SEP Acceleration at CME Driven Shocks: Numerical Simulation with Evolving Shock Geometry and Downstream Structure

Gradual solar energetic particle (SEP) events are believed to be accelerated at CME driven shocks. Shocks driven by a realistic CMEs are neither simply quasi-perpendicular nor quasi-parallel: the geometry and the shock strength may constantly change as the CME evolves. The shock is likely to be quasi-perpendicular when it forms and becomes more parallel at later stages (Tylka and Lee, 2006). The downstream sheath region immediately behind the shock has a structure of its own, which may play a significant role in the SEP acceleration process. We present numerical simulations adopting a Lagrangian scheme which is designed to handle the evolution of the shock. The scheme follows the moving magnetic field lines, which are wrapped around the expanding CME, and are pushed by the ejecta. We present numerical simulation results and discuss their implications. Tyka, A,J, and M.A. Lee, Astrophys. ~J., 646, 1319 (2006)