2026/07/10 by Yuri V. Khotyaintsev, Domenico Trotta, Daniel B. Graham · 1 voice
Physics and Astronomy · #physics.space-ph #physics.plasm-ph
Super-critical collisionless shocks are not static structures but evolve continuously as they reflect incoming ions back upstream. The physical process responsible for this non-stationarity -- whether it is dominated by wave-like corrugation of the shock surface (rippling) or by a cyclic rebuilding of the shock transition (reformation) -- remains debated. We combine Magnetospheric Multiscale (MMS) observations of a nearly perpendicular (θBn≈89^∘), supercritical (MA≈6) bow shock with high-resolution two-dimensional hybrid simulations to address this question. MMS reveals repeated ion phase-space holes and intense, localized Hall electric fields. A virtual-spacecraft analysis of the simulation reproduces these signatures and shows that they arise from a self-regulating feedback cycle: strong Hall-field ion reflection builds a reflected-ion foot, which weakens the Hall field and suppresses further reflection until the foot decays and the cycle restarts. This reformation cycle, spatially organized by the two-dimensional shock structure, explains most of the observed non-stationarity.