Global role of E-parallel to in magnetopause reconnection: An explicit demonstration

We use a global MHD simulation to compute the distribution of E-parallel to on the face of the magnetopause as represented by the last closed field li ne surface. In MHD codes, E-parallel to is a proxy for magnetic reconnectio n. Integrating E-parallel to along the topological separator line between o pen and closed magnetic field lines gives the global reconnection rate at t he magnetopause. In the case studied here, where the interplanetary magneti c field (IMF) is precisely duskward, we find the global reconnection rate t o be similar to 49 kV, comparable to potentials inferred from measurements made in the polar cap. The exercise demonstrates an application of a genera l reconnection theorem that, in effect, equates reconnection with E-paralle l to. It prepares the way for MHD imaging of reconnection in terms of conto urs of E-parallel to on the magnetopause. The result also illustrates a pro perty of parallel potentials in the global context that is not generally re cognized. Nearly the full magnetopause reconnection voltage exists on some closed field lines between the northern and southern polar caps; so that th ey leave the dawn, southern hemisphere with a sizable positive polarity and enter the dusk, northern hemisphere with a sizable negative polarity. An u nexpected finding is a substantial parallel potential (between 10 and 15 kV ) between the magnetopause and the ionosphere in northern dawn and southern dusk sectors. (Interchange "dawn" and "dusk" for dawnward IMF.) This poten tial has the polarity that accelerates electrons into the ionosphere in the dusk sector and, so, might be the origin of the "hot spot" seen there in p recipitating electrons.