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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Test-particle simulations in increasingly strong turbulence

Quasi-linear theory supposes that the energy in resonant fluctuations is small compared to that in the mean magnetic field. This is evident in the fact that the zeroth-order particle trajectories are helices about a mean field B(sub o) that is spatially uniform over many correlation lengths. However, in the solar wind it is often the case that the fluctuating part of the field is comparable in magnitude to the mean part. It is generally expected that quasi-linear theory remains viable for particles that are in resonance with a region of the fluctuation spectrum having only small energy density, but even so, care must be taken when comparing simulations to theoretical predictions. We have performed a series of test-particle simulations to explore the evolution of ion distributions in turbulent situations with varying levels of magnetic fluctuations. As delta-B/B(sub o) is increased the distinctions among absolute pitch angle (defined relative to B(sub o)), local pitch angle (defined relative to B(x)), and magnetic moment become important, some of them exhibiting periodic sloshing unrelated to the nonadiabatic processes of interest. Comparing and contrasting the various runs illustrates the phenomena that must be considered when the premise underlying quasi-linear theory are relaxed.

Pontius, D. H., Jr.↗
NASA NTRS1989

Test particle simulation study of whistler wave packets observed near Comet Giacobini-Zinner

Nonlinear interactions of water group ions with large-amplitude whistler wave packets detected at the leading edge of steepened magnetosonic waves observed near Comet Giacobini-Zinner (GZ) are studied using test particle simulations of water-ion interactions with a model wave based on GZ data. Some of the water ions are found to be decelerated in the steepened portion of the magnetosonic wave to the resonance velocity with the whistler wave packets. Through resonance and related nonlinear interaction with the large-amplitude whistler waves, the water ions become trapped by the packet. An energy balance calculation demonstrates that the trapped ions lose their kinetic energy during the trapped motion in the packet. Thus, the nonlinear trapping motion in the wave structure leads to effective energy transfer from the water group ions to the whistler wave packets in the leading edge of the steepened MHD waves.

Kaya, N.↗
NASA NTRS1996

3-D Simulations of NSTAR Ion Thruster Plasma Interactions

Described is a Particle-in-Cell with Monte Carlo Collision code developed to perform detailed three-dimensional ion thruster simulations. To capture the full kinetic behavior of ion thruster plumes, both the electrons and ions are treated as test particles. Simulation results are given of the NSTAR ion thruster under ground test and in space conditions. Numerical results are compared.

NSTAR Ion Thruster Plasma Ionization Particles↗
NASA NTRS1990

Wing galaxies: A formation mechanism of the clumpy irregular galaxy Markarian 297

In order to contribute to an understanding of collision-induced starburst activities, the authors present a detailed case study on the starburst galaxy Markarian 297 (= NGC 6052 = Arp 209; hereafter Mrk 297). This galaxy is classified as a clumpy irregular galaxy (CIG) according to its morphological properties (cf. Heidmann, 1987). Two major clumps and many small clumps are observed in the entire region of Mrk 297 (Hecquet, Coupinot, and Maucherat 1987). The overall morphology of Mrk 297 is highly chaotic and thus it seems difficult to determine possible orbits of galaxy-galaxy collision. However, the authors have serendipitously found a possible orbit during a course of numerical simulations for a radial-penetration collision between galaxies. The radial-penetration collision means that an intruder penetrates a target galaxy radially passing by its nucleus. This kind of collision is known to explain a formation mechanism of ripples around disk galaxies (Wallin and Struck-Marcell 1988). Here, the authors show that the radial-penetration collision between galaxies successfully explains both overall morphological and kinematical properties of Mrk 297. The authors made two kinds of numerical simulations for Mrk 297. One is N-body (1x10(exp 4) particles) simulations in which effects of self gravity of the stellar disk are taken into account. These simulations are used to study detailed morphological feature of Mrk 297. The response of gas clouds are also investigated in order to estimate star formation rates in such collisions. The other is test-particle simulations, which are utilized to obtain a rough picture of Mrk 297 and to analyze the velocity field of Mrk 297. The techniques of the numerical simulations are the same as those in Noguchi (1988) and Noguchi and Ishibashi (1986). In the present model, an intruding galaxy with the same mass of a target galaxy moves on a rectilinear orbit which passes the center of the target.

Taniguchi, Yoshiaki↗
NASA NTRS1981

Monte Carlo simulation of charged particles upstream of the earth's bow shock

A simple model of the earth's bow shock is presented and compared with ISEE-1 observations of diffuse ions upstream of the shock. Ion behavior is modeled in a test particle simulation using Monte Carlo techniques for a plane parallel shock at which cold solar wind ions are heated sufficiently to allow diffusion back upstream. Fitting of the model to the observations indicates that the diffusion coefficient must increase faster than linearly with velocity, and provides an upper limit to the acceleration efficiency defined as the proportion of incoming solar wind ions accelerated and observed upstream. Results suggest that astrophysical shocks are capable of accelerating ions directly from the thermal plasma without the need for a distinct preenergizing injection stage.

Ellison, D. C.↗
NASA NTRS1994

Electron collection by a highly positive satellite in the ionosphere: Test aprticle simulation

Collection of electrons by a satellite at a relatively high potential in low Earth orbit, including the effects of the satellite's orbital motion, remains a poorly understood phenomenon. Using a test particle simulation in which charged particle motion is tracked in prescribed electric fields, we calculate here the current collection and the nature of the energy distribution function of the electrons collected by the satellite, including the effects of the satellite orbital motion. Calculations of the collected current without the orbital motion show an excellent agreement with the current from the Parker-Murphy model, but with the orbital motion of the satellite the current collection is enhanced, the degree of enhancement depending on the size of the satellite sheath extending along the magnetic field line. In the latter case, the flow of electrons around the satellite shows some interesting behaviors including azimuthal E x B drift around the satellite, axial trapping along the magnetic field, and formation of field-aligned flow of electrons in the wake region. The total energy of the collected electrons is approximately equal e phi(sub 0), where phi(sub 0) is the satellite potential, but the partition of the energy into components parallel (W(sub parallel)) and perpendicular (W(sub perpendicular)) to the magnetic field shows interesting features. For the magnetic field along the polar axis Z, the energy distributions near the poles are perfectly field aligned, that is, the parallel energy (W(sub parallel) = e phi(sub 0). The perpendicular component (W(sub perpendicular)) progressively increases toward the 'equator' of the spherical satellite. For a sufficiently large bias potential phi(sub 0), the equatorial energy distributions f(W(sub parallel)) and f(W(sub perpendicular)) show fine structures having multiple peaks. These features of the distribution functions result from the chaotic motion of electrons and the associated coupling between the electron motions parallel and perpendicular to the magnetic field in strongly nonuniform electric fields near the satellite.

Singh, Nagendra↗
NASA NTRS1991

Electron acceleration at nearly perpendicular collisionless shocks. II - Reflection at curved shocks

Test particle simulations by Krauss-Varban et al. (1989), carried out for plane shocks, have confirmed previous results of Wu (1984) and Leroy and Mangeney (1984) that electrons can be effectively accelerated at nearly perpendicular shocks. This paper investigates the reflection and acceleration of electrons at a nearly perpendicular shock, using two-dimensional test-particle calculations which account for the effect of shock curvature. The computations show that reflected electron fluxes are of the order of observed fluxes. For several reasons, the combined effects of shock curvature are far less severe than anticipated.

Krauss-Varban, D.↗
NASA NTRS1993

Simulation of the prompt energization and transport of radiation belt particles during the March 24, 1991 SSC

We model the rapid (about 1 min) formation of a new electron radiation belt at L about or = 2.5 that resulted from the Storm Sudden Commencement (SSC) of March 24, 1991 as observed by the Combined Release and Radiation Effects Satellite (CRRES) satellite. Guided by the observed electric and magnetic fields, we represent the time-dependent magnetospheric electric field during the SSC by an asymmetric bipolar pulse that is associated with the compression and relaxation of the Earth's magnetic field. We follow the electrons using a relativistic guiding center code. The test-particle simulations show that electrons with energies of a few MeV at L greater than 6 were energized up to 40 MeV and transported to L about or = 2.5 during a fraction of their drift period. The energization process conserves the first adiabatic invariant and is enhanced due to resonance of the electron drift motion with the time-varying electric field. Our simulation results, with an initial W(exp -8) energy flux spectra, reproduce the observed electron drift echoes and show that the interplanetary shock impacted the magnetosphere between 1500 and 1800 MLT.

Li, Xinlin↗
NASA NTRS1984

The production of ion conics by oblique double layers

Magnetized test ions are subjected to acceleration through a numerically simulated oblique double layer in order to determine whether they emerge with velocity vectors aligned with or oblique to the ambient magnetic field. A criterion for oblique alignment, depending on the double-layer parameters and on the external magnetization, is obtained. When it is applied to observed and theoretical auroral double layers, this criterion predicts that accelerated heavy ions will be substantially less magnetic field aligned than will accelerated hydrogen ions, thus suggesting auroral double layers as a source of high-energy ion conics. Test particle simulations are also used to investigate the perpendicular heating of ions at low altitudes by the electric fields associated with moving auroral arcs. The rapid motion of small-scale structures in the arcs is suggested as a source of low-energy conical ion distributions, and the slow drifts of the entire arc forms are inferred to heat ionospheric ions.

Borovsky, J. E.↗
NASA NTRS1994

Electron acceleration at nearly perpendicular collisionless shocks. 3: Downstream distributions

Spacecraft observations at the Earth's bow shock and at interplanetary shocks have established that the largest fluxes of accelerated suprathermal electrons occur in so-called shock spike events immediately downstream of the shock ramp. Previous theoretical efforts have mainly focused on explaining upstream energetic electron beams. Here we investigate the general motion and acceleration of energetic electrons in a curved, nearly perpendicular shock by numerically integrating the orbits of solar wind halo electrons in shock fields generated by a hybrid simulation (core electron fluid and kinetic ions). Close to the angle Theta(sub Bn) = 90 degs between the upstream magnetic field and shock normal, the calculations result in a (perpendicular) temperature increase proportional to the magnetic field ratio and give the highest phase space densities in the overshoot. For a steep distribution, the temperature change can correspond to an enhancement of the distribution by several orders of magnitude. These results are in agreement with predictions from adiabatic mapping. With smaller angles Theta(sub Bn), the overshoot and downstream densities fall off quickly, because the adiabatic energy gain is less and fewer electrons transmit. The shock curvature also leads to an accumulation of electrons close to 90 degs. Without pitch angle scattering, energization is only significant within a few (approximately 5 to 10 degs) degrees of the point of tangency. However, shock spike events appear to be observed more easily and farther away from 90 degs. Given that over a region of several degrees around 90 degs the theory gives enhancements of up to approximately 4 orders of magnitude, such electrons could in principle account for the typically observed enhancements of 1 to 2 orders of magnitude, if they were distributed over Theta(sub Bn). To test the idea that scattering could efficiently redistribute the energetic electrons, we have conducted test particle simulations in which artificial pitch angle scattering is included. We find that such a process can indeed be very efficient and can explain observations of shock spike events far away from Theta(sub Bn) is approximately 90 degs. The scattering naturally leads to much higher phase space densities at smaller Theta(sub Bn) than what a local one-dimensional mapping would predict and thus can account for an observed discrepancy with adiabatic theory stated in the literature.

Krauss-Varban, D.↗
NASA NTRS1982

Nonlinear longitudinal resonance interaction of energetic charged particles and VLF waves in the magnetosphere

The longitudinal resonance of waves and energetic electrons in the Earth's magnetosphere, and the possible role this resonance may play in generating various magnetospheric phenomena are studied. The derivation of time-averaged nonlinear equations of motion for energetic particles longitudinally resonant with a whistler mode wave propagating with nonzero wave normal is considered. It is shown that the wave magnetic forces can be neglected at lower particle pitch angles, while they become equal to or larger than the wave electric forces for alpha 20 deg. The time-averaged equations of motion were used in test particle simulation which were done for a wide range of wave amplitudes, wave normals, particle pitch angles, particle parallel velocities, and in an inhomogeneous medium such as the magnetosphere. It was found that there are two classes of particles, trapped and untrapped, and that the scattering and energy exchange for those two groups exhibit significantly different behavior.

Tkalcevic, S.↗
NASA NTRS1984

Predictions of lithium interactions with earth's bow shock in the presence of wave activity

The results of a test-particle simulation studying the movement of a lithium tracer ion injected upstream of the bow shock are reported. Wave activity consists of parallel and antiparallel propagating Alfven waves characterized by a frequency power spectrum within a frequency or range of amplitudes defined separately in the upstream and downstream regions. The results show that even a moderate level of wave activity can substantially change the results obtained in the absence of waves. Among the effects observed are: (1) increased ion transmission; (2) both the average energy gain and spread about the average are increased for transmitted and reflected particles; (3) the average final pitch angle for transmitted particles tends to 90 deg, and the spread of reflected particles is reduced; and (4) the spatial dispersion of the ions on the bow shock after a single encounter is increased.

Decker, R. B.↗
NASA NTRS1985

Energetic ion acceleration at collisionless shocks

An example is presented from a test particle simulation designed to study ion acceleration at oblique turbulent shocks. For conditions appropriate at interplanetary shocks near 1 AU, it is found that a shock with theta sub B n = 60 deg is capable of producing an energy spectrum extending from 10 keV to approx. 1 MeV in approx 1 hour. In this case total energy gains result primarily from several separate episodes of shock drift acceleration, each of which occurs when particles are scattered back to the shock by magnetic fluctuations in the shock vicinity.

Decker, R. B.↗
NASA NTRS1986

Modeling of ion acceleration through drift and diffusion at interplanetary shocks

A test particle simulation designed to model ion acceleration through drift and diffusion at interplanetary shocks is described. The technique consists of integrating along exact particle orbits in a system where the angle between the shock normal and mean upstream magnetic field, the level of magnetic fluctuations, and the energy of injected particles can assume a range of values. The technique makes it possible to study time-dependent shock acceleration under conditions not amenable to analytical techniques. To illustrate the capability of the numerical model, proton acceleration was considered under conditions appropriate for interplanetary shocks at 1 AU, including large-amplitude transverse magnetic fluctuations derived from power spectra of both ambient and shock-associated MHD waves.

Decker, R. B.↗
NASA NTRS1988

Tidal interaction of small satellite galaxies with spiral primaries

The interaction of the disks of spiral galaxies and small companions is discussed. The gravitational drag effects of the disk on small satellites are of particular interest. Studies of the Andromeda Galaxy and its satellites, M32 and NGC 205, reveal the usefulness of few-body test-particle simulations in explaining many features of spiral galaxies and their satellites.

Byrd, Gene G.↗