Shock Reformation Induced by Ion-scale Whistler Waves in Quasi-perpendicular Bow Shock

Studies have long suggested that shocks can undergo cyclical self-reformation as a result of shock nonstationarity. Until now, providing solid evidence for shock reformation in spacecraft observation and identifying its generating mechanisms remain challenging. In this work, by analyzing Magnetospheric Multiscale (MMS) spacecraft observations, we unambiguously identified shock reformation occurring in a quasi-perpendicular shock. A 2-D particle-in-cell simulation reproduces and explains the observed shock reformation. It reveals two distinct stages: in the early stage, whistler waves generated by the modified two-stream instability (MTSI) dominate the foot region, while whistler precursors driven by the gradient catastrophe instability dominate the ramp. In the later stage, MTSI-driven whistlers extend to the ramp and take over the role of reducing gradients, so precursors no longer develop. Both types of whistlers can result in shock reformation: one single wave period induces the magnetic field pile-up, ion accumulation and reflection, and upstream-pointing electric field, finally evolving into a new shock front. Our results give evidence that the shock reformation in the present regime can be driven by ion-scale whistler waves and demonstrate the detailed kinetic processes how it happens, providing valuable insights into the shock dynamics.