Showing papers on "Shock wave published in 2004"

Supersonic shear imaging: a new technique for soft tissue elasticity mapping

Abstract: Supersonic shear imaging (SSI) is a new ultrasound-based technique for real-time visualization of soft tissue viscoelastic properties. Using ultrasonic focused beams, it is possible to remotely generate mechanical vibration sources radiating low-frequency, shear waves inside tissues. Relying on this concept, SSI proposes to create such a source and make it move at a supersonic speed. In analogy with the "sonic boom" created by a supersonic aircraft, the resulting shear waves will interfere constructively along a Mach cone, creating two intense plane shear waves. These waves propagate through the medium and are progressively distorted by tissue heterogeneities. An ultrafast scanner prototype is able to both generate this supersonic source and image (5000 frames/s) the propagation of the resulting shear waves. Using inversion algorithms, the shear elasticity of medium can be mapped quantitatively from this propagation movie. The SSI enables tissue elasticity mapping in less than 20 ms, even in strongly viscous medium like breast. Modalities such as shear compounding are implementable by tilting shear waves in different directions and improving the elasticity estimation. Results validating SSI in heterogeneous phantoms are presented. The first in vivo investigations made on healthy volunteers emphasize the potential clinical applicability of SSI for breast cancer detection.

Turbulent amplification of magnetic field and diffusive shock acceleration of cosmic rays

Abstract: The diffusive shock acceleration of cosmic rays by supernova remnants depends upon the generation of magnetic fluctuations by cosmic rays upstream of the shock. Strongly driven, non-resonant, nearly purely growing modes grow more rapidly than the resonant Alfven waves usually considered. Non-linear simulation shows that the magnetic field can be amplified from its seed value by orders of magnitude. The consequences for the maximum attainable cosmic ray energy in supernova remnants are explored.

Line-imaging velocimeter for shock diagnostics at the OMEGA laser facility

Abstract: A line-imaging velocity interferometer has been implemented at the OMEGA laser facility of the Laboratory for Laser Energetics, University of Rochester. This instrument is the primary diagnostic for a variety of experiments involving laser-driven shock-wave propagation, including high-pressure equation of state experiments, materials characterization experiments, shock characterization for Rayleigh–Taylor experiments, and shock timing experiments for inertial confinement fusion research. Using a laser probe beam to illuminate a target, the instrument measures shock breakout times at temporal resolutions as low as 20 ps, and spatial resolution ∼4 μm. For velocity measurements the detection limit is <0.1 km/s, and velocities of interfaces, free surfaces, and shock fronts traveling through transparent media can be measured with accuracies ∼1% over the range from 4 km/s to greater than 50 km/s. Quantitative measurements of the optical reflectance of ionizing shock fronts can also be obtained simultaneously with the velocity measurements.

Numerical simulation of the three-dimensional structure and dynamics of the non-magnetic solar chromosphere

Abstract: Numerical simulation of the three-dimensional structure and dynamics of the non-magnetic solar chromosphere

Magnetic Field Generation in Collisionless Shocks: Pattern Growth and Transport

Abstract: We present results from three-dimensional particle simulations of collisionless shock formation, with relativistic counterstreaming ion-electron plasmas. Particles are followed over many skin depths downstream of the shock. Open boundaries allow the experiments to be continued for several particle crossing times. The experiments confirm the generation of strong magnetic and electric fields by a Weibel-like kinetic streaming instability and demonstrate that the electromagnetic fields propagate far downstream of the shock. The magnetic fields are predominantly transversal and are associated with merging ion current channels. The total magnetic energy grows as the ion channels merge and as the magnetic field patterns propagate downstream. The electron populations are quickly thermalized, while the ion populations retain distinct bulk speeds in shielded ion channels and thermalize much more slowly. The results help reveal processes of importance in collisionless shocks and may help to explain the origin of the magnetic fields responsible for afterglow synchrotron/jitter radiation from gamma-ray bursts.

Molecular Cloud Formation behind Shock Waves

Abstract: Motivated by our previous paper, in which we argued for the formation of molecular clouds from large-scale flows in the diffuse Galactic interstellar medium, we examine the formation of molecular gas behind shocks in atomic gas using a one-dimensional chemical/dynamical model. In our analysis we place particular emphasis on constraints placed on the dynamical evolution by the chemistry. The most important result of this study is to stress the importance of shielding the moleculargas from the destructive effects of UVradiation. For shock ram pressures comparable to or exceeding typical local interstellar medium pressures, self-shielding controls the formation time of molecular hydrogen, but CO formation requires shielding of the interstellar radiation field by dust grains. We find that for typical parameters the molecular hydrogen fractional abundance can become significant well before CO forms. The timescale for (CO) molecular cloud formation is not set by the H2 formation rate on grains, but rather by the timescale for accumulating a sufficient column density or extinction, AV k0:7. The local ratio of atomic to molecular gas (4:1), coupled with short estimates for the lifetimes of molecular clouds (3‐5 Myr), suggests that the timescales for accumulating molecular clouds from atomic material typically must be no longer than about 12‐20 Myr. Based on the shielding requirement, this implies that the typical product of preshock density and velocity must benvk20 cm � 3 km s � 1 . In turn, depending on the shock velocity, this implies shock ram pressures that are a few times the typical estimated local turbulent gas pressure and comparable to the total pressures(gasplusmagneticpluscosmicrays).CoupledwiththerapidformationofCOonceshieldingissufficient, flow-driven formation of molecular clouds in the local interstellar medium can occur sufficiently rapidly to account for observations. We also provide detailed predictions of atomic and molecular emission and absorption that track the formation of a molecular cloud from a purely atomic medium, with a view toward helping to verify cloud formation by shock waves. However, our predictions suggest that the detection of the pre-CO stages will be challenging. Finally, we provide an analytic solution for time-dependent H2 formation that may be of use in numerical hydrodynamic calculations. Subject headingg ISM: clouds — ISM: evolution — ISM: kinematics and dynamics — ISM: molecules — shock waves — stars: formation

Axially symmetric relativistic MHD simulations of Pulsar Wind Nebulae in Supernova Remnants. On the origin of torus and jet-like features

Abstract: The structure and the evolution of Pulsar Wind Nebulae (PWNe) are studied by means of two-dimensional axisym- metric relativistic magnetohydrodynamic (RMHD) simulations. After the first imaging of the Crab Nebula with Chandra ,a growing number of objects has been found to show in the X-rays spatial features such as rings and jets, that clearly cannot be accounted for within the standard framework of one-dimensional semi-analytical models. The most promising explanation suggested so far is based on the combined effects of the latitude dependence of the pulsar wind energy flux, shaping the wind termination shock and naturally providing a higher equatorial emission, and of the wind magnetization, likely responsible for the jet collimation by hoop stresses downstream of the shock. This scenario is investigated here by following the evolution of a PWN interacting with the confining Supernova Remnant (SNR), from the free expansion to the beginning of the reverberation phase. Our results confirm the oblate shape of the wind termination shock and the formation of a polar jet with supersonic velocities (v ≈ 0.5−0.7c) for high enough values of the equatorial wind magnetization parameter (σ > 0.01).

Shock-induced star formation in a model of the Mice

Abstract: Star formation plays an important role in the fate of interacting galaxies. To date, most galactic simulations including star formation have used a density-dependent star formation rule designed to approximate a Schmidt law. Here, I present a new star formation rule which is governed by the local rate of energy dissipation in shocks. The new and old rules are compared using self-consistent simulations of NGC 4676; shock-induced star formation provides a better match to the observations of this system.

The Mouse that Soared: High-Resolution X-Ray Imaging of the Pulsar-powered Bow Shock G359.23–0.82

Abstract: We present an observation with the Chandra X-Ray Observatory of the unusual radio source G359.23-0.82 ("the Mouse"), along with updated radio timing data from the Parkes radio telescope on the coincident young pulsar J1747-2958. We find that G359.23-0.82 is a very luminous X-ray source [LX(0.5-8.0 keV) = 5 × 1034 ergs s-1 for a distance of 5 kpc] whose morphology consists of a bright head coincident with PSR J1747-2958 plus a 45'' long narrow tail whose power-law spectrum steepens with distance from the pulsar. We thus confirm that G359.23-0.82 is a bow shock pulsar wind nebula powered by PSR J1747-2958; the nebular standoff distance implies that the pulsar is moving with a Mach number of ~60, suggesting a space velocity ≈600 km s-1 through gas of density ≈0.3 cm-3. We combine the theory of ion-dominated pulsar winds with hydrodynamic simulations of pulsar bow shocks to show that a bright elongated X-ray and radio feature extending 10'' behind the pulsar represents the surface of the wind termination shock. The X-ray and radio "trails" seen in other pulsar bow shocks may similarly represent the surface of the termination shock, rather than particles in the postshock flow as is usually argued. The tail of the Mouse contains two components: a relatively broad region seen only at radio wavelengths, and a narrow region seen in both radio and X-rays. We propose that the former represents material flowing from the wind shock ahead of the pulsar's motion, while the latter corresponds to more weakly magnetized material streaming from the backward termination shock. This study represents the first consistent attempt to apply our understanding of "Crab-like" nebulae to the growing group of bow shocks around high-velocity pulsars.

A multiwavelength study of solar flare waves II. Perturbation characteristics and physical interpretation

Abstract: The study of solar flare waves - globally propagating wave-like disturbances usually observed in Hα as Moreton waves - has recently come back into focus prompted by the observation of coronal waves in the EUV with the SOHO/EIT in- strument ("EIT waves"), and in several additional wavelength channels. We study 12 flare wave events in order to determine their physical nature, using Hα, EUV, helium I, SXR and radioheliographic data. In the companion Paper I, we have presented the observational data and have discussed the morphology, spatial characteristics and the kinematics of the different flare wave signatures. The wavefronts observed in the various spectral bands were found to follow kinematical curves that are closely associated, implying that they are signatures of the same physical disturbance. In the present paper, we continue the study with a close examination of the evolution of the common perturbation that causes the different wave signatures, and with a detailed analysis of the metric type II radio bursts that were associated with all flare wave events. The basic characteristics of the waves are deceleration, perturbation profile broadening, and perturbation amplitude decrease. This behavior can be interpreted in terms of a freely propagating fast-mode MHD shock formed from a large-amplitude simple wave. It is shown that this scenario can account for all observed properties of the flare waves in the various spectral bands, as well as for the associated metric type II radio bursts.

Laser-induced cavitation bubbles for cleaning of solid surfaces

Abstract: When a high-power laser beam is focused into liquid, it results in a shock wave emission and cavitation bubble generation. Upon inserting a rigid substrate into the liquid, the bubbles migrate towards the substrate due to the Bjerknes attractive force. Due to bubble–substrate and/or bubble–free-surface interaction, a high-speed liquid jet is formed during bubble collapse, and a collapse shock wave is generated at the moment of bubble collapse near the substrate. These shock waves and liquid jet induce large forces acting on the substrate to remove particles from it. For a substrate several millimeters away from the laser focus point, the collapse shock wave and liquid jet play key roles in removal of particles. The cleaning efficiency increases with an increase of laser fluence and decreases with an increase of distance between substrate surface and laser beam focus point or depth below liquid surface. In a case of bubbles close to substrate and liquid-surface boundaries, implosion of the bubbles will give rise to shock waves and liquid jets oblique to the substrate surface with the parallel and perpendicular components of the forces onto the particles. These oblique liquid jets and shock waves result in high cleaning efficiency. A liquid, such as alcohol and commercial washing solution, as the surrounding medium, rather than air or vacuum, can reduce adhesion force and enhance cleaning efficiency.

A NUMERICAL MODEL OF A CORONAL MASS EJECTION: SHOCK DEVELOPMENT WITH IMPLICATIONS FOR THE ACCELERATION OF GeV PROTONS

Abstract: The initiation and evolution of the coronal mass ejection, which occurred on 1998 May 2 in NOAA Active Region 8210, are modeled using a fully three-dimensional, global MHD code. The initial magnetic field for the model is based on magnetogram data from the Wilcox Solar Observatory, and the solar eruption is initiated by slowly evolving the boundary conditions until a critical point is reached where the configuration loses equilibrium. At this time, the field erupts, and a flux rope is ejected that achieves a maximum speed in excess of 1000 km s-1. The shock that forms in front of the rope reaches a fast-mode Mach number in excess of 4 and a compression ratio greater than 3 by the time it has traveled a distance of 5 R☉ from the surface. For such values, diffusive shock acceleration theory predicts a distribution of solar energetic protons with a cutoff energy of about 10 GeV. For this event, there appears to be no need to introduce an additional acceleration mechanism to account for solar energetic protons with energies below 10 GeV.

Microscale Laser Shock Peening of Thin Films, Part 1: Experiment, Modeling and Simulation

Abstract: Microscale Laser Shock Peening (LSP), also known as Laser Shock Processing, is a technique that can be potentially applied to manipulate residual stress distributions in metal film structures and thus improve the fatigue performances of micro-devices made of such films. In this study, microscale LSP of copper films on single crystal silicon substrate is investigated. Before and after-process curvature measurement verifies that sizable compressive residual stress can be induced in copper thin films using microscale LSP. Improved modeling work of shock pressure is summarized and the computed shock pressure is used as loading in 3D stress/strain analysis of the layered film structure. Simulation shows that the stress/strain distribution in the metal film is close to equi-biaxial and is coupled into the silicon substrate.

Neutrino signatures of supernova forward and reverse shock propagation

Abstract: A few seconds after bounce in a core-collapse supernova, the shock wave passes the density region corresponding to resonant neutrino oscillations with the 'atmospheric' neutrino mass difference. The transient violation of the adiabaticity condition manifests itself in an observable modulation of the neutrino signal from a future galactic supernova. In addition to the shock wave propagation effects that were previously studied, a reverse shock forms when the supersonically expanding neutrino-driven wind collides with the slower earlier supernova ejecta. This implies that for some period the neutrinos pass two subsequent density discontinuities, giving rise to a 'double-dip' feature in the average neutrino energy as a function of time. We study this effect both analytically and numerically and find that it allows one to trace the positions of the forward and reverse shocks. We show that the energy dependent neutrino conversion probabilities allow one to detect oscillations even if the energy spectra of different neutrino flavours are the same as long as the fluxes differ. These features are observable in the signal for an inverted and in the νe signal for a normal neutrino mass hierarchy, provided the 13-mixing angle is 'large' ().

On the dynamics of self-sustained one-dimensional detonations: A numerical study in the shock-attached frame

Abstract: In this work we investigate the dynamics of self-sustained detonation waves that have an embedded information boundary such that the dynamics is influenced only by a finite region adjacent to the lead shock. We introduce the boundary of such a domain, which is shown to be the separatrix of the forward characteristic lines, as a generalization of the concept of a sonic locus to unsteady detonations. The concept plays a fundamental role both in steady detonations and in theories of much more frequently observed unsteady detonations. The definition has a precise mathematical form from which its relationship to known theories of detonation stability and nonlinear dynamics can be clearly identified. With a new numerical algorithm for integration of reactive Euler equations in a shock-attached frame, that we have also developed, we demonstrate the main properties of the unsteady sonic locus, such as its role as an information boundary. In addition, we introduce the so-called “nonreflecting” boundary condition at the far end of the computational domain in order to minimize the influence of the spurious reflected waves. © 2004 American Institute of Physics . [DOI: 10.1063/1.1776531]

Early afterglow emission from a reverse shock as a diagnostic tool for gamma-ray burst outflows

Abstract: The gamma-ray burst‐afterglow transition is one of the most interesting and least studied gamma-ray burst phases During this phase, the relativistic ejecta begins interacting with the surrounding matter A strong short-lived reverse shock propagates into the ejecta (provided that it is baryonic) while the forward shock begins to shape the surrounding matter into a Blandford‐McKee profile We suggest a parametrization of the early afterglow light curve and we calculate (analytically and numerically) the observed parameters that result from a reverse shock emission (in an interstellar medium environment) We present a new fingerprint of the reverse shock emission that is added to the well-known t −2 optical decay Observation of this signature would indicate that the reverse shock dominates the emission during the early afterglow The existence of a reverse shock will in turn imply that the relativistic ejecta contains a significant baryonic component This signature would also imply that the surrounding medium is an interstellar medium We further show the following (i) The reverse shock optical flash depends strongly on initial conditions of the relativistic ejecta (ii) Previous calculations have generally overestimated the strength of this optical flash (iii) If the reverse shock dominates the optical flash, then detailed observations of the early afterglow light curve would possibly enable us to determine the initial physical conditions within the relativistic ejecta and specifically to estimate its Lorentz factor and its width Ke yw ords: hydrodynamics ‐ shock waves ‐ gamma-rays: bursts

Constant-stress Hugoniostat method for following the dynamical evolution of shocked matter

Abstract: We present an alternative equilibrium molecular dynamics method---the uniaxial constant-stress Hugoniostat---for following the dynamical evolution of condensed matter subjected to shock waves. It is a natural extension of the recently developed uniaxial constant-volume Hugoniostat [Maillet et al., Phys. Rev. E 63, 016121 (2001)]. Integral feedback is employed to reach the Hugoniot (final) state of the shock process by controlling both the normal component of the stress tensor and internal energy. The finite strain rate imposed on the system is closely related to that inherent in the front of a shock wave. The method can easily identify phase transitions along the Hugoniot shock states, even those that exhibit multiple wave structures. As an example of the method, we have simulated the Hugoniot of a Lennard-Jones crystal shocked along the $⟨110⟩$ direction. The results agree well with multi-million-atom nonequilibrium molecular-dynamics simulations.

Shock Melting of a Two-Dimensional Complex (Dusty) Plasma

Abstract: Shock waves with a linear front were experimentally studied in a monolayer hexagonal Yukawa lattice which was formed from charged monodisperse plastic microspheres and levitated in the sheath of a radio-frequency discharge. It was found that the shock can cause phase transitions from a crystalline to gaslike and liquidlike states. Melting occurred in two stages. First, the lattice was compressed in the direction of shock propagation and second, the particle velocities were randomized a few lattice lines downstream. The Mach number of the shock reached 2.7.

Cosmic ray acceleration at relativistic shock waves with a realistic magnetic field structure

Abstract: The process of cosmic-ray first-order Fermi acceleration at relativistic shock waves is studied with the method of Monte Carlo simulations. The simulations are based on numerical integration of particle equations of motion in a turbulent magnetic field near the shock. In comparison to earlier studies, a few "realistic" features of the magnetic field structure are included. The upstream field consists of a mean field component inclined at some angle to the shock normal with finite-amplitude sinusoidal perturbations imposed upon it. The perturbations are assumed to be static in the local plasma rest frame. Their flat or Kolmogorov spectra are constructed with randomly drawn wavevectors from a wide range (kmin, kmax). The downstream field structure is derived from the upstream one as compressed at the shock. We present and discuss particle spectra and angular distributions obtained at mildly relativistic sub- and superluminal shocks. We show that particle spectra diverge from a simple power law; the exact shape of the spectrum depends on both the amplitude of the magnetic field perturbations and the wave power spectrum considered. Features such as spectrum hardening before the cutoff at oblique subluminal shocks and formation of power-law tails at superluminal ones are presented and discussed. The simulations have also been performed for parallel shock waves. The presence of finite-amplitude magnetic field perturbations leads to the formation of locally oblique field configurations at the shock and the respective magnetic field compressions. This results in the modification of the particle acceleration process, introducing some features present in oblique shocks, e.g., particle reflections from the shock. For the first time, we demonstrate for parallel shocks a (nonmonotonic) variation of the accelerated particle spectral index with the turbulence amplitude. At the end, a few astrophysical consequences of the results we obtained are mentioned.

Nonstationarity of quasi-perpendicular shocks: a comparison of full particle simulations with different ion to electron mass ratio

Abstract: . We have performed 3 one-dimensional full particle electromagnetic simulations of a quasi-perpendicular shock with the same Alfven Mach number MA=4.5, shock normal - magnetic field angle ΘBn=87°, and ion and electron beta (particle to magnetic field pressure) of 0.05, but with different ion to electron mass ratios (mi/me=80, 400, 1840). At high ion beta it has been shown previously that the shock is steady. At low ion beta, as in the present simulations, the shock periodically reforms itself. However, whereas at unrealistically low mass ratios the reformation is due to accumulation of specularly reflected particles at the upstream edge of the foot, at the realistic mass ratio the modified two-stream instability between the incoming solar wind ions and solar wind electrons leads to ion phase mixing and thermalization. The reformation process is thereby considerably modified. At the lowest mass ratio the Buneman instability between the solar wind electrons and the reflected ions is excited, which is stabilized at higher mass ratios.

Mach number dependence of the onset of dynamo action

Abstract: The effect of compressibility on the onset of non-helical turbulent dynamo action is investigated using both direct simulations as well as simulations with shock-capturing viscosities, keeping, however, the regular magnetic diffusivity. It is found that the critical magnetic Reynolds number increases from about 35 in the subsonic regime to about 70 in the supersonic regime. Although the shock structures are sharper in the high-resolution direct simulations compared with the low-resolution shock-capturing simulations, the magnetic field looks roughly similar in both cases and does not show any shock structures. Similarly, the onset of dynamo action is not significantly affected by the shock-capturing viscosity.

Numerical study on the rotational collapse of strongly magnetized cores of massive stars

Abstract: Hydrodynamics of the rotational collapse of strongly magnetized massive stellar cores has been studied numerically. Employing simplified microphysics and a two-dimensional nonrelativistic MHD code, we have performed a parametric research with respect to the strength of magnetic field and rotation, paying particular attention to the systematics of dynamics. We assume initially that the rotation is almost uniform and the magnetic field is constant in space and parallel to the rotation axis. The initial angular velocity and magnetic field strength span 1.7-6.8 rad s-1 and × 1012 G, respectively. We have found that the combination of rotation and magnetic field can lead to a jetlike prompt explosion in the direction of the rotational axis, which would not be produced by either of them alone. The range of the maximum angular velocity and field strength is 2.3 × 10-3 to 5.8 × 10-4 rad s-1 and 2.3 × 1015 to 5.6 × 1016 G, respectively, at the end of computations. Although the results appear to be consistent with those by LeBlanc & Wilson and Symbalisty, the magnetic fields behind the shock wave, not in the inner core, are the main driving factor of the jet in our models. The fields are amplified by the strong differential rotations in the region between the shock wave and the boundary of the inner and outer cores, enhanced further by the lateral matter motions induced either by an oblique shock wave (for a strong shock case) or possibly by the MRI (magnetorotational instability)-like instability (for a weak shock case). We have also calculated the gravitational wave forms in the quadrupole approximation. Although the wave form from a nonrotating magnetic core is qualitatively different from those from rotating cores, the amplitude is about an order of magnitude smaller. Otherwise, we have found no substantial difference in the first burst of gravitational waves among the magnetized and nonmagnetized models, since the bounce is mainly driven by the combination of the matter pressure and the centrifugal force.