It is common wisdom that collisionless shocks become nonplanar and nonstationary at sufficiently high Mach numbers. Whatever the shock structure, the upstream and downstream fluxes of the mass, momentum, and energy should be equal. These conservation laws are satisfied at low Mach numbers when the shock front is planar and stationary. When this becomes impossible, inhomogeneity and time dependence, presumably as rippling, develop. Using test particle analysis in a model shock profile, this study shows that the shock structure changes as a kind of "phase transition" when the Mach number is increased while the shock angle, the upstream beta, and the magnetic compression are kept constant.

How to cite: Gedalin, M.: A "phase transition" from a planar stationary profile to a rippled structure, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2077, https://doi.org/10.5194/egusphere-egu25-2077, 2025.

The size of collisionless shock waves is believed to play an important role in determining the maximum energy gain of particles accelerated at heliospheric and astrophysical shocks.
In this study, we use the Parker Solar Probe and Solar Orbiter gravity assists at Venus to investigate electron acceleration at the Venusian bow shock. 
The identified Venusian shock crossings are compared to terrestrial bow shock crossings with similar shock parameters using the Magnetospheric Multiscale (MMS) mission. The aim of the comparison is to uncover potential differences between the two different types of bow shocks and how the size of collisionless bow shocks affects electron acceleration. 
Preliminary results indicate a harder average spectral index (p ~ 3.5±0.6) for suprathermal electrons observed at Venus's bow shock than those found at Earth's bow shock (p ~ 4.9±0.8) for similar Mach number and shock angle.

How to cite: Lindberg, M. and Hietala, H.: Electron Acceleration Comparison of the Venusian and Terrestrial Bow Shocks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2626, https://doi.org/10.5194/egusphere-egu25-2626, 2025.

Collisionless shock waves are ubiquitous in astrophysical plasmas, from supernova remnants and planetary atmospheres to coronal mass ejections and laboratory experiments. These shocks are known to be efficient particle accelerators, crucial for understanding the origin of cosmic rays, including ultra-relativistic particles. This study presents a novel model of reinforced shock acceleration for electrons, integrating in-situ data from NASA's Magnetospheric Multiscale (MMS) and Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction (ARTEMIS) missions. Focusing on Earth's planetary environment, our analysis reveals a suprathermal electron injection threshold, demonstrating how a multiscale framework involving foreshock transient phenomena, a suprathermal seed population, and wave-particle interactions can systematically accelerate suprathermal electrons to relativistic energies. By merging theoretical advancements in astrophysical plasmas and shock physics with these multi-spacecraft observations, we address the persistent electron injection problem and explore the broader applicability of our model to other planetary environments within our solar system and beyond, including stellar and interstellar contexts.

How to cite: Raptis, S., Lalti, A., Lindberg, M., Turner, D., Caprioli, D., and Burch, J.: Revealing an unexpectedly low electron injection threshold via reinforced shock acceleration, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4552, https://doi.org/10.5194/egusphere-egu25-4552, 2025.

The solar wind and interplanetary magnetic field parameters observed in situ by the SWA and MAG instruments onboard the Solar Orbiter mission provide a unique opportunity to identify interplanetary (IP) shock waves at various distances from the Sun on the rising phase of the 25^th solar activity cycle.

In the frame of this work, to recognize IP shocks, we applied a semi-automated method on the base of quality factors comprising the solar wind velocity, density characteristics, and total interplanetary magnetic field parameters. As an example, we identified a few tens of various IP shocks that occurred in the inner heliosphere, at radial distances of 0.29–0.95 AU from the Sun. Most of them were classified as FF-type shock waves, with only a few events identified as FR-, SF- and SR-type shock waves. The semi-automatic algorithm mentioned was used to determine the time of passage of the shock wave front through the spacecraft’s location.

Moreover, we calculated the typical kinetic and magnetohydrodynamic characteristics of each identified shock wave. In particular, the radial dependences of parameters such as the density ratio (r_N), magnetic field (r_B) ratio, plasma beta (β_us), Alfvén velocity (V_A), the angle between the shock normal and the interplanetary magnetic field (Q_Bn), shock wave front velocity (V_sh), sound speed (V_s) and magnetosonic speed (V_fms) were analyzed. Additionally Alfvén (M_A) and magnetosonic (M_fms) Mach numbers were studied. Finally, the dependence of the number of identified shock waves on radial distance was also examined and compared with solar flares activity.

This work is supported by the “Long-term program of support of the Ukrainian research teams at the Polish Academy of Sciences carried out in collaboration with the U.S. National Academy of Sciences with the financial support of external partners”.

How to cite: Yakovlev, O., Dudnik, O., and Wawrzaszek, A.: Interplanetary shock waves semi-automated identified as seen by Solar Orbiter., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6223, https://doi.org/10.5194/egusphere-egu25-6223, 2025.

Shocklets are compressive, linearly polarized magnetosonic structures that have been widely observed in the Earth's foreshock. They form due to wave steepening and dispersive effects, often accompanied by whistler wave precursors. At Earth these structures are characterized by steepened upstream edges, magnetic compression below 2, and associations with hot diffuse ion distributions. Shocklets play a crucial role in energy transfer and wave-particle interactions in collisionless shocks. While most studies have focused on Earth's foreshock, some evidence suggests their presence at Venus, raising questions about their existence in other planetary foreshocks.

In this study, we investigate the presence of shocklets in Mercury's foreshock using data from the MESSENGER mission. The timescales of Hermean shocklet candidates range from 3 to 30 seconds. Our preliminary analysis reveals that shocklets at Mercury exhibit greater diversity compared to those observed at Earth. While some structures resemble typical Earth-like shocklets, characterized by a sharp leading edge with whistler wave precursors followed by a slower relaxation, we also identify ULF magnetosonic waves accompanied by high-frequency fluctuations that display initial signs of wave steepening which could correspond to an early stage of the Earth-like shocklet. Our findings highlight the complex and dynamic wave activity in Mercury's unique solar wind environment.

How to cite: Rojas-Castillo, D., Vaquero Bautista, C. A., Blanco-Cano, X., Plaschke, F., Kajdic, P., Pump, K., and Heyner, D.: Shocklets in the vicinity of Mercury, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7432, https://doi.org/10.5194/egusphere-egu25-7432, 2025.

Earth’s magnetosheath, the region between Earth’ magnetic field and the bow shock, gives rise to a plethora of transients, waves, and instabilities. Each effect changes the plasma parameter distribution, impacting the behaviour of the plasma that finally hits our magnetic field. Turbulence plays a crucial role in how energy injected into the system by transients or instabilities is transported and dissipated from large to small scales. We are investigating the role magnetosheath pressure enhancements (or so-called jets) play in adding to the turbulence in the system. These jets often plough through the system with high velocity, interacting with the surrounding environment, and slow down the further they travel. The propagation of jets is still largely unexplored, in particular when considering plasma turbulence. We aim to quantify whether jets drive turbulence in the magnetosheath plasma, and whether turbulent plasma fluctuations impact the propagation of jets. Magnetic orientation and distance to the shock are considered in the analysis in order to disentangle their impact on the effects. We are using MMS spacecraft measurements for the analysis of individual jet cases, and THEMIS spacecraft for a statistical analysis at event times when several spacecraft were close to the flow of jet events.

How to cite: Koller, F., Chen, C., and Hietala, H.: Turbulence Surrounding Magnetosheath Jets: Are Pressure Enhancement Stirring the Magnetosheath?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14042, https://doi.org/10.5194/egusphere-egu25-14042, 2025.

Localised dynamic pressure enhancements – jets – have been observed downstream of both planetary and more recently interplanetary shocks. At planetary environments, jets are known to drive enhanced particle acceleration, large-amplitude magnetic field variations and reconnecting current sheets.

The analysis of interplanetary jets observed by Wind spacecraft near the Earth showed that their properties are similar to those of magnetosheath jets. Furthermore, we found jets also at low beta, low Mach number interplanetary shocks, i.e., conditions that are rare for the Earth’s bow shock.

Following the jet identification approach we introduced for Wind data, here we now apply it to Solar Orbiter measurements made at various heliocentric distances.

How to cite: Hietala, H., Shirazul, K., Trotta, D., Webb, P., Vuorinen, L., and Koller, F.: Investigating jets downstream of interplanetary shocks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17988, https://doi.org/10.5194/egusphere-egu25-17988, 2025.

The bow shock of Mars provides a compelling example of a mass-loaded, supercritical shock. A key challenge in space plasma physics is understanding the mechanisms of particle acceleration that occur at collisionless shocks. Due to extensive studies, the terrestrial foreshock is often considered a benchmark for interactions between planetary magnetospheres and the solar wind. The MAVEN mission at Mars is offering a wealth of data, simultaneously opening a window to study the Martian foreshock in detail.

 In this context, we present new measurements of velocity distribution functions of suprathermal protons upstream of Mars' bow shock. We identify backstreaming beams aligned (FAB) and misaligned with the interplanetary magnetic field (IMF) direction for various shock geometries. FAB bulk velocities are found to be well-distributed in relation to the shock speed. Our analysis reveals that, compared to their terrestrial counterparts, Martian FABs exhibit slower sunward motion. Additionally, it appears that these FABs originate from a shock region where the IMF lines form an angle of 20-50 degrees with the shock normal— a smaller source region than that of Earth's bow shock.

 These findings rule out specular reflection as the mechanism behind beam production. Typically, terrestrial FABs are produced through a quasi-adiabatic process that preserves the first invariant to some extent. In contrast, the new Martian observations provide a valuable comparison between the foreshocks of Earth and Mars, shedding light on key differences and enhancing our understanding of these planetary phenomena.

How to cite: Meziane, K., Mazelle, C., Simon-Wedlund, C., Halekas, J., Hamza, A., Bertucci, C., Mitchell, D., and Espley, J.: Ion acceleration at Mars Bow Shock - Results from MAVEN, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12526, https://doi.org/10.5194/egusphere-egu25-12526, 2025.

Using Magnetospheric Multiscale (MMS) data from Earth orbit, an automated clustering algorithm has been employed to classify dayside MMS data into four distinct regions: magnetosphere, magnetosheath, solar wind, and ion foreshock, as detailed in Toy-Edens et al. [JGR 2024].  Applied to eight years of MMS data, over 25,000 bow shock crossings were identified from all four MMS spacecraft. Using that event database, we highlight a series of results including: new, 3-dimensional, parameterized boundary model fits for the bow shock; statistical characteristics of the quasi-parallel and quasi-perpendicular bow shock; and a new 4-point timing algorithm to systematically determine bow shock normal directions. We detail new results concerning the accuracy and performance of the shock normal results, showing that this new approach works remarkably well. We also highlight some new results of kinetic shock behavior and compare those directly to results from state-of-the-art simulations of and corresponding predictions for collisionless shocks. We end with a discussion of future work, in which we hope to train a parameterized generative model for collisionless shock crossing data as a function of upstream plasma characteristics. Our hope is to be able to apply that model to collisionless shocks beyond 1 au, validating its performance with shock observations from other systems (including Venus, Mars, Jupiter, etc.), and ultimately apply it to solar and other astrophysical shocks that are beyond our reach for in situ observations. 

How to cite: Turner, D., Toy-Edens, V., Mo, W., and Young, S.: Employing machine learning techniques for systematic and statistical studies of collisionless shocks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13753, https://doi.org/10.5194/egusphere-egu25-13753, 2025.

Interplanetary (IP) shocks can be driven in the solar wind by fast coronal mass ejections, and by the interaction of fast solar wind with slow streams of plasma. These shocks can be preceded by extended wave and suprathermal ion foreshocks perturbing large extensions of the heliosphere. In a recent study (Trotta et al., 2024 it was shown that the interaction between two interplanetary coronal mass ejections (ICMEs) can drive a forward and a reverse shock with similarities to those bounding stream interaction regions (SIRs). In this work we analyse the microstructure of this event observed by Solar Orbiter on March 8th, 2022 at 0.5 AU. We find that wave characteristics change from one ICME to the other. Inside the first ICME waves have a broad band sprectrum. In contrast, there are regions in the second ICME with very monochromatic waves. In both cases, waves are associated with proton and alpha particle distributions that show a super-Alfvenic drift, suggesting local wave generation. Larger amplitude waves due to ion reflection are found upstream of the forward shock forming an extended foreshock. Although the shock was weak,  reflected populations include alpha particles, and O⁶⁺ ions. Of particular interest is the fact that monochromatic ion cyclotron waves associated with anisotropic (Tperp >Tpar) ion distributions are found in the region between the two ICMEs. Our results show how ICME-ICME interaction can result in regions with a variety of microstructure phenomena in the inner heliosphere.

How to cite: Blanco-Cano, X., Trotta, D., Kieokaew, R., Livi, S., Hietala, H., Kajdic, P., Rojas-Castillo, D., Dimmock, A., Larosa, A., Hornury, T., Vainio, R., and Jian, L.: Microstructure at a fully formed Forward-Reverse Shock pair due to the interaction between two Coronal Mass Ejections observed at 0.5 AU., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14236, https://doi.org/10.5194/egusphere-egu25-14236, 2025.