Shock instabilities

It is well known that adiabatic shocks in ordinary gases are stable to both tranverse and longitudinal perturbations, but this need not be true if there are significant thermal effects due to chemical reactions or cooling processes. For example, detonation waves in gases are observed to form cellular structures if the chemical reaction is sufficiently temperature sensitive and a similar instability occurs in radiative shocks in the ISM if their speed exceeds 150 km s^–1. This means that interstellar shocks will be subject to this radiative instability in many cases. The temperature sensitivity of the nuclear reactions in Type I supernovae is also such that we would expect detonation waves in these objects to have a cellular structure.     

Shock Drift Electron Acceleration in a Wavy Shock Front

It is commonly believed that solar type II bursts are caused by accelerated electrons at a shock front. Holman and Pesses (1983) suggested that electrons creating type II bursts are accelerated by the shock drift mechanism. Zlobec et al. (1993) dealt with a fine structure of type II bursts (herringbones) and suggested a qualitative model where electrons are accelerated by a nearly perpendicular wavy shock front. Using this idea, we developed a model of electron acceleration by such a wavy shock front. Electrons are accelerated by the drift mechanism in the shock layer. Under simplifying assumptions it is possible to obtain an analytical solution of electron motion in the wavy shock front. The calculations show that electrons are rarely reflected more than once at the wavy shock front and that their final energy is mostly 1–3 times the initial one. Their acceleration does not depend significantly on shock spatial parameters. In the present model all electrons are eventually transmitted downstream where they form two downstream beams. Resulting spectral and angular distributions of accelerated electrons are presented and the relevance of the model to the herringbone beams is discussed.     

Shock surfing acceleration

Analytical and numerical analysis identify shock surfing acceleration as an ideal pre-energization mechanism for the slow pick-up ions at quasiperpendicular shocks. After gaining sufficient energy by shock surfing, pick-up ions undergo diffusive acceleration to reach their observed energies. Energetic ions upstream of the cometary bow shock, acceleration of solar energetic particles by magnetosonic waves in corona, ion enhancement in interplanetary shocks, generation of anomalous cosmic rays from interstellar pick-up ions at the termination shock are some of the cases where shock surfing acceleration apply. Inclusion of the lower-hybrid wave turbulence into the laminar model of shock surfing can explain the preferential acceleration of heavier particles as observed by Voyager at the termination shock. At relativistic energies, unlimited acceleration of ions is theoretically possible; because for sufficiently strong shocks main limitation of the mechanism, caused by the escape of accelerated particles downstream of the shock during acceleration no longer exists.     

Shock metamorphism on the moon

Shock metamorphism of the lunar samples is discussed. All types of lunar glasses formed by various-size collision-type impact are found as impact glass, ropy glass and agglutinates. The agglutinates bonded by crystal and glassy materials contain hydrogen and helium from the solar wind components. Lunar shocked minerals of plagioclase and silica show anomalous compositions and densities. There are typical two formation processes on planetary materials formed by shock events; that is (1) shocked quartz formed by silica-rich target rocks (esp. on evolved planets of the Earth and Mars), and (2) shocked silica with minor Al contents formed from plagioclase-rich primordial crusts of the Moon. The both shocked silica grows to coarse-grain normal crystals after high-temperature metamorphism which cannot distinguish the original main formation event of impact process.     

Formation of Coronal Shock Waves

Magnetosonic wave formation driven by an expanding cylindrical piston is numerically simulated to obtain better physical insight into the initiation and evolution of large-scale coronal waves caused by coronal eruptions. Several very basic initial configurations are employed to analyze intrinsic characteristics of MHD wave formation that do not depend on specific properties of the environment. It turns out that these simple initial configurations result in piston/wave morphologies and kinematics that reproduce common characteristics of coronal waves. In the initial stage, the wave and the expanding source region cannot be clearly resolved; i.e. a certain time is needed before the wave detaches from the piston. Thereafter, it continues to travel as what is called a “simple wave.” During the acceleration stage of the source region inflation, the wave is driven by the piston expansion, so its amplitude and phase-speed increase, whereas the wavefront profile steepens. At a given point, a discontinuity forms in the wavefront profile; i.e. the leading edge of the wave becomes shocked. The time/distance required for the shock formation is shorter for a more impulsive source-region expansion. After the piston stops, the wave amplitude and phase speed start to decrease. During the expansion, most of the source region becomes strongly rarefied, which reproduces the coronal dimming left behind the eruption. However, the density increases at the source-region boundary, and stays enhanced even after the expansion stops, which might explain stationary brightenings that are sometimes observed at the edges of the erupted coronal structure. Also, in the rear of the wave a weak density depletion develops, trailing the wave, which is sometimes observed as weak transient coronal dimming. Finally, we find a well-defined relationship between the impulsiveness of the source-region expansion and the wave amplitude and phase speed. The results for the cylindrical piston are also compared with the outcome for a planar wave that is formed by a one-dimensional piston, to find out how different geometries affect the evolution of the wave.     

Shock modelling of Planetary Nebulae

The kinematics of Planetary Nebulae are analyzed in terms of the solutions to the equations of hydrodynamic equilibrium developed by J. Cantó. We apply our analysis to the Planetary Nebulae NGC 6905 and NGC 6537. A detailed spectroscopic study of these objects reveals the existence of high nuclear velocities, together with complex kinematic structures and unusual emission line intensities. Shock ionization clearly plays a key role in these nebulae. Remarkably good agreement is obtained when comparing the synthetic maps and spectra resulting from the shock solutions with the observational data.     

Origin of Coronal Shock Waves

The basic idea of the paper is to present transparently and confront two different views on the origin of large-scale coronal shock waves, one favoring coronal mass ejections (CMEs), and the other one preferring flares. For this purpose, we first review the empirical aspects of the relationship between CMEs, flares, and shocks (as manifested by radio type II bursts and Moreton waves). Then, various physical mechanisms capable of launching MHD shocks are presented. In particular, we describe the shock wave formation caused by a three-dimensional piston, driven either by the CME expansion or by a flare-associated pressure pulse. Bearing in mind this theoretical framework, the observational characteristics of CMEs and flares are revisited to specify advantages and drawbacks of the two shock formation scenarios. Finally, we emphasize the need to document clear examples of flare-ignited large-scale waves to give insight on the relative importance of flare and CME generation mechanisms for type II bursts/Moreton waves.     

Radiative Shock Experiments At Luli

We present the set-up and the results of a supercritical radiative shock experiment performed with the LULI nanosecond laser facility. Using specific designed targets filled with xenon gaz at low pressure, the propagation of a strong shock with a radiative precursor is evidenced. The main measured quantities related to the shock (electronic density, propagation velocities, temperature, radial dimension) are presented and compared with various numerical simulations.     

The Nebular Shock Wave Model for Chondrule Formation: Shock Processing in a Particle-Gas Suspension

We present numerical simulations of the thermal and dynamical histories of solid particles (chondrules and their precursors—treated as 1-mm silicate spheres) during passage of an adiabatic shock wave through a particle-gas suspension in a minimum-mass solar nebula. The steady-state equations of energy, momentum, and mass conservation are derived and integrated for both solids and gas under a variety of shock conditions and particle number densities using the free-molecular-flow approximation. These simulations allow us to investigate both the heating and cooling of particles in a shock wave and to compare the time and distance scales associated with their processing to those expected for natural chondrules. The interactions with the particles cause the gas to achieve higher temperatures and pressures both upstream and downstream of the shock than would be reached otherwise. The cooling rates of the particles are found to be nonlinear but agree approximately with the cooling rates inferred for chondrules by laboratory simulations. The initial concentration of solids upstream of the shock controls the cooling rates and the distances over which they are processed: Lower concentrations cool more slowly and over longer distances. These simulations are consistent with the hypothesis that large-scale shocks, e.g., those due to density waves or gravitational instabilities, were the dominant mechanism for chondrule formation in the nebula.     

Shock fragmentation model for gravitational collapse

A cloud of gas collapsing under gravity will fragment.We present a new theory for this process,in which layers of shocked gas fragment due to their gravitational instability.Our model explains why angular momentum does not inhibit the collapse process.The theory predicts that the fragmentation process produces objects which are significantly smaller than most stars,implying that accretion onto the fragments plays an essential role in determining the initial masses of stars.This prediction is also consistent...     

Shock magnetism in fine particle iron

Abstract— All solid solar system bodies have been affected by impact to varying degrees, and, thus, magnetic records in these bodies may have been modified by shock events. Shock events may have overprinted all primordial magnetic records in meteorites. Shock metamorphism stages ranging from very little to extreme, when melting takes place, have been identified in meteorites. We are examining the creation and destruction of magnetic remanence associated with shock. In this paper, we develop a preliminary framework for understanding the magnetic properties of fine‐grained Fe particles (20–110 nm), which carry most of the remanent magnetization in lunar samples and, by extension, the kamacite phase in meteorite samples. Initial experiments on shock effects due to a first‐order shock‐induced crystallographic transformation are described. The first characterization of pre‐ and postshock magnetic properties for sized Fe particles and the first characterization of the transformation remanent magnetization (TMRM) associated with the face‐centered‐cubic (fcc) to body‐centered‐cubic (bcc) transformation in fine particle Fe spheres are described. This is equivalent to the 13 GPa transitions in bcc Fe. We show that the TMRM is in the same direction as the ambient magnetic field present during the shock, but is deflected from the field direction by 30–45° and that the remanence intensity is 1–2 orders of magnitude less than expected for thermoremanent magnetization (TRM) acquired during cooling through the Curie temperature. Isothermal remanence acquisition curves (RA) reveal the increasing magnetic hardness due to shock. Magnetic hysteresis loops are used to characterize the particle size and the shock‐induced magnetic anisotropy. Thermal demagnetization experiments describe the probable presence of particle size effects and the effects associated with recovery‐recrystallization due to the annealing that takes place during the thermomagnetic experiment. These observations have implications for paleofield determinations and the recognition of thermal unblocking. A TMRM mechanism could produce a shock overprint in a meteorite and might impart a significant directional feature in an asteroid magnetic signature.     

Shock Propagation Through Multiphase Media*

This paper presents two and three dimensional simulations of the interaction of shocks with media with large numbers of dense inclusions. An approximate model of the interaction of a starburst wind with the surrounding galactic ISM illustrates issues which must be addressed in global models of ISM dynamics. As a step towards developing the sub-grid model of multiphase turbulence, we define and study a form of ‘multiphase Riemann problem’. This allows us to develop macroscopic characteristics of the flows which may be compared to such subgrid models.     

Shock wave propagation in non-ideal fluids

Expressions for the variation of shock strength and its propagation in non-ideal fluids are presented by using the method developed by Whitham. The effects of the presence of rotation, rotation and magnetic field on the strength and propagation velocity of shock waves have been discussed separately. Finally, the effects of the presence of gravitation on the shock strength and on propagation velocity have been studied.     

Shock waves in a self-gravitating gas

The CCW method (see Chester, 1854; Chisnell, 1955; Whitham, 1958) has been used to investigate the propagation of diverging shock waves through an ideal gas under its own gravitation having an initial density distribution ₀ = exp(–r , where is the density at the plane/axis/origin, respectively, for plane, cylindrical, and spherical symmetry of the shock and, is non-dimensional constant, for the two situations: viz., (i) when the shock is weak and (ii) when it is strong, simultaneously. Analytical relations for shock velocity and shock strength have been obtained. Expressions for the pressure, the density and the particle velocity immediately behind the shock have been derived. Their numberical estimates for plane and cylindrical symmetry of the shock, have been computed.     

Shock acceleration of solar cosmic rays

The solar cosmic ray (SCR) acceleration by the shocks driven by coronal mass ejections is studied by taking into account the generation of Alfvén waves by accelerated particles. Detailed numerical calculations of the SCR spectra produced during the shock propagation through the solar corona have been performed within a quasi-linear approach with a realistic set of coronal parameters. The resultant SCR energy spectrum is shown to include a power-law part N ∝ ?^-γ with an index γ = 1.7–3.5 that ends with an exponential tail. The maximum SCR energy lies within the range ? _max = 0.01–10 GeV, depending on the shock velocity V _S = 750–2500 km s^?1. The decrease of the shock Alfvénic Mach number due to the increase Alfvén velocity with heliocentric distance r leads to the end of the efficient SCR acceleration when the shock size reaches R _S ≈ 4R _⊙. In this case, the diffusive SCR propagation begins to exceed the shock velocity; as a result, SCRs escape intensively from the shock vicinity. The self-consistent generation of Alfvén waves by accelerated particles is accompanied by a steepening of the particle spectrum and an increase of their maximum energy. Comparison of the calculated SCR fluxes expected near the Earth’s orbit with the available experimental data shows that the theory explains the main observed features.     

Shock acceleration of solar cosmic rays

We investigated the acceleration of solar cosmic rays (SCRs) by the shock waves produced by coronal mass ejections. We performed detailed numerical calculations of the SCR spectra produced during the shock propagation in the solar corona in terms of a model based on the diffusive transport equation using a realistic set of physical parameters for the corona. The resulting SCR energy spectrum N(ε) ∝ ε^?γ exp [? (ε/ε_max)^α] is shown to include a power-law portion with an index γ?2 that ends with an exponential tail with α ? 2.5 ? β, where β is the spectral index of the background Alfvén turbulence. The maximum SCR energy lies within the range ε_max = 1–300 MeV, depending on the shock velocity. Because of the steep spectrum of the SCRs, their backreaction on the shock structure is negligible. The decrease in the Alfvén Mach number of the shock due to the increase in the Alfvén velocity with heliocentric distance r causes the efficient SCR acceleration to terminate when the shock reaches a distance of r = 2–3R_⊙. Since the diffusive SCR propagation in this case is faster than the shock expansion, SCR particles intensively escape from the shock vicinity. A comparison of the calculated SCR fluxes expected near the Earth’s orbit with available experimental data indicates that the theory satisfactorily explains all of the main observed features.     

Shock waves on neutron star cores

A numerical model of the transient behavior of a radiation-dominated shock was calculated in order to demonstrate the relatively large initial escape of internal energy that takes place when the opacity law is a positive power of temperature, as for neutrinos,K∼T ². Attention was given to the consequences of diffusive transport of radiation versus local production of internal energy by duplicating the calculation with and without artificial viscosity. It is concluded that a shock formed on the neutron star core of an imploding supernova may radiate its internal energy in electron neutrinos more effectively than had hithertofore been considered.     

Shock metamorphism in plagioclase and selective amorphization

Plagioclase feldspar is one of the most common rock‐forming minerals on the surfaces of the Earth and other terrestrial planetary bodies, where it has been exposed to the ubiquitous process of hypervelocity impact. However, the response of plagioclase to shock metamorphism remains poorly understood. In particular, constraining the initiation and progression of shock‐induced amorphization in plagioclase (i.e., conversion to diaplectic glass) would improve our knowledge of how shock progressively deforms plagioclase. In turn, this information would enable plagioclase to be used to evaluate the shock stage of meteorites and terrestrial impactites, whenever they lack traditionally used shock indicator minerals, such as olivine and quartz. Here, we report on an electron backscatter diffraction (EBSD) study of shocked plagioclase grains in a metagranite shatter cone from the central uplift of the Manicouagan impact structure, Canada. Our study suggests that, in plagioclase, shock amorphization is initially localized either within pre‐existing twins or along lamellae, with similar characteristics to planar deformation features (PDFs) but that resemble twins in their periodicity. These lamellae likely represent specific crystallographic planes that undergo preferential structural failure under shock conditions. The orientation of preexisting twin sets that are preferentially amorphized and that of amorphous lamellae is likely favorable with respect to scattering of the local shock wave and corresponds to the “weakest” orientation for a specific shock pressure value. This observation supports a universal formation mechanism for PDFs in silicate minerals.