On the reformation at quasi- and exactly perpendicular shocks: Full particle-in-cell simulations

A full particle-in-cell (PIC) simulation study is carried out on the reformation at quasi- and exactly perpendicular collisionless shocks with a relatively low Alfven Mach number (M_A = 5). Previous self-consistent one-dimensional (1-D) hybrid and full PIC simulations have demonstrated that ion kinetics are essential for the nonstationarity of perpendicular collisionless shocks. These results showed that reflection of ions at the shock front is responsible for the periodic collapse and redevelopment of a new shock front on a timescale of the ion cyclotron period, which is called the shock reformation. Recent 2-D hybrid and full PIC simulations, however, suggested that the shock reformation does not take place at exactly perpendicular shocks with M _A ∼ 5. By contrast, another 2-D hybrid PIC simulation showed that the shock reformation persists at quasi-perpendicular shocks with M_A ∼ 5. Although these two works seem to be inconsistent with each other, the reason is not well understood because of several differences in numerical simulation conditions. Thus this paper gives a direct comparison between full PIC simulations of quasi- and exactly perpendicular shocks with almost the same condition. It is found that the time development of the shock magnetic field averaged over the shock-tangential direction shows the transition from the reformation to no-reformation phase. On the other hand, local shock magnetic field shows the evident appearance and disappearance of the shock front, and the period becomes longer in the no-reformation phase than in the reformation phase.