Modeling of Proton Acceleration in Application to a Ground Level Enhancement

Occasionally the Sun produces outbursts of relativistic ions that manifest themselves at the Earth's surface as the so-called ground level enhancements (GLEs). The conditions under which solar ions are accelerated to relativistic energies are poorly understood. Solar flares and shock waves driven by coronal mass ejections (CMEs) are considered as the possible sources of such particles. Here we present simulations of proton acceleration in a CME-driven shock. The employed simulation model accounts self-consistently for proton interactions with Alfvén waves in the shock upstream. The key feature of the modeling is that we apply a semi-empirical model of the shock (17 May 2012 GLE event) to constrain the shock parameters. We analyze the simulations with the aim to understand which plasma and shock parameters mainly influence the particle acceleration process. The simulations show that the scattering-center compression ratio is the main parameter controlling the acceleration efficiency of the shock. Moreover, the shock portions characterized by the highest values of the compression ratio can indeed accelerate protons to GLE energies, even though the particle acceleration efficiency of the shock in our simulations is significantly lower than if calculated based on the DSA theory.