Energetic particle-mediated interplanetary shocks observed by Solar Orbiter

In collisionless shocks, energetic particles can carry enough pressure to modify the upstream plasma and the shock structure itself. This represents a regime that is often invoked in theories of cosmic-ray acceleration, but rarely observed in the heliosphere. We locate and characterise interplanetary (IP) shocks where energetic particles dynamically dominate the upstream pressure. We analysed IP shocks observed by Solar Orbiter within 1 au and computed the energetic particle pressure, P_EP, from proton measurements above 10,keV. We compared these results it with the upstream thermal P_Th and magnetic P_B pressures. We identified four shocks for which P_ EP ≥ P_ Th + P_B . These events correspond to strong and fast shocks in the high-Mach-number tail of the Solar Orbiter shock population. In several cases, the P_EP increase coincides with a decreasing upstream bulk flow speed in the shock frame, with the resulting particle-mediated foreshocks extending up to sim10^5 ion inertial lengths , d_i. The extent of such an energetic particle-dominated region depends on the shock geometry. These observations provide evidence that accelerated particles can dynamically modify IP shocks. They highlight the importance of coupling among energetic particles, upstream fluctuations, and shock structure in improving our understanding of particle acceleration at collisionless shocks.