Abstract
Abstract. Large-scale two-dimensional (2-D) full particle-in-cell (PIC) simulations are carried out for studying periodic self-reformation of a supercritical collisionless perpendicular shock with an Alfvén–Mach number MA∼6. Previous self-consistent one-dimensional (1-D) hybrid and full PIC simulations have demonstrated that the periodic reflection of upstream ions at the shock front is responsible for the formation and vanishing of the shock-foot region on a timescale of the local ion cyclotron period, which was defined as the reformation of (quasi-)perpendicular shocks. The present 2-D full PIC simulations with different ion-to-electron mass ratios show that the dynamics at the shock front is strongly modified by large-amplitude ion-scale fluctuations at the shock overshoot, which are known as ripples. In the run with a small mass ratio, the simultaneous enhancement of the shock magnetic field and the reflected ions take place quasi-periodically, which is identified as the reformation. In the runs with large mass ratios, the simultaneous enhancement of the shock magnetic field and the reflected ions occur randomly in time, and the shock magnetic field is enhanced on a timescale much shorter than the ion cyclotron period. These results indicate a coupling between the shock-front ripples and electromagnetic microinstabilities in the foot region in the runs with large mass ratios. Keywords. Space plasma physics (wave–particle interactions)
Highlights
It has been well known since one-dimensional (1-D) particlein-cell (PIC) simulations in the 1970s that the “shock front” at a supercriticalperpendicular collisionless shock shows a periodic behavior. Biskamp and Welter (1972) first reported the periodic reflection of ions and the formation of a “shock foot” at a perpendicular shock, which leads to a nonstationary shock front
It has been confirmed that the reformation ofperpendicular shocks is common in 1D simulations with various ion-to-electron mass ratios, including the real one when the Alfvén–Mach number is supercritical but relatively low (MA < 10) and the ion beta is low (Scholer et al, 2003; Scholer and Matsukiyo, 2004)
The shocks excited with these parameters exhibit the periodic reformation of perpendicular shocks in1-D systems, as we show later
Summary
It has been well known since one-dimensional (1-D) particlein-cell (PIC) simulations in the 1970s that the “shock front” at a supercritical (quasi-)perpendicular collisionless shock shows a periodic behavior. Biskamp and Welter (1972) first reported the periodic reflection of ions and the formation of a “shock foot” at a perpendicular shock, which leads to a nonstationary shock front. Hellinger et al (2007) and Lembege et al (2009) reported that the periodic reformation of an exactly perpendicular shock was “suppressed” with an ion-toelectron mass ratio mi/me = 42 where the shock reformation was evident in an early phase but became less evident in a later phase, while the reformation was “absent” with mi/me = 400 In their high-resolution 2-D hybrid PIC simulation, the excitation of electron-scale whistler mode waves at the shock front was included. We aim to study the effect of the mass ratio (i.e., microinstabilities in the foot region) on the periodic self-reformation of perpendicular collisionless shocks.
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