Compressive Structures in the Foreshock of Collisionless Shocks

Abstract

Collisionless shocks are fundamental accelerators of energetic particles, yet the observations of nonlinear foreshock structures, which are essential in acceleration processes, differ significantly between interplanetary (IP) shocks and planetary bow shocks. We present a direct comparison of two high-Mach-number, quasi-parallel shocks: an IP shock observed by Solar Orbiter and the Earth’s bow shock measured by the Magnetospheric Multiscale mission during the 2024–2025 “string-of-pearls” campaign. We show that foreshock compressive structures (FCSs) initiate upstream of both shocks at similar normalized distances (≲50 ion inertial lengths, d i ) when the suprathermal (>10 keV) ion density exceeds ∼1% of the background. However, the IP shock lacks the fully evolved, high-amplitude short large-amplitude magnetic structures characteristic of the terrestrial foreshock. We demonstrate that the “growth zone” capable of sustaining these structures is spatially limited (∼135 d i ), which, due to the high speed of the propagating IP shock, corresponds to a brief observational window of <10 s. Beyond this observational constraint, we suggest an additional physical mechanism that can inhibit foreshock maturity at IP shocks. The lack of global curvature prevents the lateral supply (“cross talk”) of energetic ions from different shock regions. These findings suggest that while the fundamental physics of FCS initiation is unified across collisionless shocks, the achievement of full nonlinearity can be regulated by the unique shock geometry and upstream properties while ultimately remaining observationally challenging to identify.