  Large-scale two-dimensional (2D) full particle-in-cell simulations are
carried out for studying the relationship between the dynamics of a
perpendicular shock and microinstabilities generated at the shock foot. The
structure and dynamics of collisionless shocks are generally determined by
Alfven Mach number and plasma beta, while microinstabilities at the shock foot
are controlled by the ratio of the upstream bulk velocity to the electron
thermal velocity and the ratio of the plasma-to-cyclotron frequency. With a
fixed Alfven Mach number and plasma beta, the ratio of the upstream bulk
velocity to the electron thermal velocity is given as a function of the
ion-to-electron mass ratio. The present 2D full PIC simulations with a
relatively low Alfven Mach number (M_A ~ 6) show that the modified two-stream
instability is dominant with higher ion-to-electron mass ratios. It is also
confirmed that waves propagating downstream are more enhanced at the shock foot
near the shock ramp as the mass ratio becomes higher. The result suggests that
these waves play a role in the modification of the dynamics of collisionless
shocks through the interaction with shock front ripples.
