Particle Acceleration in shocks driven by Interplanetary Coronal Mass Ejections

Shock waves driven by the Interplanetary Coronal Mass Ejections (ICMEs) are efficient particle accelerators giving rise to Solar Energetic Particle events (SEPs). Using the Magnetohydrodynamic-Particle In Cell (MHD-PIC) module of the PLUTO code, we have studied the dynamics of energetic protons on the background of such ICME shocks. This study mostly focuses on the interaction of the particles with the ambient solar wind plasma that leads to the acceleration process. Shocks of different Alfvénic machs and magnetic orientations are studied. In a parallel shock scenario, the upstream region shows the development of magnetic fluctuations driven by the particles. The transverse magnetic power spectrum reveals the dominant role of non-resonant Bell instability in driving the solar wind upstream turbulence. These magnetic fluctuations act as scattering centers confining the particles close to the shock front which eventually lead to particle energization through Diffusive Shock Acceleration (DSA) process. The simulated parallel shock also hints at a possible action of small-scale dynamo in enhancing the downstream magnetic field. On the other hand, particles in quasi-perpendicular shock energize though the Shock Drift Acceleration (SDA) process. Operation of such a mechanism is evident from the particle velocity distribution and orientation of the magnetic field in the shock downstream region. However, the maximum energy gained by the accelerated particles in quasi- perpendicular shock is much less than the parallel shock having similar Alfvénic mach.