Showing papers on "Shock wave published in 2013"

The Two Sources of Solar Energetic Particles

Abstract: Evidence for two different physical mechanisms for acceleration of solar energetic particles (SEPs) arose 50 years ago with radio observations of type III bursts, produced by outward streaming electrons, and type II bursts from coronal and interplanetary shock waves. Since that time we have found that the former are related to “impulsive” SEP events from impulsive flares or jets. Here, resonant stochastic acceleration, related to magnetic reconnection involving open field lines, produces not only electrons but 1000-fold enhancements of 3He/4He and of (Z>50)/O. Alternatively, in “gradual” SEP events, shock waves, driven out from the Sun by coronal mass ejections (CMEs), more democratically sample ion abundances that are even used to measure the coronal abundances of the elements. Gradual events produce by far the highest SEP intensities near Earth. Sometimes residual impulsive suprathermal ions contribute to the seed population for shock acceleration, complicating the abundance picture, but this process has now been modeled theoretically. Initially, impulsive events define a point source on the Sun, selectively filling few magnetic flux tubes, while gradual events show extensive acceleration that can fill half of the inner heliosphere, beginning when the shock reaches ∼2 solar radii. Shock acceleration occurs as ions are scattered back and forth across the shock by resonant Alfven waves amplified by the accelerated protons themselves as they stream away. These waves also can produce a streaming-limited maximum SEP intensity and plateau region upstream of the shock. Behind the shock lies the large expanse of the “reservoir”, a spatially extensive trapped volume of uniform SEP intensities with invariant energy-spectral shapes where overall intensities decrease with time as the enclosing “magnetic bottle” expands adiabatically. These reservoirs now explain the slow intensity decrease that defines gradual events and was once erroneously attributed solely to slow outward diffusion of the particles. At times the reservoir from one event can contribute its abundances and even its spectra as a seed population for acceleration by a second CME-driven shock wave. Confinement of particles to magnetic flux tubes that thread their source early in events is balanced at late times by slow velocity-dependent migration through a tangled network produced by field-line random walk that is probed by SEPs from both impulsive and gradual events and even by anomalous cosmic rays from the outer heliosphere. As a practical consequence, high-energy protons from gradual SEP events can be a significant radiation hazard to astronauts and equipment in space and to the passengers of high-altitude aircraft flying polar routes.

The maximum energy of accelerated particles in relativistic collisionless shocks

Abstract: The afterglow emission from gamma-ray bursts (GRBs) is usually interpreted as synchrotron radiation from electrons accelerated at the GRB external shock that propagates with relativistic velocities into the magnetized interstellar medium. By means of multi-dimensional particle-in-cell simulations, we investigate the acceleration performance of weakly magnetized relativistic shocks, in the magnetization range 0 ? 10?1. The pre-shock magnetic field is orthogonal to the flow, as generically expected for relativistic shocks. We find that relativistic perpendicular shocks propagating in electron-positron plasmas are efficient particle accelerators if the magnetization is ? 10?3. For electron-ion plasmas, the transition to efficient acceleration occurs for ? 3 ? 10?5. Here, the acceleration process proceeds similarly for the two species, since the electrons enter the shock nearly in equipartition with the ions, as a result of strong pre-heating in the self-generated upstream turbulence. In both electron-positron and electron-ion shocks, we find that the maximum energy of the accelerated particles scales in time as ?maxt 1/2. This scaling is shallower than the so-called (and commonly assumed) Bohm limit ?maxt, and it naturally results from the small-scale nature of the Weibel turbulence generated in the shock layer. In magnetized plasmas, the energy of the accelerated particles increases until it reaches a saturation value ?sat/?0 mic 2 ~ ??1/4, where ?0 mic 2 is the mean energy per particle in the upstream bulk flow. Further energization is prevented by the fact that the self-generated turbulence is confined within a finite region of thickness ??1/2 around the shock. Our results can provide physically grounded inputs for models of non-thermal emission from a variety of astrophysical sources, with particular relevance to GRB afterglows.

A quarter century of collisionless shock research

Abstract: This review highlights conceptual issues that have both governed and reflected the direction of collisionless shock research in the past quarter century. These include MHD waves and their steepening, the MHD Rankine-Hugoniot relations, the super-critical shock transition, nonlinear oscillatory wave trains, ion sound anomalous resistivity and the resistive-dispersive transition for subcritical shocks, ion reflection and the structure of supercritical quasi-perpendicular shocks, the earth's foreshock, quasi-parallel shocks, and finally, shock acceleration processes.

Shock-Wave Phenomena and the Properties of Condensed Matter

Abstract: 1 Introduction to the Theoretical Background and Experimental Methods of Shock Physics.- 2 Elastic-Plastic Response of Solids Under Shock-Wave Loading.- 3 Yield and Strength Properties of Metals and Alloys at Elevated Temperatures.- Chapter4 Behavior of Brittle Materials under Shock-Wave Loading.- 5 Two Examples of Spatially Resolved Shock-Wave Tests.- 6 Polymorphic Transformations and Phase Transitions in Shock-Compressed Solids.- 7 Equations of State and Macrokinetics of Decomposition of Solid Explosives in Shock and Detonation Waves.- 8 Shock Waves and Extreme States of Matter.

Shock-induced plasticity in tantalum single crystals: Interatomic potentials and large-scale molecular-dynamics simulations

Abstract: We report on large-scale nonequilibrium molecular dynamics simulations of shock wave compression in tantalum single crystals. Two new embedded atom method interatomic potentials of Ta have been developed and optimized by fitting to experimental and density functional theory data. The potentials reproduce the isothermal equation of state of Ta up to 300 GPa. We examined the nature of the plastic deformation and elastic limits as functions of crystal orientation. Shock waves along (100), (110), and (111) exhibit elastic-plastic two-wave structures. Plastic deformation in shock compression along (110) is due primarily to the formation of twins that nucleate at the shock front. The strain-rate dependence of the flow stress is found to be orientation dependent, with (110) shocks exhibiting the weaker dependence. Premelting at a temperature much below that of thermodynamic melting at the shock front is observed in all three directions for shock pressures above about 180 GPa.

General-relativistic simulations of three-dimensional core-collapse supernovae

Abstract: We study the three-dimensional (3D) hydrodynamics of the post-core-bounce phase of the collapse of a 27 M_☉ star and pay special attention to the development of the standing accretion shock instability (SASI) and neutrino-driven convection. To this end, we perform 3D general-relativistic simulations with a three-species neutrino leakage scheme. The leakage scheme captures the essential aspects of neutrino cooling, heating, and lepton number exchange as predicted by radiation-hydrodynamics simulations. The 27 M_☉ progenitor was studied in 2D by Muller et al., who observed strong growth of the SASI while neutrino-driven convection was suppressed. In our 3D simulations, neutrino-driven convection grows from numerical perturbations imposed by our Cartesian grid. It becomes the dominant instability and leads to large-scale non-oscillatory deformations of the shock front. These will result in strongly aspherical explosions without the need for large-scale SASI shock oscillations. Low-l-mode SASI oscillations are present in our models, but saturate at small amplitudes that decrease with increasing neutrino heating and vigor of convection. Our results, in agreement with simpler 3D Newtonian simulations, suggest that once neutrino-driven convection is started, it is likely to become the dominant instability in 3D. Whether it is the primary instability after bounce will ultimately depend on the physical seed perturbations present in the cores of massive stars. The gravitational wave signal, which we extract and analyze for the first time from 3D general-relativistic models, will serve as an observational probe of the postbounce dynamics and, in combination with neutrinos, may allow us to determine the primary hydrodynamic instability.

Three-dimensional magnetohydrodynamic simulations of the Crab nebula

Abstract: In this paper, we give a detailed account of the first three-dimensional (3D) relativistic magnetohydrodynamic simulations of pulsar wind nebulae, with parameters most suitable for the Crab nebula. In contrast to the previous 2D simulations, we also consider pulsar winds with much stronger magnetization, up to σ ≃ few. The 3D models preserve the separation of the post-termination shock flow into the equatorial and polar components, but the polar jets are disrupted by the kink mode of the current driven instability and 'dissolve' into the main body of the nebula after propagation of several shock radii. With the exception of the region near the termination shock, the 3D models do not exhibit the strong z-pinch configuration characteristic of the 1D and 2D models. Contrary to the expectations based on 1D analytical and semi-analytical models, the 3D solutions with highly magnetized pulsar winds still produce termination shocks with radii comparable to those deduced from the observations. The reason for this is not only the randomization of magnetic field observed in the 3D solutions, but also the magnetic dissipation inside the nebula. Assuming that the particle acceleration occurs only at the termination shock, we produced synthetic maps of the Crab nebula synchrotron emission. These maps retain most of the features revealed in the previous 2D simulations, including thin wisps and the inner knot. The polarization and variability of the inner knot is in a particularly good agreement with the observations of the Crab nebula and the overall polarization of the inner nebula is also reproduced quite well. However, the polar jet is not as bright as observed, suggesting that an additional particle acceleration, presumably related to the magnetic dissipation, has to be invoked.

Heliospheric structure: The bow wave and the hydrogen wall

Abstract: Recent IBEX observations indicate that the local interstellar medium (LISM) flow speed is less than previously thought (23.2 km s–1 rather than 26 km s–1). Reasonable LISM plasma parameters indicate that the LISM flow may be either marginally super-fast magnetosonic or sub-fast magnetosonic. This raises two challenging questions: (1) Can a LISM model that is barely super-fast or sub-fast magnetosonic account for Lyα observations that rely critically on the additional absorption provided by the hydrogen wall (H-wall)? and (2) If the LISM flow is weakly super-fast magnetosonic, does the transition assume the form of a traditional shock or does neutral hydrogen (H) mediate shock dissipation and hence structure through charge exchange? Both questions are addressed using three three-dimensional self-consistently coupled magnetohydrodynamic plasma—kinetic H models with different LISM magnetic field strengths (2, 3, and 4 μG) as well as plasma and neutral H number densities. The 2 and 3 μG models are fast magnetosonic far upwind of the heliopause whereas the 4 μG model is fully subsonic. The 2 μG model admits a broad (~50-75 AU) bow-shock-like structure. The 3 μG model has a smooth super-fast-sub-fast magnetosonic transition that resembles a very broad, ~200 AU thick, bow wave. A theoretical analysis shows that the transition from a super-fast to a sub-fast magnetosonic downstream state is due to the charge exchange of fast neutral H and hot neutral H created in the supersonic solar wind and hot inner heliosheath, respectively. For both the 2 μG and the 3 μG models, the super-fast magnetosonic LISM flow passes through a critical point located where the fast magnetosonic Mach number M = 1 and Qe = γ/(γ – 1)UQm , where Qe and Qm are the plasma energy and momentum source terms due to charge exchange, U is the LISM flow speed, and γ is the plasma adiabatic index. Because the Mach number is only barely super-fast magnetosonic in the 3 μG case, the hot and fast neutral H can completely mediate the transition and impose a charge exchange length scale on the structure, making the solar-wind-LISM interaction effectively bow-shock-free. The charge exchange of fast and hot heliospheric neutral H therefore provides a primary dissipation mechanism at the weak heliospheric bow shock, in some cases effectively creating a one-shock heliosphere (i.e., a heliospheric termination shock only). Both super-fast magnetosonic models produce a sizeable H-wall. We find that (1) a sub-fast magnetosonic LISM flow cannot model the observed Lyα absorption profiles along the four sightlines considered (α Cen, 36 Oph, DK UMa, and χ1 Ori—upwind, sidewind, and downwind respectively); (2) both the super-fast magnetosonic models can account for the Lyα observations, with possibly the bow-shock-free 3 μG model being slightly favored. Subject to further modeling and comparison against further lines of sight, we conclude with the tantalizing possibility that IBEX may have discovered a class of interstellar shocks mediated by neutral H.

Shock Wave Emission by Laser Generated Bubbles

Abstract: The phenomena occurring when short pulses of laser light are focused into a liquid are reviewed from the first findings after the invention of the laser to the present state of knowledge. Dielectric breakdown with plasma and bubble formation, the breakdown shock wave, bubble dynamics with expansion and collapse, and the bubble collapse shock wave or waves are addressed. Breakdown plasma lengths as a function of laser pulse energy are given. The propagation speed of the breakdown shock wave and the related shock peak pressure as determined by high speed streak recordings for nano-, pico-, and femtosecond laser pulses focused into water are discussed. Breakdown shock wave velocities up to 5000 m/s and peak pressures up to 100 kbar are reported for these monopolar acoustic pulses for laser pulse energies up to some 10 mJ. In tissue bipolar pressure pulses are observed due to the elasticity of the medium. The widths of the shock waves reach values in the range of tens of nano seconds to beyond 100 ns. The simultaneously generated breakdown bubble gets about half of the energy of the shock wave. Bubble energy rises linearly with the laser pulse energy with different slopes depending on the laser pulse duration. Equations for bubble dynamics are given and compared with laser induced bubble dynamics. Strength and width of bubble collapse shock waves measured with PVDF and fiber optic hydrophones are presented together with the breakdown shock waves. Similar values are obtained for both collapse and breakdown in the bulk of the liquid. Shock wave emission from bubbles collapsing near boundaries (solid and elastic) is discussed together with applications in cleaning and erosion or cell destruction.

Systematic X-Ray Analysis of Radio Relic Clusters with Suzaku

Abstract: We undertook a systematic X-ray analysis of six giant radio relics in four clusters of galaxies using the Suzaku satellite. The sample included CIZA2242.8+5301, Zwcl 2341.1 0000, the South-East part of A 3667 and previously published results of the North-West part of A 3667 and A 3376. Especially, we first observed the narrow (50 kpc) relic of CIZA2242.8+5301 by the Suzaku satellite, which enabled us to reduce the projection effect. We report on X-ray detections of shocks at the positions of the relics in CIZA2242.8+5301 and A 3667 SE. At the positions of the two relics in ZWCL2341.1 0000, we did not detect shocks. From spectroscopic temperature profiles across the relic, we found that the temperature profiles exhibit significant jumps across the relics for CIZA2242.8+5301, A 3376, A 3667NW, and A 3667 SE. We estimated the Mach number from the X-ray temperature or pressure profile using the Rankine-Hugoniot jump condition, and compared it with the Mach number derived from the radio spectral index. The resulting Mach numbers (M = 1.5–3) are almost consistent with each other, while the Mach number of CIZA2242.8+5301, derived from the X-ray data, tends to be lower than that of the radio observation. These results indicate that the giant radio relics in merging clusters are related to the shock structure, as suggested by previous studies of individual clusters.

Solution of the quasi-one-dimensional linearized Euler equations using flow invariants and the Magnus expansion

Abstract: The acoustic and entropy transfer functions of quasi-one-dimensional nozzles are studied analytically for both subsonic and choked flows with and without shock waves. The present analytical study extends both the compact nozzle solution obtained by Marble & Candel (J. Sound Vib., vol. 55, 1977, pp. 225–243) and the effective nozzle length proposed by Stow, Dowling & Hynes (J. Fluid Mech., vol. 467, 2002, pp. 215–239) and by Goh & Morgans (J. Sound Vib., vol. 330, 2011, pp. 5184–5198) to non-zero frequencies for both modulus and phase through an asymptotic expansion of the linearized Euler equations. It also extends the piecewise-linear approximation of the velocity profile in the nozzle proposed by Moase, Brear & Manzie (J. Fluid Mech., vol. 585, 2007, pp. 281–304) to any arbitrary profile or equivalently any nozzle geometry. The equations are written as a function of three variables, namely the dimensionless mass, total temperature and entropy fluctuations, yielding a first-order linear system of differential equations with varying coefficients, which is solved using the Magnus expansion. The solution shows that both the modulus and the phase of the transfer functions of the nozzle have a strong dependence on the frequency. This holds for both choked flows and subsonic converging–diverging nozzles. The method is used to compare two different nozzle geometries with the same inlet and outlet Mach numbers, showing that, even if the compact solution predicts no differences between the transfer functions of the two nozzles, significant differences are found at non-zero frequencies. A parametric study is finally performed to calculate the indirect to direct noise ratio for a model combustor, showing that this ratio decreases at higher frequencies.

Ion reflection, gyration, and dissipation at supercritical shocks

Abstract: Topics on collisionless plasmas are discussed and include: (1) Ion reflection is the dominant ion dissipation mechanism at nearly perpendicular, supercritical shocks. (2) An increasing fraction of the ions incident on a supercritical shock is reflected as the Mach number increases. The actual fraction reflected can be predicted using the Rankine Hugoniot conservation relations. (3) The effective temperature associated with the dispersion in velocity space associated with ion reflection accounts for a large fraction of the temperature rise observed across supercritical, quasi perpendicular shocks.

Investigation of oxygen dissociation and vibrational relaxation at temperatures 4000–10 800 K

Abstract: The oxygen absorbance was studied at wavelengths 200–270 nm in Schumann-Runge system behind the front of a strong shock wave. Using these data, the vibrational temperature Tv behind the front of shock waves was measured at temperatures 4000–10 800 K in undiluted oxygen. Determination of Tv was based on the measurements of time histories of absorbance for two wavelengths behind the shock front and on the results of detail calculations of oxygen absorption spectrum. Solving the system of standard quasi-one-dimensional gas dynamics equations and using the measured vibrational temperature, the time evolution of oxygen concentration and other gas parameters in each experiment were calculated. Based on these data, the oxygen dissociation rate constants were obtained for thermal equilibrium and thermal non-equilibrium conditions. Furthermore, the oxygen vibrational relaxation time was also determined at high temperatures. Using the experimental data, various theoretical and empirical models of high-temperature d...

Reynolds- and Mach-number effects in canonical shock–turbulence interaction

Abstract: The interaction between isotropic turbulence and a normal shock wave is investigated through a series of direct numerical simulations at different Reynolds numbers and mean and turbulent Mach numbers. The computed data are compared to experiments and linear theory, showing that the amplification of turbulence kinetic energy across a shock wave is described well using linearized dynamics. The post-shock anisotropy of the turbulence, however, is qualitatively different from that predicted by linear analysis. The jumps in mean density and pressure are lower than the non-turbulent Rankine–Hugoniot results by a factor of the square of the turbulence intensity. It is shown that the dissipative scales of turbulence return to isotropy within about 10 convected Kolmogorov time scales, a distance that becomes very small at high Reynolds numbers. Special attention is paid to the ‘broken shock’ regime of intense turbulence, where the shock can be locally replaced by smooth compressions. Grid convergence of the probability density function of the shock jumps proves that this effect is physical, and not an artefact of the numerical scheme.

THREE-DIMENSIONAL GAS DYNAMIC SIMULATION OF THE INTERACTION BETWEEN THE EXOPLANET WASP-12b AND ITS HOST STAR

Abstract: Hubble Space Telescope transit observations in the near-UV performed in 2009 made WASP-12b one of the most "mysterious" exoplanets; the system presents an early ingress, which can be explained by the presence of optically thick matter located ahead of the planet at a distance of ~4-5 planet radii. This work follows previous attempts to explain this asymmetry with an exospheric outflow or a bow shock, induced by a planetary magnetic field, and provides a numerical solution of the early ingress, though we did not perform any radiative transfer calculation. We performed pure 3D gas dynamic simulations of the plasma interaction between WASP-12b and its host star and describe the flow pattern in the system. In particular, we show that the overfilling of the planet's Roche lobe leads to a noticeable outflow from the upper atmosphere in the direction of the and points. Due to the conservation of the angular momentum, the flow to the point is deflected in the direction of the planet's orbital motion, while the flow toward is deflected in the opposite direction, resulting in a non-axisymmetric envelope, surrounding the planet. The supersonic motion of the planet inside the stellar wind leads to the formation of a bow shock with a complex shape. The existence of the bow shock slows down the outflow through the and points, allowing us to consider a long-living flow structure that is in the steady state.

Low-velocity shocks: signatures of turbulent dissipation in diffuse irradiated gas

Abstract: Context. Large-scale motions in galaxies (supernovae explosions, galaxy collisions, galactic shear etc.) generate turbulence, which allows a fraction of the available kinetic energy to cascade down to small scales before it is dissipated. Aims. We establish and quantify the diagnostics of turbulent dissipation in mildly irradiated diffuse gas in the specific context of shock structures. Methods. We incorporated the basic physics of photon-dominated regions into a state-of-the-art steady-state shock code. We examined the chemical and emission properties of mildly irradiated (G_0 = 1) magnetised shocks in diffuse media (n_H = 10^2 to 10^4 cm^(-3)) at low- to moderate velocities (from 3 to 40 km s^(-1)). Results. The formation of some molecules relies on endoergic reactions. Their abundances in J-type shocks are enhanced by several orders of magnitude for shock velocities as low as 7 km s^(-1). Otherwise most chemical properties of J-type shocks vary over less than an order of magnitude between velocities from about 7 to about 30 km s^(-1), where H_2 dissociation sets in. C-type shocks display a more gradual molecular enhancement with increasing shock velocity. We quantified the energy flux budget (fluxes of kinetic, radiated and magnetic energies) with emphasis on the main cooling lines of the cold interstellar medium. Their sensitivity to shock velocity is such that it allows observations to constrain statistical distributions of shock velocities. We fitted various probability distribution functions (PDFs) of shock velocities to spectroscopic observations of the galaxy-wide shock in Stephan’s Quintet and of a Galactic line of sight which samples diffuse molecular gas in Chamaeleon. In both cases, low velocities bear the greatest statistical weight and the PDF is consistent with a bimodal distribution. In the very low velocity shocks (below 5 km s^(-1)), dissipation is due to ion-neutral friction and it powers H_2 low-energy transitions and atomic lines. In moderate velocity shocks (20 km s^(-1) and above), the dissipation is due to viscous heating and accounts for most of the molecular emission. In our interpretation a significant fraction of the gas in the line of sight is shocked (from 4% to 66%). For example, C^+ emission may trace shocks in UV irradiated gas where C^+ is the dominant carbon species. Conclusions. Low- and moderate velocity shocks are important in shaping the chemical composition and excitation state of the interstellar gas. This allows one to probe the statistical distribution of shock velocities in interstellar turbulence.

Transient fluid-combustion phenomena in a model scramjet

Abstract: An experimental and numerical investigation of the unsteady phenomena induced in a hydrogen-fuelled scramjet combustor under high-equivalence-ratio conditions is carried out, focusing on the processes leading up to unstart. The configuration for the study is the fuelled flow path of the HyShot II flight experiment. Experiments are performed in the HEG reflected-shock wind tunnel, and results are compared with those obtained from unsteady numerical simulations. High-speed schlieren and OH∗ chemiluminescence visualization, together with time-resolved surface pressure measurements, allow links to be drawn between the experimentally observed flow and combustion features. The transient flow structures signalling the onset of unstart are observed to take the form of an upstream-propagating shock train. Both the speed of propagation and the downstream location at which the shock train originates depend strongly on the equivalence ratio. The physical nature of the incipient shock system, however, appears to be similar for different equivalence ratios. Both experiments and computations indicate that the primary mechanism responsible for the transient behaviour is thermal choking, though localized boundary-layer separation is observed to accompany the shock system as it moves upstream. In the numerical simulations, the global choking behaviour is dictated by the limited region of maximum heat release around the shear layer between the injected hydrogen and the incoming air flow. This leads to the idea of ‘local’ thermal choking and results in a lower choking limit than is predicted by a simple integral analysis. Such localized choking makes it possible for new quasi-steady flow topologies to arise, and these are observed in both experiments and simulation. Finally, a quasi-unsteady one-dimensional analytical model is proposed to explain elements of the shock-propagation behaviour.

Cosmic-Ray-induced Filamentation Instability in Collisionless Shocks

Abstract: We used unprecedentedly large two-dimensional and three-dimensional hybrid (kinetic ions?fluid electrons) simulations of non-relativistic collisionless strong shocks in order to investigate the effects of self-consistently accelerated ions on the overall shock dynamics. The current driven by suprathermal particles streaming ahead of the shock excites modes transverse to the background magnetic field. The Lorentz force induced by these self-amplified fields tends to excavate tubular, underdense, magnetic-field-depleted cavities that are advected with the fluid and perturb the shock surface, triggering downstream turbulent motions. These motions further amplify the magnetic field, up to factors of 50-100 in knot-like structures. Once downstream, the cavities tend to be filled by hot plasma plumes that compress and stretch the magnetic fields in elongated filaments; this effect is particularly evident if the shock propagates parallel to the background field. Highly magnetized knots and filaments may provide explanations for the rapid X-ray variability observed in RX J1713.7?3946 and for the regular pattern of X-ray bright stripes detected in Tycho's supernova remnant.

Control of oblique shock wave/boundary layer interactions using plasma actuators

Abstract: Localized arc filament plasma actuators (LAFPAs) are used for shock wave/boundary layer interaction induced separation control in a Mach 2.3 flow. The boundary layer is fully turbulent with a Reynolds number based on the incompressible momentum thickness of 22,000 and shape factor of 1.37, and the impinging shock wave is generated by a 10° compression ramp. The LAFPAs are observed to have significant control authority over the interaction. The main effect is the displacement of the reflected shock and most of the interaction region upstream by approximately one boundary layer thickness (~5 mm). The initial goal of the control was to manipulate the low-frequency (St~0.03) unsteadiness associated with the interaction region. A detailed investigation of the effect of actuator placement, frequency, and duty cycle on the control authority indicates the actuators’ primary control mechanism is not the manipulation of low-frequency unsteadiness. Detailed measurements and analysis indicate that a modification to the boundary layer through heat addition by the actuators is the control mechanism, despite the extremely small power input of the actuators.

Catching the radio flare in CTA 102 - III. Core-shift and spectral analysis

Abstract: Context. The temporal and spatial spectral evolution of the jets of active galactic nuclei (AGN) can be studied with multi-frequency, multi-epoch very-long-baseline-interferometry (VLBI) observations. The combination of both morphological (kinematical) and spectral parameters can be used to derive source-intrinsic physical properties, such as the magnetic field and the nonthermal particle density. Such a study is of special interest during the high states of activity in AGNs, since VLBI observations can provide estimates of the location of the flaring site. Furthermore, we can trace the temporal variations in the source-intrinsic parameters during the flare, which may reflect the interaction between the underlying plasma and a traveling shock wave. The source CTA 102 exhibited such a radio flare around 2006.Aims. In the first two papers of this series (Papers I and II), we analyzed the single-dish light curves and the VLBI kinematics of the blazar CTA 102 and suggested a shock-shock interaction between a traveling and a standing shock wave as a possible scenario to explain the observed evolution of the component associated to the 2006 flare. In this paper we investigate the core shift and spectral evolution to test our hypothesis of a shock-shock interaction.Methods. We used eight multi-frequency Very Long Baseline Array (VLBA) observations to analyze the temporal and spatial evolution of the spectral parameters during the flare. We observed CTA 102 between May 2005 and April 2007 using the VLBA at six different frequencies spanning from 2 GHz up to 86 GHz. After the calibrated VLBA images were corrected for opacity, we performed a detailed spectral analysis. We developed methods for aligning the images and extracting the uncertainties in the spectral parameters. From the derived values we estimated the magnetic field and the density of the relativistic particles and combined those values with the kinematical changes provided from the long-term VLBA monitoring (Paper II) and single-dish measurements (Paper I).Results. The detailed analysis of the opacity shift reveals that the position of the jet core is proportional to ν -1 with some temporal variations. The value suggests possible equipartition between magnetic field energy and particle kinetic energy densities at the most compact regions. From the variation in the physical parameters we deduced that the 2006 flare in CTA 102 is connected to the ejection of a new traveling feature (t ej = 2005.9) and to the interaction between this shock wave and a stationary structure (interpreted as a recollimation shock) around 0.1 mas from the core (de-projected 18 pc at a viewing angle of ϑ = 2.6°). The source kinematics, together with the spectral and structural variations, can be described by helical motions in an overpressured jet.

Spontaneous Hot Flow Anomalies at Quasi-Parallel Shocks: 2. Hybrid Simulations

Abstract: Motivated by recent THEMIS observations, this paper uses 2.5-D electromagnetic hybrid simulations to investigate the formation of Spontaneous Hot Flow Anomalies (SHFA) upstream of quasi-parallel bow shocks during steady solar wind conditions and in the absence of discontinuities. The results show the formation of a large number of structures along and upstream of the quasi-parallel bow shock. Their outer edges exhibit density and magnetic field enhancements, while their cores exhibit drops in density, magnetic field, solar wind velocity and enhancements in ion temperature. Using virtual spacecraft in the simulation, we show that the signatures of these structures in the time series data are very similar to those of SHFAs seen in THEMIS data and conclude that they correspond to SHFAs. Examination of the simulation data shows that SHFAs form as the result of foreshock cavitons interacting with the bow shock. Foreshock cavitons in turn form due to the nonlinear evolution of ULF waves generated by the interaction of the solar wind with the backstreaming ions. Because foreshock cavitons are an inherent part of the shock dissipation process, the formation of SHFAs is also an inherent part of the dissipation process leading to a highly non-uniform plasma in the quasi-parallel magnetosheath including large scale density and magnetic field cavities.

Response of copper to shock-wave loading at temperatures up to the melting point

Abstract: The evolution of elastic-plastic shock waves as a function of the propagation distance has been studied in 99.999% purity polycrystalline copper over the 300 to 1353 K temperature range. The free surface velocity histories of shock-loaded samples 0.1 to 2.0 mm in thickness have been recorded using the velocity interferometer. The measured decay of the elastic precursor waves has been converted into relationships between the shear stress at Hugoniot elastic limit and the initial plastic strain rate. Independently of the temperature, the initial densities of mobile dislocations in a range of 2.5×106 cm−2 to 5×108 cm−2 are required to provide observed initial strain rates varied from 2.3×103 s−1 to 2×106 s−1. Above 1100 K, the shape of the elastic precursor wave changes with the appearance of a sharp spike at its front part. This change is treated in terms of nucleation and multiplication of mobile dislocations. An analysis of the rise times of the plastic shock waves has shown that for the same level of she...

Shock Heating of the Merging Galaxy Cluster A521

Abstract: A521 is an interacting galaxy cluster located at z = 0.247, hosting a low-frequency radio halo connected to an eastern radio relic. Previous Chandra observations hinted at the presence of an X-ray brightness edge at the position of the relic, which may be a shock front. We analyze a deep observation of A521 recently performed with XMM-Newton in order to probe the cluster structure up to the outermost regions covered by the radio emission. The cluster atmosphere exhibits various brightness and temperature anisotropies. In particular, two cluster cores appear to be separated by two cold fronts. We find two shock fronts, one that was suggested by Chandra and that is propagating to the east, and another to the southwestern cluster outskirt. The two main interacting clusters appear to be separated by a shock-heated region, which exhibits a spatial correlation with the radio halo. The outer edge of the radio relic coincides spatially with a shock front, suggesting that this shock is responsible for the generation of cosmic-ray electrons in the relic. The propagation direction and Mach number of the shock front derived from the gas density jump, M = 2.4 +/- 0.2, are consistent with expectations from the radio spectral index, under the assumption of Fermi I acceleration mechanism.

Plasma Waves and Instabilities

Abstract: An account is given of the waves and instabilities occurring at collisionless shocks, with attention to the mechanisms responsible for the generation of these waves. The transition region of the shock usually involves an abrupt broadband burst of electrostatic noise that extends from below the lower hybrid resonance to near the electron plasma frequency, and by a broadband burst of whistler mode EM noise below the electron cyclotron frequency. Electrostatic lower hybrid waves are also noted. Upstream of the shock, electron plasma oscillations, ion acoustic waves, and intense ULF MHD waves are often observed. The region downstream of the shock is usually very chaotic; electrostatic waves often extend long distances into the downstream region, together with whistler mode emissions.

Collisionless shock formation, spontaneous electromagnetic fluctuations and streaming instabilities

Abstract: Collisionless shocks are ubiquitous in astrophysics and in the lab. Recent numerical simulations and experiments have shown how they can arise from the encounter of two collisionless plasma shells. When the shells interpenetrate, the overlapping region turns unstable, triggering the shock formation. As a first step towards a microscopic understanding of the process, we analyze here in detail the initial instability phase. On the one hand, 2D relativistic Particle-In-Cell simulations are performed where two symmetric initially cold pair plasmas collide. On the other hand, the instabilities at work are analyzed, as well as the field at saturation and the seed field which gets amplified. For mildly relativistic motions and onward, Weibel modes govern the linear phase. We derive an expression for the duration of the linear phase in good agreement with the simulations. This saturation time constitutes indeed a lower-bound for the shock formation time.

Shock induced fluid structure interaction on a flexible wall in supersonic turbulent flow

Abstract: A detailed knowledge of the flow structure interaction in supersonic flows is important for the design of future space transportation systems. Therefore this work was devoted to the investigation of the shock wave boundary layer interaction on an elastic panel. During the wind tunnel experiments the panel deflection was measured with fast non-intrusive displacement sensors. On the flow side pressure, high-speed Schlieren photography and oil-film technique were used. The flow manipulation due to the panel deflection becomes manifest in a deformation of the impinging shock and the separation zone. The panel deflection consists of a constant and a dynamic component. The experimental results are discussed and compared to numerical results.

Highly Shocked Polymer Bonded Explosives at a Nonplanar Interface: Hot-Spot Formation Leading to Detonation

Abstract: We report reactive molecular dynamics simulations using the ReaxFF reactive force field to examine shock-induced hot-spot formation followed by detonation initiation in realistic (2.7 million atoms) models of polymer bonded explosives (PBX) with nonplanar interfaces. We considered here two energetic materials (EMs) pentaerythritol tetranitrate (PETN), a common EM for PBX, and silicon pentaerythritol tetranitrate (Si-PETN), which is so extremely sensitive that it has not been possible to characterize its shock properties experimentally. In each case the EM was embedded in a hydroxyl-terminated polybutadiene (HTPB) based polymer binder matrix to form a model of PBX that has a periodic sawtooth nonplanar interface. For the cases in which the shock wave propagates from the EM to polymer (EM→poly), we observed that a hot spot arises from shear localization at the convex polymer asperity. For the case in which the shock direction is inverted (shock wave propagates from the polymer to the EM, EM←poly), we find t...

Ion acceleration from laser-driven electrostatic shocks

Abstract: Multi-dimensional particle-in-cell simulations are used to study the generation of electrostatic shocks in plasma and the reflection of background ions to produce high-quality and high-energy ion beams. Electrostatic shocks are driven by the interaction of two plasmas with different density and/or relative drift velocity. The energy and number of ions reflected by the shock increase with increasing density ratio and relative drift velocity between the two interacting plasmas. It is shown that the interaction of intense lasers with tailored near-critical density plasmas allows for the efficient heating of the plasma electrons and steepening of the plasma profile at the critical density interface, leading to the generation of high-velocity shock structures and high-energy ion beams. Our results indicate that high-quality 200 MeV shock-accelerated ion beams required for medical applications may be obtained with current laser systems.