Locating Magnetic Reconnection at Earth ’ s Dayside Magnetopause in Global Magnetospheric Simulations

The transfer of energy and mass from the solar wind into the magnetosphere is governed by magnetic reconnection, which occurs when magnetic fields with oppositely directed components merge and nullify. This process is of critical importance to space weather, as it controls magnetospheric convection (Dungey , 1961) and energy release during geomagnetic storms and substorms (Angelopoulos et al., 2008). Conditions in the solar wind determine the efficiency and location of reconnection at the dayside magnetopause, which in turn determines the efficiency of solar wind-magnetospheric coupling. In particular, as pointed out by Dungey (1961, 1963), when the interplanetary magnetic field (IMF) is southward as sketched in Fig. 1, reconnection happens at the red box near the subsolar point, which allows the magnetic field to be stretched into the magnetotail and the transfer of energy from the solar wind is efficient. When the IMF points northward, reconnection happens near the polar cusps, and the energy transfer from the solar wind is inefficient. The examples of northward and southward IMF are special cases which are well understood because of the high degree of symmetry. However, despite the importance of this process, much less is known about where reconnection occurs for arbitrary IMF clock angles θIMF. It is not presently possible to quantitatively predict the efficiency or even the location of magnetic reconnection at Earth’s dayside magnetopause for arbitrary IMF clock angles θIMF. Understanding where reconnection occurs for different clock angles is also important to inform observational studies of magnetopause reconnection where they should go into data collection mode. Therefore, understanding magnetopause reconnection for arbitrary clock angle could directly benefit NASA’s upcoming Magnetospheric Multi-Scale (MMS) mission, a satellite mission specifically designed to study properties of magnetic reconnection at small length scales at Earth’s magnetosphere. Predicting where reconnection occurs at the dayside magnetopause is very difficult because it is a fully three-dimensional problem. For the southward IMF case in Fig. 1, the location of reconnection is relatively easy to find; it is the point in the red box on the left where open solar wind magnetic field lines meet closed terrestrial field lines, which also lies at the boundary with half-open newly reconnected field lines that map to the Earth in only one direction. This point is part of a line called the ”magnetic separator,” so-called because it separates regions of different magnetic topology (Cowley , 1973; Siscoe, 1987; Lau and Finn, 1990; Siscoe et al., 2001). When the IMF is not due northward or southward, the magnetic field structure of the magnetosphere becomes very complicated. However, there is still a magnetic field line that separates different topologies of magnetic field lines, and this separator continues to be the location where reconnection is likely to occur. Thus, identifying the dayside reconnection site requires locating the separator. The present project is to locate separators on the dayside magnetopause in global magnetospheric simulations using a technique developed by the applicant (Komar et al., submitted to J. Geophys. Res.). The