Global role of E_‖ in magnetopause reconnection: An explicit demonstration

Abstract

Abstract. We use a global MHD simulation to compute the distribution of E,, on the face of the magnetopause as represented by the last closed field line surface. In MHD codes, E,, is a proxy for magnetic reconnection. Integrating E,, along the topological separator line between open and closed magnetic field lines gives the global reconnection rate at the magnetopause. In the case studied here, where the interplanetary magnetic field (IMF) is precisely duskward, we find the global reconnection rate to be ~49 kV, comparable to potentials inferred from measurements made in the polar cap. The exercise demonstrates an application of a general reconnection theorem that, in effect, equates reconnection with E,,. It prepares the way for MHD imaging of reconnection in terms of contours of E,, on the magnetopause. The result also illustrates a property of parallel potentials in the global context that is not generally recognized. Nearly the full magnetopause reconnection voltage exists on some closed field lines between the northern and southern polar caps, so that they leave the dawn, southern hemisphere with a sizable positive polarity and enter the dusk, northern hemisphere with a sizable negative polarity. An unexpected 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 potential 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 precipitating electrons. Magnetic Reconnection at the

Multi-site observations of the association between aurora and plasma convection in the cusp/polar cap during a southeastward(<i>B<sub>y</sub></i> <u>~</u> |<i>B<sub>z</sub></i>|) IMF orientation

“…One view is that reconnection in such cases can occur where there are antiparallel components in the external magnetosheath magnetic field and the Earth's field (Gonzalez and Mozer, 1974;Cowley, 1981;Sonnerup, 1984;Taguchi et al, 1993;Kawano and Russell, 1997;Siscoe et al, 2001). According to this view, reconnection can take place over large areas of the dayside magnetopause, for example, along a tilted reconnection line traversing the subsolar region and extending to high latitudes in both hemispheres.…”

Magnetopause mapping to the ionosphere for northward IMF

“…Either way, the energy source of the auroral particle fluxes is the solar wind flow past the magnetosphere. The magnetospheric field line geometry is highly complex in the northward IMF case, as pointed out, for example, by Cowley (1973), Alexeev and Belenkaya (1983) (see also Siscoe et al, 2001), and Belenkaya (1998a, b). Frank et al (1982Frank et al ( , 1986 proposed that theta aurora is located on closed field lines and is connected with the plasma sheet boundary layer or the distant plasma sheet.…”

“…(5) gives pc =102 kV, here L y equals 8 R 1 as was demonstrated by our model calculations, and κ B =0.1. For similar solar wind parameters, but IMF in the dawn-dusk direction, Siscoe et al (2001) gave 49 kV as an estimation of the reconnection potential drop in the MHD simulations. Siscoe et al (2001) have used a global MHD simulation to compute the distribution of E on the face of the magnetopause, as represented by the last closed field line surface.…”

“…This is an advantage of our model compared to MHD models, for which it is difficult to determine the magnetopause position. For example, Siscoe et al (2001) have identified the magnetopause with the open/closed field line boundary.…”

“…For the solar wind parameters used by Siscoe et al (2001) for simulations: density 5 cm −3 , speed 350 km/s, and IMF strength 5 nT (B x =B z =0), Eq. (6) gives pc =45 kV comparable to the Siscoe et al (2001) MHD simulation results which found the global reconnection rate to be ∼49 kV.…”