Showing papers on "Shock wave published in 1974"

Large-scale effects of supernova remnants on the Galaxy - Generation and maintenance of a hot network of tunnels

Abstract: It is found that a supernova rate on the order of 1 per 50 years in the gaseous disk of our Galaxy is sufficient to generate and maintain throughout the interstellar medium a mesh of interconnected tunnels containing very low-density gas. This tunnel system would have a density of approximately 0.01 per cu cm, a temperature of about 1,000,000 K, very low magnetic field strength, tunnel radii of about 10 pc, and would occupy roughly half the interstellar volume. Such a tunnel network may already have been observed in soft X-ray emission, in ultraviolet absorption of O VI against background stars, in the seemingly chaotic distribution of local H I, and in the stringy appearance of velocity-correlated large-scale H I features.

Shock wave study of the α ⇄ ε phase transition in iron

Abstract: Plate impact experiments producing plane waves of up to 40 GPa (400 kbar) peak stress were performed using Armco iron specimens and impactors. Highly accurate time‐resolved measurements of the resulting free‐surface velocities of the specimens were obtained with the VISAR laser interferometer instrumentation system. The free‐surface velocity profiles provide new information concerning the rate effects associated with the α → e polymorphic phase transition at 13 GPa, the material strength and release wave speeds at 10 and 40 GPa, and the stress level at which the iron reverts back to the α phase on unloading. A strong magnetic field was found to produce no measurable change in the phase‐transition stress. The accuracy of the ``factor‐of‐2'' assumption relating free‐surface velocity to particle velocity in iron was also evaluated experimentally.

Nuclear Shock Waves in Heavy-Ion Collisions

Abstract: It is shown that nuclear matter is compressed during the encounter of heavy ions. If the relative velocity of the nuclei is larger than the velocity of first sound in nuclear matter (compression sound for isospin $T=0$), nuclear shock waves occur. They lead to densities which are 3-5 times higher than the nuclear equilibrium density ${\ensuremath{\rho}}_{0}$, depending on the energy of the nuclei. The implications of this phenomenon are discussed.

Behavior of the flare-produced coronal MHD wavefront and the occurrence of type II radio bursts

Abstract: The propagation of the weak MHD fast-mode shock emitted into the corona by flares at their explosive phase is computer-simulated. It is shown as the result that the shock wave is refracted towards the low Alfven velocity regions pre-existing in the corona, and the strength of the shock, which is otherwise weak, is drastically enhanced on encountering low- V A regions due to the focussing effect by refraction and also due to the lowered propagation velocity of the shock in such regions. It is expected that electron acceleration takes place in such a drastic strengthening of the shock, leading to the local excitation of plasma waves and eventually to the occurrence of radio bursts at such locations. Such locations of shock strength enhancement, when computed by using HAO realistic models of coronal density and magnetic field of the day of certain type II burst events, actually coincide roughly with the observed positions of type II bursts. Peculiar configurations of type II burst sources as well as their occurrence even beyond the horizon of the responsible flare are explained consistently by the large scale refraction and the local enhancement of the shock due to the global and local distribution of Alfven velocity in the corona. A unified interpretation is given for the occurrence of type II bursts and Moreton's wave phenomena, and also the relation of our MHD fast-mode disturbance with other flare-associated dynamical phenomena is discussed.

The formation of solar quiescent prominences by condensation

Abstract: We model the formation of solar quiescent prominences by solving numerically the non-linear, time-dependent, magnetohydrodynamic equations governing the condensation of the corona. A two-dimensional geometry is used. Gravitational and magnetic fields are included, but thermal conduction is neglected. The coronal fluid is assumed to cool by radiation and to be heated by the dissipation of mechanical energy carried by shock waves. A small, isobaric perturbation of the initial thermal and mechanical equilibrium is introduced and the fluid is allowed to relax. Because the corona with the given energy sources is thermally unstable, cooling and condensation result. When magnetic and gravitational fields are absent, condensation occurs isotropically with a strongly time-dependent growth rate, and achieves a density 18 times the initial density in 3.5 × 104 s. The rapidity of condensation is limited by hydrodynamical considerations, in contrast to the treatment of Raju (1968). When both magnetic and gravitational fields are included, the rate of condensation is inhibited and denser material falls. We conclude that: (1) condensation of coronal material due to thermal instability is possible if thermal conduction is inhibited; (2) hydrodynamical processes determine, in large part, the rate of condensation; (3) condensation can occur on a time scale compatible with the observed times of formation of quiescent prominences.

Relaxation effects caused by relative motion on shock waves in gas-bubble/liquid mixtures

Abstract: We observed a gradual change in the structure of a shock wave passing through a long tube of bubbly liquid, which we attribute to the motion of the bubbles relative to the liquid. We show that the effect of the motion on the structure of a shock wave is like that of thermal relaxation on gasdynamic shock waves: the pertinent relaxation time is the time viscous forces in the fluid take to alter the velocity of a bubble to that of the fluid. Our theory predicts certain changes in the speed of the shock wave and in its structure. We could not verify the prediction as to wave speed: in dilute mixtures it is too small to be measured. But we report experiments on the structure of the wave, which support our theoretical conclusion that the observed changes are due to the relative motion.

Interplanetary shock waves generated by solar flares

Abstract: Recent observational and theoretical studies of interplanetary shock waves associated with solar flares are reviewed. An attempt is made to outline the framework for the genesis, life and demise of these shocks. Thus, suggestions are made regarding their birth within the flare generation process, MHD wave propagation through the chromosphere and inner corona, and maturity to fully-developed coronal shock waves. Their subsequent propagation into the ambient interplanetary medium and disturbing effects within the solar wind are discussed within the context of theoretical and phenomenological models. The latter — based essentially on observations — are useful for a limited interpretation of shock geometric and kinematic characteristics. The former — upon which ultimate physical understanding depends — are used for clarification and classification of the shocks and their consequences within the solar wind. Classification of limiting cases of blast-produced shocks (as in an explosion) or longer lasting ejecta (or ‘piston’-driven shocks) will hopefully be combined with the study of the flare process itself. The theoretical approach, in spite of its contribution to clarification of various concepts, contains some fundamental limitations and requires further study. Numerical simulations, for example, depend upon a non-unique set of multi-parameter initial conditions at or near the Sun. Additionally, the subtle but important influence of magnetic fields upon energy transport processes within the solar wind has not been considered in the numerical simulation approach. Similarity solutions are limited to geometrical symmetries and have not exploited their potential beyond the special cases of the blast and the constant-velocity, piston-driven shock waves. These continuum fluid studies will probably require augmentation or even replacement by plasma kinetic theory in special situations when observations indicate the presence of anomalous transport processes. Presently, for example, efforts are directed toward identification of detailed shock structures (as in the case of Earth's bow shock) and of the disturbed solar wind (such as the piston). Further progress is expected with extensive in situ and remote monitoring of the solar wind over a wide range of heliographic radii, longitudes and latitudes.

Effects of interplanetary shock waves on energetic charged particles

Abstract: Experimental data on the influence of interplanetary perpendicular and oblique shock waves on the ambient energetic protons are presented along with a theoretical analysis of the acceleration of particles in almost perpendicular shock waves. It was found that low-energy protons can be accelerated in perpendicular shock waves by repeated crossings of the shock front up to a maximum energy given by the product of their initial energy times the ratio of the magnetic fields. High-energy protons need to stay at the shock front for longer times than low-energy protons in order to reach the same relative energy gain. In the theoretical study of proton acceleration at almost perpendicular shock waves, it was found that protons reflected at shock waves with the angle between the upstream magnetic field and the shock normal greater than about 80 deg achieve large energy gains at the shock front. The larger this angle, the higher the energy gain. However, the reflection and energization of protons at these shock waves is not 'instantaneous', neither is it a one-step process: it is performed through repeated crossings of the shock front.

A correlative study of ssc's, interplanetary shocks, and solar activity

Abstract: Ninety-three ssc's during the 4-year period from 1968 to 1971 at and near the peak of the solar activity cycle were examined Of the 93 ssc's, 81 could be associated with solar activity such as solar flares and type 2 and type 4 radio bursts Disturbances associated with 48 of the ssc's have been studied in detail by using the corresponding interplanetary (IP) magnetic field and plasma data when they were available It was found that 41 of the 48 disturbances corresponded to IP shock waves, and the remaining 7 events were tangential discontinuities Thirty per cent of the IP shocks had thick structure (ie, the magnetic field jump across the shock occurred over a distance much greater than 50 proton Larmor radii) By considering the orientations of 22 well-determined shock normals in relation to the positions of the parent flares on the solar disk, it is suggested that a typical shock front propagating out from the sun at 1 AU has a radius of curvature of the order of 1 AU

Solar wind electron temperature depressions following some interplanetary shock waves: Evidence for magnetic merging?

Abstract: An investigation of periods of anomalously low solar wind electron temperatures using 14 months of Vela 5, Vela 6, and Imp 6 measurements has shown a very strong tendency for unusually low temperatures to follow interplanetary shock waves by 10–20 hours. Of 12 low-temperature regions observed, 10 occurred in direct association with a well-defined shock-associated ssc. A detailed investigation of these events has shown that (1) the periods persist for 10 to >40 hours; (2) a distinct depression in proton temperature occurs simultaneously; (3) the electron heat flux tends to be significantly reduced, but the velocity distributions are less Maxwellian; (4) for one event the He/H concentration is found to rise significantly at about the time of the initial decrease in temperature; for most of the events the data are not continuous enough to check this association; and (5) about half of the observed shocks were followed by temperature depressions. The existence of regions with abnormally low electron temperatures is consistent with the formation of closed or nearly closed magnetic field configurations that more or less thermally isolate the electrons from the relatively hot corona. The electron component of the plasma within these regions thus cools in relation to plasma electrons embedded in open field regions as the closed or constricted regions expand away from the sun.

Waves in the Solar Atmosphere

Abstract: The study of waves in the solar atmosphere started with an attempt to explain the observed Doppler shifts (wiggly line spectra) and broadening of photospheric spectral lines. Then Biermann (1946) and Schwarzschild (1948) suggested that acoustic waves, generated in the hydrogen convection zone, supplied the non­ radiative energy needed to heat the chromosphere and corona. In 1959 Leighton (1960) discovered that most of the solar surface was covered with regions that oscillated with periods near five minutes. Although it was assumed that these observed motions and theoretically postulated waves were related, development of these two aspects has been rather independent. The heating models have concen­ trated on shock wave propagation and dissipation, without much consideration of the observed motions, while the oscillation models have concentrated on explaining the oscillations without regard for their implications for energy and momentum transfer. The early observations have been summarized by Noyes (1967), and theories of wave generation, propagation, and heating have been reviewed by Lighthill (1967), Schatzman & Souffrin (1967), and Kuperus (1969). Michalitsanos (1974) has reviewed the recent literature on the "five-minute" oscillation. Research on solar motions has recently progressed in three areas. 1. Computers have made possible the numerical integration of the nonlinear equations of motion. This has shown that the upper chromosphere and corona can be heated by the "five-minute" oscillation (and so brought observations and heating models closer together), but short-period waves are needed to heat the low chromosphere. 2. A driving mechanism, which utilizes thermal overstability of trapped subphotospheric modes, has been suggested for the oscillations. This complements penetrative convection and turbulent motions in the convection zone, which generate short­ wavelength waves. 3. Observations of oscillations and transients in active regions

The near wall jet of a normally impinging, uniform, axisymmetric, supersonic jet

Abstract: The near wall jet produced by directing a uniform axisymmetric jet of air normally onto a large flat plate has been investigated experimentally and theoretically for four jets in the Mach number range 1·64–2·77. Detailed measurements of the surface pressure and shadowgraph and surface flow pictures are presented. The results show that the mechanism which mainly determines the supersonic near wall jet is the jet-edge expansion and its reflexions from the sonic line and the wall-jet boundaries. The near wall jet is found to consist of an alternating series of expansion and recompression regions whose strengths depend on the jet Mach number and decay with distance. At Mach numbers of 2·4 and above, shock waves are observed in the first recompression region and at a Mach number of 2·77 the boundary layer separates locally. Further out, viscous effects become increasingly important and a constant-pressure shear flow is established at a distance which increases with jet Mach number. The application of the method of characteristics in an approximate manner reproduces a number of the features of the near wall jet which are observed experimentally.Pressure distributions obtained in the shock layer show that a stagnation bubble can occur and that its occurrence depends on factors such as the flow upstream of the nozzle. The wall-jet region is found to be largely independent of whether or not a bubble occurs in the shock layer.

Observations at Venus Encounter by the Plasma Science Experiment on Mariner 10

Abstract: A fully developed bow shock and magnetosheath were observed near Mercury, providing unambiguous evidence for a strong interaction between Mercury and the solar wind. Inside the sheath there is a distinct region analogous to the magnetosphere or magnetotail of Earth, populated by electrons with lower density and higher temperature than the electrons observed in the solar wind or magnetosheath. At the time of encounter, conditions were such that a perpendicular shock was observed on the inbound leg and a parallel shock was observed on the outbound leg of the trajectory, and energetic plasma electron events were detected upstream from the outbound shock crossing. The interaction is most likely not atmospheric, but the data clearly indicate that the obstacle to solar wind flow is magnetic, either intrinsic or induced. The particle fluxes and energy spectra showed large variations while the spacecraft was inside the magnetosphere, and these variations could be either spatial or temporal.

Existence and stability of strong potential double layers

Abstract: An elementary integral equation technique is used to construct strong and weak stationary shock solutions from the one-dimensional Vlasov equation. It is shown that the plasma is Penrose stable in all points in space under certain conditions.

Interacting electromagnetic shock waves in general relativity

Abstract: It is shown that the continuity conditions of Lichnerowicz must generally be relaxed in favour of the O'Brien-Synge conditions in the case of shock electromagnetic waves. This is in particular true when two electromagnetic shock waves are in collision. Gravitational impulse waves are produced as a result of the weakened conditions. An exact solution exhibiting this behaviour is derived, and the effect of the impulse waves on a measuring device are compared with experimental results of Weber.

Observations of coronal disturbances from 1 to 9 solar radii. I - First event of 1973 January 11

Abstract: H alpha, white-light and radio observations of a coronal disturbance on Jan. 11, 1973, commencing at about 0036 hr UT show that a piston-driven shock wave propagated outward through the corona to heights of at least 9 solar radii. Probably most of the expelled coronal gas originated in a coronal enhancement in the lower corona. An estimate of the kinetic energy and the mass of the expelled gas is obtained which is compatible with observations of piston-driven shock waves near the earth. Shock-wave parameters are evaluated, and a model of the disturbance is outlined.

Numerical solution of fisher's equation

Abstract: The accurate space derivative (ASD) method for the numerical treatment of nonlinear partial differential equations has been applied to the solution of Fisher's equation, a nonlinear diffusion equation describing the rate of advance of a new advantageous gene, and which is also related to certain water waves and plasma shock waves described by the Korteweg-de-Vries-Burgers equation. The numerical experiments performed indicate how from a variety of initial conditions, (including a step function, and a wave with local perturbation) the concentration of advantageous gene evolves into the travelling wave of minimal speed. For an initial superspeed wave this evolution depends on the cutting off of the right-hand tail of the wave, which is physically plausible; this condition is necessary for the convergence of the ASD method. Detailed comparisons with an analytic solution for the travelling waves illustrate the striking accuracy of the ASD method for other than very small values of the concentration. NONLINEAR DIFFUSION; EPIDEMIC WAVES; NUMERICAL SIMULATION

Reacting viscous-shock-layer solutions with multicomponent diffusion and mass injection

Abstract: Numerical solutions of the viscous-shock-layer equation where the chemistry is treated as being either frozen, equilibrium, or nonequilibrium are presented. Also the effects of the diffusion model, surface catalysis, and mass injection on surface transport and flow parameters are considered. The flow is treated as a mixture of five inert and thermally perfect species. The viscous-shock-layer equations are solved by using an implicit-difference scheme. All calculations are for hyperboloids with included angles of 20 and 45. The flight conditions are those for various altitudes and velocities in the earth's atmosphere. Data are presented to show the effects of the chemical models; diffusion models; surface catalysis; and mass injection of air on heat transfer; skin friction; shock standoff distance; wall pressure distribution; and tangential velocity, temperature, and species profiles. The results show that an equilibrium analysis can substantially overpredict the heat-transfer rates for flow conditions experienced by earth-orbital entry vehicles. Moreover, at such conditions surface catalysis significantly influences heat-transfer and flow-field properties. If a binary rather than a multicomponent diffusion model is assumed, negligible errors in most flow properties result. Quantitative results are presented that show the effect of mass injection on flow properties within and downstream of the injection region.

On the Structure of Magnetohydrodynamic Shock Waves.

Abstract: : Fast and slow magnetohydrodynamic shock waves of arbitrary strength, for general equations of state and for arbitrary non-zero viscosity matrix, are shown to possess structure. (Author)

Motions of the bow shock induced by interplanetary disturbances

Abstract: Motivated by recent observations by Greenstadt and co-workers of very high bow shock ‘normal’ velocities, we investigate the reaction of the shock to changes in solar wind conditions for different bow shock locations. At low quiescent Mach numbers the shock is found to be much more sensitive to a given slow perturbation than it is at the normally occurring high Mach numbers. Thus under these circumstances even a smooth wavelike disturbance can cause high bow shock velocities. For high Mach numbers only impulsive perturbations like tangential discontinuities or occasional interplanetary shocks can induce high bow shock velocities. The former interaction is discussed in detail. Large induced shock velocities are possible, although very strong density discontinuities are needed to produce recently estimated average shock speeds of about 85 km/sec. Thus such high average shock velocities are difficult to understand. In this context also a qualitative discussion of the importance of oscillatory modes of the magnetosphere shock system is presented. The present work suggests average bow shock velocities of the order of 10 km/sec or somewhat larger. Much higher shock speeds should be the exception rather than the rule.

Temperature deposition caused by shock interactions with material interfaces

Abstract: In shock‐wave compression experiments in which the interface temperature between two different materials is important, it is necessary to take account of more than the usual ideal shock impedance matching conditions. Several examples are discussed by means of hydrodynamic analysis of the interaction of strong shock waves with nonideal interfaces. It is demonstrated that significant residual thermal inhomogeneities exist near the interface which may be of use in understanding and measuring thermal properties of shock‐compressed materials.

Oscillation Phenomena of Pseudo-Shock Waves

Abstract: In the previous paper, we presented the diffusion model of pseudo-shock waves, by which the time-mean static pressure distributions in pseudo-shocks could be explained. In the actual flow, however, the pseudo-shock wave is not steady, but oscillates about a mean position and causes the fluctuations of the local static pressures. In this paper the oscillation phenomena of pseudo-shocks in a straight pipe were experimentally investigated and the causes and the frequencies of oscillations were discussed. As the results, it was found that the oscillation of the pseudo-shock became stronger with the increasing Mach number and the maximum wall static pressure fluctuation induced by the oscillation amounted to about 60 per cent of the difference of static pressures before and behind the shock. Moreover, it was showed that this oscillating phenomena could be explained by the interaction of the shock with the small disturbances existing in the supersonic flow upstream the shock.

Theory of Shock-Wave Ionization upon High-Velocity Impact of Micrometeorites

Abstract: The relevant processes in shock wave ionization of a solid Fe micrometeorite impinging on a W target are analyzed. The internal energy behind the shock wave in shown to depend on impact velocity w, target and meteorite density in a simple analytical form. For low impact velocities (w<7 km sec-1) the ions generated by the shock are mostly due to surface ionization. For high impact velocities [w>20 km sec-1) the number of ions can satisfactorily be explained by isentropic expansion of the shocked material to a particle density of n ≈ 1020 cm-3 whereupon the rate processes in the expanding ion cloudlet govern the residual ionization. In velocity regions where laboratory measurements can be carried out, the agreement between theory and experiment confirms the assumptions made.

Nonlinear gas oscillations in pipes. Part 2. Experiment

Abstract: Forced nonlinear acoustic oscillations near the resonant frequency of closed and open tubes are studied experimentally. In particular, the motion in tubes terminated with different orifice plates is studied, and comparison is made with second- and third-order theories of the motion which contain an adjustable end-wall reflexion coefficient. It is found that oscillations at resonance in an open tube exhibit remarkably large amplitudes despite the fact that in some cases shock waves are emitted from the open end. For oscillations at resonance in a closed tube, the effect of substituting an orifice plate for the solid end wall is to reduce the amplitude and thicken the compressive portion of the shock waves which occur under these conditions. In both the open-tube and closed-tube experiments the reflexion coefficients which are evaluated by fitting theory to experiment are found to increase with increasing amplitude, in agreement with the observations of previous investigators (Ingard & Ising 1967). In fact, for the open end the same linear dependence upon amplitude is observed, but the constant of proportionality is different. Qualitative differences are observed between the reflexion coefficients of a given orifice at the open-end and the closed-end resonant frequencies; at the open-end frequency the reflexion from the given orifice is less ideal than at the closed-end frequency. The implications of reflexion coefficients dependent on the wave forms are discussed.