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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 163 records · Page 9
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NASA NTRS1981

Non-thermal O/1D/ produced by dissociative recombination of O2/+/ - A theoretical model and observational results

Thermal and nonthermal O(1D) number density profiles are calculated. The two populations are assumed to be coupled by a thermalization cross-section which determines the loss and production in the nonthermal and thermal populations, respectively. The sources, sinks and transport of the two populations are used to model volume emission rate profiles at 6300 A. The 6300 A brightness measured by the Visible Airglow Experiment is then used to establish the presence of the nonthermal population and to determine the thermalization cross-section.

Schmitt, G. A.↗
NASA NTRS1982

Observational constraints for a theoretical model describing the soft X-ray flare

High-resolution solar flare X-ray spectra have recently been obtained from X-ray spectrometer experiments flown on an Air Force spacecraft (P78-1) launched on 1979 February 24. Interpretation of the spectra has produced new results concerning the physical conditions and time behavior of the thermal soft X-ray emitting plasma at temperatures near 20,000,000 K. It is argued that soft and hard X-ray events are not causally related to each other, but are simply two different manifestations of flare energy release. They probably occur in different plasma volumes. The source of the preflare plasma appears to be in the cooler parts of the solar atmosphere, perhaps transition region loops with initial temperatures of 100,000 K and densities of 10 to the 11th per cu cm. Continuous energy input, rather than sequential activation of loops, is required to explain the observations. Compression coupled with chromospheric ablation may produce the high densities in coronal flare loops.

Feldman, U.↗
NASA NTRS1981

Theoretical models of X-ray emission from rich clusters of galaxies

After assessing the theory of cluster gas with polytropic equations of state, and discussing the prediction of local polytropic indices in the gas distribution, attention is given to the consequences of modifying the equation of state by thermal cooling in the central regions of the cluster. It is suggested that, if the equation of state can be understood locally, an attempt may be made at (1) predicting the X-ray luminosities of the clusters at distant cosmological epochs and (2) measuring the evolution of the cluster's potential, through observation of the evolution of X-ray luminosity and surface brightness.

Cowie, L. L.↗
NASA NTRS1982

BL Herculis stars - Theoretical models for field variables

Type II Cepheids with periods between 1 and 3 days, commonly designated as Bl Herculis stars, have been modeled here with the aim of interpreting the wide variety of light curves observed among the field variables. Previously modeled globular cluster members are used as standard calibration objects. The major finding is that only a small range of luminosities is capable of generating a large variety of light curve types at a given period. For a mass of approximately 0.60 solar mass, the models are able to reproduce the observed mean luminosities, dispersion of mean luminosities, periods, light amplitudes, light asymmetries, and phases of secondary features in the light curves of known BL Her stars. It is possible that the metal-rich variables (which are found only in the field) have luminosities lower than those of most metal-poor variables. The present revised mass for BL Her, a metal-rich object, is not significantly different from the mean mass of the metal-poor variables.

Carson, R.↗
NASA NTRS1983

A simple theoretical model for calculating and parameterizing the ionospheric photoelectron flux

A method for calculating the ionospheric photoelectron flux is developed which uses the concept of average electron energy loss to simplify the calculation of the degraded electron spectrum. This method requires only a knowledge of the total inelastic electron impact cross sections and can be used for the calculation of all secondary ion and excited state production rates. The simple calculation reduces the computing time by a factor of 10 and considerably reduces storage requirements. It is found that the ionospheric photoelectron flux in the local equilibrium region is directly proportional to the attenuated solar EUV flux, which is a function only of the total neutral column density, and is independent of the neutral density composition. Thus, it is shown that electron impact cross section can be chosen so that this method may also be used to parameterize the measured ionospheric photoelectron fluxes.

Richards, P. G.↗
NASA NTRS1984

A theoretical model for calculation of molecular stopping power

A modified local plasma model is established. The Gordon-Kim's molecular charged density model is employed to obtain a formula to evaluate the stopping power of many useful molecular systems. The stopping power of H2 and He gas was calculated for incident proton energy ranging from 100 keV to 2.5 MeV. The stopping power of O2, N2, and water vapor was also calculated for incident proton energy ranging from 40 keV. to 2.5 MeV. Good agreement with experimental data was obtained. A discussion of molecular effects leading to department from Bragg's rule is presented. The equipartition rule and the effect of nuclear momentum recoiling in stopping power are also discussed.

Xu, Y. J.↗
NASA NTRS1984

Very slow classical Cepheids - Theoretical models with periods longer than 50 days

Systematics of the light curves of classical Cepheids with the longest known periods have been investigated with the help of full-amplitude models of pulsating stellar envelopes. For periods exceeding about 60 days, flat-topped light curves of the S Vul type are found to replace the smooth, asymmetric light curves characteristic of the slightly faster Cepheids. Predicted light and velocity amplitudes (although not the predicted radius amplitudes) agree well with observations. Variables with fluctuating light minima are observed to lie well off the mean period-luminosity relation, as are a few other (more stable?) variables with similarly long periods. The explanation for the long periods is probably low effective temperature rather than a low stellar mass. Because of the abnormal slowness of the classical Cepheids with periods longer than about 100 days, it is recommended that these variables not be used to calibrate the mean period-luminosity relation. Analogies between the slow classical Cepheids and the slow Population II Cepheids are drawn.

Carson, T. R.↗
NASA NTRS1984

Theoretical Modeling for Numerical Weather Prediction

The goal is to utilize predictability theory and numerical experimentation to identify and understand some of the dynamical processes which must be modeled more realistically if large-scale numerical weather predictions are to be improved. The emphasis is on the use of relatively simple models to exlore the properties of physically comprehensive general circulation models (GCM's). A global linear quasi-geostrophic model and the Goddard Laboratory for Atmospheric Sciences (GLAS) GCM were used to investigate several mechanisms which are responsible for the decay of large-scale forecast skill in mid-latitude numerical weather predictions. Five-day forecasts for an ensemble of cases were made using First GARP Global Experiment data. It was found that forecast skill depends crucially on the specification of the stationary forcing. A lack of stationary forcing leads to spurious westwad propagation of the ultralong waves. Forecasts made with stationary forcings derived from climatological data are superior to those using forcings inferred from observations immediately preceding the forecast period. Interhemispheric forecast differences were analyzed, and the model errors were compared to errors of a simple persistence-damped-to-climatology scheme and to errors of the GLAS GCM.

Somerville, R. C. J.↗
NASA NTRS1984

Evaluation of the radar response to land surfaces and volumes: Examination of theoretical models, target statistics, and applications

Four areas of L-band radar remote sensing of terrain were examined: (1) the behavior of the radar backscatter coefficient of distributed surface and volumes as a function of the targets' dielectric and geometric parameters and as a fnction of their physical parameters; (2) the correspondence of the angular behavior of the relative backscatter coefficient as extracted from SIR-B digital imagery and truck mounted L-band scatterometer measurements for about 100 fields; (3) the statistical behavior of SIR-B image density for targets that appear homogeneous on Thematic Mapper (TM) optical imagery and/or color IR photography; and (4) the applicability of SIR-B imagery both alone and in conjunction with TM imagery for the classification and monitoring of land cover and renewable resources.

Ulaby, F. T.↗
NASA NTRS1984

Theoretical model of impact damage in structural ceramics

This paper presents a mechanistically consistent model of impact damage based on elastic failures due to tensile and shear overloading. An elastic axisymmetric finite element model is used to determine the dynamic stresses generated by a single particle impact. Local failures in a finite element are assumed to occur when the primary/secondary principal stresses or the maximum shear stress reach critical tensile or shear stresses, respectively. The succession of failed elements thus models macrocrack growth. Sliding motions of cracks, which closed during unloading, are resisted by friction and the unrecovered deformation represents the 'plastic deformation' reported in the literature. The predicted ring cracks on the contact surface, as well as the cone cracks, median cracks, radial cracks, lateral cracks, and damage-induced porous zones in the interior of hot-pressed silicon nitride plates, matched those observed experimentally. The finite element model also predicted the uplifting of the free surface surrounding the impact site.

Liaw, B. M.↗
NASA NTRS1984

Theoretical model of the solar corona during sunspot minimum. I - Quasi-static approximation

The model of the solar corona (1.5 to 5 solar radius) during sunspot minimum is constructed. It is suggested that pole-equator asymmetry is caused by the interaction between global azimuthal electric currents in the corona and potential magnetic fields originated below. The asymmetry between the two hemispheres, which can be due to the magnetic quadrupole, is considered in detail. It is shown that an additional quadrupole term in the magnetic field representation affects the position of polar coronal hole boundaries differently in the two hemispheres. As a result, the outflow from the two polar regions is found to be different.

Osherovich, V. A.↗
NASA NTRS1985

A time-dependent theoretical model of the polar wind Preliminary results

The coupled, time-dependent continuity, momentum, and energy equations of a two-ion O(+) and H(+) quasi-neutral plasma were solved in order to extend understanding of polar wind behavior. This numerical code allows studies of the time dependent behavior of polar wind-type flows into and out of the ionosphere. Initial studies indicate that the typical time constants for electron and ion temperature changes are of the order of minutes and tens of minutes, respectively. The response time of the minor high altitude ion O(+) is less than an hour, whereas that of the major ion, H(+), is many hours. The initial test runs also demonstrate the fact that temporary supersonic flows of both O(+) and H(+) are possible, especially in the presence of significant ion heating.

Gombosi, T. I.↗
NASA NTRS1986

Theoretical models of single-element LTSA antenna elements

The spectral domain computations for slotline guide wavelength and characteristic impedance provide the basic parameters that are need to calculate the radiation pattern of an Linearly Tapered Slot Antenna (LTSA) by using the half-plane Green's function and a stepped approximation to the LTSA structure. However, the spectral domain computations are somewhat time consuming and not suitable for computer aided design application. Therefore, the results of several computation were fitted by formulas that are readily evaluated on a calculator or computer. Antenna measurements were continued, and an attempt was made to predict the truncation effect by three different methods.

Source record↗
NASA NTRS1985

Tone excited jets. V - A theoretical model and comparison with experiment

A mathematical model of tone-excited jets is developed which consists of two major components: a mathematical description of the process by which the intrinsic instability waves of the jet are excited by the upstream tones; and the modeling of the nonlinear interaction between the mean flow of the jet, the excited large-scale instability waves or turbulence structure, and the fine scale turbulence. It is assumed that each of these jet flow components can be characterized by a few parameters, which are then related by conservation equations that are supplemented by closure models. This quasi-linear model's results are compared with experimental measurements, and good agreement is obtained over a wide range of excitation frequencies and excitation levels.

Tam, C. K. W.↗
NASA NTRS1986

Theoretical models of free-free microwave emission from solar magnetic loops

The free-free microwave emission is calculated from a series of model magnetic loops. The loops are surrounded by a cooler external plasma, as required by recent simultaneous X ray and microwave observations, and a narrow transition zone separating the loops from the external plasma. To be consistent with the observational results, upper limits on the density and temperature scale lengths in the transition zone are found to be 360 km and 250 km, respectively. The models which best produce agreement with X ray and microwave observations also yielded emission measure curves which agree well with observational emission measure curves for solar active regions.

Brosius, Jeffrey W.↗
NASA NTRS1987

Theoretical modelling of the fine structures in sunspots

Until a decade ago most solar physicists thought of a sunspot as the upper end of a giant flux tube floating vertically. The existence of umbral dots and penumbral grains has been known for several decades. On the basis of available observations, they seem to be regions of photospheric intensity with upflowing gas motion and magnetic fields much weaker than in the surrounding sunspot surface. It has also been suggested that the differences in the appearances of umbral dots and granular cells are caused by the highly nonlinear nature of the convection problem in the presense of strong magnetic fields. The main ideas are presented here without any equations. It can be shown that a pocket of field free gas surrounded by a vertical magnetic field in the presence of gravity takes up the shape of a tapering column ending at a vertex at the top. Some convection is expected to take place in the trapped field free gas, whereas the magnetic field around it makes those regions stable against convection. Eventually the apex of the tapering column reaches the photospheric surface where the bulging of the magnetic field makes the field no longer able to close on the field free gas and trap it underneath.

Choudhuri, Arnab Rai↗
NASA NTRS1988

Theoretical models of free-free microwave emission from solar magnetic loops

The free-free microwave emission is calculated from a series of model magnetic loops. The loops are surrounded by a cooler external plasma, as required by recent simultaneous X ray and microwave observations, and a narrow transition zone separating the loops from the external plasma. To be consistent with the observational results, upper limits on the density and temperature scale lengths in the transition zone are found to be 360 km and 250 km, respectively. The models which best produce agreement with X-ray and microwave observations also yielded emission measure curves which agree well with observational emission measure curves for solar active regions.

Brosius, Jeffrey W.↗