Global magnetohydrodynamic simulation of the 15 March 2013 coronal mass ejection event--Interpretation of the 30-80MeV proton flux

The coronal mass ejection (CME) event on 15 March 2013 is one of the few solar events in Cycle 24 that produced a large solar energetic particle (SEP) event and severe geomagnetic activity. Observations of SEP from the ACE spacecraft show a complex time-intensity SEP profile that is not easily understood with current empirical SEP models. In this study, we employ a global three-dimensional (3-D) magnetohydrodynamic (MHD) simulation to help interpret the observations. The simulation is based on the H3DMHD code and incorporates extrapolations of photospheric magnetic field as the inner boundary condition at a solar radial distance (r) of 2.5 solar radii. A Gaussian-shaped velocity pulse is imposed at the inner boundary as a proxy for the complex physical conditions that initiated the CME. It is found that the time-intensity profile of the high-energy (>10MeV) SEPs can be explained by the evolution of the CME-driven shock and its interaction with the heliospheric current sheet and the nonuniform solar wind. We also demonstrate in more detail that the simulated fast-mode shock Mach number at the magnetically connected shock location is well correlated (rcc≥0.7) with the concurrent 30-80MeV proton flux. A better correlation occurs when the 30-80MeV proton flux is scaled by r-1.4(rcc=0.87). When scaled by r-2.8, the correlation for 10-30MeV proton flux improves significantly from rcc=0.12 to rcc=0.73, with 1h delay. The present study suggests that (1) sector boundary can act as an obstacle to the propagation of SEPs; (2) the background solar wind is an important factor in the variation of IP shock strength and thus plays an important role in manipulation of SEP flux; (3) at least 50% of the variance in SEP flux can be explained by the fast-mode shock Mach number. This study demonstrates that global MHD simulation, despite the limitation implied by its physics-based ideal fluid continuum assumption, can be a viable tool for SEP data analysis. Key Points Sector boundary can act as an obstacle to the propagation of SEPs Background solar wind is an important factor in the variation of shock strength At least 50% of the variance in SEP flux can be explained by the shock Mach number