Nonthermal Electrons at High Mach Number Shocks: Electron Shock Surfing Acceleration

We study the suprathermal electron acceleration mechanism in a perpendicular magnetosonic shock wave in a high Mach number regime by using a particle-in-cell simulation. We find that shock surfing/surftron acceleration producing the suprathermal electrons occurs in the shock transition region where a series of large amplitude electrostatic solitary waves (ESWs) are excited by Buneman instability under the interaction between the reflected ions and the incoming electrons. It is shown that the electrons are likely to be trapped by ESWs, and during the trapping phase they can be effectively accelerated by the shock motional/convection electric field. We discuss that suprathermal electrons can be accelerated up to $m_i c^2 (v_0/c)$, where $m_i c^2$ is the ion rest mass energy and $v_0$ is the shock upstream flow velocity. Furthermore, some of these suprathermal electrons may be effectively trapped for infinitely long time when Alfv\'en Mach number $M_A$ exceeds several 10, and they are accelerated up to the shock potential energy determined by the global shock size.