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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Study of fatigue durability of advanced composite materials under conditions of accelerated loading

The effect of temperature on the tension-tension fatigue life of the T300/5208 graphite/epoxy angle-ply laminate system was investigated in an effort to develop an acceptable and reliable method of accelerated loading. Typical S log sub 10 N curves were determined experimentally at 25 C, 75 C, and 115 C. The time-temperature superposition principle was employed to find the shift factors of uniaxial fatigue strength, and a general linear equation of S log sub 10 N for shifting purpose was established. The combined techniques of cyclic creep measurements and optical microscopy upon fatigue failure allow some assessment of the possible physical basis of S log 10 N curve shifting. Before fatigue, the laminates at all test temperatures and stress levels undergo a unique damage mechanism during fatigue loading. It is concluded that an accelerated loading method is feasible.

Shih, H. M.↗
NASA NTRS1982

Modeling lateral acceleration effects on pilot performance

Attendant to the direct side force maneuver of a Vectored Force Fighter is the transverse acceleration imposed on the pilot. This lateral acceleration (Gy), when combind with a positive Gz stress, is a potential source of pilot tracking performance impairment. A research effort to investigate these performance decrements includes experimental as well as anaytical pilot performance modeling using the Optimal Control Model.

Korn, J.↗
NASA NTRS1981

Radio evidence for shock acceleration of electrons in the solar corona

It is pointed out that the new class of kilometer-wavelength solar radio bursts observed with the ISEE-3 Radio Astronomy Experiment occurs at the reported times of type II events, which are indicative of a shock wave. An examination of records from the Culgoora Radio Observatory reveals that the associated type II bursts have fast drift elements emanating from them; that is, a herringbone structure is formed. It is proposed that this new class of bursts is a long-wavelength continuation of the herringbone structure, and it is thought probable that the electrons producing the radio emission are accelerated by shocks. These new events are referred to as shock-accelerated events, and their characteristics are discussed.

Cane, H. V.↗
NASA NTRS1981

Acceleration response of fuselage sidewall panels on a twin-engine, light aircraft

A response analysis is carried out to determine the predictability of sidewall accelerations in aircraft, to investigate whether it is necessary to model the exterior pressure as a traveling wave as opposed to a standing wave, and to determine the importance of parameters describing the exterior pressure and sidewall panel. Prediction of the acceleration response is found to be improved by including traveling wave and spatial variation effects of the exterior pressure in the model. Variations with propeller rpm are found to be important for the longer panel.

Roussos, L. A.↗
NASA NTRS1981

Observations of the ionization states of energetic particles accelerated in solar flares

The ionization states and spectra of 0.3 to 2.4 MeV/nuc He, C, O and Fe are measured in a survey of ten solar flare particle events. He(plus) is found to be present in all events, which indicates the common presence of energetic He(plus) in the source material from which solar particles are accelerated. The distribution functions of He(plus), He(plus plus) and heavier elements are represented by simple exponentials of the particle speed times its rigidity to a power n, where n is between 0 and 1, and equal e-folding values. Results are consistent with a model where ions are accelerated in the corona by multi-dimensional shocks out of a population taken from both hot and cold coronal regions.

Gloeckler, G.↗
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 NTRS1981

The kilometric radio emission spectrum - Relationship to auroral acceleration processes

Using current theories for the generation of auroral kilometric radiation, the possibilities for obtaining information on the auroral acceleration processes from the spectrum of this radiation are discussed. It is shown, for example, that under certain conditions the low-frequency cutoff provides a direct indication of the upper altitude limit of the acceleration region. What is more, certain discrete features in the kilometric radiation spectrum show a very close similarity to whistler-mode emissions, thereby suggesting that the basic instability mechanisms are quite similar. Certain of these discrete features have frequency drifts which can be interpreted in terms of the propagation of shock-like disturbances along the auroral field lines at velocities near the ion-acoustic speed.

Gurnett, D. A.↗
NASA NTRS1981

Electron and ion acceleration by strong electrostatic turbulence

It is noted that, for waves observed on auroral field lines, ion trapping may be important, affecting models of transverse ion acceleration. The strength of the electrostatic ion cyclotron turbulence observed in the auroral zone may be attributed to the large convective growth rate that is associated with this mode. These waves may be destabilized by the enhancement in the field-aligned current that results from the onset of a magnetospheric substorm. The rise in the field-aligned current is associated with the passage of shear mode Alfven waves between the magnetosphere and ionosphere. When these Alfven waves have a perpendicular wavelength comparable to the ion gyroradius, they carry a parallel electric field that can accelerate the plasma in the topside auroral ionosphere, destabilizing the electrostatic ion cyclotron mode.

Lysak, R. L.↗
NASA NTRS1982

Acceleration mechanisms for energetic particles in the earth's magnetosphere

By analyzing data on energetic particle fluxes measured simultaneously with detector systems on several earth satellites, signatures of different acceleration mechanisms for these particles were searched for. One of the samples is an event observed on ATS-6 and IMP-7. IMP-7 was in the dusk quarter at 38 earth radii while ATS-6 was located at local midnight at a distance of 6.6 earth radii. Although the flux variations as observed on the two spacecraft both showed 1.5 min periodicity, there was a 40-second time lag with IMP-7 behind. The results indicate that the particles are accelerated by magnetic field line annihilation, with the x-point located at about 10 earth radii.

Schiferl, S.↗
NASA NTRS1982

Particle drift, diffusion, and acceleration at shocks

The gradient and curvature drifts implicit in change of the ambient magnetic field at a hydromagnetic shock wave are incorporated into the diffusive theory of shock acceleration of charged particles. The conventional jump condition at the shock is modified by a term incorporating the large drift along the shock plane. This term vanished identically for one-dimensional systems, but must be included in general for shocks which are finite in transverse extent or which have transverse structure. It is found that the effect of the drift is such that the transverse drift rate is proportional to the acceleration rate, and for perpendicular shocks is exactly equal to the rate of change of energy in the V x B electric field observed in the shock frame. This establishes a connection with the 'shock drift' models which neglect diffusion.

Jokipii, J. R.↗
NASA NTRS1982

The role of betatron acceleration in complex solar bursts

An explanation in terms of betatron acceleration is attempted for two prominent features of solar hard X-ray emission profiles: the multiplicity manifested by complex impulsive events, and the intense, long-enduring radiation typical of two-stage or extended events. The model employs the basic characteristics of a simple magnetic trap. The observed spectral evolution can be represented effectively by a set of characteristics derived in a thick-target representation. The application to multiple-spike and two-stage bursts is studied, and it is found that impulsive events show no signs of betatron acceleration. However, during the first few minutes of the second stage, the nonimpulsive X-ray emission appears to be consistent with the predictions of the model. Several intense extended events are analyzed and the betatron process is established as a viable candidate for producing the early second-stage emission.

Karpen, J. T.↗
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

Electron acceleration and radiation signatures in loop coronal transients

It is proposed that in loop coronal transients an erupting loop moves away from the solar surface, with a velocity exceeding the local Alfven speed, pushing against the overlying magnetic fields and driving a shock in the front of the moving part of the loop. Lower hybrid waves are excited at the shock front and propagate radially toward the center of the loop with phase velocity along the magnetic field that exceeds the thermal velocity. The lower hybrid waves stochastically accelerate the tail of the electron distribution inside the loop. The manner in which the accelerated electrons are trapped in the moving loop are discussed, and their radiation signature is estimated. It is suggested that plasma radiation can explain the power observed in stationary and moving type IV bursts.

Vlahos, L.↗
NASA NTRS1982

Effect of physical training in cool and hot environments on +Gz acceleration tolerance in women

Acceleration tolerance, plasma volume, and maximal oxygen uptake were measured in 15 healthy women before and after submaximal isotonic exercise training periods in cool and hot environments. The women were divided on the basis of age, maximal oxygen uptake, and +Gz tolerance into three groups: a group that exercised in heat (40.6 C), a group that exercised at a lower temperature (18.7 C), and a sedentary control group that functioned in the cool environment. There was no significant change in the +Gz tolerance in any group after training, and terminal heart rates were similar within each group. It is concluded that induction of moderate acclimation responses without increases in sweat rate or resting plasma volume has no influence on +Gz acceleration tolerance in women.

Brock, P. J.↗
NASA NTRS1982

Chronic acceleration and brain density

Tests carried out on rabbits show that the effect of chronic acceleration is not uniform among the various tissues studied. Although body mass is reduced by the treatment, as expected, no change is apparent in brain mass or in the density of cerebrospinal fluid. Acceleration-induced changes are encountered in tissue density, the myocardium exhibiting a transient increase followed by an exponential decrease toward a limit and the brain showing an arithmetic increase in density with continued exposure to 2.5 G. The data are seen as suggesting that a specific brain load is not a regulated phenomenon and that no physiological processes occur to attenuate the increased load imposed by the hyperdynamic environment. An equation is derived indicating that the stimulus potential per unit of brain load increases with body size, even though brain density decreases and cerebrospinal fluid density increases.

Hoffman, L. F.↗
NASA NTRS1982

Energetic protons accelerated at corotating shocks - Pioneer 10 and 11 observations from 1 to 6 AU

Vector helium magnetometer and proton telescope data are used to examine the relationship between low energy proton increases and corotating interaction regions (CIRs). A general correlation is noted between the maximum CIR field intensity and the maximum proton count rate. The minimum proton flux, located between the two proton maxima, appears to be correlated with the maximum field strength of the CIR. The evidence presented strongly supports shock acceleration as the primary source of the 1-MeV protons. A schematic figure incorporating many of the features deduced in the present study is given to illustrate the relationship between energetic protons, forward and reverse shocks, and the interplanetary magnetic field structure. The predictions of various theories and mechanisms for interplanetary nucleon acceleration are discussed in light of the experimental results presented.

Tsurutani, B. T.↗
NASA NTRS1982

The acceleration of charged particles in interplanetary shock waves

Consideration of the theoretical and observational literature on energetic ion acceleration in interplanetary shock waves is the basis for the present discussion of the shock acceleration of the solar wind plasma and particle transport effects. It is suggested that ISEE data be used to construct data sets for shock events that extend continuously from solar wind to galactic cosmic ray energies, including data for electrons, protons, alphas and ions with Z values greater than 2.0, and that the temporal and spatial evolution of two- and three-dimensional particle distribution functions be studied by means of two or more spacecraft.

Pesses, M. E.↗
NASA NTRS1982

The dynamics of accelerating coronal bullets

Results are presented of computer simulations of the jets that accelerate through the corona at velocities of 50 to 400 km/s. Particular emphasis is placed on the sensitivity of the induced acceleration to the form in which energy is put into the system. A comparison is made between the observed and predicted physical characteristics of the high-velocity bullets; the potential contribution of the bullets to the mass and energy balance of the solar corona is considered. It is found that the velocity and temperature evolution of the bullets can be modeled successfully by assuming energy input in the form of an external force, pushing continuously on the ejected material. From the physical characteristics of the model bullets and the energy input required to reproduce the observations, it is concluded that the bullets may constitute a significant fraction of the coronal mass flux but only a negligible component of the coronal energy budget.

Karpen, J. T.↗