A Unifying Model of Substorms: Evolving Magnetic Field Line Shape in the Magnetotail

During the growth phase of substorms, there is dayside reconnection which leads to a buildup of lobe flux in the tail. The result is that the closed magnetotail field lines become stretched and a strong minimum in B occurs in the center of the neutral sheet (NS) at about 12 Re . The stretched closed field lines have two important features that occur in the regions neighboring the NS to the north and south. First, there is an outward magnetic field gradient away from the NS. Secondly, there are four inflection points on each field line, two north of the NS and two south of the NS, such that the curvature between the inner and outer inflection points is opposite to that of a dipolar field. Consequently, in these two regions surrounding the NS, which we call the north and south current disruption zones (DZN and DZS), the gradient and curvature drifts of the ions are both eastward, opposite to the strong westward flow of the essentially unmagnetized ions in the NS, which are “free-streaming” in the dawn-to-dusk electric field. The width in the Y-direction of the field line distortion region is likely less than 10 Re east and west from the NS central axis, and the distortion becomes stronger as the growth stage proceeds. This size is compatible with the ~70 degree width proposed for the substorm current wedge (SCW). If the finite width magnetotail convection (FWMC) model proposed in 1993 by Spence and Kivelson is applied to the DZs, it becomes obvious that a positive charge accumulation occurs on the east (dawn) side of each DZ and a negative charge accumulation on the west (dusk) side. These are the charge accumulations that drive the SCW field-aligned currents that cause the auroral westward substorm electrojet in the ionosphere. Even more importantly, this 3-layer geometry consisting of the DZN, NS, and DZS regions is characterized by strong shears in the ion flows at the two NS/DZ interfaces. As a result, the Kelvin-Helmholtz (KH) instability grows steadily in these interface regions as the growth phase proceeds. Although the conditions for pure MHD are clearly violated in these regions, one can use the MHD KH formulation as a guide and infer that the more easily excited kink mode will first occur, followed by the sausage mode as conditions become more disturbed just prior to the expansive phase. The sausage mode quite naturally leads to very thin regions in the NS where reconnection occurs. It is likely that multiple reconnections, i.e. multiple onset activation zones, occur within the two large NS-DZ interface regions, which probably extend at least from X ~ -7 to -25 Re. This simple model thus removes the debate between current disruption and reconnection advocates, since the eastward gradient and curvature drifts producing the current disruption zones lead to the shears that cause the KH instability which in turn produces the reconnection sites in the NS regions thinned by the KH waves. The model also is clearly compatible with the extensive wave activity observed both in the ionosphere and the inner plasmasheet prior to, during, and following substorm onset.