Periodic self-reformation of rippled perpendicular collisionless shocks in two dimensions

Abstract

Abstract. Large-scale two-dimensional (2-D) full particlein-cell (PIC) simulations are carried out for studying periodic self-reformation of a supercritical collisionless perpendicular shock with an Alfvén-Mach number M A ∼ 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 ionto-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.

Implications of weak rippling of the shock ramp on the pattern of the electromagnetic field and ion distributions

“…Even multispacecraft observations provide information only about a small part of the shock surface. Simulations (Umeda & Daicho 2018;Omidi et al 2021) show that rippling is only approximately monochromatic. Finite width of the spectrum would further enhance gyrotropization and isotropization by adding randomness in the gyrophases of the ion which are mixed at a fixed spatial position behind the ramp.…”

“…Even multispacecraft observations provide information only about a small part of the shock surface. Simulations (Umeda & Daicho 2018; Omidi et al. 2021) show that rippling is only approximately monochromatic.…”

“…These changes do not appear abruptly when the Mach number exceeds some critical value but gradually become more and more pronounced when the Mach number increases (Ofman & Gedalin 2013). Rippling and time dependence (reformation), as well as generation of the propagating upstream whistlers Hull et al 2012;Ramírez Vélez et al 2012;Wilson et al 2012Wilson et al , 2017) may be interrelated (Burgess et al 2016;Gingell et al 2017;Umeda & Daicho 2018;Omidi et al 2021). At present, it is not quite clear what causes rippling.…”

PIC simulations of stable surface waves on a subcritical fast magnetosonic shock front

“…Shocks in collisionless plasma, in which effects due to Coulomb collisions between charged particles are negligible compared to collective electromagnetic forces, have been studied in the laboratory [1][2][3][4][5], in the Solar system [6], and by means of numerical simulations [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Shocks are important structures for the dissipation of energy in collisionless plasma (See [16] for a recent review).…”

“…Resolving their full dynamics is more expensive but it allows particle-in-cell (PIC) simulations to model high-frequency processes, where electron dynamics matters. Several PIC simulation studies have addressed the interplay of subcritical [21] and supercritical collisionless shocks [8,12,17] with the shock-modified upstream plasma. These studies covered a wide range of magnetic field strengths and orientations relative to the shock normal.…”