Search NASA⌕ Search

SEARCH · Search NASA

Results for “collisionless”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Dynamic modeling of spacecraft in a collisionless plasma

A new computational model is described which can simulate the charging of complex geometrical objects in three dimensions. Two sample calculations are presented. In the first problem, the capacitance to infinity of a complex object similar to a satellite with solar array paddles is calculated. The second problem concerns the dynamical charging of a conducting cube partially covered with a thin dielectric film. In this calculation, the photoemission results in differential charging of the object.

Katz, I.↗
NASA NTRS1979

Collisionless galaxy simulations

Three-dimensional fully self-consistent computer models were used to determine the evolution of galaxies consisting of 100 000 simulation stars. Comparison of two-dimensional simulations with three-dimensional simulations showed only a very slight stabilizing effect due to the additional degree of freedom. The addition of a fully self-consistent, nonrotating, exponential core/halo component resulted in considerable stabilization. A second series of computer experiments was performed to determine the collapse and relaxation of initially spherical, uniform density and uniform velocity dispersion stellar systems. The evolution of the system was followed for various amounts of angular momentum in solid body rotation. For initally low values of the angular momentum satisfying the Ostriker-Peebles stability criterion, the systems quickly relax to an axisymmetric shape and resemble elliptical galaxies in appearance. For larger values of the initial angular momentum bars develop and the systems undergo a much more drastic evolution.

Hohl, F.↗
NASA NTRS1978

Collisionless effects on the spectrum of secondary auroral electrons at low altitudes

A common feature of all rocket measurements of the differential flux of primary and secondary electrons under auroral activity is that between 30 and 85 eV, where the measurements overlap in energy, the electron flux data can be fitted by a certain power law. In the present paper, it is shown that the existence of plasma waves even in a region where they are nonresonant with the ambient particles can significantly modify the observed flux power law.

Papadopoulos, K.↗
NASA NTRS1979

Depletion of satellite atoms in a collisionless exosphere by radiation pressure

Perturbations of hydrogen atoms in Keplerian orbits are examined by analyzing the rates of change of the classical orbital elements. There are three main effects: high inclination orbits with eccentricities e at least 0.4 are forced toward the ecliptic plane within a few weeks, the perigees of direct orbits drift rapidly toward stable positions roughly westward of the planet, and satellite orbits in or near such a stable point rapidly lower their perigees and the satellite's life is ended by a collision in the atmosphere. Thus there are effects tending to diminish the number of highly eccentric orbits with distant apogees in six principal directions. The various lifetimes are compared for a sample of initial elements.

Chamberlain, J. W.↗
NASA NTRS1980

Dynamics of collisionless systems

The three dimensional dynamics of rotating stellar systems were studied. A comparison of various mathematical models of flat galaxies is presented. The effects of self-gravity upon a flat galaxy undergoing a tidal encounter with another galaxy were investigated.

Zang, T. A.↗
NASA NTRS1981

Abundance enhancements in cosmic rays produced by collisionless shocks

It is shown that shocks preferentially accelerate partially ionized heavy elements over protons. Within the framework of a previously published model of injection, the spectra of different ion species are calculated from thermal to ultrarelativistic energies. For typical astrophysical parameters, the predicted enhancement is in qualitative agreement with observations for a 10 to the 6th K preshock plasma; ions with Z greater than 10 are enhanced by about an order of magnitude or so. The continuing increase with energy of the heavy-element abundances into the air-shower regime is discussed.

Eichler, D.↗
NASA NTRS1982

Measurement of the potential drop across the earth's collisionless bow shock

The normal component of the dc electric field measured on ISEE-1 ordinarily exhibits an enhancement of a few mV/m over both upstream and downstream values at the earth's bow shock. Using the measured relative velocity between the shock and the spacecraft (from the ISEE-1/2 time delay in the magnetometer data), it is possible to transform the observed E enhancement to a potential drop (delta phi). For a subcritical shock the potential drop is found to be very close to the measured change of particle kinetic energy (delta phi, approximately 280 V on day 330, 1977), whereas for a supercritical shock the potential drop is only a fraction of the measured change of kinetic energy (delta phi, approximately 140 V on day 324, 1977).

Formisano, V.↗
NASA NTRS1983

Spiky ion acoustic waves in collisionless auroral plasma

A theory of spiky electric fields in 'inverted V' precipitation regions is elaborated and compared with recent spacecraft observations of solitary waves and double layers. A prediction from the theory is that the electric fields propagate along the magnetic field as perturbed ion acoustic solitons that intensify by exchanging momentum with reflected particles. The solitons have minimum scale lengths of approximately 100 m and maximum electric potential and field amplitudes of 1-10 V and 1-10 mV/m. They propagate at the local ion acoustic speed and are Doppler-shifted by the drift speed of upward flowing cold ions. Both rarefactive and compressive solitons with, respectively, negative and positive electric potentials are possible. It is noted that upward propagating compressive modes intensify when the upward flow of ionospheric ions exceeds approximately 10 times the local ion acoustic speed. The kinematic and dynamic properties of rarefactive solitons are found to be consistent with recent observations.

Lotko, W.↗
NASA NTRS1983

Structure of perpendicular shocks in collisionless plasma

The macrostructure of perpendicular shocks in the supercritical regime is investigated theoretically. The coupling between the shock precursor (foot region) associated with reflected ions and the magnetic ramp associated with electron ohmic heating is analyzed using a model in which the ions are treated as a multifluid and the electrons as a massless fluid. The results are found to be in good agreement with a number of features exhibited by recent numerical simulations of perpendicular shocks, including the morphology of the reflected-gyrating ion stream, the presence of potential and magnetic field overshoots, and the Mach number dependence of the shock structure.

Leroy, M. M.↗
NASA NTRS1983

On the relationship between collisionless shock structure and energetic particle acceleration

Recent experimental research on bow shock structure and theoretical studies of quasi-parallel shock structure and shock acceleration of energetic particles were reviewed, to point out the relationship between structure and particle acceleration. The phenomenological distinction between quasi-parallel and quasi-perpendicular shocks that has emerged from bow shock research; present efforts to extend this work to interplanetary shocks; theories of particle acceleration by shocks; and particle acceleration to shock structures using multiple fluid models were discussed.

Kennel, C. F.↗
NASA NTRS1984

Collisionless Shock Waves in the Solar Terrestrial Environment

Natural shocks, magnetohydrodynamics, shock extensions, global asymmetries, shock profiles, quasiperpendicular supercritical process; ion dynamics and field structure, numerical simulations, ion reflection machinery, electrons, instabilities, phenomenology, and ion acceleration were described.

Source record↗
NASA NTRS1983

Collisionless electron shocks in electron-beam-plasma systems

One-dimensional Vlasov simulations show that when an electron beam is suddenly injected into a plasma, a fast-moving monotonic shock forms during the early stage of the transient plasma response. In the shock, the ions are nearly immobile. The shock appears to be an electrostatic electron-beam-plasma mode. The shock evolves from an initial positive potential perturbation, which is supported by an ion burst. The steepening of the perturbation into a shock is characterized by an electron-beam-plasma mode.

Singh, N.↗
NASA NTRS1984

Collisionless dissipation in quasi-perpendicular shocks

Microscopic dissipation processes in quasi-perpendicular shocks are studied by two-dimensional plasma simulations in which electrons and ions are treated as particles moving in self-consistent electric and magnetic fields. Cross-field currents induce substantial turbulence at the shock front reducing the reflected ion fraction, increasing the bulk ion temperature behind the shock, doubling the average magnetic ramp thickness, and enhancing the upstream field aligned electron heat flow. The short scale length magnetic fluctuations observed in the bow shock are probably associated with this turbulence.

Forslund, D. W.↗
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 NTRS1984

Electron precipitation in solar flares - Collisionless effects

A large fraction of the electrons which are accelerated during the impulsive phase of solar flares stream towards the chromosphere and are unstable to the growth of plasma waves. The linear and nonlinear evolution of plasma waves as a function of time is analyzed with a set of rate equations that follows, in time, the nonlinearly coupled system of plasma waves-ion fluctuations. As an outcome of the fast transfer of wave energy from the beam to the ambient plasma, nonthermal electron tails are formed which can stabilize the anomalous Doppler resonance instability responsible for the pitch angle scattering of the beam electrons. The non-collisional losses of the precipitating electrons are estimated, and the observational implication of these results are discussed.

Vlahos, L.↗
NASA NTRS1985

Ion and electron heating at collisionless shocks near the critical Mach number

The present study has the objective to document more fully a somewhat surprising heating pattern for 10 of the bow shock crossings investigated by Russell et al. (1982). A description of the detailed evolution of the actual distribution functions across the subcritical shocks is also provided. It is pointed out that the shocks examined are all very near the theoretical one-fluid critical Mach number. None of the shocks has a significant magnetic foot or overshoot. It is argued that the observed heating is not due to reflected ions but rather to physical processes characteristic of subcritical shocks.

Thomsen, M. F.↗
NASA NTRS1985

Return currents in solar flares - Collisionless effects

If the primary, precipitating electrons in a solar flare are unstable to beam plasma interactions, it is shown that strong Langmuir turbulence can seriously modify the way in which a return current is carried by the background plasma. In particular, the return (or reverse) current will not be carried by the bulk of the electrons, but by a small number of high velocity electrons. For beam/plasma densities greater than 0.01, this can reduce the effects of collisions on the return current. For higher density beams where the return current could be unstable to current driven instabilities, the effects of strong turbulence anomalous resistivity is shown to prevent the appearance of such instabilities. Again in this regime, how the return current is carried is determined by the beam generated strong turbulence.

Rowland, H. L.↗