Compressive Structures in the Foreshock of Collisionless Shocks

(Solar Orbiter Nugget #88 by S. Raptis¹, D. Trotta², Drew L. Turner¹, Xóchitl Blanco-Cano³, Heli Hietala⁴, Tomas Karlsson⁵, Immanuel Christopher Jebaraj⁶, Ivan Y. Vasko⁷, Adnane Osmane⁸, Kazue Takahashi¹, David Lario⁹, Lynn B. Wilson III⁹, Gregory G. Howes¹⁰, and Robert F. Wimmer-Schweingruber¹¹)

Introduction

Collisionless shocks are among the most efficient particle accelerators in the universe [1]. A key factor that controls their behavior is the angle between the upstream magnetic field and the shock’s normal vector. In quasi-parallel shocks, this angle is small, allowing particles reflected at the shock to stream far upstream along magnetic field lines. This escaping population forms the foreshock region, generating waves and nonlinear structures that can, in turn, feedback on the shock itself extending its influence. Understanding quasi-parallel shocks is crucial, as they are more efficient at accelerating particles than their quasi-perpendicular counterparts, as observed at planetary bow shocks, interplanetary (IP) shocks, and even supernova remnant shocks [2–4].

Despite this common framework, quasi-parallel shocks do not all look alike with in-situ observations. Planetary foreshocks, such as Earth’s, often exhibit a rich spectrum of large-amplitude compressive magnetic structures, while foreshocks of IP shocks tend to appear comparatively calmer. Comparing these two environments is challenging since high-Mach-number quasi-parallel IP shocks are intrinsically rare, making well-resolved observations of their foreshock regions exceptional. The IP shock event studied here, observed by Solar Orbiter [5], therefore provides a unique opportunity to examine these processes under conditions that are seldom accessible.

The goal of this study is to explore this apparent discrepancy by examining a high-Mach-number quasi-parallel IP shock observed by Solar Orbiter and comparing it with Earth’s bow shock using the Magnetospheric Multiscale (MMS) mission [6] in a “string-of-pearls” configuration. This coordinated setup allows us to probe how similar physical processes unfold under very different conditions.

Methods

A direct comparison between these two environments requires careful treatment of geometry and motion. Earth’s bow shock is curved and nearly stationary in the spacecraft frame, whereas IP shocks are typically planar and propagate rapidly through the solar wind. While we cannot change shock’s geometry, we can place both systems on equal footing. To do that, the observed time series were transformed into spatial profiles along the shock-normal direction, expressed in units of ion inertial length (d_i). This conversion allows structures upstream of the shock to be compared in terms of physical scale rather than observation time. In addition, the contribution of suprathermal ions was quantified by integrating particle fluxes above 10 keV, providing an estimate of the energetic population responsible for driving upstream wave activity.

Figure 1 presents an overview of the event. It shows the magnetic field, plasma parameters, and energetic particle fluxes measured by Solar Orbiter across the IP shock and similar plasma conditions for Earth’s quasi-parallel shock observed by MMS. The shock transition is clearly visible (Solar Orbiter and MMS1), with a turbulent upstream region populated by compressive magnetic fluctuations shown in both cases. The figure highlights both the variability of the foreshock and the relatively short time interval over which these structures are observed.

Figure 1 Time-series data for the collisionless shock observations and associated foreshock for Solar Orbiter and MMS, in RTN and GSE coordinates, respectively. Panels (a)–(c), (f)–(h), (k)–(m) show the magnetic field (nT), plasma density (cm−3), and bulk ion velocity (km s−1). Energetic and suprathermal ion fluxes (keV) are displayed in panels (d), (i), (n) from EPT+STEP (Solar Orbiter) and FEEPS (MMS), while the lower-energy ion energy fluxes (eV) are shown in (e), (j), (o) from the Proton-Alpha Sensor and FPI. Shaded regions in the time-series panels denote different plasma environments denoted as “solar wind” (green), “far foreshock” region (orange), and “close foreshock” (blue). The bottom row (p)–(u) details a foreshock of collisionless shock observed by Solar Orbiter and one of the compressive structures observed by MMS1. Panels (p), (q), (s), (t) show zoomed-in time series of the magnetic field (nT) and density (n). The wider interval of the zoomed-in panels is shaded in purple in the main time-series panels. The corresponding hodograms in the L–M plane (nT) are shown in (r), (u), with the green and red dots corresponding to start and end points, respectively. The shaded area of Solar Orbiter observations (p), (q) and MMS (s), (t) correspond to 1 and 10 s, respectively. Vertical black dashed lines denote the shock time for both the IP and the bow shock.

Results

Looking at the overview time series (Figure 1), some key differences are already revealed. Upstream of the IP shock, magnetic fluctuations are present but remain relatively modest in amplitude and duration compared to those typically observed at Earth’s bow shock. However, once the data are expressed in spatial coordinates, a more unified picture emerges. Both environments show the formation of foreshock compressive structures at comparable distances upstream of the shock. These structures appear when the density of suprathermal ions exceeds roughly 1% of the background solar wind, indicating a potential common threshold for wave growth.

This side-by-side comparison in the shock-normal frame is shown in Figure 2. There, we can see that the region where these structures develop (“growth zone”), extends over only about ~100-150 ion inertial lengths in both cases. Within this narrow region, fluctuations begin to significantly steepen and evolve, allowing full compressive structures to occur at <50 dᵢ. Very importantly, observations within these scales correspond to hours of upstream measurements for Earth’s bow shock, while for an IP shock, due to the relative speed of the shock with respect to the spacecraft they only reflect ~10 seconds of data.

Figure 2. Comparative analysis of magnetic field and suprathermal particle properties across collisionless shocks. Panel (a) shows Solar Orbiter IP shock observations, while panel (b) presents MMS1 measurements of Earth’s bow shock. For each shock event, the upper plots display the magnetic field magnitude |B| (red line) and normalized suprathermal particle density (blue line) as functions of distance along the shock normal. The lower plots show the magnetic field SD using a 15-data point moving window, which quantifies local variability levels. The x-axis represents distance in upstream ion inertial lengths (di) from the shock crossing (S = 0). The background magnetic field is defined as the average value from observations taken at >100 di. Red shaded regions highlight the presence of compressive structures occurring at the same relative distance of <50 di. The two horizontal axes on MMS plots indicate the spatial distance computed with the calculated shock speed along the normal and with the spacecraft speed, representing the upper and lower bounds of the distance from the shock, respectively, while the relative location of MMS4 bounds this in context to Figure 1. More information and details about the suprathermal density and their profiles are detailed in Appendix B of the associated article

Conclusions & Discussion

Our study suggests that the underlying physics of wave generation at quasi-parallel shocks is largely universal. In both IP and planetary shocks, suprathermal ions escaping upstream can trigger the formation of compressive structures at similar spatial scales originating from the intrinsic foreshock wave field.

What differs is the environment in which these structures evolve. We suggest that Earth’s bow shock, with its curved geometry, can allow suprathermal particles to spread laterally and interact with neighboring regions of the shock. This “cross-talk” can further sustain and amplifies upstream waves, enabling them to grow into large nonlinear structures. In contrast, the nearly planar geometry of IP shocks can limit this lateral transport. As a result, well-developed foreshock structures are both intrinsically rarer due to the geometry of IP shocks but also particularly observationally challenging to capture. This is further compounded by the scarcity of high-Mach-number quasi-parallel IP shocks, making events like this one particularly valuable for testing our understanding of collisionless shocks.

Overall, these findings reconcile the apparent differences between the two systems. While quasi-parallel shocks across the heliosphere share the same basic mechanisms, geometry and dynamics can influence how, and whether, those processes appear in observations.

This nugget is based on the following paper: Raptis et al., ApJL, 1000 L55 (2026)

Acknowledgements

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.

Affiliations

(1) Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA

(2) European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada, Madrid, Spain

(3) Departamento de Ciencias Espaciales, Instituto de Geofísica, Universidad Nacional Autónoma de México, Mexico City, Mexico

(4) Department of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK

(5) Division of Space and Plasma Physics—KTH Royal Institute of Technology, Stockholm, Sweden

(6) Department of Physics and Astronomy, University of Turku, 20500 Turku, Finland

(7) William B. Hanson Center for Space Sciences, University of Texas at Dallas, Richardson, TX, USA

(8) Department of Physics, University of Helsinki, Helsinki, Finland

(9) NASA Goddard Space Flight Center, Greenbelt, MD, USA

(10) Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52242, USA

(11) Institute of Experimental and Applied Physics, Kiel University, Leibnizstrasse 11, D-24118 Kiel, Germany

References

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