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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Inclusive inelastic scattering of heavy ions in the independent particle model

We consider the inclusive inelastic scattering of heavy ions using the Glauber (1959) model and the independent particle approximation. Inclusive inelastic distributions for projectile excitation of the target and total inelastic scattering, where all projectile and target excited states are summed, are discussed using closure. The total inelastic distribution, when integrated, is shown to be equivalent to the absorption cross section, found from applying the optical theorem to the elastic scattering amplitude in the coherent approximation. Calculations are presented for several heavy-ion pairs, using realistic nuclear densities in a large mass number approximation.

Cucinotta, Francis A.↗
NASA NTRS1993

A test of the applicability of independent scattering to high albedo planetary regoliths

We show that 'independent scattering' is a useful approximation for high albedo particles whose size and packing density are similar to typical particles in the lunar regolith. Laboratory measurements of the intensity and linear polarization of light scattered from a laboratory sample of glass spheres of know size and composition are compared to radiative transfer calculations of the same observable quantities. Mie scattering is integrated over the size distribution of the particles to determine the mean phase and polarization functions, or phase matrix, of the particle. Assuming that the particles scatter independently, the 'doubling method' is used to rigorously calculate multiple scattering in an optically thick layer of these anisotropically scattering particles. All of the major features 'predicted' in the calculated intensity (double peaks at small phase angles) and polarization (negative branch at small phase angles, large positive peak near 20 degrees phase, and small polarization elsewhere) are observed in the laboratory measurements, with good quantitative agreement indicated at phase angles less than 90 degrees. Even though the particles are supported by physical contact with each other in the sample, as are the particles in planetary regoliths, the independent scattering calculation yields a good approximation to both the intensity and polarization. The physical parameters input to the calculation are only the size distribution of the particles and their complex index of refraction (composition). Significant advantages of this approach are that the phase matrix is calculated from basic physical principles and both the intensity and polarization are determined simultaneously. This model may have broad applications to the interpretation of photometry, spectroscopy, and polarimetry of the icy regoliths of high albedo satellites. The intent of this effort is to perform a controlled experiment that tests the utility of the independent scattering assumption

Goguen, J. D.↗
NASA NTRS1994

Asymmetry parameters of the phase function for densely packed scattering grains

Spatial correlation among densely packed particles can substantially change their single-scattering properties, thus making questionable the applicability of the independent scattering approximation in calculations of light scattering by planetary regoliths. The same problem arises in geophysics in light scattering computations for snow, frosts, and bare soil. In this paper, we use a dense-medium light-scattering theory based on the introduction of the static structure factor to calculate asymmetry parameters of the phase function for densely packed particles with real refractive indices 1.31 and 1.66, approximating water ice and soil particles, respectively, and imaginary refractive indices 0, 0.01, and 0.3. For sparsely distributed, independently scattering grains, the calculated asymmetry parameters are always positive and always larger than those for densely packed particles. For densely packed grains, the asymmetry parameters may be negative but only for radius-to-wavelength ratios from about 0.1 to about 0.4. With decreasing particle size, the calculated asymmetry parameters tend to zero independently of the compaction state. In the geometrical optics regime, the asymmetry parameters for densely packed scatterers are positive and very close to those for independently scattering grains. These results may have important implications for remote sensing of the Earth and solid planetary surfaces. In particular, it is demonstrated that negative asymmetry parameters derived with some approximate multiple-scattering theories may be physically irrelevant and can be the result of using an inaccurate bidirectional reflection function combined with the ill-conditionally of the inverse scattering problem.

Mishchenko, Michael I.↗
NASA NTRS2018

An Overview of Methods for Deriving the Radiative Transfer Theory from the Maxwell Equations. II: Approach Based on the Dyson and Bethe-Salpeter Equations

In this paper, the vector radiative transfer equation is derived by means of the vector integral Foldy equations describing the electromagnetic scattering by a group of particles. By assuming that in a discrete random medium the positions of the particles are statistically independent and by applying the Twersky approximation to the order-of-scattering expansion of the total field, we derive the Dyson equation for the coherent field and the ladder approximated Bethe–Salpeter equation for the dyadic correlation function. Then, under the far-field assumption for sparsely distributed particles, the Dyson equation is reduced to the Foldy integral equation for the coherent field, while the iterated solution of the Bethe–Salpeter equation ultimately yields the vector radiative transfer equation.

Electromagnetic scattering↗
NASA NTRS1974

Cosmic ray anisotropies late in a solar flare event

The detailed relationship between the anisotropy characteristics observed during late times in the decay of a solar flare event and the interplanetary magnetic field parameters is investigated. The anisotropy always is from 45 deg east of the earth-sun line. This direction is approximately perpendicular to the nominal Archimedean spiral, independent of the particle energy. The amplitude of the anisotropy increases as the magnetic field azimuthal direction shows greater departure from the radial direction. These results are discussed in terms of current ideas about solar particle propagation in the interplanetary space.

Allum, F. R.↗
NASA NTRS1979

Diffuse reflectance of the ocean - The theory of its augmentation by chlorophyll a fluorescence at 685 nm

The radiative transfer equation is modified to include the effect of fluorescent substances and solved in the quasi-single scattering approximation for a homogeneous ocean containing fluorescent particles with wavelength independent quantum efficiency and a Gaussian shaped emission line. The results are applied to the in vivo fluorescence of chlorophyll a (in phytoplankton) in the ocean to determine if the observed quantum efficiencies are large enough to explain the enhancement of the ocean's diffuse reflectance near 685 nm in chlorophyll rich waters without resorting to anomalous dispersion. The computations indicate that the required efficiencies are sufficiently low to account completely for the enhanced reflectance. The validity of the theory is further demonstrated by deriving values for the upwelling irradiance attenuation coefficient at 685 nm which are in close agreement with the observations.

Gordon, H. R.↗
NASA NTRS1980

Singly-ionized helium in the driver gas of an interplanetary shock wave

The interplanetary shock wave observed on Jan. 29, 1977 by the HELIOS-1 plasma instruments shows an unusual feature: in the cold tenuous piston plasma following this shock, there appears a third peak in the energy per charge (E/q) spectra, in addition to the normal proton and alpha-particle peaks. The peak is located at E/q ? 4 and persists for about 14 hours, with slowly varying intensities. Independent simultaneous measurement of these particles' charge yields a value of approximately 1. These ions are thought to be He-4(+) ions travelling with the same speed as protons and alpha particles. The occurrence of He-4(+) indicates the possibility that during eruptive prominences or other solar transients, 'cold' chromospheric plasma might escape from the sun without undergoing the normal coronal heating process.

Schwenn, R.↗
NASA NTRS1980

Calculations of the variability of ice cloud radiative properties at selected solar wavelengths

This study shows that there is surprising little difference in values of reflectance, absorptance, and transmittance for many of the intermediate-size particle spectra. Particle size distributions with mode radii ranging from approximately 50 to 300 microns, irrespective of particle shape and nearly independent of the choice of size distribution representation, give relatively similar flux values. The very small particle sizes, however, have significantly larger values of reflectance and transmittance with corresponding smaller values of absorptance than do the larger particle sizes. The very large particle modes produce very small values of reflectance and transmittance along with very large values of absorptance. Such variations are particularly noticeable when plotted as a function of wavelength.

Welch, R. M.↗
NASA NTRS1992

A particle model for magnetotail neutral sheet equilibria

A particle model of the magnetotail neutral sheet self-consistent structure was developed by assuming that the thickness of the neutral sheet is much less than the thickness of the plasma sheet. This made it possible to approximate the neutral sheet as an x-independent structure. The role of nonlinear particle motions in determining the conductivity and the global current sheet structure is investigated.

Burkhart, G. R.↗
NASA NTRS1984

Atomic inner-shell transitions

Atomic inner-shell processes have quite different characteristics, in several important aspects, from processes in the optical regime. Energies are large, e.g., the 1s binding energy reaches 100 keV at Z = 87; relativistic and quantum-electrodynamic effects therefore are strong. Radiationless transitions vastly dominate over photon emission in most cases. Isolated inner-shell vacancies have pronounced single-particle character, with correlations generally contributing only approximately 1 eV to the 1s and 2p binding energies; the structure of such systems is thus well tractable by independent-particle self-consistent-field atomic models. For systems containing multiple deep inner-shell vacancies, or for highly stripped ions, the importance of relativistic intermediate coupling and configuration interaction becomes pronounced. Cancellation of the Coulomb interaction can lead to strong manifestations of the Breit interaction in such phenomena as multiplet splitting and hypersatellite X-ray shifts. Unique opportunities arise for the test of theory.

Crasemann, B.↗
NASA NTRS1993

A Quantitative Test of the Applicability of Independent Scattering to High Albedo Planetary Regoliths

To test the hypothesis that the independent scattering calculation widely used to model radiative transfer in atmospheres and clouds will give a useful approximation to the intensity and linear polarization of visible light scattered from an optically thick surface of transparent particles, laboratory measurements are compared to the independent scattering calculation for a surface of spherical particles with known optical constants and size distribution. Because the shape, size distribution, and optical constants of the particles are known, the independent scattering calculation is completely determined and the only remaining unknown is the net effect of the close packing of the particles in the laboratory sample surface...

Goguen, Jay D.↗
NASA NTRS1994

Effect of meteoroid rotation on atmospheric entry heating and meteor beginning height

The beginning height of a meteor varies by approximately 10 km depending on the rotation state of a spherical meteoroid as long as the body sustains a temperature gradient. Such gradients build up in friable stony meteoroids larger than approximately 0.1 cm and in iron meteoroids that are approximately 1 cm or larger. The height where 100 microns or smaller zodiacal dust particles ablate is nearly independent of rotation. Stony particles in the 100 microns size range ablate near 120 km while both larger and smaller meteoroids penetrate deeper. All calculations were made with a nominal speed of 30 km/s and a zenithal distance equal to z(sub r) = 0 deg. Larger meteoroids cool their surface by conduction to the interior while smaller meteoroids decelerate significantly due to their large surface area to mass ratio so that the surface receives a lower energy flux. Some micro-meteoroids smaller than approximately 10 microns may escape abalation altogether at the nominal velocity. The effect of rotation on particle temperature during atmospheric flight is significant on meteoroids larger than 0.1 cm but may be negligible on micro-meteorites that are typically 10-50 microns in diameter.

Adolfsson, L. G.↗
NASA NTRS1978

The interplanetary scattering mean free path from 1 to 3 x 1000 MV

The paper reports a statistical study of published intensity-time profiles of proton and electron solar particle events from 1967 to 1974. The purpose of the study was to examine systematically the temporal and rigidity dependence of the interplanetary scattering mean free path from approximately 1000 to 3000 MV. The observed t-max, the time from release of particles at the sun to the time of maximum flux at the spacecraft, were interpreted in terms of a propagation model to obtain the average radial scattering mean-free path. This path (1) appears to be species independent when it is derived from proton and electron solar particle events, (2) varies less than a factor of 2 from solar maximum to solar minimum, and (3) is nearly rigidity independent below approximately 500 MV. The path values obtained at low rigidities are inconsistent with path values derived theoretically from the interplanetary magnetic field fluctuations.

Zwickl, R. D.↗
NASA NTRS2010

Ballistics Model for Particles on a Horizontal Plane in a Vacuum Propelled by a Vertically Impinging Gas Jet

A simple trajectory model has been developed and is presented. The particle trajectory path is estimated by computing the vertical position as a function of the horizontal position using a constant horizontal velocity and a vertical acceleration approximated as a power law. The vertical particle position is then found by solving the differential equation of motion using a double integral of vertical acceleration divided by the square of the horizontal velocity, integrated over the horizontal position. The input parameters are: x(sub 0) and y(sub 0), the initial particle starting point; the derivative of the trajectory at x(sub 0) and y(sub 0), s(sub 0) = s(x(sub 0))= dx(y)/dy conditional expectation y = y((sub 0); and b where bx(sub 0)/y(sub 0) is the final trajectory angle before gravity pulls the particle down. The final parameter v(sub 0) is an approximation to a constant horizontal velocity. This model is time independent, providing vertical position x as a function of horizontal distance y: x(y) = (x(sub 0) + s(sub 0) (y-y(sub 0))) + bx(sub 0) -(s(sub 0)y(sub 0) ((y - y(sub 0)/y(sub 0) - ln((y/y(sub 0)))-((g(y-y(sub 0)(exp 2))/ 2((v(sub 0)(exp 2). The first term on the right in the above equation is due to simple ballistics and a spherically expanding gas so that the trajectory is a straight line intersecting (0,0), which is the point at the center of the gas impingement on the surface. The second term on the right is due to vertical acceleration, which may be positive or negative. The last term on the right is the gravity term, which for a particle with velocities less than escape velocity will eventually bring the particle back to the ground. The parameters b, s(sub 0), and in some cases v(sub 0), are taken from an interpolation of similar parameters determined from a CFD simulation matrix, coupled with complete particle trajectory simulations.

Lane, J. E.↗
NASA NTRS1981

Elemental composition of solar energetic particles

The Low Energy Telescopes on the Voyager spacecraft are used to measure the elemental composition (2 or = Z or = 28) and energy spectra (5 to 15 MeV/nucleon) of solar energetic particles (SEPs) in seven large flare events. Four flare events are selected which have SEP abundance ratios approximately independent of energy/nucleon. The abundances for these events are compared from flare to flare and are compared to solar abundances from other sources: spectroscopy of the photosphere and corona, and solar wind measurements. The four flare average SEP composition is significantly different from the solar composition determined by photospheric spectroscopy. The average SEP composition is in agreement with solar wind abundance results and with a number of recent coronal abundance measurements. The evidence for a common depletion of oxygen in SEPs, the corona and the solar wind relative to the photosphere suggest that the SEPs originate in the corona and that both the SEPs and solar wind sample a coronal composition which is significantly and persistently different from that of the photosphere.

Cook, W. R., III↗
NASA NTRS1984

Elemental composition of solar energetic particles

The Low Energy Telescopes on the Voyager spacecraft have been used to measure the elemental composition (Z = 2-28) and energy spectra (5-15 MeV per nucleon) of solar energetic particles (SEPs) in seven large flare events. Four flare events were selected which have SEP abundance ratios approximately independent of energy per nucleon. For these selected flare events, SEP composition results may be described by an average composition plus a systematic flare-to-flare deviation about the average. The four-flare average SEP composition is systematically different from the solar composition determined by photospheric spectroscopy. These systematic composition differences are apparently not due to SEP propagation or acceleration effects. In contrast, the four-flare average SEP composition is in agreement with measured solar wind abundances and with a number of recent spectroscopic coronal abundance measurements. These findings suggest that SEPs originate in the corona, and that both SEPs and the solar wind sample a coronal composition which is significantly and persistently different from that measured for the photosphere.

Cook, W. R.↗
NASA NTRS1983

A quantitative test of solar modulation theory - The proton, helium, and electron spectra from 1965 through 1979

The predictions of a conventional, spherically symmetric model of solar modulation have been compared with the measured spectra of positively and negatively charged galactic cosmic-ray particles at 1 AU throughout the 1965-1976 solar cycle and through the enhanced modulation of 1979. For the proton/helium, proton/electron, and helium/electron flux ratios, there is remarkably good agreement between theory and experiment, except for small differences in 1965 and 1969. Possible systematic experimental errors are discussed, and it is concluded that: (1) the 11 year modulation process is largely independent of the sign of the particle charge; and (2) the assumption of steady state is a fairly good approximation for long term modulation.

Evenson, P.↗