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At least 1,333 records · Page 74

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NASA NTRS1979

High temporal resolution observations of electron heating at the bow shock

Results deduced from highly time-resolved electron plasma profiles of earth's bow shock obtained with fast-plasma-experiment instrumentation on ISEE 1 and 2 are presented. Emphasis is placed on those bow-shock crossings that occurred during periods of high-data-rate transmission, so that the detailed structure of the bow-shock transition for electrons is discerned. The measurements indicate that electron thermalization and density compression are generally synchronized, although exceptions to this rule occur. In a few examples where direct comparison with magnetic-field measurements is possible, the electron-temperature and density profiles at the bow shock are found to be nearly identical to that of the field intensity. The measurements also reveal an interesting feature of the bow-shock profile, viz., an electron-pressure overshoot lasting several tens of seconds and generally followed by an undershoot, which gives the shock profile the appearance of a damped wave.

Bame, S. J.↗

NASA NTRS1979

An equipotential model for auroral arcs - The theory of two-dimensional laminar electrostatic shocks

The two-dimensional current-driven shock theory of Swift (1976) is reformulated in a way that removes previous restrictions on the shock thickness in relation to an ion gyrodiameter and also makes it possible to include effects of finite gyrational energy of the ions. The theory is applied to a shock model consisting of cold streaming electrons of magnetospheric origin and streaming ions of ionospheric origin, and it is shown that shock widths will be slightly less than the gyrodiameter of an ion whose energy is equal to the maximum potential difference across the shock. It is also shown that ions in passing through the shock may gain gyrational energy on the order of 10% of the parallel and the E x B energy gain. The theory also requires that the electron beam flux be largest on field lines where the precipitating electron experiences the largest energy gain.

Swift, D. W.↗

NASA NTRS1979

Molecule formation and infrared emission in fast interstellar shocks. I Physical processes

The paper analyzes the structure of fast shocks incident upon interstellar gas of ambient density from 10 to the 7th per cu cm, while focusing on the problems of formation and destruction of molecules and infrared emission in the cooling, neutral post shock gas. It is noted that such fast shocks initially dissociate almost all preexisting molecules. Discussion covers the physical processes which determine the post shock structure between 10 to the 4 and 10 to the 2 K. It is shown that the chemistry of important molecular coolants H2, CO, OH, and H2O, as well as HD and CH, is reduced to a relatively small set of gas phase and grain surface reactions. Also, the chemistry follows the slow conversion of atomic hydrogen into H2, which primarily occurs on grain surfaces. The dependence of this H2 formation rate on grain and gas temperatures is examined and the survival of grains behind fast shocks is discussed. Post shock heating and cooling rates are calculated and an appropriate, analytic, universal cooling function is developed for molecules other than hydrogen which includes opacities from both the dust and the lines.

Hollenbach, D.↗

NASA NTRS1980

Conditions for acceleration of energetic ions greater than 30 keV associated with the earth's bow shock

A statistical analysis of particles (greater than 30 keV/charge) upstream of the earth's bow shock is conducted and shows that the rate of occurrence of upstream particle events is relative to the angle between the magnetic field and the shock normal at the shock intersection point as well as relative to the angle between the magnetic field and the radial direction (i.e., the sun-earth line). In addition, the occurrence rate of upstream particle events relative to the bow shock connection time of a field line convected with the solar wind is presented for a model bow shock. A linear dependence of the diffusion coefficient on energy per charge is apparent with the value of the mean free path of a 30-keV proton found to be about 4 earth radii, and the free escape boundary to be at about 30 earth radii in front of the bow shock.

Scholer, M.↗

NASA NTRS1979

Particle acceleration by propagating interplanetary shocks

The process of charged particle acceleration in interplanetary shocks has been simulated for typical parameters. Since space probe observations of charged particle fluxes are the principle means of inferring the action of an acceleration mechanism, the simulation was designed to follow particles backwards in time from a given observing point, usually 1 AU. This allowed a full diagnosis of which observed particles had interacted with an oncoming shock, where the interacting occurred, and by how much the energy had been changed by the shock interaction. Simple assumptions about the preshock energy spectrum allow the construction of a full temporal profile of the expected intensity, anisotropy, and energy spectrum. The simulations are apparently capable of reproducing the main features of the less than 10 MeV/nuc ion flux enhancements observed at interplanetary shocks using only a laminar interplanetary magnetic field, Archimidean spiral, and a spherical oblique shock with typical speed, strength, and shock normal-magnetic field angle.

Armstrong, T. P.↗

NASA NTRS1980

Comparison of experimental and computational shock structure in a transonic compressor rotor

Measurement of passage shock strength in a transonic compressor rotor using a gas fluorescent technique revealed an unexpected variation in shock strength in the radial direction. An axisymmetric idealization would normally predict that the passage shock strength would gradually weaken when moving radially inward until disappearing at the sonic radius. However, the measurements indicated a sharp peak in strength at the nominal sonic radius. Blade boundary layer separation originating at this point accounts for about one half of the total rotor losses. A numerical computation of the three-dimensional inviscid flow, using time-marching techniques, has accurately predicted in general the radial and tangential variations in passage shock strength and in particular the sharp pressure peak at the nominal sonic radius. The overall shock strength was somewhat overpredicted, but this overprediction may be the result of boundary layer separation in the experiment. This paper presents comparisons between the optical density measurements and computational results and in addition a short analytical discussion which demonstrates that the sharp shock strength rise may occur in many transonic compressor rotors.

Haymann-Haber, G.↗

NASA NTRS1980

The solar wind interaction with Venus - Pioneer Venus observations of bow shock location and structure

Pioneer Venus observations are used in carrying out a study of the location and structure of the Venus bow shock. The trace of the shock in the solar wind aberrated terminator plane is almost circular at an altitude of 1.38 Venus radii independent of interplanetary magnetic field orientation with an extrapolated subsolar height of 0.38 Venus radii. Gas dynamic relations and scaling of the terrestrial analogue are used in determining the effective impenetrable obstacle altitude from the mean shock surface with the conclusion that it lies beneath the observed height of the ionopause. The short-term variability in shock position is similar to that found at the earth; over the long-term bow shock, altitude varies by up to approximately 35% in phase with the solar cycle for reasons other than changing solar wind Mach number. In contrast to ionopause position, which is shown to be well determined by external pressure measurements, it is found that bow shock altitude is only weakly dependent on ionopause height and solar wind dynamic pressure.

Slavin, J. A.↗

NASA NTRS1981

Solar radio bursts of spectral type II, coronal shocks, and optical coronal transients

An examination is presented of the association of solar radio bursts of spectral type II and coronal shocks with solar flare ejecta observed in H-alpha, the green coronal line, and white-light coronagraphs. It is suggested that fast-moving optical coronal transients should for the most part be identified with piston-type phenomena well behind the outward-traveling shock waves that generate type II radio bursts. A general model is presented which relates type II radio bursts and coronal shocks to optically observed ejecta and consists of three main velocity regimes: (1) a quasi-hemispherical shock wave moving outward from the flare at speeds of 1000-2000 km/sec and Alfven Mach number of about 1.5; (2) the velocity of the piston driving the shock, on the order of 0.8 that of the shock; and (3) the regime of the slower-moving H-alpha ejecta, with velocities of 300-500 km/sec.

Maxwell, A.↗

NASA NTRS1981

Thermal shock resistance of silicon oxynitride

The thermal shock resistance of Si2N2O refractory material was studied. The thermal expansion coeff. is 3.55x10 to the -6th power at 20 to 800 C and 2.86x10 to the -6th power m/m/deg at 20 to 200 C. The breaking loads are high at high stress. Young's modulus E and the shear modulus G decrease linearly with increasing porosity. For dense material E sub o approx. = 216,500 N/mm2 and G approx = 90,600 N/mm2. The Vickers hardness of the dense material is comparable to that of sapphire. The results on thermal shock show that R, the breaking load, stays constant for T T sub c, the first cracks appear and R decreases sharply for T=T sub c. As the severity of the thermal shock is increased at T T sub c, a small no. of new, large-size cracks appears. The shock's cumulative effect is negligible, and repeated shocks do not change the cracks. The low values of the thermal expansion coefficient and Young's modulus and the high tension breaking load are considered. Sintered Si2N2O with 5% MgO shows excellent cracking resistance under thermal shock.

Glandus, J. C.↗

NASA NTRS1981

Acceleration of electrons by interplanetary shocks

Ion acceleration is a well known phenomenon at both interplanetary shocks, while there have been only a few reports concerning an acceleration of electrons at interplanetary shocks. The considered investigation reports that electron acceleration occurs at many shocks. The instrumentation used is discussed, taking into account the ISEE 3 (International Sun Earth Explorer) spacecraft and the electrostatic analyzer employed for the measurement of electrons. Changes in the electron distribution function caused by shocks are found to be quite varied. Some events show a simple step to a higher flux level, some show fluctuations as the flux rises, and some have one or more pulses at the time of the shock. Attention is given to data obtained on July 26, 1979, December 25, 1978, and November 12, 1978. A calculation of shock geometries is also reported.

Potter, D. W.↗

NASA NTRS1981

First-order Fermi acceleration of the diffuse ion population near the earth's bow shock

The flux of 30-65 keV particles observed by the ISEE-3 200 earth radii upstream is shown to be an upstream escape of the energetic ions in the earth's bow shock. A formal solution to the transport equation for the distribution function of energetic particles upstream from an isotropic monoenergetic source of particles/sq cm at a plane shock where the plasma changes speed is found, and escape conditions are defined. The efficiency of the acceleration is calculated to depend on the charge/particle, and fluxes near and far upstream of the shock are described analytically. Any model which takes into account shock acceleration by diffusive scattering with significant escape losses produces the observed spectrum close to the shock. The escape loss upstream is demonstrated to control the spectrum and the variation of flux and anisotropy with distance from the shock.

Forman, M. A.↗

NASA NTRS1982

Motions of the cloud medium behind large scale galactic shocks

Mechanisms of decelerating the cloud medium in the large-scale galactic shock are studied. It is shown that the process of cloud-cloud collisions, which results in diffusive momentum transport and hence gives rise to the turbulent viscosity, is very effective in slowing down the cloud medium so that the postshock velocity of the intercloud medium can be matched within a short distance behind the shock front. The drag force exerted by the slow-moving intercloud medium alone is simply not enough to effectively decelerate the cloud medium in the shock. By the use of the results of Shu et al (1972), the internal structure of the shock of the cloud medium is analyzed by including turbulent viscous effects. The thickness of the shock is found to be on the order of 100 pc if the turbulent viscosity is taken proportional to the mean free path of the cloud-cloud collisions. The phase transition takes place in an even thinner layer on the order of 10 pc immediately after the viscous shock front of the intercloud medium.

Yuan, C.↗

NASA NTRS1982

Heating of the corona and solar wind by switch-on shocks

The possibility is examined that the corona is heated by a train of weak switch-on shocks which are formed in the chromosphere from a train of Alfven waves, and which subsequently enter the corona from below. New results for the shock train propagation and dissipation and the resultant coronal heating are derived. It is shown that most of the energy in the shock train can be dissipated within one or two solar radii above the coronal base. A train of switch-on shocks therefore represents a viable coronal heating mechanism. The results are generalized to switch-on shocks in the solar wind. It is shown that such shocks can dissipate rapidly, but it is concluded that they are not the dominant factor governing the evolution of the solar wind turbulence.

Hollweg, J. V.↗

NASA NTRS1982

Coupled hydromagnetic wave excitation and ion acceleration upstream of the earth's bow shock

A self-consistent theory is set forth for the excitation of hydromagnetic waves and the acceleration of 'diffuse' ions upstream of the earth's bow shock in the quasi-equilibrium that results when the solar wind velocity and the interplanetary magnetic field are nearly parallel. For the waves, the quasi-equilibrium derives from a balance between excitation by the ions, which stream relative to the solar wind plasma, and convective loss to the magnetosheath. For the diffuse ions, the quasi-equilibrium derives from a balance between injection at the shock front, confinement to the foreshock by pitch angle scattering on the waves, and acceleration by compression at the shock front. It also results from loss to the magnetosheath, loss due to escape upstream of the foreshock, and loss via diffusion perpendicular to the average magnetic field onto field lines that do not connect to the shock front. Diffusion equations describing the ion transport and wave kinetic equations describing the hydromagnetic wave transport are solved self-consistently to give analytical expressions. These describe (1) the differential wave intensity spectrum as a function of frequency and distance from the bow shock and (2) the ion omnidirectional distribution functions and anisotropies as functions of energy and distance from the bow shock.

Lee, M. A.↗

NASA NTRS1982

Three-dimensional shock structure in a transonic flutter cascade

Rapid double-pulse holography was employed to obtain detailed, two-dimensional images of the shock forming during simulated flutter in a transonic flowfield. The experiment comprised a linear cascade of airfoils externally oscillated in torsion and viewed tangentially at the shock surface. Three biconvex airfoils were subjected to harmonic pitching motion about the midchord axis at a frequency of 0.53 while immersed in a Mach 0.81 flow. Failure to produce observable shocks led to use of choked flow with a Mach number near one, of which 50 holograms were taken. The images revealed a narrow shock surface with a spanwise variation in the shock properties. The method is concluded to be useful for examining transonic flowfield shocks in the presence of airfoil flutter.

Boldman, D. R.↗

NASA NTRS1982

Transient phenomena of shock-induced turbulent separation for a spikebody and stalling airfoil at transonic and supersonic speeds

The time-dependent, compressible, Reynolds-averaged, full Navier-Stokes equations are applied to solve an axisymmetric flow around a forward-facing stepbody (spikebody) at supersonic speeds and a stalling airfoil at transonic speeds. Important transient and unsteady phenomena, not yet well understood, are examined, and significant new findings of the present solution to the phenomena are discussed. The phenomena described in detail are as follows: The evolution of the shock wave pressure built up by the impact of the pressure waves, one from the trailing edge; the separation of the flow as influenced by the shock wave; the location of the reversed flow, the separation point, and the reattachment point; and the transient (or unsteady) phenomena of the flow pulsation, oscillation, and stalling of the body and airfoil wake flow. The numerical results show that the transient flow instability is caused by a supersonic jet induced in the separation bubble by the shock-bifurcation (lambda shock) mechanism between the separation shock and the reflected shock. Pulsation and stall phenomena are caused by a sudden increase in the leading-edge pressure due to the jet and the separation bubble interacting along the stagnation point flow.

Yoshikawa, K. K.↗

NASA NTRS1981

Shock waves, atmospheric structure and mass loss in Miras

Large amplitude shock waves are observed to be present in the atmospheres of the Mira variables: spectral line doubling with Delta v 30 km/s is present in infrared spectra. Even the visible spectra contain some evidence for such shocks. These shocks are sufficiently large to clearly dominate the energy balance of the atmosphere. Mira variables also show symptoms of substantial mass loss rates: they are strong maser and infrared continuum sources and have strong circumstellar absorption features. The pulsation induced shocks which are seen to be present are obvious suspects for causing or enhancing the mass loss from these stars. The Miras thus present an ideal case for the study of dynamical effects on atmospheric structure, since both the dynamics and the results are clearly observable. The results are given of calculations of the thermalization and cooling of the shock heated material passing through shock fronts whose properties were selected to be consistent with both the isothermal models and the spectroscopic observations.

Willson, L. A.↗

NASA NTRS1983

Electron velocity distributions near the earth's bow shock

New information is presented on the general characteristics of electron distribution functions upstream, within, and downstream of the earth's bow shock, thereby providing new insights into the instabilities in collisionless shocks. The results presented are from a survey of electron velocity distributions measured near the earth's bow shock between October 1977 and December 1978 using the Los Alamos/Garching plasma instrumentation aboard ISEE 2. A wide variety of distribution shapes is found within the different plasma regions in close proximity to the bow shock. It is found that these shapes can be classified into general types that are characteristic of three different plasma regions, namely the upstream region or electron foreshock, the shock proper where most of the heating occurs, and the downstream region or the magnetosheath. Evidence is provided that field-aligned, rather than cross-field, instabilities are the major source of electron dissipation in the earth's bow shock.

Feldman, W. C.↗