The topology of the subsolar magnetopause under northward IMF conditions: Global simulations and MMS observations

Abstract:

The topology of magnetopause reconnection under northward IMF conditions remains poorly understood due to the difficulty of inferring global properties of the magnetic field from local spacecraft observations. Does reconnection occur for nearly anti-parallel magnetic fields poleward of the cusps, or is component reconnection possible near the subsolar magnetopause? In the literature, observations of electron anisotropy and heat flux have been used to support both scenarios. In this talk, we revisit this problem, taking advantage of two recent developments: 1) the availability of new tools for tracking three-dimensional X lines in global simulations, 2) the availability of 7.5 ms electron moments from the FPI instrument on MMS. Using the Space Weather Modeling Framework, we perform global simulations of a MMS subsolar magnetopause crossing under steady northward IMF conditions. The simulations show that the topology of the dayside magnetopause is consistent with the superposition of a uniform field with a dipole: a topological separator extends across the dayside magnetopause, terminating at three-dimensional magnetic nulls in the cusps. Due to the combination of IMF Bx > 0 and By < 0, the separator is displaced about 10 RE toward the positive Y flank, but the current sheet extends away from the separator all the way to the subsolar magnetopause. Thus, the simulations predict that MMS would see a sub-ion scale current sheet at the subsolar magnetopause, with an open magnetic topology (i.e., field lines that come in from the solar wind and connect to the southern ionosphere). MMS observations confirm this prediction, demonstrating that the structure of the subsolar magnetopause under northward IMF conditions is similar in many ways to the structure of the electron-scale current sheets previously observed by MMS near Electron Dissipation Regions.