  A full particle simulation study is carried out on a perpendicular
collisionless shock with a relatively low Alfven Mach number (M_A=5). In the
present study, we have performed a two-dimensional (2D) electromagnetic full
particle simulation with a "shock-rest-frame model". The simulation domain is
taken to be larger than the ion inertial length in order to include full
kinetics of both electrons and ions. The present simulation result has
confirmed the transition of shock structures from the cyclic self-reformation
to the quasi-stationary shock front. During the transition, electrons and ions
are thermalized in the direction parallel to the shock magnetic field. Ions are
thermalized by low-frequency electromagnetic waves (or rippled structures)
excited by strong ion temperature anisotropy at the shock foot, while electrons
are thermalized by high-frequency electromagnetic waves (or whistler mode
waves) excited by electron temperature anisotropy at the shock overshoot. Ion
acoustic waves are also excited at the shock overshoot where the electron
parallel temperature becomes higher than the ion parallel temperature. We
expect that ion acoustic waves are responsible for parallel diffusion of both
electrons and ions, and that a cross-scale coupling between an ion-scale
mesoscopic instability and an electron-scale microscopic instability is
important for structures and dynamics of a collisionless perpendicular shock.
