Impact of the rippling of a perpendicular shock front on ion dynamics

摘要

Both hybrid/full particle simulations and recent experimental results have clearly evidenced that the front of a supercritical quasi-perpendicular shock can be rippled. Recent two-dimensional simulations have focused on two different types of shock front rippling: (1) one characterized by a small spatial scale along the front is supported by lower hybrid wave activity, (2)the other characterized by a large spatial scale along the front is supported by the emission of large amplitude nonlinear whistler waves. These two rippled shock fronts are self-consistently observed when the static magnetic field is perpendicular to(so called B0-OUT case)or within(so called B0-IN case)the simulation plane, respectively. On the other hand, several studies have been made on the reflection and energization of incoming ions with a shock but most have been restricted to a one dimensional shock profile only(no rippling effects). Herein, two-dimensional test particle simulations based on strictly perpendicular shock profiles chosen at a fixed time in twodimensional Particle-in-cell(PIC)simulations, are performed in order to investigate the impact of the shock front ripples on incident ion(H+)dynamics. The acceleration mechanisms and energy spectra of the test-ions(described by shell distributions with different initial kinetic energy)interacting with a rippled shock front are analyzed in detail. Both B0-OUT and B0-IN cases are considered separately; in each case, y-averaged(front rippling excluded)and non-averaged(front rippling included)profiles will be analyzed. Present results show that: (1)the incident ions suffer both shock drift acceleration(SDA)and shock surfing acceleration(SSA)mechanisms. Moreover, a striking feature is that SSA ions not only are identified at the ramp but also within the foot which confirms previous 1-D simulation results; (2)the percentage of SSA ions increases with initial kinetic energy, a feature which persists well with a rippled shock front; (3)furthermore, the ripples increase the porosity of the shock front, and more directly transmitted(DT)ions are produced; these strongly affect the relative percentage of the different identified classes of ions(SSA, SDA and DT ions), their average kinetic energy and their relative contribution to the resulting downstream energy spectra; (4)one key impact of the ripples is a strong diffusion of ions(in particular through the frontiers of their injection angle domains and in phase space which are blurred out)which leads to a mixing of the different ion classes. This diffusion increases with the size of the spatial scale of the front ripples; (5)through this diffusion, an ion belonging to a given category(SSA, SDA, or DT)in y-averaged case changes class in non-averaged case without one-to-one correspondence.