002JA009612.
6 Saito M H, Hau L N, Hung C C, et al. Spatial profile of magnetic field in the near-Earth plasma sheet prior to dipolarization by THEMIS: Feature of minimum B. Geophys Res Lett, 2010, 37(8): L08106 DOI:10.1029/2010GL042813.
7 Spence H E, Kivelson M G. Contributions of the low-latitude boundary layer to the finite width magnetotail convection model. J Geophys Res, 1993, 98(A9): 15487–15496 DOI:10.1029/93JA01531.
8 Zhou X Y, Tsurutani B T. Interplanetary shock triggering of nightside geomagnetic activity: Substorms, pseudobreakups, and quiescent events. J Geophys Res, 2001, 106(A9): 18957–18967 DOI:10.1029/2000JA003028.
9 Morley S K, Freeman M P. On the association between northward turnings of the interplanetary magnetic field and substorm onsets. Geophys Res Lett, 2007, 34(8): L08104 DOI:10.1029/2006GL028891.
10 Lee L C, Albano R K, Kan J R. Kelvin-Helmholtz Instability in the magnetopause-boundary layer region. J Geophys Res, 1981, 86(A1): 54–58 DOI:10.1029/JA086iA01p00054.
11 Nakagawa T, Nishida A. Southward magnetic field in the neutral sheet produced by wavy motions propagating in the dawn-dusk direction. Geophys Res Lett, 1989, 16(11): 1265 DOI:10.1029/GL016i011p01265.
12 B��chner J, Kuska J P. Sausage mode instability of thin current sheets as a cause of magnetospheric substorms. Ann Geophys, 1999, 17(5): 604–612 DOI:10.1007/s00585-999-0604-5.
13 Parks G K. Physics of Space Plasmas: An Introduction, 2nd edn. Boulder, Colorado: Westview Press, 2004 : 278–280
14 Erickson G M, Maynard N C, Burke W J, et al. Electromagnetics of substorm onsets in the near-geosynchronous plasma sheet. J Geophys Res, 2000, 105(A11): 25265–25290 DOI:10.1029/1999JA000424.
15 Newell P T, Gjerloev J W. SuperMAG-based partial ring current indices. J Geophys Res, 2012, 117(A5): A05215 DOI:10.1029/2012JA017586.
16 Kadokura A, Yukimatu A S, Ejiri M, et al. Detailed analysis of a substorm event on 6 and 7 June 1989, 1, Growth phase evolution of nightside auroral activities and ionospheric convection toward expansion phase onset. J Geophys Res, 2002, 107(A12): 1479 DOI:10.1029/2001JA009127
Friend Links
Related Journals
Related Links