Heliospheric shocks offer the unique advantage of being directly accessible by in situ measurements. Missions, such as Solar Orbiter, STEREO, and Parker Solar Probe, have deepened our knowledge of interplanetary shocks and the associated regions, while MMS, Cluster, THEMIS, Cassini, Maven, and others have similarly enhanced our knowledge of planetary bow shocks.
High-performance computing has also played a critical role in filling key knowledge gaps, enabling global and local simulations to provide insights into the nature of collisionless shocks.
Despite these efforts, many questions remain open. In particular, we still do not fully understand the mechanisms associated with certain aspects of particle heating and acceleration, wave generation, wave-particle interaction, and energy redistribution at shocks. The interplay of collisionless shocks and pre-existing plasma turbulence also remains poorly understood. Additionally, details about the formation and impact of transient structures, such as hot flow anomalies, foreshock bubbles, cavitons, spontaneous hot flow anomalies, magnetosheath jets, etc., are still unknown.
We thus welcome observational, numerical, and theoretical works that explore plasma processes at collisionless shocks and surrounding regions.
We investigate the diffusive shock acceleration of partially ionized ions by introducing helium-, carbon-, and iron-like ions at solar abundances into two-dimensional hybrid (kinetic ions/fluid electrons) simulations of nonrelativistic collisionless shocks. We find that heavy ions are preferentially accelerated, with energy transfer to helium comparable to or exceeding that of hydrogen, enhancing shock acceleration efficiency. Moreover, accelerated helium ions play a role in magnetic field amplification and in controlling the ensuing spectra of accelerated particles. We also show how efficient particle acceleration modifies the shock compression ratio, which has implications for the predicted arrival times of coronal mass ejections at Earth.
How to cite: Caprioli, D., Orusa, L., Cernetic, M., Haggerty, C. C., and Ostler, B.: Acceleration of He and heavy ions at collisionless shocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22242, https://doi.org/10.5194/egusphere-egu26-22242, 2026.
Interplanetary (IP) shocks are driven by solar activity and provide a unique in situ laboratory for studying particle acceleration. With its high-resolution measurements in the suprathermal range (above ~10 keV), Solar Orbiter opens a new window on how particles are energized out of the thermal population.
We focus on energetic proton production at IP shocks observed by Solar Orbiter, presenting results from a systematic calculation of proton acceleration efficiency and discussing its variability across 150 events observed since launch. We then highlight a subset of particularly strong shocks where the energetic particle pressure exceeds the combined magnetic and thermal pressure, a regime with direct relevance to cosmic-ray shocks in astrophysical environments. For these shocks, we examine the details of the local plasma and magnetic field conditions, with focus on upstream fluctuations and their role in particle acceleration. Together, these results provide new insight into how shocks accelerate particles across both heliospheric and broader astrophysical environments.
How to cite: Trotta, D., Giacalone, J., Lario, D., Raptis, S., Turner, D. L., Mostafavi, P., Hietala, H., Reville, B., Pezzi, O., and Wimmer-Schweingruber, R.: Energetic Protons at Solar Orbiter Shocks: Efficiency, Variability, and Energetic Particle Pressure-Dominated Events, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13398, https://doi.org/10.5194/egusphere-egu26-13398, 2026.
Diffusive shock acceleration is widely accepted as the primary mechanism to generate high-energy particles in supernova remnant shocks but faces challenges with efficiently accelerating low-energy particles, known as the injection problem. Shock stochastic drift acceleration presents a promising pre-acceleration mechanism, in which the whistler waves in shock transition region can be essential in scattering and energizing low-energy electrons, aligning well with observations (Amano et al., 2022). However, the physical origin of these waves within the shock transition layer has not been fully understood.
In our study, we investigate the generation of whistler-mode waves by shock-reflected electrons at quasi-perpendicular collisionless shocks. Using Liouville mapping, we construct the electron velocity distribution function in the shock, which allows us to explicitly capture the phase-space features of mirror-reflected electrons near the upstream edge of the shock transition region. Based on the constructed distribution, we perform a linear instability analysis using a semi-analytical method (Kennel & Wong, 1967) to examine the whistler wave generation by the mirror-reflected electrons.
We find that the reflected electrons can excite two distinct instabilities with different propagation directions when both the upstream electron beta βe and Alfvén Mach number in the de Hoffmann-Teller frame MA/cosθBn are sufficiently large, where MA is Alfvén Mach number and θBn is the angle between the upstream magnetic field and the shock normal. In the parameter regime of Earth's bow shock, the instability threshold is approximately MA/cosθBn>∼50. Since such shocks are super-critical with respect to the whistler critical Mach number, the excited waves cannot propagate upstream and instead accumulate within the shock transition layer.
Furthermore, we find that the pitch-angle scattering by the generated waves may trigger secondary instabilities on the same branch. We suggest that the sequence of instabilities likely happening within the shock transition layer can efficiently scatter the reflected electrons over a broad range of pitch angles. We propose that this sequence of self-generated instabilities enables the confinement of the reflected electrons within the shock transition region. Such self-confinement provides the key ingredient of stochastic shock drift acceleration, which then offers a plausible mechanism for the electron injection into diffusive shock acceleration.
How to cite: Wang, R. and Amano, T.: Generation of Whistler Waves by Reflected Electrons and Their Self-Confinement at Quasi-Perpendicular Shocks , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15879, https://doi.org/10.5194/egusphere-egu26-15879, 2026.
Collisionless shocks are often modelled as smooth, planar surfaces - but many show organized corrugations that steer how particles get accelerated and how they radiate. We present a simple, linear magnetohydrodynamic (MHD) model that treats the shock as an evolving interface. This lets us separate two things: (1) the shock’s own properties and geometry, and (2) the statistics of the upstream turbulence that hits it. With this separation, we obtain a
direct map from incoming fluctuations to the corrugation patterns they create, including their drift speed and coherence. In our model, the interface acts like an “impedance” that focuses broad-band turbulent power into fast-mode waves that skim along the shock. The shock responds most strongly when the wave’s normal group speed is small (a sharp, single-peaked response). Corrugation strength increases with compression, while the shock geometry and plasma beta control how long these patterns persist. The framework makes testable predictions linking upstream turbulence and shock shape to fine
structure in electromagnetic signals from heliospheric and supernova-remnant shocks.
How to cite: Jebaraj, I. C., Malkov, M., Wijsen, N., Pomoell, J., Krasnoselskikh, V., Dresing, N., and Vainio, R.: Turbulence-driven corrugation of fast-mode shock waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8354, https://doi.org/10.5194/egusphere-egu26-8354, 2026.
Foreshock transients are formed when a solar wind directional discontinuity interacts with the reflected solar wind particles upstream of Earth's bow shock. Their sizes can be multiple Earth radii, and they can drive significant wave activity and accelerate particles in Earth's magnetosphere. In this study, we aim to determine which solar wind context hosts the most discontinuities and is the most favorable for foreshock transient formation. We consider quiet solar wind and large-scale structures like coronal mass ejections and high speed streams. To identify discontinuities in the solar wind, we use 1-second resolution magnetic field data from the Advanced Composition Explorer (ACE) spacecraft. We also use solar wind measurements from ACE to study properties around the discontinuities like direction of the convective electric field and solar wind cone angle, that can reveal whether a discontinuity is more likely to form a foreshock transient once it reaches the near-Earth space. In this work we use the definition "wave storm" to describe multi-hour intervals when ultra-low frequency wave activity on Earth is continuously increased. We will assess whether the occurrence rate of discontinuities and favorable conditions for foreshock transient formation in these large-scale structures are connected to wave storm occurrence and intensity.
How to cite: Lipsanen, V., Turc, L., Ojuva, M., Hoilijoki, S., Dahani, S., Tao, S., Kalliokoski, M., and Kilpua, E.: Statistical study of directional discontinuities: solar wind context and relevant properties for foreshock transient formation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11231, https://doi.org/10.5194/egusphere-egu26-11231, 2026.
Short Large-Amplitude Magnetic Structures (SLAMS) are isolated non-linear magnetic field signatures frequently observed in the foreshock of quasi-parallel shocks. They are believed to form due to a non-linear growth of ultra-low frequency (ULF) waves, but the detailed formation mechanisms are still highly uncertain. SLAMS have been suggested to be important for the formation of the quasi-parallel shock, and understanding the nature of these structures is hence important in order to fully understand the physics of collisionless shocks.
The majority of SLAMS (about 80%) are right-hand polarized in the spacecraft frame, corresponding to a left-hand polarization in the plasma frame. This polarization is opposite from that of the ULF waves from which they are believed to grow. The reason for this polarization change is unknown. Not all SLAMS are however right-hand polarized in the spacecraft frame. About 20% are left-hand polarized, and the reason for these two different groups of SLAMS and their underlying formation mechanisms are also unknown.
In this work, we make a statistical analysis of the polarization properties of SLAMS in the foreshock of Earth using data from the Cluster mission. The aim is to investigate the difference between right-hand and left-hand polarized SLAMS, studying differences in the ion distribution and the general properties of the plasma environment. This can give clues about the underlying formation mechanisms. We also study the evolution of the polarization as the ULF waves grow into SLAMS.
How to cite: Bergman, S., Karlsson, T., and Wong Chan, T. K.: The polarization of Short Large-Amplitude Magnetic Structures (SLAMS) in the foreshock of Earth, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3425, https://doi.org/10.5194/egusphere-egu26-3425, 2026.
The properties of the region upstream of planetary bow shocks depend strongly on the direction of the interplanetary magnetic field. For the quasi-parallel bow shock, part of the solar wind ions is reflected back upstream from the shock and this reflected ion population triggers instabilities resulting in a turbulent region. Within this region, Short Large-Amplitude Magnetic Structures (SLAMS) can frequently be found, which are suggested to play a pivotal role in the formation of planetary bow shocks. Yet many properties of SLAMS are not well known at Earth and even less so at other planets.
SLAMS are identified by three criteria. First, a magnetic field amplitude twice the background magnetic field is required. Second, SLAMS should exhibit an elliptic polarization so that it can be differentiated from a shock oscillation. Last, it takes place upstream of the bow shock.
Here we present results on the occurrence and other properties of SLAMS at different planetary foreshocks including Mars, Saturn and Mercury using different space missions. The results presented here can also offer comparative insights with SLAMS found at Earth for exploring potential dependencies on system size, and other magnetospheric and solar wind parameters.
How to cite: Wong Chan, (. T. K., Karlsson, T., and Bergman, S.: Statistical study of SLAMS at different planetary foreshocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7354, https://doi.org/10.5194/egusphere-egu26-7354, 2026.
Ultralow frequency (ULF) waves in the Hermean foreshock are believed to be driven by the interaction of solar wind ions reflected from the bow shock with the original solar wind beam. The direction of the interplanetary magnetic field (IMF) determines what regions are accessible to the reflected ions, and where therefore ULF waves are growing to an observable amplitude. Short Large-Amplitude Magnetic Structures (SLAMS) have been suggested to form when the growth of the ULF waves enter a non-linear, explosive stage. We use observations of foreshock ULF waves and SLAMS based on MESSENGER magnetic field data to investigate the relation between the two types of phenomena. We study the spatial extent of both SLAMS and ULF waves for different IMF directions, and relate them to the angle qBn between the IMF and the bow shock normal. At Earth, a majority of SLAMS have an elliptical polarization in the opposite sense to the ULF waves. We investigate whether this is the case also at Mercury, and also check if there is a continuous change of distribution of polarization organized by amplitude of SLAMS and structures with an amplitude intermediate between ULF waves and SLAMS, sometimes known as ‘shocklets’. The results are discussed in terms of possible similarities between terrestrial and Hermean SLAMS formation mechanisms, with a particular focus on possible extensions of these studies to be performed by the upcoming BepiColombo mission.
How to cite: Karlsson, T., Bergman, S., and Wong Chan, T. K.: The relation between Ultralow Frequency (ULF) waves and Short Large Amplitude Magnetic Structures (SLAMS) in the Mercury foreshock, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1541, https://doi.org/10.5194/egusphere-egu26-1541, 2026.
The foreshock is a large region of space upstream of a collisionless shock characterised by the presence of particles of solar wind origin that have been reflected at the shock. The interaction between these particles and the pristine solar wind can also generate ultra-low frequency (ULF) waves that are advected toward the quasi-parallel bow shock, that is, the part of the bow shock where the interplanetary magnetic field (IMF) and shock normal are almost parallel. The interaction between the ULF waves and the shock cause the quasi-parallel bow shock and the magnetosheath downstream of it to become turbulent and dynamic, which in turn can lead to the formation of transient structures such as magnetosheath jets. During intervals of quasi-radial IMF at Earth, the quasi-parallel bow shock is upstream of the dayside magnetosheath, and the dynamics of the quasi-parallel bow shock and magnetosheath have larger potential to have geoeffective consequences. Understanding the properties of the foreshock is therefore important, but studying the overall structure of this extended region using point measurements by spacecraft is difficult.
In this study, we present the first ever global 6D (3D+3V) hybrid-Vlasov simulation of near-Earth space with quasi-radial IMF conditions, featuring a high-resolution foreshock. We introduce the new criterion that was used to identify the foreshock for the purpose of applying adaptive mesh refinement, and elaborate on some of the technical challenges that needed to be overcome to make the simulation possible. We investigate the effects of ULF waves on the velocity distributions in different parts of the foreshock. Finally, we probe the velocity distributions inside magnetosheath jets in order to study their kinetic nature.
How to cite: Suni, J., Palmroth, M., Turc, L., Ojuva, M., Kotipalo, L., Alho, M., and Ganse, U.: 6D hybrid-Vlasov simulation of a high-resolution foreshock during quasi-radial IMF: First Vlasiator results, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1794, https://doi.org/10.5194/egusphere-egu26-1794, 2026.
Collisionless low Mach number shocks are abundant in astrophysical and space plasma environments, exhibiting complex wave activity and wave-particle interactions. In this paper, we present 2D Particle-in-Cell (PIC) simulations of quasi-perpendicular nonrelativistic low Mach number shocks, with a specific focus on studying electrostatic waves in the shock ramp and the precursor regions. In these shocks, an ion-scale oblique whistler wave creates a configuration with two hot counter-streaming electron beams, which drive unstable electron acoustic waves (EAWs) that can turn into electrostatic solitary waves (ESWs) at the late stage of their evolution. By conducting simulations with periodic boundaries, we show that EAW properties agree with linear dispersion analysis. The characteristics of ESWs in shock simulations, including their wavelength and amplitude, depend on the shock velocity. When extrapolated to shocks with realistic velocities, the ESW wavelength is reduced to one tenth of the electron skin depth and the ESW amplitude is anticipated to surpass that of the quasi-static electric field by more than a factor of 100. These theoretical predictions may explain a discrepancy, between PIC and satellite measurements, in the relative amplitude of high- and low-frequency electric field fluctuations.
How to cite: Bohdan, A., Tran, A., Sironi, L., and Wilson, L. B.: Electrostatic Waves and Electron Holes in Simulations of Low-Mach Quasi-perpendicular Shocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2987, https://doi.org/10.5194/egusphere-egu26-2987, 2026.
Collisionless shocks, such as planetary bow shocks and interplanetary shocks, can cause a wide range of ion kinetic instabilities in their downstream region. These phenomena (in particular mirror mode, ion cyclotron, and firehose instabilities) are sensitive to upstream solar-wind conditions. Changing the Mach number, from low to high, is expected to modify the balance between wave activity, transient structures, and turbulent fluctuations. However, a systematic comparative picture across Mach number regimes is still lacking.
We investigate the emergence and behaviour of ion kinetic instabilities across shock crossings spanning a broad range of Alfvénic and magnetosonic Mach numbers under selected solar wind conditions. We focus on the role of upstream parameters such as plasma beta, alpha-to-proton abundance ratio, or upstream interplanetary magnetic field fluctuations in shaping downstream instability behaviour. The analysis is based on MMS terrestrial bow shock crossings and cross-checked against interplanetary shocks by Wind and Solar Orbiter, enabling us to disentangle local planetary bow shock effects from more universal shock-driven processes. This study aims to clarify which instabilities dominate under different upstream conditions and how they contribute to plasma variability and energy redistribution downstream of collisionless shocks.
How to cite: Koller, F., Hietala, H., Vuorinen, L., and Lindberg, M.: Ion Plasma Stability Downstream of Collisionless Shocks Across the Mach Number Regimes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12052, https://doi.org/10.5194/egusphere-egu26-12052, 2026.
Collisionless shocks play a key role in space and astrophysical plasmas, enabling the conversion of large-scale kinetic energy into heat and non-thermal particle populations without relying on binary Coulomb collisions. Instead, these shocks are sustained by collective effects such as wave-particle interactions that are inherently kinetic and often nonlinear. Despite significant observational and theoretical efforts, the precise mechanisms and spatial localization of energy dissipation in collisionless shocks remain debated. While it is widely accepted that dissipation occurs within the shock ramp, simulations and observations have shown that energy conversion may also extend upstream and downstream, involving shock structures such as the foot and overshoot. Observational studies using Magnetospheric Multiscale (MMS) have further highlighted that ion heating are often concentrated in the ramp and foot, while electron heating may remain nearly constant or increase only under specific conditions such as enhanced wave activity in the transition region.
We analyze high-resolution measurements from the MMS mission across multiple quasi-perpendicular bow shock crossings. We quantify energy contribution of different particle species within a vicinity of the shock ramp to analyze energy transfer among electrons, ions and electromagnetic fields within collisionless shocks. By correcting the measured ion and electron distribution functions for instrumental effects, we isolate the energy contributions of each species and examine how they vary throughout the shock structure. We calculate the theoretically expected values for thermal energy from mass conservation principles and Rankine-Hugoniot conditions to analyze the observed deviation from adiabatic behavior in collisionless shocks. Finally, we discuss how energy transfer between species depends on various shock parameters.
How to cite: Villaflor, V., Bohdan, A., and Jenko, F.: Energy dissipation in collisionless shocks: MMS observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3632, https://doi.org/10.5194/egusphere-egu26-3632, 2026.
Understanding ion energization during the interaction between an Interplanetary (IP) shock and a Bow shock remains an important and intriguing problem in space plasma physics. In this context, we present hybrid particle-in-cell simulations using the 2D EPOCH code to investigate particle acceleration during supercritical collisionless shocks interactions. In order to estimate the level of particle energization in two shocks interaction, we consider two cases. First, we present an example of particle acceleration induced by an isolated bow shock resulting from the solar wind-Earth’s magnetosphere interaction. Second, we present a case study of a Coronal Mass Ejection (CME)–driven IP shock interaction with the Earth’s bow shock for both quasi-parallel and quasi-perpendicular geometries. During the interaction of two shocks, ions undergo multiple reflections between the converging magnetic fields, enabling efficient energy gain through Fermi acceleration. By modelling the system using hybrid simulations, we can further observe how this acceleration is modified and enhanced in the presence of ion-kinetic scale structures and non-stationary developed self-consistently at both shocks. As expected, the shock–shock configuration produces substantially stronger ion energization than a single isolated collisionless shock. Our simulations show that as the two shocks approach and overlap, their highly structured magnetic ramps, reflected-ion populations, and upstream waves interfere, producing time-dependent variations in shock thickness, amplitude, and position. By analyzing ion velocity distributions, bulk flow, temperature, and electromagnetic fields, we characterize key features of the interaction region, including shock evolution, reformation, ion reflection, and particle energization. These results provide new insight into how shock–shock interactions influence the turbulent shock transition and enhance ion acceleration compared with a single shock.
How to cite: Imtiaz, N., Gingell, I., and Steinvall, K.: Kinetic Simulations of Particle Acceleration in Collisionless Supercritical Shock-Shock Interaction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4068, https://doi.org/10.5194/egusphere-egu26-4068, 2026.
Interplanetary shocks are ubiquitous in the heliosphere in relation or not with solar events such as Stream Interaction Regions and Coronal Mass Ejections. However, their interactions with planetary environments remain poorly understood.
In this study, we performed hybrid-PIC simulations of the interaction between interplanetary shocks and a realistic near-Earth environment system. We firstly focused on two aspects: (i) the self-consistent generation of a realistic bow shock–magnetosheath–magnetopause system, and (ii) a stand-alone analysis of the self-consistent evolution of a high-speed stream throughout an interplanetary medium in different scenarios. The latter included quasi-perpendicular, quasi-parallel, and Parker spiral-based scenarios, in order to highlight their profoundly diverse dynamics, as well as the possible formation of large upstream instabilities, such as foreshocks.
Finally, we present preliminary findings on the interaction between the realistic near-Earth environment and an interplanetary shock-front and shock-sheath in a quasi-perpendicular scenario.
How to cite: Cazzola, E., Fontaine, D., and Savoini, P.: 3D hybrid simulations of self-consistent IP shocks and their interaction with Earth, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5366, https://doi.org/10.5194/egusphere-egu26-5366, 2026.
Despite decades of study, questions about particle acceleration and energy cascades within the heliosphere remain open. Understanding these turbulent processes is key to understanding solar wind plasma dynamics. Interplanetary shocks provide a natural laboratory for investigating turbulent properties across the shock in upstream and downstream regions. However, conventional single-point spacecraft observations make it difficult to distinguish spatial from temporal variations, limiting direct comparisons with theoretical models.
We used the Wind, ACE, and DSCOVR missions, which are located at the L1 Lagrange point, to study turbulent scales perpendicular and parallel to the interplanetary (IP) shock normal. We estimate correlation lengths and effective Reynold numbers from the autocorrelation functions (ACFs). We show that these turbulent parameters decrease from upstream to downstream. However, an extended statistical analysis of tens of IP shocks showed little or no systematic decrease in correlation length across shocks. This can be related to the limitations of single-point measurements and to the small upstream and downstream intervals for the estimation of ACFs. To partially overcome these limitations, we identified cases in which the same turbulent structures were first observed upstream by Wind and then downstream by MMS near the subsolar point. This enabled us to better compare the properties of turbulence across the Earth’s bow shock. This multi-spacecraft configuration improves constraint on the spatial evolution of turbulence across shocks, allowing for more reliable estimates of correlation scales.
How to cite: Abushzada, I., Pitna, A., Nemecek, Z., and Safrankova, J.: Evolution of Solar-Wind Turbulence Correlation Lengths Across Interplanetary Shocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8041, https://doi.org/10.5194/egusphere-egu26-8041, 2026.
Shocklets are nonlinear compressive magnetosonic structures formed by the steepening of ultra-low-frequency (ULF) waves due to dispersive effects in collisionless foreshocks. At Earth, they are characterized by sharp upstream edges, moderate magnetic compression, and frequent whistler wave precursors.
We investigate shocklet-like structures in Mercury’s foreshock using 20 Hz magnetic field observations from the MESSENGER mission. The analysis targets upstream intervals with broadband ULF activity at frequencies of 2 Hz and below, including both low-frequency (≲0.3 Hz) and higher-frequency (~1–2 Hz) fluctuations analogous to waves known to evolve into shocklets at Earth.
More than 200 candidate events are identified and classified into two main categories based on waveform morphology and polarization. The first consists of Earth-like shocklets, exhibiting sharp leading edges, clear magnetic compression, linear or elliptical polarization, and frequent whistler precursors. The second, more prevalent category comprises ULF magnetosonic waves with superposed higher-frequency fluctuations, displaying weaker steepening and less clear polarization.
These observations indicate that similar wave-steepening processes operate at Mercury and Earth. However, Mercury’s weaker bow shock and therefore a reduced foreshock turbulence could favor multiscale wave coexistence and a broader diversity of shocklet-like structures.
How to cite: Rojas Castillo, D., Vaquero Bautista, C., Blanco Cano, X., Plashcke, F., Pump, K., Kajdic, P., and Heyner, D.: Shocklet-like Structures in Mercury’s Foreshock: New Evidence from MESSENGER, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8322, https://doi.org/10.5194/egusphere-egu26-8322, 2026.
In the transition region of a collisionless shock, the magnetic field strength generally reaches a larger value than the eventual downstream one. The formation of this magnetic overshoot plays an important role in, e.g., regulating the ion reflection. The magnitude of the overshoot varies spatially and temporarily along the shock front, necessitating several measurements to quantify it.
Catalogues of in situ shock crossing observations are now readily available from across the whole heliosphere, enabling large statistical studies. Here we combine measurements from Earth, Mars, Saturn as well as interplanetary shocks in the inner heliosphere, to investigate how the magnitude of the overshoot depends on the upstream parameters.
Consistent with previous studies, we find that there is a clear relationship with the upstream Alfvén Mach number and the magnitude of the overshoot relative to the upstream field strength. In the low Mach number (< 4) range, however, there appears to be an intriguing difference between planetary and interplanetary shocks. In contrast to past studies, we find that the overshoot does not depend on the upstream plasma beta for a given Alfvén Mach number.
How to cite: Hietala, H., Hoang, A., Lindberg, M., Sattiraju, T., Koller, F., and Vuorinen, L.: Magnetic overshoots at heliospheric shocks: parameter studies, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11398, https://doi.org/10.5194/egusphere-egu26-11398, 2026.
The interaction of solar wind directional discontinuities with a shock can give rise to large-scale transient structures such as hot flow anomalies and foreshock bubbles. These transients play an important role in particle acceleration, and, when they are formed at Earth's bow shock, can have a global impact on near-Earth space. The properties of foreshock transients upstream of the shock have been extensively studied, and a number of recent studies have started taking a closer look at their signatures in the magnetosheath. There however often remain ambiguities as to whether a structure is observed upstream or downstream of the shock, which cannot be resolved with single-point or closely-spaced multi-spacecraft observations. Foreshock transient properties strongly depart from typical solar wind values, and vary widely from one event to another. It can therefore be difficult to conclude with certainty on which side of the shock a set of observations is made without having reference upstream measurements. To complicate matters further, the bow shock moves in response to the foreshock transient’s varying properties, which can lead to boundary crossings embedded within the transient's observations. In this work, we leverage 2D global numerical simulations performed with the hybrid-Vlasov Vlasiator model to get a global view of the interaction of foreshock transients with Earth's bow shock and compare their properties upstream and downstream of the shock. We investigate how foreshock transient signatures change as they are processed through the shock and compare ion energy spectrograms and velocity distribution functions on both sides of the shock. We aim to identify signatures which could be then used to distinguish between upstream and downstream observations. We test our findings on events previously reported in the literature. Determining whether a foreshock transient is observed before or after its interaction with the shock is crucial to evaluate its impact on the magnetosphere, as a change in e.g. dynamic pressure variations can lead to a different amplitude in the response of the magnetopause.
How to cite: Turc, L., Dahani, S., Suni, J., Tao, S., Kalliokoski, M., Lipsanen, V., Ojuva, M., and Palmroth, M.: Foreshock transient signatures upstream and downstream of the shock, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11250, https://doi.org/10.5194/egusphere-egu26-11250, 2026.
Past kinetic simulations and spacecraft observations have shown that traveling foreshocks (TFs) are bounded by either foreshock compressional boundaries (FCBs) or foreshock bubbles (FBs). Here we present four TFs with a different kind of structure appearing at one of their edges. Two of them, observed by the Cluster mission, are bounded by a hot flow anomaly (HFA). In one case, the HFA was observed only by the spacecraft closest to the bow shock, while the other three probes observed an FCB. In addition, two other TFs were observed by the MMS spacecraft to be delimited by a structure that we call HFA-like FCB. In the spacecraft data, these structures present signatures similar to those of HFAs: dips in magnetic field magnitude and solar wind density, decelerated and deflected plasma flow and increased temperature. However, a detailed inspection of these events reveals the absence of heating of the SW beam. Instead, the beam almost disappears inside these events and the plasma moments are strongly influenced by the suprathermal particles. We suggest that HFA-like FCBs are related to the evolution and structure of the directional discontinuities of the interplanetary magnetic field whose thickness is larger than the gyroradious of suprathermal ions. We also show that individual TFs may appear together with several different types of transient upstream mesoscale structures, which brings up a question about their combined effect on regions downstream of the bow shock.
How to cite: Kajdič, P., Blanco Cano, X., Rojas Castillo, D., and Omidi, N.: Different Transient Phenomena at the Edges of Traveling Foreshocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8229, https://doi.org/10.5194/egusphere-egu26-8229, 2026.
The ion foreshock is the region of space where solar wind ions interacting with Earth’s bowshock can reach after being reflected in the bowshock instead of penetrating into the megnetosheath. The foreshock forms wherever the interplanetary magnetic field is quasit parallel to the bowshock’s normal and connected to it. In the foreshock, the ions reflected at the bowshock form a population that propagates in the upstream direction and make the Velocity Distribution Function (VDF) unstable, giving rise to Ultra Low Frequency (ULF) waves.
The Magnetospheric Multiscale Mission (MMS) in it orbit around the earth periodically probes the ion foreshock. The on-board Fast Plasma Investigation (FPI) instrument provides full ion VDFs of this region, a velocity distribution function comprised of the contributions of all ion populations present, including different ion species and the back-streaming protons characteristic of the ion foreshock. Here we present a method based on Gaussian Mixture Models (GMM) that we use to decompose full ion VDF into partial VDFS corresponding to the different ion populations. In this way, we can isolate the VDF of only the back-propagating foreshock protons, that can be used to study how they contribute to the instability of the full VDF and the excitation of propagation of ULF waves throughout the foreshock.
We demonstrate the ability of this method to find separate population-specific VDFs and apply it to a case study where ULF waves are observed in association to a diffuse back-streaming proton distribution function.
How to cite: Albert, I. F., Toledo-Redondo, S., Graham, D., Khotyaintsev, Y., Norgren, C., Montagud-Camps, V., and Castilla-Tevar, A.: Characterization of back-streaming proton VDFs in the Earth's bowshock using a Gaussian Mixture Model., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13565, https://doi.org/10.5194/egusphere-egu26-13565, 2026.
The solar wind interaction with Mercury’s magnetic field generates a bow shock in front of the planet. As at Earth, the region upstream of the shock that is magnetically connected to it, and known as foreshock, is permeated by a variety of waves. The characteristic frequencies and wave properties so far reported are (i) high frequency 2 Hz whistler waves (similar to the 1 Hz waves at Earth), (ii) intermediate frequency of 0.8 Hz, whose properties and formation mechanism remains unknown and (iii) lower frequency compressive waves in the 0.3 Hz range (corresponding to the large amplitude 30-s waves observed at Earth’s foreshock). The existence of ultra-low frequency waves indicates that backstreaming ions are able to drive instabilities as in the terrestrial case. However, simultaneous occurrence of different modes at Earth is not often observed. In this work we use Messenger magnetic field data to study some examples of extended regions at Mercury’s foreshock where multiple wave modes at frequencies 2, 0.8 and 0.3 Hz co-exist. The waves can maintain coherence over long intervals of time which may be related to the fact that the shock is weaker with Mach numbers in the range 2-5, and so that less backstreaming ions and density gradients are expected. Future work using plasma data from the BepiColombo mission are needed to understand in more detail wave generation and evolution in Mercury’s environment.
How to cite: Blanco-Cano, X., Plaschke, F., Kajdic, P., Rojas-Castillo, D., Pump, K., Heyner, D., Vaquero-Bautista, C. A., Erinfolami, F., Poh, G., Karlsson, T., and Le, G.: Multi-mode wave observations at Mercury’s Foreshock, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13602, https://doi.org/10.5194/egusphere-egu26-13602, 2026.
