A Double-Disruption Substorm Model - The Growth Phase
Tuesday, 16 December 2014
When the IMF turns from Bz- to Bz+, dayside merging forms open lobe field lines at low latitudes. These lobe lines are populated with shocked solar wind and dayside magnetospheric plasma from the reconnection inflow. As those lobe flux tubes pass tailward over the polar caps, they are also populated with outflow from the north and south polar cap ionospheres. As the lobe lines move tailward, they acquire a convex curvature that blocks the westward-flowing cross-tail current (XTJ). This constitutes the first stage of XTJ disruption, and it begins less than 10 min after the frontside merging.The disrupted XTJ closes dawn-to-dusk in the transition plasmasheet (TPS), where it produces a downward FAC to the ionosphere. This causes the proton arc, which is seen for the period from about 10 – 80 min after frontside merging begins at time t=0. The lobe lines eventually reconnect well downtail at about t=30 minutes. The middle section that closes the lobe lines has concave curvature and is called the Neutral Sheet (NSh). The resulting stretched field lines thus have a central NSh which separates the two convex-curvature regions to the north and south, regions which are called the Disruption Zones (DZs); the overall combination of the NDZ, NSh and SDZ is called the Stretched Plasmasheet (SPS). As the SPS continues to grow and the stretched lines are pulled earthward to relieve the magnetic tension, the filling of the NSh occurs both from the DTNL with the higher energy magnetospheric particle population on the lobe lines, but eventually also at about 25 earth radii when the polar cap ionospheric outflow (PCO) component finally reaches the NSh. A NSh FAC system forms, from which electrons flow down to the auroral ionosphere to create the pre-onset arc, starting at about t=65 min. At the same time, the Lyons-Speiser mechanism is initiated in the inner NSh, causing the PCO ions to become trapped and accelerated in the inner NSh region. Eventually, when the SPS grows earthward sufficiently that the NSh reaches the outer radiation belt at about t=85 min, the ionospheric conductivity has grown sufficiently that the XTJ disrupted by the DZs changes its dawn-to-dusk closure by travelling through the ionosphere. This second stage of disruption is the Substorm Current Wedge (SCW). Onset follows at about t=88 min.