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 Letter, by analyzing magnetospheric multiscale observations, we unambiguously identified shock reformation occurring in a quasiperpendicular shock. A 2D PIC simulation reproduces and explains the observed shock reformation. The result reveals that the shock reformation is driven by the ion-scale whistler waves, manifested in two distinct types: whistler precursor and modified two-stream instability whistlers. An interesting finding is that the two mechanisms compete and dominate the reformation at early and later stages, respectively. Our results not only provide evidence of ion-scale whistlers as mechanism driving shock reformation, but also demonstrate the detailed kinetic processes how it happens, offering valuable insights into shock dynamics.

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• Laboratory studies of space & astrophysical plasmas
• Plasma instabilities
• Plasma waves
• Space & astrophysical plasma