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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Constraints on Cosmic-ray Acceleration Efficiency in Balmer Shocks of Two Young Type Ia Supernova Remnants in the Large Magellanic Cloud

We present results from an optical study of two young Balmer-dominated remnants of SNIa in the Large Magellanic Cloud, 0509-67.5 and 0519-69.0, in an attempt to search for signatures of efficient cosmic-ray (CR) acceleration. Here, we combine proper motion measurements with corresponding optical spectroscopic measurements of the Hα line at multiple rim positions from VLT/FORS2 and SALT/RSS and compare our results to published Balmer shock models. Analyses of the optical spectra result in broad Hα widths in the range of 1800–4000 km s -1 for 12 separate Balmer-dominated filaments that show no evidence for forbidden line emission; the corresponding shock speeds from proper motion measurements from HST span a range of 1700–8500 km s -1 . Our measured values of shock speeds and broad Hα widths in 0509-67.5 and 0519-69.0 are fit well with a Balmer shock model that does not include effects of efficient CR acceleration. We determine an upper limit of 7%/χ (95% confidence) on the CR acceleration efficiency for our ensemble of data points, where χ is the ionization fraction of the pre-shock gas. The upper limits on the individual remnants are 6%/χ (0509-67.5) and 11%/χ (0519-69.0). Finally, these upper limits are below the integrated CR acceleration efficiency in the Tycho supernova remnant, where the shocks predominantly show little Hα emission, indicating that Balmer-dominated shocks are not efficient CR accelerators.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Electron Heating in 2D Particle-in-cell Simulations of Quasi-perpendicular Low-beta Shocks

Abstract We measure the thermal electron energization in 1D and 2D particle-in-cell simulations of quasi-perpendicular, low-beta ( β p = 0.25) collisionless ion–electron shocks with mass ratio m i / m e = 200, fast Mach number M ms = 1 –4, and upstream magnetic field angle θ Bn = 55°–85° from the shock normal n ˆ . It is known that shock electron heating is described by an ambipolar, B -parallel electric potential jump, Δ ϕ ∥ , that scales roughly linearly with the electron temperature jump. Our simulations have Δ ϕ ∥ / ( 0.5 m i u sh 2 ) ∼ 0.1 –0.2 in units of the pre-shock ions’ bulk kinetic energy, in agreement with prior measurements and simulations. Different ways to measure ϕ ∥ , including the use of de Hoffmann–Teller frame fields, agree to tens-of-percent accuracy. Neglecting off-diagonal electron pressure tensor terms can lead to a systematic underestimate of ϕ ∥ in our low- β p shocks. We further focus on two θ Bn = 65° shocks: a M s = 4 ( M A = 1.8 ) case with a long, 30 d i precursor of whistler waves along n ˆ , and a M s = 7 ( M A = 3.2 ) case with a shorter, 5 d i precursor of whistlers oblique to both n ˆ and B ; d i is the ion skin depth. Within the precursors, ϕ ∥ has a secular rise toward the shock along multiple whistler wavelengths and also has localized spikes within magnetic troughs. In a 1D simulation of the M s = 4 , θ Bn = 65° case, ϕ ∥ shows a weak dependence on the electron plasma-to-cyclotron frequency ratio ω pe /Ω ce , and ϕ ∥ decreases by a factor of 2 as m i / m e is raised to the true proton–electron value of 1836.

Tran, Aaron (ORCID:0000000334834890)↗
NASA NTRS1970

Dispersion discontinuities of strong collisionless shocks

Linear fluid equations are used to estimate wave train properties of strong collisionless shocks. Fast shocks exhibit several dispersion changes with increasing Mach number. For perpendicular propagation into a finite-beta plasma, an ion cyclotron radius trailing wave train exists only for (M sub F)2 is smaller than 2. Oblique fast shocks have a leading ion inertia wave train if M sub A is smaller than root of M(+)/M(-) cos theta/2 and a trailing electron inertia train if M sub A is greater than root of M(+)/M(-) cos theta/2. If the downstream sound speed exceeds the flow speed, linear wave theory predicts a trailing ion acoustic structure which probably resides within the magnetic shock. For a turbulent shock model in which an effective electron-ion collision frequency exceeds the lower hybrid frequency, ions decouple from the magnetic field; the shock wave train now trails with electron inertia and electron gyroradius lengths. Comparisons of this turbulent model and observations on the earth's bow shock are made.

Coroniti, F. V.↗
NASA NTRS1970

Laminar wave train structure of collisionless magnetic slow shocks

The laminar wave train structure of collisionless magnetic slow shocks is investigated using two fluid hydromagnetics with ion cyclotron radius dispersion. For shock strengths less than the maximally strong switch-off shock, in the shock leading edge dispersive steepening forms a magnetic field gradient, while in the downstream flow dispersive propagation forms a trailing wave train; dispersion scale lengths are the ion inertial length if beta is smaller than 1 and the ion cyclotron radius if beta is greater than 1. In the switch-off slow shock leading edge, dispersion only produced rotations of the magnetic field direction; the gradient of the magnetic field magnitude, and hence the shock steepening length, is determined solely by resistive diffusion. The switch-off shock structure consists of a long trailing of magnetic rotations which are gradually damped by resistivity.

Coroniti, F. V.↗
NASA NTRS1972

Interplanetary shock waves and the structure of solar wind disturbances

Observations and theoretical models of interplanetary shock waves are reviewed, with emphasis on the large-scale characteristics of the associated solar wind disturbances and on the relationship of these disturbances to solar activity. The sum of observational knowledge indicates that shock waves propagate through the solar wind along a broad, roughly spherical front, ahead of plasma and magnetic field ejected from solar flares. Typically, the shock front reaches 1 AU about two days after its flare origin, and is of intermediate strength. Not all large flares produce observable interplanetary shock waves; the best indicator of shock production appears to be the generation of both type 2 and type 4 radio bursts by a flare. Theoretical models of shock propagation in the solar wind can account for the typically observed shock strength, transit time, and shape.

Hundhausen, A. J.↗
NASA NTRS1972

Shock pair observation

On day 84, 1969, the HEOS 1 satellite observed a shock pair connected with a plasma bulk velocity increase from 400 to approximately 750 km/sec. Both shocks were fast shocks. The forward shock had a Mach number of 1.7, the reverse shock had M(fast) = 1.4. The time interval between the two shocks was 7 hrs, 10 min. The time delay between HEOS 1 and Explorer 35 reverse shock observation (20 + or - 6 min) agrees with the computed time delay (11 + or - 4 min).

Chao, J. K.↗
NASA NTRS1974

Shapes of strong shock fronts in an inhomogeneous solar wind

The shapes expected for solar-flare-produced strong shock fronts in the solar wind have been calculated, large-scale variations in the ambient medium being taken into account. It has been shown that for reasonable ambient solar wind conditions the mean and the standard deviation of the east-west shock normal angle are in agreement with experimental observations including shocks of all strengths. The results further suggest that near a high-speed stream it is difficult to distinguish between corotating shocks and flare-associated shocks on the basis of the shock normal alone. Although the calculated shapes are outside the range of validity of the linear approximation, these results indicate that the variations in the ambient solar wind may account for large deviations of shock normals from the radial direction.

Heinemann, M. A.↗
NASA NTRS1974

Jupiter entry simulation with the ANAA shock tube

An annular arc accelerator (ANAA) shock tube has been built which produces shock velocities and pressures that simulate entry into the atmosphere of Jupiter. The ANAA driver deposits the energy of an arc discharge into a flowing gas, which then expands and cools without any delay for the opening of a diaphragm. A flow transducer, trigger system, and spark gap switches have been developed to coordinate the flow from a high-pressure helium driver with the discharge from a 300-kJ capacitor bank. Shock velocities up to 47 km/sec have been produced in 1.0 torr of hydrogen with the ANAA shock tube, compared with 35 km/sec velocity produced in a conical arc driver with three times the available energy. Attenuation with the ANAA shock tube is comparable to that of a conical arc driver shock tube, and initial spectroscopic measurements indicate that an impurity-free test slug is formed behind the shock wave.

Leibowitz, L. P.↗
NASA NTRS1975

Bow shock and magnetosheath waves at Mercury

Mariner 10 measurements at the Mercury bow shock provide examples where the magnetic field is approximately parallel or perpendicular to the bow shock normal. Upstream of a broad irregular parallel shock, left hand circularly polarized waves are observed which cut off very sharply at approximately 4 Hz. Upstream of a perpendicular shock, right hand circularly polarized waves are observed which persist up to the Nyquist frequency of 12 Ha. Determination of the wave propagation vector as a function of frequency helps conclusively identify the waves as whistler mode waves propagating from the shock. The magnetosheath downstream of the parallel shock is disturbed more than that downstream of the perpendicular shock particularly below 1 Hz. In the latter case regular left hand polarized waves observed slightly above the proton gyrofrequency are identified as ion cyclotron waves with wavelength approximately 300 km which are Doppler shifted up to their observed frequency.

Fairfield, D. H.↗
NASA NTRS1974

Structure of the terrestrial bow shock

Preliminary results of a bow shock study which isolates various parameters are reported. The main findings are successful isolation of shock structures by parameter set, and correlated diagnostic behavior. Conventional shock structural nomenclature is first modified to suit the results of spacecraft observations and to provide the terminology needed in the remainder of the paper. Several examples of shock morphology for a wide range of plasma states are then displayed, as seen with various diagnostics. The bow shock is described as a system in the solar wind. Advantages of the bow shock as an object of shock investigation are pointed out, and a few aims of future study are outlined.

Greenstadt, E. W.↗
NASA NTRS1975

From regolith to rock by shock

The purpose of this study is to relate the various mechanisms by which material is shock-lithified in terrestrial analogs of the lunar regolith to specific conditions of cratering, thereby making more specific the possible conditions of formation of rock from regolith by shock processes on the lunar surface. A model for shock-lithification of terrestrial and lunar regolith is proposed in which air or an air-water mixture initially in the pores of terrestrial soil affects the behavior of a soil-air-water system under shock-loading. Shock compression of porous terrestrial regolith by relatively small impact events give rise to three pressure regimes: (1) regime 1 - at pressures below 100 kb, material is compacted and weakly shock-lithified; (2) regime 2 - at pressures between 100 and 200 kb, material may be fragmented if the induced pore pressure exceeds the strength of the weak lithification mechanisms; and (3) at pressure above 200 kb, material is strongly lithified but may greatly expand in volume due to the pressure of pore gases. The three other regimes (below 50 kb, between 50 and 100 kb, and above 100 kb) associated with shock compression of lunar regolith are identified and discussed.

Kieffer, S. W.↗
NASA NTRS1976

Three dimensional aspects of interplanetary shock waves

Most of the interplanetary shock waves observed with 1 AU of the sun originate from some short lived solar event, such as a solar flare, and then propagate out as a more-or-less spherical shock wave until they leave the solar system. Beyond 1 AU another class of interplanetary shock wave becomes common--the corotating shock pair formed by the interaction of long lived solar wind streams. The three dimensional geometry of these two classes of interplanetary shocks is discussed. Also discussed are how these geometries can be statistically studied with an out-of-the-ecliptic mission. Diagrams of shock wave propagation are shown. Also given are numerical examples of shock wave propagation.

Siscoe, G. L.↗
NASA NTRS1975

Motion of shocks through interplanetary streams

A model for the motion of flare-generated shocks through interplanetary streams is presented which illustrates the effects of a stream-shock interaction on the shock strength and geometry. It is a gasdynamic calculation based on Whitham's (1958) method and on an empirical approximation for the relevant characteristics of streams. The results show that the Mach number of a shock can decrease appreciably to near unity in the interaction region ahead of streams and that the interaction of a spherically symmetric shock with a spiral-shaped corotating stream can cause significant distortions of the initial shock-front geometry. The geometry of a shock discussed by Lepping and Chao (1972) is qualitatively explained by this model.

Burlaga, L. F.↗
NASA NTRS1976

Shock-loading meteoritic b.c.c. metal above the pressure transition - Remanent-magnetization stability and microstructure

The paper describes the remanent-magnetization stability and microstructure associated with samples of b.c.c. Fe-Ni from the Odessa meteorite which were shock loaded at 200, 400, 600, 800, and 1000 kbar. The samples include those which were transformation hardened as well as those which were altered by varying thermal levels associated only with the shock levels themselves. Demagnetization curves for an unshocked Odessa sample and for samples shocked at each level are presented along with saturation isothermal remanence demagnetization curves for samples shocked at 600 and 1000 kbar. It is found that: (1) the 200-kbar sample was shock-hardened with no significant thermal effects; (2) stability to demagnetization decreased in the 400- and 600-kbar specimens, indicating that recovery took place; and (3) the specimens shocked at 800 and 100 kbar had the greatest demagnetization stability. These three levels of shock effects are shown to be delimited by discrete levels of microhardness, microstructure, and stability of remanence to demagnetization. It is concluded that the first level is simply a consequence of antiferromagnetic-ferromagnetic reversal and that the other two levels contain a thermal component.

Wasilewski, P.↗
NASA NTRS1976

Interplanetary shocks seen by Ames plasma probe on Pioneer 6 and 7

Interplanetary shocks and discontinuities observed by the Ames Research Center plasma probe on Pioneer 6 and 7 are analyzed with Goddard Space Flight Center magnetometer data. Several shock normals are used for the MHD model of a shock where the mixed data shock normals, which use plasma and magnetic-field data, give the best agreement with the theoretical requirements. The requirements are the satisfaction of the Rankine-Hugoniot conservation equations across the shock where angles (predicted theoretically from the conservation equations) between certain combinations of plasma and magnetic-field data vectors in the shock normals are explicitly checked. The results for the August 29, 1966, shock are compared with previous results.

Abraham-Shrauner, B.↗
NASA NTRS1976

Bow shock and magnetosheath waves at Mercury

Mariner 10 measurements at the Mercury bow shock provide examples where the magnetic field is approximately parallel or perpendicular to the bow shock normal. Upstream of a broad irregular parallel shock, left-hand circularly polarized waves are observed which cut off very sharply at about 4 Hz. Upstream of a perpendicular shock, righ-hand circularly polarized waves are observed which persist up to the Nyquist frequency of 12 Hz. Determination of the wave propagation vector as a function of frequency helps to identify conclusively the waves as whistler mode waves propagating from the shock. The magnetosheath downstream of the parallel shock is disturbed more than that downstream of the perpendicular shock, particularly below 1 Hz. In the latter case, regular left-hand-polarized waves observed slightly above the proton gyrofrequency are identified as ion cyclotron waves with wavelengths of about 300 km which have been Doppler shifted up to their observed frequency.

Fairfield, D. H.↗
NASA NTRS1976

Shock-induced fine-grained recrystallization of olivine - Evidence against subsolidus reduction of Fe/2+/

Electron spin resonance (ESR) studies have been carried out on three single grains of terrestrial olivine (Fo90) shock loaded along the 010 line to peak pressures of 280, 330, and 440 kbar. The results indicate that neither metallic Fe similar to that observed in returned lunar soils nor paramagnetic Fe(3+) caused by oxidation of Fe(2+) has been produced in these shock experiments. Trace amounts of Mn (2+) have been detected in both shocked and unshocked olivine. The ESR signals of Mn(2+) show spectral features which are found to correlate with the degree of shock-induced recrystallization observed petrographically. The increasing mass fraction of recrystallized olivine correlates with increasing shock pressures. This phenomenon is modelled assuming it results from the progressive effect of the shock-induced transformation of the olivine to a yet unknown high-pressure phase and its subsequent reversion to the low-pressure olivine phase. The mass fraction of recrystallized material is predicted to be nearly linear with shock pressure.

Ahrens, T. J.↗
NASA NTRS1977

An implicit shock-fitting scheme for unsteady transonic flow computations

The alternating-direction implicit scheme developed by NASA Ames for unsteady transonic flows has been modified to include a shock-fitting algorithm as well as an analytically stretched coordinate system. The shock-fitting procedure treats shock waves as discontinuities normal to the free stream. Improvements in shock position and the unsteady pressure distributions are obtained by this modification. The various types of shock motion observed experimentally by Tijdeman are well simulated in calculations using the modified computational scheme. The method of detecting shock wave formation and the procedure for fitting a moving shock wave are illustrated. Results for a pulsating parabolic arc airfoil and for an NACA 64A006 airfoil with oscillating quarter-chord flap are presented and discussed.

Yu, N. J.↗