SEARCH · Search NASA

Results for “Model fitting”

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.

SourceResearch connectionFrom yearThrough year

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

At least 919 records · Page 51

All sources NASA NTRS DOE OSTI

NASA NTRS1984

Rotation and the internal structures of the major planets and their inner satellites

Measurements of the rotational periods coupled with those of the masses, the mean radii, and the shapes or the gravitational moments (J2 and J4) enable important constraints to be placed on the internal structures of some remote bodies. Values of J2 for Uranus and Neptune have been calculated from the observed precession rates of the narrow eccentric and inclined Uranian rings and of the orbit of Triton, Neptune's massive satellite. Recent observations of the motions of spots have yielded reliable rotational periods for these planets. These observations are used to show that Uranus and Neptune may have quite different internal structures. The shapes of satellites that are close to their primaries may yield information on the degree of internal differentiation of these bodies. Io, Mimas, Enceladus, and Miranda are of interest in this respect. Residuals in the observed precession rates of the Uranian rings, about 0.005 deg/day, that cannot be accounted for by the best-fit model of J2 and J4 may be related directly to observed irregular variations in ring width of magnitude over 2 km and may indicate the existence of shepherding satellites with mass ratios of over 10 to the -10th. If this is the case, then the effects of these satellites on the precession rates of the rings will result in an appreciable uncertainty in the value of J4 for Uranus.

Dermott, S. F.↗

NASA NTRS1985

Quantitative analysis of planetary reflectance spectra with principal components analysis

A technique is presented for quantitative analysis of planetary reflectance spectra as mixtures of particles on microscopic and macroscopic scales using principal components analysis. This technique allows for determination of the endmembers being mixed, their abundance, and the scale of mixing, as well as other physical parameters. Eighteen lunar telescopic reflectance spectra of the Copernicus crater region, from 600 nm to 1800 nm in wavelength, are modeled in terms of five likely endmembers: mare basalt, mature mare soil, anorthosite, mature highland soil, and clinopyroxene. These endmembers were chosen from a similar analysis of 92 lunar soil and rock samples. The models fit the data to within 2 percent rms. It is found that the goodness of fit is marginally better for intimate mixing over macroscopic mixing.

Johnson, P. E.↗

NASA NTRS1985

Colors of reflection nebulae. II - The excitation of extended red emission

New BVI color-difference measurements are reported for the reflection nebulae NGC 7023, NGC 2068, and CED 201. The data are obtained through BVI imaging with a CCD detector and are analyzed together with previously published measurements for the reflection nebula NGC 2023. All four nebulae are substantially redder in the (V, I) range than expected on the basis of dust scattering alone, a result of the presence of extended emission in the I band. The relative strength of the I excess reaches a maximum in nebular regions where the color in the (B, V) range is bluest. These facts are interpreted in terms of a model which explains the extended I emission as a luminescence process excited by UV radiation from the illuminating star(s). Model fits identify the wavelength region between 1800 and 2500 A of the illuminating star's spectrum as the source of the energy of excitation. It appears possible that the 2200 A band in the dust-extinction curve is a gate for the excitation energy, and it is likely that the I excess emission and the extended near-IR emission recently discovered in reflection nebulae are similar in nature and origin.

Witt, A. N.↗

NASA NTRS1986

Unsteady design-point flow phenomena in transonic compressors

High-frequency response probes which had previously been used exclusively in the MIT Blowndown Facility were successfully employed in two conventional steady state axial flow compressor facilities to investigate the unsteady flowfields of highly loaded transonic compressors at design point operation. Laser anemometry measurements taken simultaneously with the high response data were also analyzed. The time averaged high response data of static and total pressure agreed quite well with the conventional steady state instrumentation except for flow angle which showed a large spread in values at all radii regardless of the type of instrumentation used. In addition, the time resolved measurements confirmed earlier test results obtained in the MIT Blowdown Facility for the same compressor. The results of these tests have further revealed that the flowfields of highly loaded transonic compressors are heavily influenced by unsteady flow phenomena. The high response measurements exhibited large variations in the blade to blade flow and in the blade passage flow. The observed unsteadiness in the blade wakes is explained in terms of the rotor blades' shed vorticity in periodic vortex streets. The wakes were modeled as two-dimensional vortex streets with finite size cores. The model fit the data quite well as it was able to reproduce the average wake shape and bi-modal probability density distributions seen in the laser anemometry data. The presence of vortex streets in the blade wakes also explains the large blade to blade fluctuations seen by the high response probes which is simply due to the intermittent sampling of the vortex street as it is swept past a stationary probe.

Gertz, J. B.↗

NASA NTRS1986

Einstein observations of the soft X-ray background close to the Galactic plane

A map of the soft X-ray emission along a Galactic meridian within + or - 3 deg of the Galactic plane is presented. An excess of diffuse M-band emission is found at low latitudes which exhibits a resolved, double-peaked profile that is roughly symmetric about b = 0 deg with a half-width about 1-2 deg. The integrated intensity derived for that band for this component is roughly 3-4 times the dip in intensity expected due to absorption of the extragalactic background in the plane at these energies. Assuming that this excess is characteristic of the whole Galactic plane, model fits are performed so as to derive constraints on the scale height and volume emissivity of the X-ray emitting material. The present results are compared with those inferred from observations of the harder 2-10 keV background.

Kahn, S. M.↗

NASA NTRS1986

A continued program of planetary study at the University of Texas McDonald Observatory

The beginning of eclipses of the Pluto-Charon system was detected. The onset of coma formation of P/Halley at 5.4 au was detected and evidence of sublimation at 4.8 au when CN emission was detected. Extensive spatial maps of the gas in the comae of comets Halley and Giacobini-Zinner were obtained in fall 1985. Halley was time variable, and Giacobini-Zinner was depleted in C2 and C3 relative to CN. Comet Kopff was shown to have a pre-perihelion brightness maximum of its gas, consistent with mantle development if the comet is a high obliquity object. New Haser model scale lengths for CN, C3, and C2 were determined using results from the Faint Comet Survey. Spectra of 12 asteroids in unusual orbits showed no evidence of any comet-like emission features. In particular, 3200 Phaethon (1983 TB) has no gas or dust coma, in spite of the similarity of its orbit with the Geminid meteor stream. Data were analyzed on Saturn's H2 and CH4 bands for the recent southern summer using a Tomasko-Doose type of haze distribution. This haze model fits the data moderately well, giving a CH4 mixing ratio of (4.2 + or 0.4)x003. Simple functions were found to approximate the collision-induced rotation-translation thermal opacity of H2.

Smith, H. J.↗

NASA NTRS1987

Uranus and Neptune: Questions and possible answers

Uranus and Neptune form a special class of planetary objects; intermediate in mass and composition between the giant hydrogen-rich planets of Jupiter and Saturn, and the small, rocky terrestrial planets, their structure and composition are not only of intrinsic importance, but also should provide information regarding the nature of the protoplanetary nebula and the processes of planetary formation. A detailed set of theoretical models of these planets within the framework of two and three shell models was costructed. The ratio of ice to rock was varied. The three shell model fits the data on the two planets best.

Reynolds, R. T.↗

NASA NTRS1987

The origin of the diffuse galactic IR/submm emission: Revisited after IRAS

Balloon observations are compared with Infrared Astronomy Satellite observations. There was good agreement for the longitudinal profiles. However, the dust emission observed by IRAS, contrary to the balloon observations which show dust emission only within the absolute value of b is equal to or less than 3 degrees, extends all the way to the galactic pole. The model fits were repeated using more recent parameters for the distribution of interstellar matter in the galactic disk and central region. The IR luminosities are derived for the revised galactic distance scale of solar radius - 8.5 Kpc. A total IR luminosity of 1.2 E10 solar luminosity is obtained, which is about one third of the estimated stellar luminosity of the Galaxy. The dust emission spectrum lambdaI(sub lambda) attains it maximum at 100 microns. A secondary maximum in the dust emission spectrum occurs at 10 microns, which contains 15% of the total IR luminosity of the Galaxy. The galactic dust emission spectrum was compared with the dust emission spectra of external IRAS galaxies. The warm dust luminosity relates to the present OB star formation rate, while flux densities observed at longer submm wavelengths are dominated by cold dust emission and thus can be used to estimate gas masses.

Cox, P.↗

NASA NTRS1987

Simultaneous measurements of sunspot umbral oscillations in the photosphere, chromosphere, and transition region

Measurements of umbral oscillations in a sunspot were made simultaneously from space (with the SMM/UVSP instrument) in the C IV transition-region line and from the ground (with the tower telescope at NSO/sunspot) in spectral lines formed in the photosphere and chromosphere. The power spectra of velocity and intensity variations show multiple peaks in the 3 min band (4.5-10 mHz). A strong oscillation at 5.5 mHz is coherent between the chromosphere and transition region. Another strong oscillation mode at 7.5 mHz is coherent between the photosphere and transition region and appears to have a node in the chromosphere. The rms velocity in the 3 min band is a little over 12 km/sec in both the chromosphere and transition region, but the kinetic energy density is lower in the transition region (by a factor of 10 or more) due to the lower mass density there. These measurements of amplitude and phase of the waves at different heights provided a new, independent method of testing or fitting models of the vertical temperature distribution in the umbral chromosphere and transition regions.

Thomas, John H.↗

NASA NTRS1987

The 1 May 1982 stellar occultation by Uranus and the rings Observations from Mount Stromlo Observatory

The Klemola et al. (1981) object KME 15 has been observed by the Mount Stromlo 1.9-m telescope during its occultation by the Uranian rings on May 1, 1982. In addition to deriving accurate midtimes for rings 6, 5, 4, alpha, beta, eta, gamma, delta, and epsilon, on the basis of model fits to the immersion and emersion ring profiles, normal optical depths and equivalent widths are obtained for all save ring 6. The numerical inversion of immersion and emersion atmosphere occultation values has yielded vertical temperature profiles that exhibit mean temperatures in the vicinity of 130 K and a local maximum near the 8-microbar pressure level.

French, Richard G.↗

NASA NTRS1987

Observations of the Jovian UV aurora by Voyager

The observations of the Jovian aurora made by the Voyager UV spectrometer (UVS) during 1979 are analyzed, with special consideration given to the model fitting process. Several different estimates of the function specifying auroral arc geometry were tried, with the Broadfoot et al. (1981) estimate, found by probing with the tip of the UVS slit, giving the best agreement; this agreement is slightly better than the agreement with the Io torus footprint computed by Roederer et al. (1977). The results suggest that the UVS observations are more sensitive to the surface field geometry than are the Voyager flyby in situ observations. The results of a study of intensity maximum positions indicate that the particles exciting the spatially variable portion of the aurora are drifting west, implying either that these particles are electrons or that they are positive ions drifting east more slowly than the corotation lag of the Io torus region carries them to the west. The latter case is most consistent with the high-energy charged particle measurements.

Herbert, Floyd↗

NASA NTRS1988

Recursive Algorithm For Linear Regression

Order of model determined easily. Linear-regression algorithhm includes recursive equations for coefficients of model of increased order. Algorithm eliminates duplicative calculations, facilitates search for minimum order of linear-regression model fitting set of data satisfactory.

Varanasi, S. V.↗

NASA NTRS1987

Nature of the stratospheric haze on Uranus - Evidence for condensed hydrocarbons

The characteristics and origin of lower-stratosphere haze on Uranus are investigated on the basis of high-phase-angle images obtained at 430-600 nm with the wide-angle and narrow-angle cameras of Voyager 2 during its encounter with Uranus in January 1986. The data-reduction and model-fitting procedures are explained in detail, and the results are presented in extensive tables and graphs. The data are found to be best matched by a haze consisting of particles of modal radius 130 + or - 20 nm and number density 2 + or - 1 per cu cm at the 44-mbar level; such aerosols could be formed by the stratospheric condensation of photochemically produced hydrocarbon gases (locally formed diacetylene and ethane, acetylene, and diacetylene formed at higher altitudes). A total aerosol production rate of (2-15) x 10 to the -17th g/sq cm sec is estimated.

Pollack, James B.↗

NASA NTRS1988

Periodicities in gamma-ray burst light curves

Only one case is known where a gamma-ray burst (GRB) light curve indisputably exhibits a periodic behavior. However, a number of GRB light curves have claims for periodicity of varying degrees of plausibility. A vital part of each claim is some mathematical calculation of the significance of the period when compared to some appropriate hypothesis where no modulation is present. For the period search techniques of Fourier transforms, periodograms, and model fitting, well known statistical procedures allow the significance to be evaluated. However, a fourth period search technique, so called 'pattern recognition', does not have any published means of estimating confidence levels. Here, a series of Monte Carlo calculations is presented which will allow evaluation of a period's significance. These results are then applied to periods proposed for 16 bursts.

Schaefer, B. E.↗

NASA NTRS1988

VLA observations of Uranus at 1.3-20 cm

Observations of Uranus, obtained with resolution 0.5-1.2 arcsec at wavelengths 1.3, 2, 6, and 20 cm using the A and B configurations of the VLA in June-July 1982, October 1983, and February 1984, are reported. The disk-averaged brightness temperatures (DABTs) are determined by model fitting, and the results are presented in extensive graphs and contour maps and characterized in detail. Findings discussed include: (1) an overall spectrum which is relatively flat above 6 cm, (2) 1.3-6-cm brightness which is concentrated nearer to the pole than to the subsolar point, and (3) small changes in DABT from 1982 to 1983/1984 (consistent with an explanation based on a pole-equator temperature gradient).

De Pater, Imke↗

NASA NTRS1988

The high-latitude F supergiant IRAS 18095 + 2704 - A proto-planetary nebula

The paper reports the discovery of a new high-latitude F supergiant, IRAS 18095 + 2704, which shows a large excess in the far-infrared. Ground-based observations have identified it as a V = 10.4 mag F3 Ib star which displays light and velocity variability. Comparison with the post-asymptotic giant branch (AGB) evolution model of Volk and Kwok suggests that it is an excellent candidate for a protoplanetary nebula. Model fitting of the spectrum of 18095 + 2704 from 0.35 to 100 microns suggests that it evolved from the AGB approximately 265 yr ago and had a mass-loss rate of 0.00003 solar mass/yr at the end of the AGB.

Hrivnak, Bruce J.↗

NASA NTRS1988

A comprehensive study of the X-ray structure and spectrum of IC 443

This paper presents the results of a comprehensive study of the X-ray emission from the supernova remnant IC 443, using the solid state spectrometer, IPC, and high-resolution images of the Einstein Observatory, and the medium-energy detector of the HEAO 1 A-2 experiment. A soft X-ray appearance was observed, highly atypical of a supernova remnant in the adiabatic phase, with little correlation between X-ray and optical or radio features. The best-fit models of the low-energy X-ray spectrum of the brightest area of the remnant suggest either that the remnant has not yet attained ionization equilibrium or that the X-rays arise in a multiphase medium. Pronounced soft X-ray spectral differences across the remnant are accounted for by variations in absorption by an intervening molecular cloud. The analysis suggests that, despite the atypical X-ray appearance, the X-ray emission in IC 443 is probably confined to a thin (0.1 pc) shell.

Petre, R.↗

NASA NTRS1989

Far infrared, submm and mm spectroscopy of the galactic center: Radio arc and +20/+50 km s (exp -1) clouds

The observations of the par improved spectroscopy and the molecular clouds at the galactic center are reported. The results show: the spatial distributions of C(II) 158 microns and molecular line radiation and of the thermal radio continuum emission in the arched filaments of the radio arc are similar; about 2 x 10(exp 4) of the solar mass, or 10 percent of the total gas mass in the radio arc, are contained in C(+) regions; the H(+)/C(+) regions are probably located at the surfaces of the dense molecular clouds in the arc. Profiles, fluxes and spatial distributions of the C(II) fine structure and CO rotational lines are reported. It is demonstrated that the data does not fit models in which the neutral interstellar clouds in the arc are ionized by shocks or by magnetohydrodynamic phenomena. Moreover, that the high temperatures and densities derived previously from NH3 and CS observations may not be characteristic of the bulk of the molecular gas.

Genzel, R.↗