3D Hall-MHD Simulations of Mercury’s Dayside Magnetopause Reconnection and Its Impact on the Global Magnetospheric Dynamics

Abstract:

Observations from NASA’s MESSENGER spacecraft reveal that Mercury has a miniature magnetosphere arising from the interaction of its dipolar intrinsic field with the inner heliosphere solar wind. Compared to the terrestrial magnetosphere, Mercury’s magnetosphere appears to be more dynamic in that the typical timescales for global plasma and magnetic flux circulation are much shorter, and the dayside magnetopause reconnection occurs at faster rates and under a wider range of magnetic shear angles. As a product of multiple X-line reconnection, flux transfer events (FTEs) are found to arise much more frequently with occurrence rates of about 50 times higher than detected at Earth. MESSENGER observations suggest that aside from the apparent difference in system size, the large differences in reconnection-driven dynamics between Mercury’s magnetosphere and the Earth’s are likely related to the upstream solar wind conditions. In order to obtain a quantitative, global understanding of how magnetopause reconnection occurs at Mercury and its large-scale consequences, we have used the BATSRUS Hall-MHD code with a high resolution grid that resolves ion kinetic scales to simulate Mercury’s magnetopause dynamics under a variety of upstream solar wind and IMF conditions. Flux ropes are found to form in all of our time-dependent Hall MHD simulations under steady solar wind conditions, but their properties, such as occurrence rate and spatial structure, vary depending on the upstream parameters. We have developed techniques to identify flux ropes in our simulations and extract their magnetic field and plasma properties that can be compared directly with MESSENGER observations of FTEs. The set of carefully designed Hall MHD simulations allow us to examine how the properties of FTEs depend on such parameters as the solar wind Alfvénic Mach number, IMF orientation, and the magnetosheath plasma . With the global model, we also evaluate the contribution of FTEs to the global circulation of plasma and magnetic flux at Mercury and how it might vary in response to changes in the external conditions.