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At least 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Acceleration of convergence of vector sequences

A general approach to the construction of convergence acceleration methods for vector sequence is proposed. Using this approach, one can generate some known methods, such as the minimal polynomial extrapolation, the reduced rank extrapolation, and the topological epsilon algorithm, and also some new ones. Some of the new methods are easier to implement than the known methods and are observed to have similar numerical properties. The convergence analysis of these new methods is carried out, and it is shown that they are especially suitable for accelerating the convergence of vector sequences that are obtained when one solves linear systems of equations iterative. A stability analysis is also given, and numerical examples are provided. The convergence and stability properties of the topological epsilon algorithm are likewise given.

Sidi, A.↗
NASA NTRS1986

Energetic ions upstream of planetary bow shocks: Fermi acceleration or leakage

Spacecraft observations of Jupiter, Saturn, and Earth planetary bow shocks are assessed. For Jupiter, the unique composition of magnetospheric plasma, where oxygen and sulfur ions are major components, and the simultaneous presence of MeV electrons enable identification of the upstream particles as originating from within the magnetosphere. The observations at Saturn are phenomonologically similar, but no unique tracer element exists which can identify the origin. Earth observations show that substantial energetic particle activity occurs within the Earth's plasma sheet when upstream ions and electrons are observed in the interplanetary medium. Observations of the three planets are discussed to compare in-situ acceleration in the foreshock region vs. leakage of already accelerated ions from the parent magnetosphere. It is concluded that the evidence favors magnetospheric leakage.

Krimigis, S. M.↗
NASA NTRS1986

Shock acceleration of electrons and ions in solar flares

The simultaneous first-order Fermi shock acceleration of electrons, protons, and alpha particles are compared to observations of solar energetic particle events. For each event, a unique shock compression ratio in the range approx. 1.6 to 3 produces spectra in good agreement with observation. The range in compression ratios predicts that the more than 5 orders of magnitude spread in electron to proton intensity ratios observed at MeV energies is compressed to about 3 orders of magnitude at an assumed injection energy of 100 keV. The remaining spread can be accounted for with a modest range of injection conditions. The model predicts that the acceleration time to a given energy will be approximately equal for electrons and protons and, for reasonable solar parameters, can be on the order of 1 second to approx. 100 MeV.

Ellison, Donald C.↗
NASA NTRS1987

Numerical modeling of on-orbit propellant motion resulting from an impulsive acceleration

In-space docking and separation maneuvers of spacecraft that have large fluid mass fractions may cause undesirable spacecraft motion in response to the impulsive-acceleration-induced fluid motion. An example of this potential low gravity fluid management problem arose during the development of the shuttle/Centaur vehicle. Experimentally verified numerical modeling techniques were developed to establish the propellant dynamics, and subsequent vehicle motion, associated with the separation of the Centaur vehicle from the shuttle orbiter cargo bay. Although the shuttle/Centaur development activity was suspended, the numerical modeling techniques are available to predict on-orbit liquid motion resulting from impulsive accelerations for other missions and spacecraft.

Aydelott, John C.↗
NASA NTRS1987

Fast ion mass spectrometry and charged particle spectrography investigations of transverse ion acceleration and beam-plasma interactions

Ion acceleration transverse to the magnetic field in the topside ionosphere was investigated. Transverse acceleration is believed to be responsible for the upward-moving conical ion distributions commonly observed along auroral field lines at altitudes from several hundred to several thousand kilometers. Of primary concern in this investigation is the extent of these conic events in space and time. Theoretical predictions indicate very rapid initial heating rates, depending on the ion species. These same theories predict that the events will occur within a narrow vertical region of only a few hundred kilometers. Thus an instrument with very high spatial and temporal resolution was required; further, since different heating rates were predicted for different ions, it was necessary to obtain composition as well as velocity space distributions. The fast ion mass spectrometer (FIMS) was designed to meet these criteria. This instrument and its operation is discussed.

Gibson, W. C.↗
NASA NTRS1986

Shock acceleration of nucleons at 16 deg and above solar latitude associated with interplanetary corotating interaction regions

The analysis of magnetic field and nuclei measurements in the 1 to 10 MeV/nucleon range for protons and helium at about 16 deg N solar latitude near solar cycle minimum (1975-76) shows both: (1) that nucleons are accelerated locally in corotating shocks, and (2) that the associated recurring corotating interaction regions (CIRs) at about 16 deg latitude have approximately the same temporal and spatial parameters found for CIRs in the equatorial plane at radial distances less than 5 AU. The evidence indicates that both the CIR structures and nucleon acceleration by shocks extend to intermediate solar latitudes and are likely to be observed during the Ulysses Mission out of the ecliptic plane.

Simpson, J. A.↗
NASA NTRS1986

Nonlinear interfaces for acceleration-commanded control of spacecraft and manipulators

Nominal command generation in real time for the control of manipulators or of maneuvering spacecraft is hampered by the nonlinearity of the equations of motion. Likewise the real time tracking of a computed nominal trajectory in the presence of disturbances requires the computation of time-varying Jacobians of the motion. An alternative approach is the formulation of acceleration-commanded control laws in appropriately chosen generalized advantageous to design dedicated circuit interfaces to perform the required transformation. It is also possible to guarantee that actuator and sensor saturation limits are not exceeded, by means of feedback-biased circuits that implement automatic overload limitation of acceleration commands. Recent developments following this 'hardware computation' point of view will be discussed.

Dwyer, T. A. W., III↗
NASA NTRS1986

An acceleration mechanism for cometary plasma tails

Cometary plasma tail formation by the interaction between the solar wind plasma flow and the plasma at the head of the coma is discussed using the unipolar electric generation theory. The plasma in the 'plasma tail' is almost directly accelerated from the cometary ionopause along the sun-nucleus line where the tail current flows. For steady state solar wind conditions, the cometary 'plasma tail' velocity distribution is obtained self-consistently. The solution of a kinetic equation gives the velocity of the cometary plasma as a function of the cometary tail position. The characteristic length is 3 x 10 to the 6th km when the plasma density near the nucleus of the comet is 10 to the 6th/cu cm and the component of the interplanetary magnetic field perpendicular to the solar wind flow is 3 nT. The tailward cometary plasma is finally accelerated to the speed of the solar wind. The theory is compared with the observational plasma velocities in the tails of comet Bennett (1970II) and Comet Halley (1985).

Minami, S.↗
NASA NTRS1986

Radio evidence for a delayed acceleration process in solar flares

Observations of six delayed solar radio bursts at dm and mm frequencies are analyzed. The events included five Type II bursts. The data comprise 17 GHz interferometer data and ISEE-3 and SMM hard X-ray spectrometry data which peaked 0.5-1.0 hr after the main radio bursts. The data indicate the electrons with energies in the MeV range continue to be excited for tens of minutes after the impulsive phase acceleration. The continuing acceleration occurs in a large magnetic structure extending to at least 200,000 km altitude. The radio signals arise from a columnar source, the microwave signals being emitted near a leg or legs and meterwave emissions originating from the top of the magnetic structure.

Kai, K.↗
NASA NTRS1986

The identification of solar He-3-rich events and the study of particle acceleration at the sun

Kilometric radio observations are to associate type III radio bursts with solar He-3-rich events and to identify and study the sources of those events at the sun and the transport of the particles outward toward earth. The events exhibit an impulsive behavior that earns them a natural role in the event classification scheme based upon the time scale of the particle acceleration process. Multiple He-3-rich events are observed frequently from a single active region where they provide a convenient signature to test theories of particle storage and transport. These multiple events act as impulsive probes of the condition of the interplanetary medium. Their profiles contrast with the source-acceleration-induced profiles of the long-duration events from the same active region that are interspersed among them.

Reames, D. V.↗
NASA NTRS1987

Motions in fluids caused by microgravitational acceleration and their modification by relative rotation

The primary reason for conducting many materials science experiments in space is to minimize or eliminate undesirable effects that might result because of convective motions in fluids which are driven by buoyancy effects. Of particular interest are the low-frequency accelerations caused by the earth's gravity gradient field, spacecraft attitude motions, and atmospheric drag, since it is known that these can give rise to sustained fluid motion. In order to gain a limited understanding of the effects of these accelerations their magnitude and orientation have been examined, and the Stokes' motion of a spherical particle in a fluid has been calculated for various types of spacecraft attitude motions. In addition, the effect of slowly rotating the experimental system relative to the spacecraft (in order to increase the residence time of the particle in the neighborhood of its initial position) is assessed.

Alexander, J. Iwan D.↗
NASA NTRS1986

Observations of coherent transverse ion acceleration

DE-1 retarding ion mass spectrometer (RIMS) observations of transverse O(+) acceleration in the topside ionosphere are reported and analyzed. The operation and capabilities of RIMS are reviewed, and the data are presented graphically and characterized in detail. Torus or ring O(+) distributions with radii 10 km/s are observed, consistent with coherent transverse acceleration to 10-eV energies in the bulk-plasma reference frame, and conical hot tails at energies above the 50-eV RIMS maximum are noted. Possible mechanisms for these phenomena are discussed.

Moore, T. E.↗
NASA NTRS1986

Acceleration of energetic oxygen (E greater than 137 keV) in the storm-time ring current

Measurements obtained with the medium-energy particle analyzer of the equatorial-orbit AMPTE/CCE satellite during a geomagnetic storm on September 4-7, 1984 are reported and analyzed, with a focus on the high-energy populations (H, He, and CNO-group ions with E greater than 56, 72, and 137 keV, respectively). During the main phase of the storm, ring-current-region increases in O(+) intensity by factors up to 2000 are observed at L = 2.5-7.0, and the component at L = 3.5-5.5 is attributed to a 1.5-earth-radius inward displacement of the prestorm energetic-oxygen population followed by betatron acceleration. Alternative oxygen sources and/or acceleration mechanisms to account for the component at L = 6.5-8 and pitch angle 90 deg are discussed.

Lui, A. T. Y.↗
NASA NTRS1986

A comparison of plasma waves produced by ion accelerators in the F-region ionosphere

Ion beams injected into the ionosphere are known to produce waves related to the normal modes of the plasma. The spectra of plasma waves produced during four sounding rocket experiments are examined. The experimental conditions were somewhat different during each experiment. The accelerated ion was either Xe(+) or Ar(+) and the experimental geometry, described by the separation vector between the plasma wave receiver and the ion accelerator, was either parallel or perpendicular to the geomagnetic field.

Kintner, P. M.↗
NASA NTRS1986

Ion acceleration in laboratory plasmas

Recent experiments which investigate ion acceleration in 'collisionless' laboratory plasmas are described. The experiments fall into two classes - those involving stationary and those involving time dependent processes.Among the stationary structures which accelerate ions are sheaths, presheaths, double layers, multiple double layers and ambipolar potentials. RF is an example of time dependent processes. Both classes of experiments are considered. Experiments with 'collisionless' plasmas in multidipole triple plasma-type devices and novel phenomena associated with the use of Ion Cyclotron Resonance Heating in the inhomogeneous magnetic field of a tandem mirror 'fusion' plasma are described. New RF phenomenon include RF pitch angle scattering, RF electron pumping, and ICRF ponderomotive force.

Hershkowitz, Noah↗
NASA NTRS1986

Ion acceleration in expanding ionospheric plasmas

Plasma expansion along the ambient magnetic field in regions of density gradients provides a mechanism for accelerating ions. A brief review of the basic phenomenon of plasma expansion is given. Estimates of the energies of the accelerated ions in an expanding ionospheric plasma along geomagnetic flux tubes are obtained by solving the time-dependent hydrodynamic equations. It is found that, over certain altitude ranges, each ion species can be the most energetic; the maximum energies of the different ions are found to be limited to less than about 10 eV for H(+), 5 eV for He(+), and less than about 1.5 eV for O(+).

Singh, Nagendra↗
NASA NTRS1986

Pion-decay radiation and two-phase acceleration in the June 3, 1982 solar flare

The June 3, 1982 flare is unique in the wealth of observed neutron, gamma-ray and energetic-particle emission that it produced. Using calculations of high-energy emissions to fit the various time-dependent gamma-ray fluxes, a self-consistent interaction model for the June 3 flare is constructed in which the observed fluxes are produced by two distinct particle populations with different acceleration and interaction time histories as well as different but time-independent energy spectra. The two populations are associated with first- and second-phase particle acceleration, respectively.

Ramaty, R.↗
NASA NTRS1987

Positrons from accelerated particle interactions

Positron production from the decay of radioactive nuclei produced in nuclear interactions of accelerated particles is treated in detail. Laboratory data as well as theoretical considerations are used to construct energy-dependent cross sections for the production of a large number of radioactive positron emitters resulting from proton and alpha-particle interactions with ambient cosmic matter. Using these cross sections, positron production rates are calculated for a variety of energetic particle spectra, assuming solar abundances for both the energetic particles and the ambient medium. These results can be used for the study of astrophysical sites which emit annihilation radiation. In particular, the results have been applied to solar flares, where the observed 0.511 MeV line is shown to be due to positrons resulting from accelerated particle reactions.

Kozlovsky, B.↗