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At least 343 records · Page 19
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NASA NTRS1988

Accretion shock geometries in the magnetic variables

The first self consistent shock models for the AM Herculis-type systems successfully identified the dominant physical processes and their signatures. These homogenous shock models predict unpolarized, Rayleigh-Jeans optical spectra with sharp cutoffs and rising polarizations as the shocks become optically thin in the ultraviolet. However, the observed energy distributions are generally flat with intermediate polarizations over a broad optical band. These and other observational evidence support a non-homogenous accretion profile which may extend over a considerable fraction of the stellar surface. Both the fundamental assumptions underlying the canonical 1-D shock model and the extension of this model to inhomogenous accretion shocks were identified, for both radial and linear structures. The observational evidence was also examined for tall shocks and little evidence was found for relative shock heights in excess of h/R(1) greater than or equal to 0.1. For several systems, upper limits to the shock height can be obtained from either x ray or optical data. These lie in the region h/R(1) is approximately 0.01 and are in general agreement with the current physical picture for these systems. The quasi-periodic optical variations observed in several magnetic variables may eventually prove to be a major aid in further understanding their accretion shock geometries.

Stockman, H. S.↗
NASA NTRS1990

Effects of shock on the stability of hypersonic boundary layers

A set of linearized shock boundary conditions is derived, which is then imposed at the shock to account for the interaction of the shock wave with the boundary/shock layer instability wave; these boundary conditions are used to study the effect of shock on hypersonic boundary layer stability under the assumption of quasi-parallel flow. The result show that the shock has little effect on the boundary layer instability (subsonic first and second mode disturbances) when the shock is located outside the boundary layer edge. When the shock is located near the boundary layer edge, it exerts a stabilizing influence on the first and second modes. The shock also induces unstable supersonic modes with oscillatory structure in the shock layer, but these modes grow slower than the subsonic modes.

Chang, Chau-Lyan↗
NASA NTRS1990

Heliospheric shocks and catastrophe theory

Various configurations of forward and reverse shocks that occur in the outer heliosphere can be classified using catastrophe theory. The existence of a forward shock is associated with a local maximum of a polynomial, and the existence of a reverse shock is associated with a local minimum of a polynomial. A configuration with N forward shocks and N reverse shocks corresponds to a polynomial with N maxima and N minima. The formation of forward and reverse shocks corresponds to the creation of maxima and minima of a polynomial, which is described by the separatrices of the catastrophes. The coalescence of two forward (reverse) shocks corresponds to the situation when two maxima (minima) of a polynomial have equal values, and the interaction of a forward shock with a reverse shock corresponds to a polynomial with a local maximum equal to a local minimum; these situations are described by the Maxwell sets of the appropriate catastrophes.

Burlaga, L. F.↗
NASA NTRS1991

The interaction of collisionless shocks in astrophysical plasmas

The interaction between collisionless shocks, such as may arise in a number of situations in space plasmas, is examined by means of hybrid numerical simulations. The production of energetic particles by shock collisions is investigated. When the shocks are quasi-perpendicular, ions with energies greater than 10E(0) are produced, where E(0) is the kinetic energy of the unshocked ions. The effect is optimized when the two colliding shocks have roughly equal strengths. As the shocks become closer to quasi-parallel, the number and energy of the accelerated ions increases. Energies in excess of 30E(0) are obtained in the quasi-parallel regime, with a small fall-off when the shocks are exactly parallel. For quasi-parallel collisions, the ion energization can be attributed to the interaction of hot ions with both strong electromagnetic waves and the motional electric field located at the shocks. These results are applied to shock collisions in the distant solar wind, at planetary bow shocks, as well as to the impulsive phase of solar flares.

Cargill, P. J.↗
NASA NTRS1991

Shock metamorphism of ordinary chondrites

This study proposes a revised petrographic classification of progressive stages of shock metamorphism of 26 ordinary chondrites. Six stages of shock (S1 to S6) are defined on the basis of shock effects in olivine and plagioclase as recognized by thin section microscopy, and the characteristic shock effects of each shock stage are described. It is concluded that shock effects and the sequence of progressively increasing degrees of shock metamorphosis are very similar in H, L, and LL groups. Differences in the frequency distribution of shock stages are relatively minor. It is suggested that the collisional histories of the H, L, and LL parent bodies were similar. Petrologic type-3 chondrites are deficient in stages S4 and S6 and, with increasing petrologic type, the frequency of stages S4 to S6 increases. It is suggested that the more porous and volatile-rich Type-3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type. Stage S3 is the most abundant in nearly all petrologic types.

Stoeffler, Dieter↗
NASA NTRS1992

Multiple switch-off slow shock solutions

A new electromagnetic quasi-neutral time-dependent implicit hybrid model for simulating the slow-mode switch-off shock is developed, in which the electrons are treated as a finite-mass adiabatic fluid (gamma sub e = 5/3), whose motion along the shock normal provides local charge neutrality. The model is used to examine the effects of various boundary conditions applied during shock formation. It is shown that, for each upstream condition, two distinctly different slow-shock structures can be obtained: a low-shock structure with a trailing magnetic wave (TMW) and a slow-shock structure without a TMW, depending strongly on the boundary conditions for the electric and magnetic fields at the physical piston which generates the shock. The results indicate that the slow-shock structure with a TMW is nonlinearly unstable with respect to changes in the upstream magnetic field, suggesting that the downstream structure of slow shocks need not be associated with a TMW.

Vu, H. X.↗
NASA NTRS1993

Signatures of shock drivers in the solar wind and their dependence on the solar source location

Solar wind and energetic ion observations following 40 interplanetary shocks with well-established solar source locations have been examined in order to determine whether signatures characteristic of the coronal material forming the shock driver are present. The signatures considered include magnetic-field-aligned bidirectional ion flows observed by the ISEE 3 and IMP 8 spacecraft; bidirectional solar wind electron heat fluxes; solar wind plasma proton and electron temperature depressions; low-beta plasma; enhanced, low-variance magnetic fields; and energetic ion depressions. Several shock driver signatures are commonly observed following shocks originating from within about 50 deg of central meridian, and are generally absent for other events. We conclude that shock drivers generally extend up to about 100 deg in longitude, centered on the solar source longitude. Since shocks from central meridian events are not usually associated with all the shock driver signatures examined, the absence of a driver cannot be confirmed from consideration of one of these signatures alone. We also find evidence that a few bidirectional energetic ion and solar wind electron heat flux events following shocks (in particular from far eastern sources) may occur on open field lines outside of shock drivers.

Richardson, I. G.↗
NASA NTRS1994

On the dynamics of a shock-bubble interaction

We present a detailed numerical study of the interaction of a weak shock wave with an isolated cylindrical gas inhomogenity. Such interactions have been studied experimentally in an attempt to elucidate the mechanisms whereby shock waves propagating through random media enhance mixing. Our study concentrates on the early phases of the interaction process which are dominated by repeated refractions of acoustic fronts at the bubble interface. Specifically, we have reproduced two of the experiments performed by Haas and Sturtevant : M(sub s) = 1.22 planar shock wave, moving through air, impinges on a cylindrical bubble which contains either helium or Refrigerant 22. These flows are modelled using the two-dimensional, compressible Euler equations for a two component fluid (air-helium or air-Refrigerant 22). Although simulations of shock wave phenomena are now fairly commonplace, they are mostly restricted to single component flows. Unfortunately, multi-component extensions of successful single component schemes often suffer from spurious oscillations which are generated at material interfaces. Here we avoid such problems by employing a novel, nonconservative shock-capturing scheme. In addition, we have utilized a sophisticated adaptive mesh refinement algorithm which enables extremely high resolution simulations to be performed relatively cheaply. Thus we have been able to reproduce numerically all the intricate mechanisms that were observed experimentally (e.g., transitions from regular to irregular refraction, cusp formation and shock wave focusing, multi-shock and Mach shock structures, jet formation, etc.), and we can now present an updated description for the dynamics of a shock-bubble interaction.

Quirk, James J.↗
NASA NTRS1994

Cosmic ray decreases and shock structure: A multispacecraft study

We examine greater than 60-MeV/amu ion data from three spacecraft (IMP 8 and Helios 1 and 2) at the time of a number of short term (less than 20-day duration) cosmic ray decreases (greater than 1 GeV) detected by ground-based neutron monitors in the years 1976 to 1979. The multispacecraft data allow us to investigate the structure of the modulation region and in particular the relative importance, as a function of location, of the shock and shock driver (ejecta) in causing the reduction in particle densities. Although the shocks contributing to cosmic ray decrease often have particle enhancements associated with them in the greater than 60-meV/amu data, this is not the case for three of the events discussed in this paper where a shock-associated decrease is also evident. Whereas the shock can cause an increase or decrease at low (i.e., less than neutron monitor) energies, the reduction of particle densities in the driver, if it is intercepted, is usually evident at all energies. Thus the overall shape of a decrease at greater than 60 MeV/amu depends primarily on whether the ejecta is intercepted. We find that the particle density inside ejecta increases with increasing radical distance from the Sun. In many of the events in this study, entry and exit of ejecta are accompanied by abrupt changes in the decrease and recovery rates which indicate that the effect of the ejecta is local. In contrast, the effect of the shock lasts many days after the shock has passed by and is evident at large angular distances from the longitude of the solar source, i.e., the effect of the shock is nonlocal. Within 1 AU there seems to be no radial dependence of the shock effect. One cosmic ray decrease seen at Earth, which had an unusual profile, can be understood if the median plane of the ejecta was inclined to the ecliptic.

Cane, H. V.↗
NASA NTRS1995

Wave and ion evolution downstream of quasi-perpendicular bow shocks

Distribution functions of ions heated in quasi-perpendicular bow shocks have a large perpendicular temperature anisotropy that provides free energy for the growth of Alfven ion cyclotron (AIC) waves and mirror waves. Both types of waves have been observed in the Earth's magnetosheath downstream of quasi-perpendicular shocks. We use a two-dimensional hybrid simulations to give a self-consistent description of the evolution of the wave spectra downstream of quasi-perpendicular shocks. Both mirror and AIC waves are identified in the simulated magnetosheath. They are generated at or near the shock front and convected away from it by the sheath plasma. Near the shock, the waves have a broad spectrum, but downstream of the shock, shorter-wavelength modes are heavily damped and only longer-wavelength modes persist. The characteristics of these surviving modes can be predicted with reasonable accuracy by linear kinetic theory appropriate for downstream conditions. We also follow the evolution of the ion distribution function. The shocked ions that provide the free energy for wave growth have a two-component distribution function. The halo is initially gyrophase-bunched and extremely anisotropic. Within a relatively short distance downstream of the shock (of the order of 10 ion inertial lengths), wave-particle interactions remove these features from the halo and reduce the anisotropy of the distribution to near-threshold levels for the mirror and AIC instabilities. A similar evolution has been observed for ions at the Earth's bow shock.

Mckean, M. E.↗
NASA NTRS1995

Time-dependent simulation of oblique MHD cosmic-ray shocks using the two-fluid model

Using a new, second-order accurate numerical method we present dynamical simulations of oblique MHD cosmic-ray (CR)-modified plane shock evolution. Most of the calculations are done with a two-fluid model for diffusive shock acceleration, but we provide also comparisons between a typical shock computed that way against calculations carried out using the more complete, momentum-dependent, diffusion-advection equation. We also illustrate a test showing that these simulations evolve to dynamical equilibria consistent with previously published steady state analytic calculations for such shocks. In order to improve understanding of the dynamical role of magnetic fields in shocks modified by CR pressure we have explored for time asymptotic states the parameter space of upstream fast mode Mach number, M(sub f), and plasma beta. We compile the results into maps of dynamical steady state CR acceleration efficiency, epsilon(sub c). We have run simulations using constant, and nonisotropic, obliquity (and hence spatially) dependent forms of the diffusion coefficient kappa. Comparison of the results shows that while the final steady states achieved are the same in each case, the history of CR-MHD shocks can be strongly modified by variations in kappa and, therefore, in the acceleration timescale. Also, the coupling of CR and MHD in low beta, oblique shocks substantially influences the transient density spike that forms in strongly CR-modified shocks. We find that inside the density spike a MHD slow mode wave can be generated that eventually steepens into a shock. A strong layer develops within the density spike, driven by MHD stresses. We conjecture that currents in the shear layer could, in nonplanar flows, results in enhanced particle accretion through drift acceleration.

Frank, Adam↗
NASA NTRS1994

Determining the standoff distance of the bow shock: Mach number dependence and use of models

We explore the factors that determine the bow shock standoff distance. These factors include the parameters of the solar wind, as well as the size and shape of the obstacle. In this report we develop a semiempirical Mach number relation for the bow shock standoff distance in order to take into account the shock's behavior at low Mach numbers. This is done by determining which properties of the shock are most important in controlling the standoff distance and using this knowledge to modify the current Mach number relation. While the present relation has proven useful at higher Mach numbers, it has lacked effectiveness at the low Mach number limit. We also analyze the bow shock dependence upon the size and shape of the obstacle, noting that it is most appropriate to compare the standoff distance of the bow shock to the radius of curvature of the obstacle, as opposed to the distance from the focus of the object to the nose. Last, we focus our attention on the use of bow shock models in determining the standoff distance. We note that the physical behavior of the shock must correctly be taken into account, specifically the behavior as a function of solar wind dynamic pressure; otherwise, erroneous results can be obtained for the bow shock standoff distance.

Farris, M. H.↗
NASA NTRS1995

Three-dimensional position and shape of the bow shock and their variation with Alfvenic, sonic and magnetosonic Mach numbers and interplanetary magnetic field orientation

A large set of bow shock crossings (i.e., 1392) observed by 17 spacecraft has been used to explore the three-dimensional shape and location of the Earth's bow shock and its dependence on solar wind and interplanetary magnetic field (IMF) conditions. This study investigates deviations from gas dynamic flow models associated with the magnetic terms in the magnetohydrodynamic (MHD) equations. Empirical models predicting the statistical position and shape of the bow shock for arbitrary values of the solar wind pressure, IMF, and Alfvenic Mach number (M(sub A)) have been derived. The resulting data set has been used to fit three-dimensional bow shock surfaces and to explore the variations in these surfaces with sonic (M(sub S)), Alfvenic (M(sub A)) and magnetosonic (M(sub MS)) Mach numbers. Analysis reveals that among the three Mach numbers, M(sub A) provides the best ordering of the least square bow shock curves. The subsolar shock is observed to move Earthward while the flanks flare outward in response to decreasing M(sub A); the net change represents a 6-10% effect. Variations due to changes in the IMF orientation were investigated by rotating the crossings into geocentric interplanetary medium coordinates. Past studies have suggested that the north-south extent of the bow shock surface exceeds the east-west dimension due to asymmetries in the fast mode Mach cone. This study confirms such a north-south versus east-west asymmetry and quantifies its variation with M(sub S), M(sub A), M(sub MS), and IMF orientation. A 2-7% effect is measured, with the asymmetry being more pronounced at low Mach numbers. Combining the bow shock models with the magnetopause model of Roelof and Sibeck (1993), variations in the magnetosheath thickness at different local times are explored. The ratio of the bow shock size to the magnetopause size at the subpolar point is found to be 1.46; at dawn and dusk, the ratios are found to be 1.89 and 1.93, respectively. The subsolar magnetosheath thickness is used to derive the polytropic index gamma according to the empirical relation of Spreiter et al. (1966). The resulting gamma = 2.3 suggests the empirical formula is inadequate to describe the MHD interaction between the solar wind and the magnetosphere.

Peredo, M.↗
NASA NTRS1993

Steady state and dynamical structure of a cosmic-ray-modified termination shock

A hydrodynamic model is developed for the structure of a cosmic-ray-modified termination shock. The model is based on the two-fluid equations of diffuse shock acceleration (Drury and Volk, 1981). Both the steady state structure of the shock and its interaction with outer heliospheric disturbances are considered. Under the assumption that the solar wind is decelerated by diffusing interstellar cosmic rates, it is shown that the natural state of the termination shock is a gradual deceleration and compression, followed by a discontinuous jump to a downstream state which is dominated by the pressure contribution of the cosmic rays. A representative model is calculated for the steady state which incorporates both interstellar cosmic ray mediation and diffusively accelerated anomalous ions through a proposed thermal leakage mechanism. The interaction of large-scale disturbances with the equilibrium termination shock model is shown to result in some unusual downstream structure, including transmitted shocks and cosmic-ray-modified contact discontinuities. The structure observed may be connected to the 2-kHz outer heliospheric radio emission (Cairns et al., 1992a, b). The time-dependent simulations also demonstrate that interaction with solar wind compressible turbulence (e.g., traveling interplanetary shocks, etc.) could induce the termination shock to continually fluctuate between cosmic-ray-dominated and gas-dynamic states. This fluctuation may represent a partial explanation of the galactic cosmic ray modulation effect and illustrates that the Pioneer and Voyager satellites will encounter an evolving shock whose structure and dynamic properties are strongly influence by the mediation of interstellar and anomalous cosmic rays.

Donohue, D. J.↗
NASA NTRS1995

Energization in regions of CIRs unconnected to shocks are probably not the result of cross-field transport

Corotating energetic ion populations (CEIPs) associated with the forward and reverse shocks of corotating interaction regions (CIRs) are observed in CIRs at places where models say are magnetically unconnected to either shock. Such disconnections between CEIPs and shocks are common and have been documented with data from Pioneers 10 and 11 and confirmed with data from Ulysses. They pose a problem for models that account for these CEIPs in terms of ion energization at the shocks followed by ion propagation along field lines. Two possible resolutions to this problem have been suggested: diffusion of the ions across field lines and extension of the ion energization process to regions beyond the shock waves. Here we quantitatively examine the first of these possibilities. We give the Green's function solution to the convection-diffusion equation applied to idealized CIR geometry, with a source at the reverse shock -- the main producer of CEIPs. Two kinds of diffusion are considered: resonant diffusion and stochastic field line diffusion. We find that for resonant diffusion the computed ratio is many orders of magnitudes below the observed ratio. For stochastic field line diffusion, the computed ratio approximately equals the observed ratio if a diffusion coefficient appropriate to the free solar wind is used. It is several orders of magnitude below the observed ratio, however, if a diffusion coefficient appropriate to CIRs is used. We conclude that cross-field diffusion probably does not account for the presence of energetic ions in regions of CIRs that are magnetically unconnected to its shock waves. We suggest that the alternative possibility -- the energetic ions in regions magnetically unconnected to shocks result from an acceleration process that is independent of shocks -- be pursued to the point where quantitative tests can be performed.

Intriligator, Devrie S.↗
NASA NTRS1995

Electron impact ionization rates for interstellar neutral H and He atoms near interplanetary shocks: Ulysses observations

During average solar wind flow conditions at 1 AU, ionization rates of interstellar neutrals that penetrate into the inner heliosphere are dominated by charge exchange with solar wind protons for H atoms, and by photoionization for He atoms. During occurrences of strong, coronal mass ejection (CME)-driven interplanetary shock waves near 1 AU, electron impact ionization can make substantial, if not dominating, contributions to interstellar neutral ionization rates in the regions downstream of the shocks. However, electron impact ionization is expected to be relatively less important with increasing heliocentric distance because of the decrease in electron temperature. Ulysses encountered many CME-driven shocks during its journey to and beyond Jupiter, and in addition, encountered a number of strong corotating interaction region (CIR) shocks. These shocks generally occur only beyond approximately 2 AU. Many of the CIR shocks were very strong rivalling the Earth's bow shock in electron heating. We have compared electron impact ionization rates calculated from electron velocity distributions measured downstream from CIR shocks using the Ulysses SWOOPS experiment to charge-exchange rates calculated from measured proton number fluxes and the photoionization rate estimated from an assumed solar photon spectrum typical of solar maximum conditions. We find that, although normally the ratio of electron-impact ionization rates to charge-exchange (for H) and to photoionization (for He) rates amounts to only about one and a few tens of percent, respectively, downstream of some of the stronger CIR shocks they amount to more than 10% and greater than 100%, respectively.

Feldman, W. C.↗
NASA NTRS1996

Interaction of a shock with a longitudinal vortex

In this paper we study the shock/longitudinal vortex interaction problem in axisymmetric geometry. Linearized analysis for small vortex strength is performed, and compared with results from a high order axisymmetric shock-fitted Euler solution obtained for this purpose. It is confirmed that for weak vortices, predictions from linear theory agree well with results from nonlinear numerical simulations at the shock location. To handle very strong longitudinal vortices, which may ultimately break the shock, we use an axisymmetric high order essentially non-oscillatory (ENO) shock capturing scheme. Comparison of shock-captured and shock-fitted results are performed in their regions of common validity. We also study the vortex breakdown as a function of Mach number ranging from 1.3 to 10, thus extending the range of existing results. For vortex strengths above a critical value. a triple point forms on the shock and a secondary shock forms to provide the necessary deceleration so that the fluid velocity can adjust to downstream conditions at the shock.

Erlebacher, Gordon↗
NASA NTRS2003

The Shock Structure of Supernova Remnant IC443

We present and discuss ISO observations of IC443, a supernova remnant interacting with a molecular cloud. An SWS spectrum centered on molecular hydrogen clump R10E (RA(2000) = 6 17 7.6, Decl(2000) = 22 25 34.6) is dominated by strong [SiII] (34 microns) emission and the pure rotational transitions of molecular hydrogen ranging from 0-0 S(1) to 0-0 S(13). Fits to these H$-2$ lines imply a large column (approx. 7E19 cm$ {-2)$) of warm (T approx. 700 K) gas and an ortho/para ratio for hydrogen near 3. LWS Fabry-Perot spectra of [OI] (63 microns) and [CII] (158 microns) at positions R10E and C (RA(2000) = 6 17 42.8, Decl(2000) = 22 21 38.1) find broad (approx. 75 km/s), blue-shifted (-40 km/s) line profiles; their similarity strongly suggests a common, shock-generated origin for these two lines. The surprisingly large [CII]/[OI] ratio (approx. 0.1 to 0.2) confirms previous observations with the Kuiper Airborne Observatory. These [CII] and [OI] line intensities, the [SiII] intensity (above), and LWS grating measurements of OH (119 microns) and [OI] (145 microns) are all readily fit by a single, fast J-shock model. Although the [OI] (63) emission can alternatively be produced by a slow C-shock, this ensemble of lines can not be produced by such a shock and provides strong evidence for the existence of a J-shock. A 24-arcmin strip map shows that this far-infrared line emission is spatially correlated with the H$-2$ 1-0 S(1) emission, which most likely arises in an associated C-shock. In addition to this spatially correlated shock emission, the strip map identifies extended [CII] and [OI] emission with a significantly larger line ratio (approx. 0.6); this 'background' component is compared with current J-shock, C-shock, photo-dissociation region (PDR), and X-ray dissociation region (XDR) models in an effort to explain its origin.

Haas, Michael R.↗