New Behavior of X lines Inside a Thick Near-Earth Plasma Sheet and Evolution of the Reconnection into Lobe Merging

It is proposed that short-lived reconnection can occur in the center of a thick near-earth nightside plasma sheet and evolve into lobe merging. For reconnection to occur there must be outflow on both the earthward and tailward sides of an X line. The outflow on the earthward side is provided by the well-known azimuthal convection around the earth. The inner bondary of azimuthal convection is at the Alfven shielding layer, and the outer boundary is at the transition to flux tubes which are too tail-like to convect to the dayside due to the geometry of the magnetosphere. The X line is located at the outer boundary of this azimuthal convection channel; hence reconnection creates azimuthally convecting flux tubes from tail-like flux tubes by removing plasma and energy. The outflow from the tailward side of the X line occurs by earthward motion of the X line (that is there is tailward flow in a reference frame moving with the X line) and the reconnected flux forms a magnetic island on the tailward side. It is argued that the above type of reconnection is suppressed during the growth phase of substorms because the azimuthal outflow on the earthward side is blocked due to the changing topology. Both the shrinking of the dayside magnetopause and the increasing tail-likeness of nightside field lines support the idea that the outer boundary of the azimuthal convection channel moves earthward away from any newly-forming X line. When the rate of change of the topology becomes small, due to either a northward turning of the IMF or an approach to a steady-state, the above nightside reconnection is no-longer suppressed. The expansion phase commences with reconnection of the above type (earthward propagation of the X line and formation of a magnetic island). Two possible options for the further development of the expansion phase are considered. In the first, the above reconnection at the X line eats through the plasma sheet and the configuration evolves into the usual lobe-merging picture. In the second, the earthward-moving X line reaches the Alfven layer. The magnetic island coalesces with nearer-earth field lines creating a rapid inflow into the nightside and producing the outflow conditions for a second X line to form near the tailward end of the island. Reconnection at this second X line eats through the plasma sheet and the configuration evolves into the usual lobe-merging picture.