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NASA NTRS1993

Plasma waves as indicators of the termination shock

The plasma wave receivers on the Voyager spacecraft will likely provide indicators of both the actual crossing of the termination shock as well as precursors of the shock crossing. Since the electron foreshock can extend considerable distances upstream of the termination shock, the detection of these waves can provide as many as several weeks warning that a crossing of the termination shock is imminent. Electrostatic turbulence associated with planetary bow shocks themselves is also an expected feature of the solar wind termination shock and will provide an important signature with which to identify the shock and to provide information on its thickness and fundamental processes. Both upstream Langmuir waves and electrostatic wave turbulence can often be found in conjunction with interplanetary shocks, although the generally weaker nature of these shocks often leads to weaker plasma wave signatures than observed at planetary bow shocks. We demonstrate with Voyager observations that the amplitudes expected for each of these phenomena are well within the range of detectability by the Voyager plasma wave receiver even for termination shock distances exceeding 100 AU.

Kurth, W. S.↗
NASA NTRS1994

Motion of the heliospheric termination shock. 2: Energy loss due to energetic particle acceleration

We present a generalization of earlier analysis of the motion of the heliospheric termination shock in response to heliospheric disturbances (Barnes, 1993) (paper 1), to allow jump conditions that include an energy sink at the shock front. The motivation for this study is that acceleration of the anomalous cosmic ray component may in fact represent such a sink. We have idealized the situation by assuming an infinitely thin shock parameterized by a quantity lambda(0 less than or equals lambda less than or equals 1), defined as the fraction of solar wind energy that is lost due to acceleration of the energetic particle component. If the sink is strong (consuming, say, 50% or more of the incident solar wind energy), the model leads to the following principal conclusions: (1) the shocked plasma would be much denser and cooler than in the standard gasdynamic case, thereby leading to more favorable conditions for direct observation of the shocked plasma; (2) the equilibrium shock position would be slightly farther (less than 10%) from the Sun than in the standard model; (3) as in the gasdynamic case, the shock would normally be in motion, so that the instantaneous position of the termination shock is not determined by interstellar conditions but by the recent history of the wind that has passed through the termination shock; and (4) the response of the shock to upstream disturbances would be similar to the response in the gasdynamic case, but the speed of the new termination shock would be somewhat smaller (probably by a f actor of 4 or less). We estimate that this speed is typically, approximately a few tens of kilometers per second corresponding to an inward or outward excursion of order of less than 1 to several AU, rather less than conventional estimates of several tens of AU.

Barnes, Aaron↗
NASA NTRS1994

Interaction of global merged interaction region shock with the heliopause and its relation to the 2- and 3-kHz radio emissions

We use the Voyager 2 plasma and magnetic field data together with a one-fluid magnetohydrodynamics model to study the interactions of the 1991 global merge interaction region (GMIR) shock with the termination shock and the heliopause. The 1991 GMIR is an extraordinarily large global solar wind structure in radial, longitudinal, and latitudinal extents. It has a strong shock at the leading edge. After its penetration through the termination shock, the GMIR shock first propagates through the subsonic solar wind, then interacts with the heliopause. The interaction produces a transmitted shock propagating outward in the interstellar medium, and a reflected shock propagating backward in the subsonic solar wind. We identify the reflected shock and the transmitted shock as the possible source of the radio noise detected at Voyagers (Gurnett, et al., 1993). The plasma frequency behind the reflected and the transmitted shock can be responsible for the 2- and 3-kHz radio emissions, respectively. The two bands of radio noise are emitted from sources on both sides of the heliopause starting at about the same time. If the emissions picked up by Voyager are due to f(sub P) radiation, then the heliopause is located at R is approximately = 130 AU. If the emissions are due to 2f(sub P) radiation, the R is approximately 150 AU. Because the relative speed of the interstellar plasma with respect to the sun appears to be sub-Alfvenic, it is very unlikely there is a fast-mode bow shock of the heliosphere.

Whang, Y. C.↗
NASA NTRS1995

Galileo flybys of Earth: The nature of the distant shock

We report on observations of the Earth's bow shock at unprecedentedly large downtail distances, some as remote as 360 R(sub E). Suprisingly, we find that even at these large distances, the bow shock signature remains clear. The cases we report are among the weakest shocks ever clearly identified. These shocks reveal patterns of field changes remarkably similar to those observed for stronger shocks. Indeed, several of the shocks could serve as textbook examples because they occur in unusually quiet and steady solar wind conditions. The quasi- perpendicular shocks are, in some cases preceded by whistler wave trains. Several of the shocks which have normal vectors in the transitional region between quasiperpendicular and quasiparallel are associated with large amplitude wave disturbances in the downstream plasma. Although low Mach number, these shocks are the source of large amplitude turbulence. We show a range of fits to the locations of the shock crossings, and argue that, for steady solar wind conditions, there is no sign of tail flapping. The displacement of the shock from the tail axis appears to be governed by the interplanetary magnetic field orientation as previously reported for Venus.

Kivelson, M. G.↗
NASA NTRS1995

Global aspects of the motion of the heliospheric termination shock: A gasdynamic model

The heliospheric termination shock is expected to move in response to variation in upstream solar wind conditions. Using numerical techniques, we extend an earlier strictly one-dimensional analytic gas dynamic model of shock motion to two dimensions, to investigate the qualitative features of global behavior of shock motion, and the consequences of latitudinal variation in dynamic pressure. The boundary conditions of the calculation are given by the solar wind parameters as a function of latitude and time on an inner spherical boundary, and a constant pressure (roughly simulating the effect of the local interstellar medium) on an outer boundary. Density variations, specified at the inner boundary as a function of time, are convected into the termination shock. Immediately after the interaction, the shock moves with speeds given by the earlier analytic model. However, as the termination shock propagates outward (or inward), it begins to slow down. After about 2 to 10 years, depending on details of boundary conditions, the signal from the shock interaction has reached the outer boundary and propagates inward to the position of the termination shock, strongly affecting the behavior of the shock. Assuming no further disturbances in the solar wind, the termination shock will reach its new equilibrium after some tens of years. In reality, large-scale variations in solar wind dynamic pressure occur on time scales short in comparison with the eleven year solar cycle, so that one expects that the termination shock is never in an equilibrium position, but rather oscillates inward and outward; this oscillation will vary with heliographic latitude. The effects of a variety of types of solar wind disturbances are investigated and summarized.

Ratkiewicz, R.↗
NASA NTRS1998

Direct Acceleration of Pickup Ions at The Solar Wind Termination Shock: The Production of Anomalous Cosmic Rays

We have modeled the injection and acceleration of pickup ions at the solar wind termination shock and investigated the parameters needed to produce the observed Anomalous Cosmic Ray (ACR) fluxes. A non-linear Monte Carlo technique was employed, which in effect solves the Boltzmann equation and is not restricted to near-isotropic particle distribution functions. This technique models the injection of thermal and pickup ions, the acceleration of these ions, and the determination of the shock structure under the influence of the accelerated ions. The essential effects of injection are treated in a mostly self-consistent manner, including effects from shock obliquity, cross- field diffusion, and pitch-angle scattering. Using recent determinations of pickup ion densities, we are able to match the absolute flux of hydrogen in the ACRs by assuming that pickup ion scattering mean free paths, at the termination shock, are much less than an AU and that modestly strong cross-field diffusion occurs. Simultaneously, we match the flux ratios He(+)/H(+) or O(+)/H(+) to within a factor approx. 5. If the conditions of strong scattering apply, no pre-termination-shock injection phase is required and the injection and acceleration of pickup ions at the termination shock is totally analogous to the injection and acceleration of ions at highly oblique interplanetary shocks recently observed by the Ulysses spacecraft. The fact that ACR fluxes can be modeled with standard shock assumptions suggests that the much-discussed "injection problem" for highly oblique shocks stems from incomplete (either mathematical or computer) modeling of these shocks rather than from any actual difficulty shocks may have in injecting and accelerating thermal or quasi-thermal particles.

Ellison, Donald C.↗
NASA NTRS2006

Multi-Spacecraft Observations of Interplanetary Shock Accelerated Particle Events

We use simultaneous measurements from the Wind and ACE spacecraft to determine the spatial properties of both interplanetary (IP) shocks and the shock-associated energetic particle events. We combine plasma, magnetic field and energetic particle data from ACE and Wind for 124 energetic storm particle (ESP) events from 1998 to 2003 and examine the spatial and temporal variations of these events in the Earth's vicinity. We find that even though the two spacecraft were occasionally separated by more than 400 RE, the plasma, field, and energetic particle time-intensity profiles during the events were very similar. In addition, we find that the ion composition and energy spectra in individual IP shock events are identical at the two spacecraft locations. We also use the fitted shock velocity along the normal from ACE and estimate the shock transit time to Wind location. In general, there is poor agreement between the estimated transit time and the actual measured transit time. Hence, our assumptions that a) the IP shock at 1 AU propagates radially, and/or b) the IP shock is spherically symmetric at 1 AU are not valid. In this paper, we will also study, for the first time, the anisotropy measurements of low-energy IP shock-associated ions at both ACE and Wind. We will then compare these new anisotropy analyses with locally measured shock parameters and identify possible signatures of different shock acceleration processes as predicted by the first-order Fermi and shock-drift models.

Ho, G. C.↗
NASA NTRS2009

One-Dimensional Scanning Approach to Shock Sensing

Measurement tools for high speed air flow are sought both in industry and academia. Particular interest is shown in air flows that exhibit aerodynamic shocks. Shocks are accompanied by sudden changes in density, pressure, and temperature. Optical detection and characterization of such shocks can be difficult because the medium is normally transparent air. A variety of techniques to analyze these flows are available, but they often require large windows and optical components as in the case of Schlieren measurements and/or large operating powers which precludes their use for in-flight monitoring and applications. The one-dimensional scanning approach in this work is a compact low power technique that can be used to non-intrusively detect shocks. The shock is detected by analyzing the optical pattern generated by a small diameter laser beam as it passes through the shock. The optical properties of a shock result in diffraction and spreading of the beam as well as interference fringes. To investigate the feasibility of this technique a shock is simulated by a 426 m diameter optical fiber. Analysis of results revealed a direct correlation between the optical fiber or shock location and the beam s diffraction pattern. A plot of the width of the diffraction pattern vs. optical fiber location reveals that the width of the diffraction pattern was maximized when the laser beam is directed at the center of the optical fiber. This work indicates that the one-dimensional scanning approach may be able to determine the location of an actual shock. Near and far field effects associated with a small diameter laser beam striking an optical fiber used as a simulated shock are investigated allowing a proper one-dimensional scanning beam technique.

Tokars, Roger↗
NASA NTRS2012

X-Ray Emission from Supernovae in Dense Circumstellar Matter Environments: A Search for Collisionless Shock

The optical light curve of some supernovae (SNe) may be powered by the outward diffusion of the energy deposited by the explosion shock (so-called shock breakout) in optically thick (tau approx > 30) circumstellar matter (CSM). Recently, it was shown that the radiation-mediated and -dominated shock in an optically thick wind must transform into 8. collisionless shock and can produce hard X-rays. The X-rays are expected to peak at late times, relative to maximum visible light. Here we report on a search, using Swift-XRT and Chandra, for X-ray emission from 28 SNe that belong to classes whose progenitors are suspected to be embedded in dense CSM. Our sample includes 19 type-IIn SNe, one type-Ibn SN and ei~ht hydrogen-poor super-luminous SNe (SLSN-I; SN 2005ap like). Two SNe (SN 2006jc and SN 2010jl) have X-ray properties that are roughly consistent with the expectation for X-rays from a collisionless shock in optically thick CSl\l. Therefore, we suggest that their optical light curves are powered by shock breakout in CSM. We show that two other events (SN 2010al and SN 2011ht) were too X-ray bright during the SN maximum optical light to be explained by the shock breakout model. We conclude that the light curves of some, but not all, type-IIn/Ibn SNe are powered by shock breakout in CSM. For the rest of the SNe in our sample, including all the SLSN-I events, our X-ray limits are not deep enough and were typically obtained at too early times (i.e., near the SN maximum light) to conclude about their nature. Late time X-ray observations are required in order to further test if these SNe are indeed embedded in dense CSM. We review the conditions required for a shock breakOut in a wind profile. We argue that the time scale, relative to maximum light, for the SN to peak in X-rays is a probe of the column density and the density profile above the shock region. The optical light curves of SNe, for which the X-ray emission peaks at late times, are likely powered by the diffusion of shock energy from a dense CSM. We note that if the CSM density profile falls faster than a constant-rate wind density profile, then X-rays may escape at earlier times than estimated for the wind profile case. Furthermore, if the CSM have a region in which the density profile is very steep, relative to a steady wind density profile, or the CSM is neutral, then the radio free-free absorption may be low enough, and radio emission may be detected.

Ofek, E.O↗
NASA NTRS2013

X-ray Emission from Supernovae in Dense Circumstellar Matter Environments: a Search for Collisionless Shocks

The optical light curve of some supernovae (SNe) may be powered by the outward diffusion of the energy deposited by the explosion shock (the so-called shock breakout) in optically thick (Tau approx > 30) circumstellar matter (CSM). Recently, it was shown that the radiation-mediated and radiation-dominated shock in an optically thick wind must transform into a collisionless shock and can produce hard X-rays. The X-rays are expected to peak at late times, relative to maximum visible light. Here we report on a search, using Swift/XRT and Chandra, for X-ray emission from 28 SNe that belong to classes whose progenitors are suspected to be embedded in dense CSM. Our sample includes 19 Type IIn SNe, one Type Ibn SN, and eight hydrogen-poor superluminous SNe (SLSN-I such as SN 2005ap). Two SNe (SN 2006jc and SN 2010jl) have X-ray properties that are roughly consistent with the expectation for X-rays from a collisionless shock in optically thick CSM. However, the X-ray emission from SN 2006jc can also be explained as originating in an optically thin region. Thus, we propose that the optical light curve of SN 2010jl is powered by shock breakout in CSM. We suggest that two other events (SN 2010al and SN 2011ht) were too X-ray bright during the SN maximum optical light to be explained by the shock-breakout model.We conclude that the light curves of some, but not all, SNe IIn/Ibn are powered by shock breakout in CSM. For the rest of the SNe in our sample, including all of the SLSN-I events, our X-ray limits are not deep enough and were typically obtained too early (i.e., near the SN maximum light) for definitive conclusions about their nature. Late-time X-ray observations are required in order to further test whether these SNe are indeed embedded in dense CSM. We review the conditions required for a shock breakout in a wind profile. We argue that the timescale, relative to maximum light, for the SN to peak in X-rays is a probe of the column density and the density profile above the shock region. In SNe whose X-ray emission slowly rises, and peaks at late times, the optical light curve is likely powered by the diffusion of shock energy in a dense CSM. We note that if the CSM density profile falls faster than a constant-rate wind-density profile, then X-rays may escape at earlier times than estimated for the wind-profile case. Furthermore, if the CSM has a region in which the density profile is very steep relative to a steady wind-density profile, or if the CSM is neutral, then the radio free-free absorption may be sufficiently low for radio emission to be detected.

matter↗
NASA NTRS1960

Shock Losses in Transonic Compressor Blade Rows

The rather extensive study of the shock losses in transonic compressors can he summarized by the following remarks: 1. A simple flow model can be used to estimate shock losses at the design point for transonic compressor blade rows and results iii reasonable correlation of loss data. It is indicated that shock losses can constitute a sizable portion of the total losses in it transonic compressor rotor. This includes all blade elements at which sonic or higher relative velocities are obtained. 2. Shock losses can he shown to exist across the blade passage (free-stream loss) and by the method of superposition with the blade profile losses result in an estimated design total loss coefficient. 3. The shock configuration was experimentally determined by the rapid pressure rise between the blades as measured by the use of barium titanate crystals. At the minimum loss operating conditions the shock is very similar to that assumed in the simple How model. 4. Shock losses obtained from a more detailed flow model were compared with the losses obtained by the simple flow model. Measured loss distribution from blade to blade closely approaches the analytical shock loss distribution. The measured distribution shows the effect of a shock boundary layer interaction. 5. The analytical method (from the detailed flow model) of determining the shock location ahead of the blade seems to apply reasonably well over a range of incidence angles. The analytical shock losses do not vary a great deal with blade element incidence angles.

Miller, Genevieve R.↗
NASA NTRS2017

Measuring Shock Stage of ltokawa Regolith Grains by Electron Back-Scattered Diffraction and Synchrotron X-Ray Diffraction

We have been analyzing Itokawa samples in order to definitively establish the degree of shock experienced by the regolith of asteroid Itokawa, and to devise a bridge between shock determinations by standard light optical petrography, crystal structures as determined by electron and X-ray diffraction techniques. We are making measurements of olivine crystal structures and using these to elucidate critical regolith impact processes. We use electron back-scattered diffraction (EBSD) and synchrotron X-ray diffraction (SXRD). We are comparing the Itokawa samples to L and LL chondrite meteorites chosen to span the shock scale experienced by Itokawa, specifically Chainpur (LL3.4, Shock Stage 1), Semarkona (LL3.00, S2), Kilabo (LL6, S3), NWA100 (L6, S4) and Chelyabinsk (LL5, S4). In SXRD we measure the line broadening of olivine reflections as a measure of shock stage. In this presentation we concentrate on the EBSD work. We employed JSC's Supra 55 variable pressure FEG-SEM and Bruker EBSD system. We are not seeking actual strain values, but rather indirect strain-related measurements such as extent of intra-grain lattice rotation, and determining whether shock state "standards" (meteorite samples of accepted shock state, and appropriate small grain size) show strain measurements that may be statistically differentiated, using a sampling of particles (number and size range) typical of asteroid regoliths. Using our system we determined that a column pressure of 9 Pa and no C-coating on the sample was optimal. We varied camera exposure time and gain to optimize mapping performance, concluding that 320x240 pattern pixilation, frame averaging of 3, 15 kV, and low extractor voltage yielded an acceptable balance of hit rate (>90%), speed (11 fps) and map quality using an exposure time of 30 ms (gain 650). We found that there was no strong effect of step size on Grain Orientation Spread (GOS) and Grain Reference Orientation Deviation angle (GROD-a) distribution; there was some effect on grain average Kernel Average Misorientation (KAM) (reduced with smaller step size for the same grain), as expected. We monitored GOS, Maximum Orientation Spread (MOS) and GROD-a differences between whole olivine grains and sub-sampled areas, and found that there were significant differences between the whole grain dataset and subsets, as well as between subsets, likely due to sampling-related "noise". Also, in general (and logically) whole grains exhibit greater degrees of cumulative lattice rotation. Sampling size affects the apparent strain character of the grain, at least as measured by GOS, MOS and GROD-a. There were differences in the distribution frequencies of GOS and MOS between shock stages, and in plots of MOS and GOS vs. grain diameter. These results are generally consistent with those reported this year. However, it is unknown whether the differences between samples of different shock states exceeds the clustering of these values to the extent that shock stage determinations can still be made with confidence. We are investigating this by examination of meteorites with higher shock stage 4 to 5. Our research will improve our understanding of how small, primitive solar system bodies formed and evolved, and improve understanding of the processes that determine the history and future of habitability of environments on other solar system bodies. The results will directly enrich the ongoing asteroid and comet exploration missions by NASA and JAXA, and broaden our understanding of the origin and evolution of small bodies in the early solar system, and elucidate the nature of asteroid and comet regolith.

Zolensky, Michael↗
NASA NTRS2019

An ALMA+ACA measurement of the shock in the Bullet Cluster

Context: The thermal Sunyaev-Zeldovich (SZ) effect presents a relatively new tool for characterizing galaxy cluster merger shocks, traditionally studied through X-ray observations. Widely regarded as the “textbook example” of a cluster merger bow shock, the western, most-prominent shock front in the Bullet Cluster (1E0657-56) represents the ideal test case for such an SZ study. Aims: We aim to characterize the shock properties using deep, high-resolution interferometric SZ effect observations in combination with priors from an independent X-ray analysis. Methods: Our analysis technique relies on the reconstruction of a parametric model for the SZ signal by directly and jointly fitting data from the Atacama Large Millimeter/submillimeter Array (ALMA) and Atacama Compact Array (ACA) in Fourier space. Results: The ALMA+ACA data are primarily sensitive to the electron pressure difference across the shock front. To estimate the shock Mach number M, this difference can be combined with the value for the upstream electron pressure derived from an independent Chandra X-ray analysis. In the case of instantaneous electron-ion temperature equilibration, we find M = 2.08 (+0.12,−0.12), in ≈ 2.4σ tension with the independent constraint from Chandra, M(X)= 2.74 ± 0.25. The assumption of purely adiabatic electron temperature change across the shock leads to M = 2.53 (+0.33,−0.25), in better agreement with the X-ray estimate M(X) = 2.57 ± 0.23 derived for the same heating scenario. Conclusion: We have demonstrated that interferometric observations of the thermal SZ effect provide constraints on the properties of the shock in the Bullet Cluster that are highly complementary to X-ray observations. The combination of X-ray and SZ data yields a powerful probe of the shock properties, capable of measuring M and addressing the question of electron-ion equilibration in cluster shocks. Our analysis is however limited by systematics related to the overall cluster geometry and the complexity of the post-shock gas distribution. To overcome these limitations, a simultaneous, joint-likelihood analysis of SZ and X-ray data is needed.

Luca Di Mascolo↗
NASA NTRS2022

Snapping Shrimp Have Helmets That Protect Their Brains By Dampening Shock Waves

Shock waves are supersonic high-amplitude pressure waves that cause barotrauma when they transfer kinetic energy to the tissues of animals.1–4 Snapping shrimp (Alpheidae) produce shock waves and are exposed to them frequently, so we asked if these animals have evolved mechanisms of physical protection against them. Snapping shrimp generate shock waves by closing their snapping claws rapidly enough to form cavitation bubbles that release energy as an audible ‘‘snap’’ and a shock wave when they collapse.5–8 We tested if snapping shrimp are protected from shock waves by a helmet-like extension of their exoskeleton termed the orbital hood. Using behavioral trials, we found shock wave exposure slowed shelterseeking and caused a loss of motor control in Alpheus heterochaelis from which we had removed orbital hoods but did not significantly affect behavior in shrimp with unaltered orbital hoods. Shock waves thus have the potential to harm snapping shrimp but may not do so under natural conditions because of protection provided to shrimp by their orbital hoods. Using pressure recordings, we discovered the orbital hoods of A. heterochaelis dampen shock waves. Sealing the anterior openings of orbital hoods diminished how much they altered the magnitudes of shock waves, which suggests these helmet-like structures dampen shock waves by trapping and expelling water so that kinetic energy is redirected and released away from the heads of shrimp. Our results indicate orbital hoods mitigate blast-induced neurotrauma in snapping shrimp by dampening shock waves, making them the first biological armor system known to have such a function.

Alexandra C.N. Kingston↗
DOE OSTI2024

Shock equation of state experiments in MgO up to 1.5 TPa and the effects of optical depth on temperature determination

Laser-driven shock compression enables an experimental study of phase transitions at unprecedented pressures and temperatures. One example is the shock Hugoniot of magnesium oxide (MgO), which crosses the B1–B2-liquid triple point at 400–600 GPa, 10 000–13 000 K (0.86–1.12 eV). MgO is a major component within the mantles of terrestrial planets and has long been a focus of high-pressure research. Here, we combine time-resolved velocimetry and pyrometry measurements with a decaying shock platform to obtain pressure–temperature data on MgO from 300 to 1500 GPa and 9000 to 50 000 K. Pressure–temperature–density Hugoniot data are reported at 1500 GPa. These data represent the near-instantaneous response of an MgO [100] single crystal to shock compression. We report on a prominent temperature anomaly between 400 and 460 GPa, in general agreement with previous shock studies, and draw comparison with equation-of-state models. We provide a detailed analysis of the decaying shock compression platform, including a treatment of a pressure-dependent optical depth near the shock front. We show that if the optical depth of the shocked material is larger than 1 μm, treating the shock front as an optically thick gray body will lead to a noticeable overestimation of the shock temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Amplification of turbulence through multiple planar shocks

We study the amplification of isotropic, incompressible turbulence through multiple planar, collisional shocks, using analytical linear theory. There are two limiting cases we explore. The first assumes shocks occur rapidly in time such that the turbulence does not evolve between shocks. Whereas the second case allows enough time for turbulence to isotropize between each shock. For the latter case, through a quasi-equation-of-state, we show that the weak multishock limit is agnostic to the distinction between thermal and vortical turbulent pressures, like an isotropic volumetric compression. When turbulence does not return to isotropy between shocks, the generated anisotropy—itself a function of shock strength—can feedback on amplification by further shocks, altering choices for maximal or minimal amplification. In addition for this case, we find that amplification is sensitive to the shock ordering. As a result, we map how choices of shock strength can impact these amplification differences due to ordering, finding, for example, shock pairs which lead to identical mean postshock fields (density, temperature, pressure) but maximally distinct turbulent amplification.

Astrophysical fluid dynamics↗
DOE OSTI2025

Planar Collisionless Shock Simulations with the Semi-implicit Particle-in-cell Model FLEKS

This study investigates the applicability of the semi-implicit particle-in-cell code FLexible Exascale Kinetic Simulator (FLEKS) to heliospheric shock simulations. We examine one- and two-dimensional local planar shock simulations, initialized using MHD states with upstream conditions representative of plasmas in the hypersonic, β ∼ 1 regime, for both quasi-perpendicular and quasi-parallel configurations. The refined algorithm in FLEKS proves robust, enabling accurate shock simulations with a grid resolution on the order of the electron inertial length d e . Our simulations successfully capture key shock features, including shock structures (foot, ramp, overshoot, and undershoot), upstream and downstream waves (fast magnetosonic, whistler, Alfvén ion-cyclotron, and mirror modes), and non-Maxwellian particle distributions. Crucially, we find that at least two spatial dimensions are critical for accurately reproducing downstream-wave physics in quasi-perpendicular shocks and capturing the complex dynamics of quasi-parallel shocks, including surface rippling, shocklets, short, large-amplitude magnetic structures, magnetic reconnection, and jets. Furthermore, our parameter studies demonstrate the impact of mass ratio and grid resolution on shock physics. This work provides valuable guidance for selecting appropriate physical and numerical parameters for shock simulations using a semi-implicit PIC method, paving the way for incorporating kinetic shock processes into large-scale collisionless plasma simulations with the MHD-AEPIC model.

plasma astrophysics↗
NASA NTRS2023

Multi-Spacecraft Observations of Shocklets at an Interplanetary Shock

Interplanetary (IP) shocks are fundamental building blocks of the heliosphere, and the possibility to observe them in situ is crucial to address important aspects of energy conversion for a variety of astrophysical systems. Steepened waves known as shocklets are known to be important structures of planetary bow shocks, but they are very rarely observed related to IP shocks. We present here the first multi-spacecraft observations of shocklets observed by upstream of an unusually strong IP shock observed on 3 No v ember 2021 by several spacecraft at L1 and near-Earth solar wind. The same shock was detected also by radially aligned Solar Orbiter at 0.8 AU from the Sun, but no shocklets were identified from its data, introducing the possibility to study the environment in which shocklets developed. The Wind spacecraft has been used to characterize the shocklets, associated with pre-conditioning of the shock upstream by decelerating incoming plasma in the shock normal direction. Finally, using the Wind observations together with ACE and DSCOVR spacecraft at L1, as well as THEMIS B and THEMIS C in the near-Earth solar wind, the portion of interplanetary space filled with shocklets is addressed, and a lower limit for its extent is estimated to be of about 110 R E in the shock normal direction and 25 R E in the directions transverse to the shock normal. Using multiple spacecraft also reveals that for this strong IP shock, shocklets are observed for a large range of local obliquity estimates (9° –64°).

plasmas↗