The formation of a collisionless shock is the result of a balance between nonlinear steepening and processes that counteract this steepening. Dispersive shocks are shocks in which dispersive processes counterbalance the front steepening and are formed when the dispersive spatial scale exceeds scales associated with resistive processes. Oblique dispersive shocks are characterized by a phase standing wave precursor adjacent to the magnetic ramp. The whistler critical Mach number is defined as the maximum Mach number for which a linear whistler wave can phase stand upstream of the shock front. It was widely accepted that if the Mach number exceeds , linear whistler waves propagating along the shock normal are not able to "phase stand" in the upstream flow, and "& mldr;the shock will be initiated by a monotonic ramp." (Kennel et al., 1985, https://doi.org/10.1029/gm034p0001). In this study, we present results of numerical simulations and observations of shocks with that reveal the occurrence of an alternative scenario. For both the shock resulting from kinetic particle-in-cell simulations and that observed by MMS, the propagation direction of the precursor deviates from the shock normal direction. As a result, the velocity of the surface of constant phase along the shock normal exceeds the phase speed of these waves. It is shown that the propagation of the surface of constant phase along the shock normal occurs at a velocity that is nearly equal to the shock speed. Hence, these waves are "phase standing along the shock normal" in spite of .

A magnetohydrodynamic (MHD) shock is a discontinuity through which plasma flows, while the mass, momentum and energy are conserved, and the entropy increases. In a collisionless system, the entropy is strictly constant along the phase-space trajectory. The MHD required entropy increase is attributed to dissipative processes causing relaxation to local thermodynamic equilibrium. The search for such dissipative processes in the shock observations usually explores variations of the entropy density or entropy per particle across the shock front. We show that both quantities may increase or decrease across the shock front even when the entropy is strictly conserved. Entropy production should be assessed using the entropy flux density.

A number of mechanisms have been suggested to operate within the terrestrial bow shock to redistribute energy contained in the incoming solar wind flow. The majority of mechanisms involve the generation of turbulence while some are based on particle motion alone. In this paper, we investigate the possible occurrence of the Electron Trajectory Instability, that results from short scale electric field gradients. Spike-like bipolar features in electric field measurements are a commonly observed signature within the terrestrial bow shock. They are usually associated with the passage of electrostatic solitary waves associated with phase space holes in the particle distribution. Using electric field measurements, we compare different interferometric methods to determine the propagation direction, velocity, and spatial scale of these features. Based on these results, it appears that the instability criterion for the Electron Trajectory Instability is fulfilled and the electron trajectories will diverge in the presence of these structures.

Diffusive shock acceleration, at shocks from coronal mass ejections to supernova-remnant blast waves, presupposes a scattering wave field that the accelerated particles themselves maintain. This self-regulation has not been resolved in situ. We report Parker Solar Probe observations of a fast ( 2800 km/s), near-parallel interplanetary shock at 0.24 AU on 2023 March 13 and separate its upstream wave field into four families, a classification not made before at a fast shock near the Sun. Right-hand and left-hand circularly polarized families over a common wavenumber band, with a field-aligned linearly polarized family, are cyclotron-resonant with the suprathermal-to-MeV protons streaming from the shock: the beam drives the field that scatters it, and the measured mean free path, half the precursor scale, leaves the beam anisotropic enough to sustain the drive. Outside this loop lies a weak, oblique, linearly polarized component, a few per cent of the wave power, resolved here for the first time at an in situ foreshock. Its in-phase density and field-magnitude fluctuations identify the compressive part as fast magnetosonic and shift the cyclotron-resonance energies of the resonant families by up to 13

Particle acceleration and radiation are fundamental cosmic processes that significantly contribute to the universe’s energy density, driven by phenomena ranging from solar flares to supernova explosions. Shock waves, prevalent across various spatial scales, play a key role in converting kinetic energy into plasma heating and particle acceleration. Recent advancements from missions such as the Parker Solar Probe (PSP) have provided unprecedented insights into the dynamics of shock waves within the heliosphere, thereby enhancing our understanding of these critical energy conversion mechanisms.In this talk, I will present findings from two recent studies that leverage the PSP’s unique proximity to the Sun and its advanced, high-fidelity instrumentation. First, we analyzed one of the fastest shocks ever observed on March 13, 2023, revealing the efficient acceleration of electrons up to and exceeding 6 MeV and the collective acceleration of ions from the thermal solar wind. Second, we made the surprising discovery of synchrotron radiation emanating from ultra-relativistic electrons in both a quasi-parallel and a quasi-perpendicular shock, with the quasi-parallel shock exhibiting significantly higher radiation intensities due to more effective electron acceleration. These results are consistent not just with theoretical models of strong cosmic shocks, but also observations. This offers an unprecedented opportunity to bridge in situ heliospheric observations with remote observations of phenomena such as supernova remnants.

In the concluding work of a series of studies, the disk's spiral structure of the Milky Way is anew considered as a Lin-Shu-type density-wave pattern. Asa step forward, a sample of similar to 500000 stars from a total of 656161 sources within 3 kpc from the Sun and +/- 250 pc from the disk's mean plane identified in the Gaia EDR3 is examined. The parameters of Galactic rotation and solar peculiar motion corrected for the effects of 3D density waves, and the radial, azimuthal, and vertical components of the streaming motion of stars due to the spiral arms are derived from the measured velocities of objects. The parameters of the waves are also estimated. Two almost equal scales of periodic compression/rarefaction irregularity of the streaming velocity field with the radial and vertical wavelengths of <= 2 kpc in the form of a spiral density wave propagating in the disk at the Sun are revealed, which are reasonably related to the global spiral pattern. The Sun is located between the central part and the inner edge of the Orion (Local) wave arm. The spiral pattern in the kinematics of stars is consistent with the spatial distribution of the nearby arm segments traced by young populations. In agreement with the original Lin and Shu proposal, the gravitational field associated with the arms is a small deviation from the total gravitational field of the Galaxy that is axisymmetric in the mean. The very existence of the spiral arrangement in the velocity field suggests that the localized disk is gravitationally unstable.

Collisionless shock waves (CSWs) in plasma, prevalent in diverse astrophysical contexts, are key to understanding cosmic particle acceleration. These shock waves, observable in environments from heliospheric planetary bow shocks to supernova remnants (SNRs), efficiently convert kinetic energy to thermal energy and accelerate particles to sub-relativistic and relativistic energies. A particular focus is on electrons accelerated by these shocks, as they generate electromagnetic radiation, making astrophysical shocks like SNRs observable. Despite their significance, gaps remain in our understanding of the dynamic mechanisms behind these universal accelerators, underscoring the necessity for in-depth, direct in situ measurements. Heliospheric shocks offer a unique opportunity for such in situ studies, particularly those that are strong and fast, potentially mirroring SNR shocks. This study highlights the groundbreaking in situ observations of the fastest heliospheric shock wave yet, traveling at nearly 1% the speed of light, captured by the pioneering Parker Solar Probe. Positioned just 0.23 astronomical units from the Sun, the probe directly measured the acceleration of electrons and ions to high energies amidst intense electromagnetic activity. A landmark discovery was the acceleration of electrons to ultra-relativistic speeds, with energies reaching up to 6 Million electron volts (MeV). This observation not only provides unprecedented insights into the mechanisms of particle acceleration in CSWs but also bridges the gap in our understanding of similar processes in more distant astrophysical phenomena like SNRs. The findings from the Parker Solar Probe open new avenues for exploring and comprehending the intricate processes of cosmic particle acceleration.

Large-amplitude electrostatic fluctuations are routinely observed by spacecraft upon traversal of collisionless shocks in the heliosphere. Kinetic simulations of shocks have struggled to reproduce the amplitude of such fluctuations, complicating efforts to understand their influence on energy dissipation and shock structure. In this paper, one-dimensional particle-in-cell simulations with realistic proton-to-electron mass ratio are used to show that in cases with upstream electron temperature T _e exceeding the ion temperature T _i , the magnitude of the fluctuations increases with the electron plasma-to-cyclotron frequency ratio ω _pe /Ω _ce , reaching realistic values at ω _pe /Ω _ce ≳ 30. The large-amplitude fluctuations in the simulations are shown to be associated with electrostatic solitary structures, such as ion phase-space holes. In the cases where upstream temperature ratio is reversed, the magnitude of the fluctuations remains small.

It is common wisdom that collisionless shocks become non-planar and non-stationary at sufficiently high Mach numbers. Whatever the shock structure, the upstream and downstream fluxes of the mass, momentum and energy should be equal. At low Mach numbers, these conservation laws are satisfied when the shock front is planar and stationary. When this becomes impossible, inhomogeneity and time dependence, presumably in the form of rippling, develop. In this study, we show that the shock structure changes as a kind of ‘phase transition’ when the Mach number is increased while other shock parameters are kept constant.

A collisionless shock structure results from the nonlinear interaction between charged particles and electromagnetic fields. Yet, a collisionless shock is globally governed by the mass, momentum and energy conservation requirements. A stable shock structure must ensure that the fluxes of the conserved quantities are constant on average, and, therefore, is determined by this necessity. Here, we study an observed high upstream temperature high Mach number shock and show that the conservation laws cannot be fulfilled unless the shock is spatially inhomogeneous along the shock front and time-dependent.

Electron heating and acceleration in collisionless shocks is a long-standing problem. Rapid isotropization of heated electrons cannot be explained solely by the cross-shock potential. In addition, the macroscopic cross-shock potential prevents efficient reflection and injection into the diffusive acceleration regime. Recent observations have shown that small-scale electric fields are present in the shock front, together with the large-scale cross-shock potential. These small-scale fields have been found also in the upstream and downstream regions. Electron heating in shocks is produced by the combined action of the large- and small-scale fields. The large-scale potential determines the energy transferred to the electrons. The small-scale electrostatic fields scatter electrons. Here we study the scattering of electrons on the typical waveforms, namely solitary bipolar spikes and wavepackets. The main effect is the generation of backstreaming electrons with large pitch angles. It is found that wavepackets are more efficient in electron reflection in the interaction of electrons both with a single spike and with multiple spikes.

Abstract High‐speed jets (HSJs) are commonly observed in the Earth's magnetosheath. The HSJs can drive shock‐like bow waves when compressing the ambient plasma, which are important for the HSJ's evolution and the energization of charged particles. Here we present the first two‐dimensional hybrid simulation of the formation and evolution of jet‐driven bow waves. The simulated bow waves exhibit localized enhanced magnetic field and ion density, with their peaks separated by the order of ion inertial length. The bow waves are formed when a super‐magnetosonic HSJ encounters a magnetic structure with the magnetic field nearly perpendicular to the HSJ's velocity. The magnetic field structure acts as an obstacle to deflect and decelerate the jet, causing the pile up of ions on the jet side and the compression of the magnetic structure on the downstream side. Our study explains the observed properties of bow waves, and helps to better understand the evolution of HSJs.

Collisionless shock waves, ubiquitous in the Universe, are crucial for particle acceleration in various astrophysical systems. Currently, the heliosphere is the only natural environment available for their in situ study. In this work, we showcase the collective acceleration of electrons and ions by one of the fastest in situ shocks ever recorded, observed by the pioneering Parker Solar Probe at only 34.5 million km from the Sun. Our analysis of this unprecedented, near-parallel shock shows electron acceleration up to 6 MeV amidst intense multiscale electromagnetic wave emissions. We also present evidence of a variable shock structure capable of injecting and accelerating ions from the solar wind to high energies through a self-consistent process. The exceptional capability of the probe’s instruments to measure electromagnetic fields in a shock traveling at 1% the speed of light has enabled us, for the first time, to confirm that the structure of a strong heliospheric shock aligns with theoretical models of strong shocks observed in astrophysical environments. This alignment offers viable avenues for understanding astrophysical shock processes and the self-consistent acceleration of charged particles.

In typical heliospheric collisionless shocks most of the mass, momentum and energy are carried by ions. Therefore, the shock structure should be most affected by ions. With the increase of the Mach number, ion reflection becomes more and more important, and reflected ions participate in shaping the shock profile. Ion reflection at the collisionless shock is a non-local process: the reflected–transmitted ions re-enter the shock front far from the reflection point. The direction and the magnitude of this shift depend on the shock angle. The distance between the reflection point and the re-entry point is of the order of the upstream ion convective gyroradius and exceeds the shock width. The non-locality of ion reflection may have implications for shock rippling since reflected ions may carry perturbations along the shock front.

In a collisionless shock the energy of the directed flow is converted to heating and acceleration of charged particles, and to magnetic compression. In low-Mach number shocks the downstream ion distribution is made of directly transmitted ions. In higher-Mach number shocks ion reflection is important. With the increase of the Mach number, rippling develops, which is expected to affect ion dynamics. Using ion tracing in a model shock front, downstream distributions of ions are analyzed and compared for a planar stationary shock with an overshoot and a similar shock with ripples propagating along the shock front. It is shown that rippling results in the distributions, which are substantially broader and more diffuse in the phase space. Gyrotropization is sped up. Rippling is able to generate backstreaming ions, which are absent in the planar stationary case.

A collisionless shock is a self-organized structure where fields and particle distributions are mutually adjusted to ensure a stable mass, momentum and energy transfer from the upstream to the downstream region. This adjustment may involve rippling, reformation or whatever else is needed to maintain the shock. The fields inside the shock front are produced due to the motion of charged particles, which is in turn governed by the fields. The overshoot arises due to the deceleration of the ion flow by the increasing magnetic field, so that the drop of the dynamic pressure should be compensated by the increase of the magnetic pressure. The role of the overshoot is to regulate ion reflection, thus properly adjusting the downstream ion temperature and kinetic pressure and also speeding up the collisionless relaxation and reducing the anisotropy of the eventually gyrotropized distributions.

Using ion tracing in a model shock front we study heating of thermal (Maxwellian) and superthermal (Vasyliunas–Siscoe) populations of protons, singly charged helium, and alpha particles. It is found that heating of thermal and superthermal populations is different, mainly because of substantially higher ion reflection in the superthermal populations. Accordingly, the temperature increase of initially superthermal populations is substantially higher than that of the thermal ions. Heating per mass decreases with the increase of the mass-to-charge ratio because of the reduced effect of the cross-shock potential and, accordingly, weaker ion reflection. The findings are supported by two-dimensional hybrid simulations.

The aim of this study is to compare observations of the magnetic field structure of observed quasi-parallel collisionless shock fronts with the results obtained analytically. A two-fluid analytic model of the shock front structure was derived under the assumptions that the shock is stationary and planar. The ion and electron kinetic pressures were assumed to be scalar, and polytropic state equations were used. The results of this analytical approach show that the shock magnetic field has an oscillatory structure. Venus Express (VEX) observations of the Venusian bow shock have been used to validate these theoretical findings. The Venusian bow shock and corresponding foreshock are significantly smaller than those of Earth. Thus, observations of the underlying structure of the quasi-parallel shock at Venus are not masked by the presence of high-amplitude waves and nonlinear structures originating in the foreshock. It is shown that the structure of the shock front, as observed by VEX, has a very strong similarity to the structure obtained analytically.

The structure of a collisionless shock affects ion motion in the shock front and is affected by the formed ion distribution. In high-Mach-number shocks, a significant fraction of incident ions are reflected by the macroscopic electric and magnetic fields in the shock front. Ions are non-specularly reflected by the combined electric deceleration and magnetic deflection. Here, a first analytical description of the non-specular reflection is presented. The contribution of the increasing magnetic field is evaluated and shown to enhance reflection. The distribution of non-specularly reflected ions ahead of the ramp is calculated and their velocities at the re-entry to the shock are found numerically. Dependence on the angle between the shock normal and the upstream magnetic field vector is illustrated.

&lt;p&gt;Collisionless shocks exist across diverse plasma environments. Examples are supernova remnants, comets, near planets, and interplanetary (IP) shocks in the solar wind. As the shock Mach number increases, so does the complexity of the ion distribution functions at the shock front due to features such as whistler precursors, ion reflection, shock ripples, and nonstationarity.&amp;#160;&lt;/p&gt; &lt;p&gt;Experimental studies of ion dynamics at supercritical high Mach (&gt;5) number shocks are typically conducted using planetary bow shock crossings since the Mach number of these shocks are higher while the shock speeds with respect to the observing spacecraft are lower. As a result, it is easier to resolve complex features in the ion velocity distribution function. For these reasons, studies concentrating on ion reflection at IP shocks are rare. However, comparisons with IP shocks are interesting since they have a much larger curvature radius and can be accompanied by more energetic particles.&lt;/p&gt; &lt;p&gt;In this work, we analyze a quasi-perpendicular shock observed by Solar Orbiter (SolO) on 30 October 2021 with a Mach number of around 7; this is much higher than the typical values of SolO IP shocks, which are between 1-3. For this event, we observed clear signatures in the upstream ion distribution function of reflected ions with energies extending to around 15 keV, which is lower than reported by other studies. The shock also demonstrates a non-planar feature, which may indicate shock rippling. In addition, whistler precursors are also found immediately upstream locally within the shock foot. We present these experimental results and a comparison with test-particle analysis and numerical modeling results.&lt;/p&gt;