A1 Refereed original research article in a scientific journal
Compressive Structures in the Foreshock of Collisionless Shocks
Authors: Raptis, Savvas; Trotta, Domenico; Turner, Drew L.; Blanco-Cano, Xóchitl; Hietala, Heli; Karlsson, Tomas; Jebaraj, Immanuel Christopher; Vasko, Ivan Y.; Osmane, Adnane; Takahashi, Kazue; Lario, David; Wilson, Lynn B.; Howes, Gregory G.; Wimmer-Schweingruber, Robert F.
Publisher: Institute of Physics Publishing
Publication year: 2026
Journal: Astrophysical Journal Letters
Article number: L55
Volume: 1000
Issue: 2
ISSN: 2041-8205
eISSN: 2041-8213
DOI: https://doi.org/10.3847/2041-8213/ae53e5
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://doi.org/10.3847/2041-8213/ae53e5
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/523364103
Self-archived copy's licence: CC BY
Self-archived copy's version: Publisher`s PDF
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, di) 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 di), 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.
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Funding information in the publication:
S.R. acknowledges funding from the MMS Early Career Award 80NSSC25K7353 and the Magnetospheric Multiscale (MMS) mission of NASA’s Science Directorate Heliophysics Division via subcontract to the Southwest Research Institute (NNG04EB99C). S.R. also acknowledges the support of the International Space Sciences Institute (ISSI) team 555, “Impact of Upstream Mesoscale Transients on the Near-Earth Environment,” and the useful discussions with Ahmad Lalti. X.B.C. thanks PAPIIT DGAPA IN106724 and SECIHTI CBF-2023-2024-852 grants. H.H. is supported by the Royal Society Award URF\R1\180671. URF\R \251031. I.C.J. acknowledges support from the Research Council of Finland (X-Scale, grant No. 371569). K.T. was supported by the Parker Solar Probe project under contract NNN06AA01C. R.F.W.-S. acknowledges support of Solar Orbiter’s EPD by DLR grant 50OT2002. We also acknowledge support from ESA through the Science Faculty—Funding reference ESA-SCI-E-LE-170.
