Kinetic modeling of low Mach number collisionless shocks in galaxy clusters
- Dr. Jacek NIEMIEC
Collisionless shocks are found in a number of astrophysical objects, ranging in size from the Earth's bow shock through solar flares, termination shock of the solar wind, supernova remnant shocks, and merger shocks in galaxy clusters. In the latter case, low Mach number (Ms << 10) shocks are found propagating in a high beta (β > 1) plasmas, where β is the ratio of thermal to magnetic pressure. Observations in X-ray and radio show that cluster shocks are electron accelerators to non-thermal energies. However, the mechanism of particle energization is poorly known. Recent studies with kinetic PIC simulations suggested the so-called shock-drift acceleration followed by particle-wave interactions in the shock upstream as a mechanism of electron injection in high-β regime. However, other works demonstrated the importance of the multi-scale shock structure which includes ion-scale shock-rippling fluctuations that can significantly alter the injection mechanism. Here we present preliminary results of our first-principles 2D large-scale PIC simulations of shocks for conditions of high-β plasmas, as appropriate to galaxy clusters. The aim is to investigate the multi-scale electron injection physics. We demonstrate the slowly growing rippling modes to emerge after a few tens of ion gyro times and their appearance is marked with an increase in accelerated electron energy. This suggests an energization mechanism that is different from processes identified so far in low Mach number shocks. The micro-physics of these processes is now under detailed investigation whose results will be presented at the conference.