Particle Acceleration by Magnetic Islands in a Strong Turbulent Shock

A stochastic particle energization by magnetic reconnection in a strong turbulent shock is discussed. The strong shocks, whose Mach numbers are greater than several tens, are often observed in planetary shocks in the outer Heliosphere such as Saturn's bow shock, and would be also expected in high-speed interplanetary shocks in the inner Heliosphere. The study of these strong shocks is important to bridge the gap between the earth's bow shock with a relatively low Mach number less than 10 and the supernova remnant shocks with a very high Mach number more than 100. Based on our two- and three-dimensional, particle-in-cell simulations of high Mach number shocks, we discuss that a highly turbulent state associated with many magnetic islands/clumps can be formed in the shock foot region. An effective temperature anisotropy due to the incoming and reflected ions excites the Weibel instability, and the Weibel generated magnetic fields form many current sheets with anti-parallel magnetic fields. In the highly turbulent plasma state, magnetic reconnection can happen, which in turn leads to the multiple interaction of electrons with the magnetic islands, and the strong electron acceleration can be observed. We argue that the above stochastic particle energization is one of keys to understand the shock injection physics in high Mach number magnetosonic shocks.