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All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

Analysis of the DFP/AFCS Systems for Compensating Gravity Distortions on the 70-Meter Antenna

This paper presents the theoretical computations showing the expected performances for both systems. The basic analysis tool is a Physical Optics reflector analysis code that was ported to a parallel computer for faster execution times. There are several steps involved in computing the RF performance of the various systems. 1 . A model of the RF distortions of the main reflector is required. This model is based upon measured holography maps of the 70-meter antenna obtained at 3 elevation angles. The holography maps are then processed (using an appropriate gravity mechanical model of the dish) to provide surface distortion maps at all elevation angles. 2. From the surface distortion maps, ray optics is used to determine the theoretical shape of the DFP that will exactly phase compensate the distortions. 3. From the theoretical shape and a NASTRAN mechanical model of the plate, the actuator positions that generate a surface that provides the best RMS fit to the theoretical model are selected. Using the actuator positions and the NASTRAN model provides an accurate description of the actual mirror shape. 4. Starting from the mechanical drawings of the feed, a computed RF feed pattern is generated. This pattern is expanded into a set of spherical wave modes so that a complete near field analysis of the reflector system can be obtained. 5. For the array feed, the excitation coefficients that provide the maximum gain are computed using a phase conjugate technique. The basic experimental geometry consisted of a dual shaped 70-meter antenna system; a refocusing ellipse, a DFP and an array feed system. To provide physical insight to the systems performance, focal plane field plots are presented at several elevations. Curves of predicted performance are shown for the DFP system, monopulse tracking system, AFCS and combined DFP/AFCS system. The calculated results show that the combined DFP/AFCS system is capable of recovering the majority of the gain lost due to gravity distortion.

Imbriale, William A.↗
NASA NTRS1994

Reanalysis of X-ray emission from M87. 1: The single-phase medium

We reanalyze the density and temperature profiles of the X-ray-emitting gas around M87, assuming a spherically symmetric, single-phase model. For given assumed density and temperature profiles, we predict Einstein High Resolution Imager (HRI) and Imaging Proportional Counter (IPC) surface brightness distribution as well as Focal Point Crystal Spectrometer (FPCS) line fluxes, which we compare with the data. We find that a good fit to these data can be obtained and that, with a suitable adjustment of the abundances, the equivalent width of the iron complex at 7 keV as observed by wide-beam instruments can also be explained. In contrast to this, the Einstein Solid State Spectrometer (SSS) spectrum and optically determined mass profiles cannot be accounted for simultaneously with the previous X-ray data. We show that the disagreement of the Solid State Spectrometer with our models is due to an inconsistency of the Solid State Spectrometer data with that of the other X-ray instruments. Because of this, we find no evidence for the existence of absorption above the Galactic value, contrary to an earlier claim based on Solid State Spectrometer data. The disagreement of our models with optical mass data is due either to the assumption of a single-phase model, thus rendering the model unacceptable, or possibly to maximal systematic errors in the FPCS data. We also show that thermal conduction at the Spitzer rate is important in our models, although this conclusion is of limited importance in view of the poor fit of our models to the data.

Tsai, John C.↗
NASA NTRS1995

Models of far-IR sources at W3-IRS4 and W3-IRS5

Far-IR continuum maps made with the KAO of W3 at 47 and 95 micrometers show peaks identified with the mid-IR sources IRS4 and IRS5 and extended emission identified with the radio source W3A. We have taken the steepest radial scan profiles from the peaks at IRS4 and IRS5 to represent the objects as spherical clouds. Spherically symmetric models were created in an attempt to match the observed profiles. Radiative transfer dust cloud models heated by central protostars or stars do not match extended emission in the 47 micrometers scan profile for any assumed density distribution for either source. However, both sources can be approximately fit by power law density profiles and ad hoc temperature profiles which are much less steep than those by the single source radiative transfer models. One possible physical explanation for the shallow temperature gradients suggested by the data is heating by a distribution of luminosity sources.

Campbell, M. F.↗
NASA NTRS1973

Approximations to the mean surface of Mars and Mars atmosphere using Mariner 9 occultations

When the Mariner 9 spacecraft passed behind Mars in its orbiting phase, the radio beam was cut off by some physical feature. These occultations occurred during the months of November and December 1971 and again in April and May 1972. The fairly good global spread of these data was used to estimate the shape of the Martian globe, as definable by mathematical shapes (ellipsoidal and spherical harmonic). The refraction profiles allowed the estimation of pressure near the occultation point; these, corrected back to an arbitrary fixed value of pressure, chosen to be 6.1 mb, were used as radii to an isobaric surface. Preliminary results showed a figure displacement from the mass center with a best fit southeast by 2.85 km. There was also an equatorial displacement in the direction of about 100 deg W. Atmospheric approximations for the mass center to 6.1-mb pressure distances were fit to an ellipsoid.

Cain, D. L.↗
NASA NTRS1988

The X-ray emission of normal elliptical galaxies - Steady state cooling flow models

Steady state, spherical, homogeneous cooling flow models are compared to the observed X-ray luminosities, spectra, and surface-brightness profiles of elliptical galaxies. Galaxies without heavy halos and SNe have temperatures which are too low and X-ray surface brightness profiles which are too rapidly decreasing with radius to fit the observations. Galaxies with heavy halos and SNe have luminosities which are too large except for the highest luminosity ellipticals. Galaxies without heavy halos but with SNe have temperatures which are probably too cool in the outer parts. Galaxies with heavy halos but without SNe, where the gas is heated by the motions of gas-losing stars and by infall and compression in a heavy halo potential, are favored. This suggests that elliptical galaxies have heavy halos of optically dark material.

Sarazin, Craig L.↗
NASA NTRS2001

MTF Driven by Plasma Liner Dynamically Formed by the Merging of Plasma Jets: An Overview

One approach for standoff delivery of the momentum flux for compressing the target in MTF consists of using a spherical array of plasma jets to form a spherical plasma shell imploding towards the center of a magnetized plasma, a compact toroid (Figure 1). A 3-year experiment (PLX-1) to explore the physics of forming a 2-D plasma liner (shell) by merging plasma jets is described. An overview showing how this 3-year project (PLX-1) fits into the program plan at the national and international level for realizing MTF for energy and propulsion is discussed. Assuming that there will be a parallel program in demonstrating and establishing the underlying physics principles of MTF using whatever liner is appropriate (e.g. a solid liner) with a goal of demonstrating breakeven by 2010, the current research effort at NASA MSFC attempts to complement such a program by addressing the issues of practical embodiment of MTF for propulsion. Successful conclusion of PLX-1 will be followed by a Physics Feasibility Experiment (PLX-2) for the Plasma Liner Driven MTF.

Thio, Y. C. Francis↗
NASA NTRS1990

Non-LTE model calculations for SN 1987A and the extragalactic distance scale

This paper presents model atmospheres for the first week of SN 1987A, based on the luminosity and density/velocity structure from hydrodynamic models of Woosley (1988). The models account for line blanketing, expansion, sphericity, and departures from LTE in hydrogen and helium and differ from previously published efforts because they represent ab initio calculations, i.e., they contain essentially no free parameters. The formation of the UV spectrum is dominated by the effects of line blanketing. In the absorption troughs, the Balmer line profiles were fit well by these models, but the observed emissions are significantly stronger than predicted, perhaps due to clumping. The generally good agreement between the present synthetic spectra and observations provides independent support for the overall accuracy of the hydrodynamic models of Woosley. The question of the accuracy of the Baade-Wesselink method is addressed in a detailed discussion of its approximations. While the application of the standard method produces a distance within an uncertainty of 20 percent in the case of SN 1987A, systematic errors up to a factor of 2 are possible, particularly if the precursor was a red supergiant.

Schmutz, W.↗
NASA NTRS1987

Spherical harmonic expansion of the Levitus Sea surface topography

Prior information for the stationary sea surface topography (SST) may be needed in altimetric solutions that intend to simultaneously improve the gravity field and determine the SST. For this purpose the oceanographically derived SST estimates are represented by a spherical harmonic expansion. The spherical harmonic coefficients are computed from a least squares adjustment of the data covering the majority of the oceanic regions of the world. Several tests are made to determine the optimum maximum degree of solution and the best configuration of the geometry of the data in order to obtain a solution that fits the data and also provides a good spectral representation of the SST.

Engelis, Theodossios↗
NASA NTRS1992

The remarkable twisted disk of NGC 4753 and the shapes of galactic halos

The complex dust lanes in the S0 galaxy NGC 4753 are shown to be consistent with a disk that is strongly twisted by differential precession. Yet another peculiar S0 can therefore be explained as the result of an accretion event. An evolving disk model is fitted to the observed distribution. This disk is inclined by 15 deg relative to the galaxy's equatorial plane and twisted such that its line of nodes changes smoothly by 3.8 pi over a factor of seven in radius. The model indicates that most of the galaxy's mass is unseen, is nearly spherically distributed, and has a nearly scale-free spatial distribution. The ellipticity of the total galactic mass distribution must be constant to within 20 percent over the radial extent of the twisted disk - a conclusion which may significantly constrain galaxy formation mechanisms.

Steiman-Cameron, Thomas Y.↗
NASA NTRS2006

Multi-Spacecraft Observations of Interplanetary Shock Accelerated Particle Events

We use simultaneous measurements from the Wind and ACE spacecraft to determine the spatial properties of both interplanetary (IP) shocks and the shock-associated energetic particle events. We combine plasma, magnetic field and energetic particle data from ACE and Wind for 124 energetic storm particle (ESP) events from 1998 to 2003 and examine the spatial and temporal variations of these events in the Earth's vicinity. We find that even though the two spacecraft were occasionally separated by more than 400 RE, the plasma, field, and energetic particle time-intensity profiles during the events were very similar. In addition, we find that the ion composition and energy spectra in individual IP shock events are identical at the two spacecraft locations. We also use the fitted shock velocity along the normal from ACE and estimate the shock transit time to Wind location. In general, there is poor agreement between the estimated transit time and the actual measured transit time. Hence, our assumptions that a) the IP shock at 1 AU propagates radially, and/or b) the IP shock is spherically symmetric at 1 AU are not valid. In this paper, we will also study, for the first time, the anisotropy measurements of low-energy IP shock-associated ions at both ACE and Wind. We will then compare these new anisotropy analyses with locally measured shock parameters and identify possible signatures of different shock acceleration processes as predicted by the first-order Fermi and shock-drift models.

Ho, G. C.↗
NASA NTRS1993

Flexure and the role of inplane force around coronae on Venus

Large coronae on Venus, such as Artemis and Latona, are rimmed by conspicuous trenches and associated outer rises. Sandwell and Schubert have observed that these systems resemble terrestrial subduction zones in planform and have succeeded in fitting an elastic plate bending equation to the inferred flexural topography. However, the first zero crossing bending moments required are -2.5 x 10(exp 17) N for Artemis and -5.0 x 10(exp 16) N for Latona. Since these moments are similar in magnitude to those of subducting slabs on Earth, a rollback subduction mechanism was proposed to explain the flexure around the largest coronae, although a differential thermal subsidence model is sufficient to account for the topography around some coronae. The purpose is to investigate the effect of inplane force as a possible alternative to large applied moments in producing flexure at Artemis and Latona. The close correlation of gravity to topography on Venus implies the absence of a low viscosity zone and the strong coupling of the lithosphere to mantle convection. If coronae are the surface manifestations of mantle plumes, they may be the sites of active convective stress coupling. As the upwelling reaches the lithosphere, it spreads radially outward, inducing shear tractions on the base of the plate. In addition, the hot, expanding corona may load the surrounding plate horizonally. Both the basal shear stresses and radial loading can be treated as an equivalent compressive inplane force in the mechanical lithosphere, which contributes to the bending of the outlying plate. Using a model that relates inplane force to the measured gravity anomalies, a rough value of the inplane force at Artemis was calculated. Recent Pioneer Venus spherical harmonic gravity models indicate a geoid anomaly of about 75 m over Artemis, which corresponds to an estimated inplane force on the order of -1x10(exp 13) N/m. The gravity model is unable to resolve Latona, but an inplane force of similar dimensions is assumed. The maximum possible inplane force based on the expected rheology can be constrained by using the approximate 5 K/km thermal gradient inferred from the best fit 30 km elastic plate at Artemis and Latona. For a dry olivine flow law in the upper mantle, the compressional load limit of the 60 km thick mechanical lithosphere is -4 x 10(exp 13) N/m. This value is equivalent to a load of -8 x 10(exp 13) N/m on a 30 km thick elastic plate.

Brown, C. David↗
NASA NTRS2012

Magnetization of the Lunar Crust

Magnetic fields measured by the satellite Lunar Prospector show large scale features resulting from remanently magnetized crust. Vector data synthesized at satellite altitude from a spherical harmonic model of the lunar crustal field, and the radial component of the magnetometer data, have been used to produce spatially continuous global magnetization models for the lunar crust. The magnetization is expressed in terms of localized basis functions, with a magnetization solution selected having the smallest root-mean square magnetization for a given fit to the data, controlled by a damping parameter. Suites of magnetization models for layers with thicknesses between 10 and 50 km are able to reproduce much of the input data, with global misfits of less than 0.5 nT (within the uncertainties of the data), and some surface field estimates. The magnetization distributions show robust magnitudes for a range of model thicknesses and damping parameters, however the magnetization direction is unconstrained. These global models suggest that magnetized sources of the lunar crust can be represented by a 30 km thick magnetized layer. Average magnetization values in magnetized regions are 30-40 mA/m, similar to the measured magnetizations of the Apollo samples and significantly weaker than crustal magnetizations for Mars and the Earth. These are the first global magnetization models for the Moon, providing lower bounds on the magnitude of lunar crustal magnetization in the absence of multiple sample returns, and can be used to predict the crustal contribution to the lunar magnetic field at a particular location.

Lunar↗
NASA NTRS1975

Recovery of atmospheric refractivity profiles from simulated satellite-to-satellite tracking data

Techniques for recovering atmospheric refractivity profiles from simulated satellite-to-satellite tracking data are documented. Examples are given using the geometric configuration of the ATS-6/NIMBUS-6 Tracking Experiment. The underlying refractivity model for the lower atmosphere has the spherically symmetric form N = exp P(s) where P(s) is a polynomial in the normalized height s. For the simulation used, the Herglotz-Wiechert technique recovered values which were 0.4% and 40% different from the input values at the surface and at a height of 33 kilometers, respectively. Using the same input data, the model fitting technique recovered refractivity values 0.05% and 1% different from the input values at the surface and at a height of 50 kilometers, respectively. It is also shown that if ionospheric and water vapor effects can be properly modelled or effectively removed from the data, pressure and temperature distributions can be obtained.

Murray, C. W., Jr.↗
NASA NTRS1980

Physical characteristics of cometary dust from optical studies

Observations of the sunlight scattered and thermal emission from cometary dust, which may be used to infer the physical properties of the dust grains, are reviewed. Consideration is given to the observed wavelength dependence of the scattered light from cometary coma and tails, the average scattering function of the dust grains, the average grain Bond albedo, the polarization of the scattered light, and grain temperatures deduced from thermal infrared emission. The thermal properties of dust grains are illustrated for models based on magnetite or olivine grain materials, with consideration given to the variation of thermal properties with particle radius and heliocentric distance. Comparison of the models with observations indicates that a disordered or amorphous olivine composition can give a reasonable fit to the data for appropriate grain sizes and temperatures. The observations acquired are noted to indicate an optically important particle size of 1 micron, with silicate particles not larger than a few microns usually present although pure silicate grains can not be responsible for the thermal emission, and the cometary dust grains are most likely not spherical. Further observations needed in the infrared are indicated.

Hanner, M. S.↗
NASA NTRS1982

Generation of off-axis aspherics

A method for precise generation of optical surfaces is described. The work is turned about one axis, while a grinding head or cutting tool is gradually moved across by an arm turning about a second axis. This geometry can be used to directly generate spherical surfaces or aspherical surfaces presently termed 'hulahoids'. The properties of these surfaces are analyzed to determine how well large paraboloids and their off-axis segments can be generated. Very close approximations are possible. For example, 1.5-m segments of a 10-m paraboloid differ by only 0.6 microns peak-to-peak from the best fit hulahoid when the focal ratio is f/2, and 60 microns when it is f/0.4. For accurate generation the cutting tool may need numerically controlled travel of 1 mm or less, with the position set as a function of the swing arm angle.

Angel, J. R. P.↗
NASA NTRS1981

Simulation of the magnetic structure of the inner heliosphere by means of non-spherical source surface

A new method for mapping the Sun's magnetic field B from the photosphere through the corona and interplanetary space is presented. The method entails the derivation of B from a scalar potential within a current-free annular volume bounded inside by photosphere and outside by a prescribed nonspherical source surface to which B is made (as nearly as possible) perpendicular. As usual we obtain the potential for the part of B that arises from currents inside the Sun by fitting an expansion to the observed line-of-sight component of B at the photosphere. A second least-squares fit is introduced to obtain the part of B that arises from currents outside the source surface. Comparisons are made between this model and observed coronal and interplanetary structures. There is evidence that observation data underestimate the strength of photospheric polar magnetic fields.

Levine, R. H.↗
NASA NTRS1988

Relativistic jet models for the BL Lacertae object Mrk 421 during three epochs of observation

Coordinated observation of the nearby BL Lacertae object Mrk 421 obtained during May 1980, January 1984, and March 1984 are described. These observations give a time-frozen picture of the continuous spectrum of Mrk 421 at X-ray, ultraviolet, optical, and radio wavelengths. The observed spectra have been fitted to an inhomogeneous relativistic jet model. In general, the models reproduce the data well. Many of the observed differences during the three epochs can be attributed to variations in the opening angle of the jet and in the angle that the jet makes to the line of sight. The jet models obtained here are compared with the homogeneous, spherically symmetric, synchrotron self-Compton models for this source. The models are also compared with the relativistic jet models obtained for other active galactic nuclei.

Mufson, S. L.↗
NASA NTRS1992

Gamma-ray bursts in the Galactic halo

It is shown here that significant isotropy can be achieved for the angular and luminosity distributions of gamma-ray bursts with a spherically symmetric Galactic halo model for the bursts if the halo extends out beyond 100 kpc. Large halo core radii enhance isotropy, although consistency with observation is possible for core radii as small as 5 kpc if the halo radius is sufficiently large. The intrinsic luminosity distribution of gamma-ray bursts must be treated as a free parameter to fit the observations. If gamma-ray bursts are from the halo, they are likely to be old population H neutron stars, because models based on pulsars escaping from the Galactic plane have strong anisotropies.

Brainerd, J. J.↗