Search NASA⌕ Search

SEARCH · Search NASA

Results for “background model”

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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

Selective norepinephrine reuptake inhibition as a human model of orthostatic intolerance

BACKGROUND: Observations in patients with functional mutations of the norepinephrine transporter (NET) gene suggest that impaired norepinephrine uptake may contribute to idiopathic orthostatic intolerance. METHODS AND RESULTS: We studied the effect of the selective NET blocker reboxetine and placebo in a randomized, double-blind, crossover fashion on cardiovascular responses to cold pressor testing, handgrip testing, and a graded head-up tilt test (HUT) in 18 healthy subjects. In a subset, we determined isoproterenol and phenylephrine sensitivities. Subjects ingested 8 mg reboxetine or placebo 12 hours and 1 hour before testing. In the supine position, heart rate was 65+/-2 bpm with placebo and 71+/-3 bpm with reboxetine. At 75 degrees HUT, heart rate was 84+/-3 and 119+/-4 bpm with placebo and with reboxetine (P<0.0001). Mean arterial pressure was 85+/-2 with placebo and 91+/-2 mm Hg with reboxetine while supine (P<0.01) and 88+/-2 mm Hg and 90+/-3 mm Hg at 75 degrees HUT. Blood pressure responses to cold pressor and handgrip testing were attenuated with reboxetine. Reboxetine increased the sensitivity to the chronotropic effect of isoproterenol and the pressor effect of phenylephrine. Vasovagal reactions occurred in 9 subjects on placebo and in 1 subject on reboxetine. CONCLUSIONS: Selective NET blockade creates a phenotype that resembles idiopathic orthostatic intolerance. This observation supports the hypothesis that disordered norepinephrine uptake mechanisms can contribute to human cardiovascular disease. Our study also suggests that NET inhibition might be useful in preventing vasovagal reactions.

Non-NASA Center↗
NASA NTRS1981

Numerical simulation of the generation of electrostatic turbulence in the magnetotail

A two-dimensional plasma model is used to investigate the development of electrostatic turbulence in a magnetized plasma from plasma instabilities. The simulation consists of following the motion of 100,000 ions in their self-consistent electrostatic field. The electrons are treated as a constant neutralizing background. The instabilities modeled are driven by a ring-type velocity distribution and by interpenetrating ion beams in a time-variable magnetic field. Instability growth times are of the order of an ion gyroperiod in the case of the ring distribution and of the order of an ion plasma period in the case of the beam simulation. Maximum potential differences generated are of the order of the ion kinetic energies. These simulations demonstrate the cascade of wave energy to long wavelengths, thus showing the E x B turbulence can be generated from plasma microinstabilities. After the free energy feeding, the instabilities are exhausted, and wave energy at wavelengths less than an ion gyrodiameter decays quickly to equilibrium levels, while longer wavelength modes persist for much longer times. In one model with a time dependent, but spatially uniform, magnetic field the electric field energy at long wavelengths appeared to increase as a result of the increase of the magnetic field.

Swift, D. W.↗
NASA NTRS1988

Models of the quasar population. I - A new luminosity function

A new functional form for the quasar luminosity function is tested using recent observational results for both bright and faint quasar count and redshift distributions. The form is of a fairly general type based on three free parameters and allows for quasars to undergo a combination of luminosity evolution and luminosity-dependent density evolution; an advantage to this approach is that it does not constrain quasars to follow a single type of evolution. Models of pure luminosity evolution or luminosity-dependent density evolution can be constructed, but the apparent magnitude distribution of observed quasars is best fitted by a combination model. The combination model also gives the correct redshift distribution for quasars with redshifts less than three and predicts that quasars brighter than B = 22 provide a 2-10 keV X-ray flux that is equal to 32 percent of the observed X-ray background. However, the model is flawed in that it predicts more high-redshift quasars than are observed.

Heisler, Julia↗
NASA NTRS1997

Meta-Modeling: A Knowledge-Based Approach to Facilitating Model Construction and Reuse

In this paper, we introduce a new modeling approach called meta-modeling and illustrate its practical applicability to the construction of physically-based ecosystem process models. As a critical adjunct to modeling codes meta-modeling requires explicit specification of certain background information related to the construction and conceptual underpinnings of a model. This information formalizes the heretofore tacit relationship between the mathematical modeling code and the underlying real-world phenomena being investigated, and gives insight into the process by which the model was constructed. We show how the explicit availability of such information can make models more understandable and reusable and less subject to misinterpretation. In particular, background information enables potential users to better interpret an implemented ecosystem model without direct assistance from the model author. Additionally, we show how the discipline involved in specifying background information leads to improved management of model complexity and fewer implementation errors. We illustrate the meta-modeling approach in the context of the Scientists' Intelligent Graphical Modeling Assistant (SIGMA) a new model construction environment. As the user constructs a model using SIGMA the system adds appropriate background information that ties the executable model to the underlying physical phenomena under investigation. Not only does this information improve the understandability of the final model it also serves to reduce the overall time and programming expertise necessary to initially build and subsequently modify models. Furthermore, SIGMA's use of background knowledge helps eliminate coding errors resulting from scientific and dimensional inconsistencies that are otherwise difficult to avoid when building complex models. As a. demonstration of SIGMA's utility, the system was used to reimplement and extend a well-known forest ecosystem dynamics model: Forest-BGC.

Keller, Richard M.↗
NASA NTRS2020

Charge Exchange, from the Sky to the Laboratory: A Method to Determine State-Selective Cross-Sections for Improved Modeling

Charge exchange (CX) is a semi-resonant recombination process that can lead to spectral line emission in the X-ray band. It occurs in nearly any environment where hot plasma and cold gas interact: in the solar system, in comets and planetary atmospheres, and likely astrophysically, in, for example, supernova remnants and galaxy clusters. It also contributes to the soft X-ray background. Accurate spectral modeling of CX is thus critical to properly interpreting our astrophysical observations, but the commonly used CX models in popular spectral fitting packages often rely on scaling equations and may not accurately describe observations or laboratory measurements. This paper introduces a method that can be applied to high-resolution CX spectra to directly extract state-selective CX cross-sections for electron capture, a key parameter for properly simulating the resulting CX spectrum.

Gabriele L. Betancourt-Martinez↗
NASA NTRS1991

A Compton reflection model for the cosmic X-ray and gamma-ray backgrounds

The gamma-ray spectrum of an AGN in the Compton reflection model is calculated and the contribution of such sources to the cosmic gamma-ray background is considered. The spectrum is composed of three parts: an X-ray power law, a broad hump from roughly 10 to 120 keV due to reprocessing of the power-law X-ray spectrum by cold gas, and gamma rays which are due to the inverse-Compton scattering of X-rays. The contribution of such sources to the cosmic gamma-ray background is computed and it is found that they provide an excellent fit to the entire background spectrum from 3 keV to 10 MeV. Constraints on the physical conditions in these sources are derived based on these results and the possibility of observing such sources with GRO is considered.

Rogers, R. D.↗
NASA NTRS1977

Photochemistry of tropospheric ozone

A photochemical equilibrium model of minor constituent chemistry in the troposphere was constructed to investigate the photochemistry of tropospheric ozone. We find that photochemical production must be supplemented by additional sources to account for observed ozone levels in clean air, while photochemical loss is more important than surface destruction as an ozone loss mechanism. Photochemically produced ozone is sensitive to the NO(x) background in the model, exhibiting a maximum of about 0.5 ppb of NO(x).

Stewart, R. W.↗
NASA NTRSDate not supplied

Global Solar Activity Data Portal for Studying 3D Dynamics and Activity of the Sun

The main problems with understanding and predicting solar activity are tightly linked to limitations in describing the global evolution of the Sun from the deep interior to the corona. Because of the complexity of interactions in a wide range of dynamical, turbulent, and spatial scales and dramatic changes in thermodynamic and magnetic field conditions, only physics-based models can provide essential background to generate reliable solar activity forecasts. However, performing accurate model calibration and estimating uncertainties is often challenging due to the unavailability of a long time series of observations. To mitigate these limitations, we have developed the Global Solar Activity (GSA) Data Portal, which enables convenient access to a variety of modern and historical data. The portal supports a dynamic visualization for 1D time series (such as the sunspot number and solar irradiance) and quick-look visualization for 2D datasets (e.g., synoptic magnetograms, the solar internal rotation, and flows). The GSA portal includes a search engine that enables data retrieval from user-specified data sources and time intervals. In this presentation, we will discuss the current and upcoming capabilities of the data portal and its potential applications for space weather studies.

SMD↗
NASA NTRS2012

The Influence of Pickup Protons, from Interstellar Neutral Hydrogen, on the Propagation of Interplanetary Shocks from the Halloween 2003 Solar Events to ACE and Ulysses: A 3-D MHD Modeling Study

We describe our 3-D, time ]dependent, MHD solar wind model that we recently modified to include the physics of pickup protons from interstellar neutral hydrogen. The model has a time-dependent lower boundary condition, at 0.1 AU, that is driven by source surface map files through an empirical interface module. We describe the empirical interface and its parameter tuning to maximize model agreement with background (quiet) solar wind observations at ACE. We then give results of a simulation study of the famous Halloween 2003 series of solar events. We began with shock inputs from the Fearless Forecast real ]time shock arrival prediction study, and then we iteratively adjusted input shock speeds to obtain agreement between observed and simulated shock arrival times at ACE. We then extended the model grid to 5.5 AU and compared those simulation results with Ulysses observations at 5.2 AU. Next we undertook the more difficult tuning of shock speeds and locations to get matching shock arrival times at both ACE and Ulysses. Then we ran this last case again with neutral hydrogen density set to zero, to identify the effect of pickup ions. We show that the speed of interplanetary shocks propagating from the Sun to Ulysses is reduced by the effects of pickup protons. We plan to make further improvements to the model as we continue our benchmarking process to 10 AU, comparing our results with Cassini observations, and eventually on to 100 AU, comparing our results with Voyager 1 and 2 observations.

Detman, T. R.↗
NASA NTRS1987

Numerical simulation of the generation of turbulence from cometary ion pick-up

Observations of magnetic field fluctuations near Comet Halley have revealed a rapid development of a Kolmogoroff-like turbulence spectrum extending from below 0.01 Hz to above 0.1 Hz. Spectra obtained far from the comet have a strong peak in power near the Doppler-shifted ion-cyclotron frequency of singly ionized water. Closer to the comet, the spectrum at higher frequencies is enhanced in power level over the background solar wind spectrum by approximately an order of magnitude. The equations of incompressible MHD are solved using a two-dimensional 256 x 256 mode spectral method code to simulate this spectral evolution as an inertial range turbulent cascade. The initial conditions contained a constant magnetic field and a single coherent wave mode at a low wave number. The solar wind turbulence was modeled by a background noise spectrum having a Kolmogoroff spectral index. The coherent mode decayed into an inertial range spectrum with Kolmogoroff slope within a few eddy-turnover times. Both the time scale and the increase in power level of the turbulence seen in the simulation are in accord with the Giotto observations.

Goldstein, M. L.↗
NASA NTRS1992

Martens-Kuin models of normal and inverse polarity filament eruptions and coronal mass ejections

An analysis is made of the Martens-Kuin filament eruption model in relation to observations of coronal mass ejections (CMEs). The field lines of this model are plotted in the vacuum or infinite resistivity approximation with two background fields. The first is the dipole background field of the model and the second is the potential streamer model of Low. The Martens-Kuin model predicts that, as the filament erupts, the overlying coronal magnetic field lines rise in a manner inconsistent with observations of CMEs associated with eruptive filaments. This model and, by generalization the whole class of so-called Kuperus-Raadu configurations in which a neutral point occurs below the filament, are of questionable utility for CME modeling. An alternate case is considered in which the directions of currents in the Martens-Kuin model are reversed resulting in a so-called normal polarity configuration of the filament magnetic field. The background field lines now distort to support the filament and help eject it. While the vacuum field results make this configuration appear very promising, a full two- or more-dimensional MHD simulations is required to properly analyze the dynamics resulting from this configuration.

Smith, D. F.↗
NASA NTRS2011

Searches for Periodic Neutrino Emission from Binary Systems with 22 and 40 Strings of IceCube

Recent observations of GeV /TeV photon emission from several X-ray binaries have sparked a renewed interest in these objects as galactic particle accelerators. In spite of the available multi-wavelength data, their acceleration mechanisms are not determined, and the nature of the accelerated particles (hadrons or leptons) is unknown. While much evidence favors leptonic emission, it is very likely that a hadronic component is also accelerated in the jets of these binary systems. The observation of neutrino emission would be clear evidence for the presence of a hadronic component in the outflow of these sources. In this paper we look for periodic neutrino emission from binary systems. Such modulation, observed in the photon flux, would be caused by the geometry of these systems. The results of two searches are presented that differ in the treatment of the spectral shape and phase of the emission. The 'generic' search allows parameters to vary freely and best fit values, in a 'model-dependent' search, predictions are used to constrain these parameters. We use the IceCube data taken from May 31, 2007 to April 5, 2008 with its 22-string configuration, and from April 5, 2008 and May 20, 2009 with its 40-string configuration. For the generic search and the 40 string sample, we find that the most significant source in the catalog of 7 binary stars is Cygnus X-3 with a 1.8% probability after trials (2.10" sigma one-sided) of being produced by statistical fluctuations of the background. The model-dependent method tested a range of system geometries - the inclination and the massive star's disk size - for LS I+61 deg 303, no significant excess was found.

Abassi, R.↗
NASA NTRS1987

Evolution of the axial electron cyclotron maser instability, with applications to solar microwave spikes

The nonlinear evolution of cyclotron radiation from streaming and gyrating electrons in an external magnetic field is analyzed. The nonlinear dynamics of both the fields and the particles are treated fully relativistically and self-consistently. The model includes a background plasma and electrostatic effects. The analytical and numerical results show that a substantial portion of the beam particle energy can be converted to electromagnetic wave energy at frequencies far above the electron cyclotron frequency. In general, the excited radiation can propagate parallel to the magnetic field and, hence, escape gyrothermal absorption at higher cyclotron harmonics. The high-frequency Doppler-shifted cyclotron instability can have saturation efficiencies far higher than those associated with well-known instabilities of the electron cyclotron maser type. Although the analysis is general, the possibility of using this model to explain the intense radio emission observed from the sun is explored in detail.

Vlahos, Loukas↗
NASA NTRS1981

Precursor active galaxies and the cosmic X-ray background

Consideration is given to Carr's theory (1980) that most cosmic X-ray background is produced by thermal X-ray emission associated with black hole accretion disks for compact objects. It is posited that the onset of significant X-rays occurs during galaxy formation when accretion disks are formed by the interaction of young galaxies and massive black holes of pregalactic origin. The study suggests that most cosmic X-ray background could be produced from distant sources with a low ratio of optical to X-ray luminosity. Precursor active galaxy objects involving a massive black hole surrounded by a hot optically thin accretion disk radiating near the Eddington luminosity limit and with an effective temperature of about 2 x 10 to the 9th K would fulfill the condition and produce the correct background spectrum and intensity. This model also associates cosmic X-ray background and nonthermal gamma-ray background to active galaxies, thus implying that massive black holes are associated with their internal dynamics.

Boldt, E.↗
NASA NTRS2014

Fractional Order Modeling of Atmospheric Turbulence - A More Accurate Modeling Methodology for Aero Vehicles

The presentation covers a recently developed methodology to model atmospheric turbulence as disturbances for aero vehicle gust loads and for controls development like flutter and inlet shock position. The approach models atmospheric turbulence in their natural fractional order form, which provides for more accuracy compared to traditional methods like the Dryden model, especially for high speed vehicle. The presentation provides a historical background on atmospheric turbulence modeling and the approaches utilized for air vehicles. This is followed by the motivation and the methodology utilized to develop the atmospheric turbulence fractional order modeling approach. Some examples covering the application of this method are also provided, followed by concluding remarks.

Atmospheric Turbulence Modeling↗
NASA NTRS1991

An analog retina model for detecting dim moving objects against a bright moving background

We are interested in applications that require the ability to track a dim target against a bright, moving background. Since the target signal will be less than or comparable to the variations in the background signal intensity, sophisticated techniques must be employed to detect the target. We present an analog retina model that adapts to the motion of the background in order to enhance targets that have a velocity difference with respect to the background. Computer simulation results and our preliminary concept of an analog 'Z' focal plane implementation are also presented.

Searfus, R. M.↗
NASA NTRS2005

Modeling of the EUV and X-Ray Emission Spectra Induced by the Solar Winds Ions in the Heliosphere

We have carried out investigation of the EUV and X-ray emission spectra induced in interaction between the Solar Wind (SW) and interstellar neutral gas. The spectra of most important SW ions have been computed for the charge-exchange mechanism of X-ray emission using new accurate spectroscopic data from recent laboratory measurements and theoretical calculations. Total spectra have been constructed as a sum of spectra induced in the charge-exchange collisions by individual O(exp q+), C(exp q+), N(exp q+), Ne(exp q+), Mg (exp q+) and Fe(exp q+) ions. Calculations have been performed for X-ray emission from the heliospheric hydrogen and helium gas. X-ray maps of the heliosphere have been computed. The power density of X-ray sources in the heliospheric ecliptic plane is shown for the H gas and for the He gas. Distances from the Sun (0,0) are given in AU. The helium cone is clear seen in the X-ray map of the charge-exchange emission induced by the solar wind. X-ray emission spectra detected by the Chandra X-ray telescope from the "dark" side of Moon has been identified as a X-ray background emission induced by the solar wind from the geocorona. Spectra and intensities of this charge-exchange X-rays have been compared with the heliospheric component of the X-ray background. Observations and modeling of the SW spectra induced from the geocorona indicate a strong presence of emission lines of highly charged oxygen ions. Anisotropy in distribution of heliospheric X-rays has been predicted and calculated for the regions of the fast and slow solar winds.

Kharchenko, Vasili↗
NASA NTRS1980

The correlation of the cosmic X-ray background with the light of galaxies

Models for the origin of the cosmic X-ray background (CXB) radiation attribute it to numerous weak, unresolved discrete sources (e.g., active galactic nuclei), to some diffuse emission mechanism (e.g., thermal bremmstrahlung from a hot intergalactic plasma), or to an unresolved population of discrete sources which evolve substantially with redshift (e.g., quasars). It is shown that the comparison of the CXB flux variations with flux variations in the light from galaxies can measure the relative contribution of these possible sources of the CXB. Applying the technique to available Uhuru data, it is found that the absence of correlation between the optical flux variations and the X-ray flux variations sets an upper limit of approximately 50% on the fraction of the CXB originating with any classes of X-ray sources substantially represented among bright (apparent photographic magnitude less than or equal to 15.5) galaxies. The data are consistent with nearly all of the CXB being due to diffuse emission or to a class of sources whose density increases rapidly with redshift.

Turner, E. L.↗