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 type 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. The simulation reveals that shock dynamics in the foot and ramp region generate two types of ion-scale whistler waves, respectively, each of which can drive shock reformation. Within one single wave period, the wave induces the magnetic field pile-up, ion accumulation and reflection, and upstream-pointing electric field, finally evolving into a new shock front. An interesting finding is that different shock dynamics compete and dominate the reformation at different stages. Our results not only provide evidence that the shock reformation in the present regime can be driven by ion-scale whistler waves, but also demonstrate the detailed kinetic processes how it happens, offering valuable insights into shock dynamics.