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NASA NTRS2013

XIPE the X-Ray Imaging Polarimetry Explorer

X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 in the 210 keV band in 105 s for pointed observations, and 0.6 for an X10 class solar flare in the 1535 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin 14.7 arcmin. The spectral resolution is 20 at 6 keV and the time resolution is 8 s. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut fr extraterrestrische Physik (MPE, Germany). XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 of the net observing time.

Astronomy X-ray Polarimetry↗
NASA NTRS1975

Error analysis of Dobson spectrophotometer measurements of the total ozone content

A study of techniques for measuring atmospheric ozone is reported. This study represents the second phase of a program designed to improve techniques for the measurement of atmospheric ozone. This phase of the program studied the sensitivity of Dobson direct sun measurements and the ozone amounts inferred from those measurements to variation in the atmospheric temperature profile. The study used the plane - parallel Monte-Carlo model developed and tested under the initial phase of this program, and a series of standard model atmospheres.

Holland, A. C.↗
NASA NTRS2019

LongRunMIP - Motivation and Design for a Large Collection of Millennial-Length AO-GCM Simulations

LongRunMIP is the first collection of millennial-length simulations of complex coupled climate models and enables investigations of how these models equilibrate in response to radiative perturbations.We present a model intercomparison project, LongRunMIP, the first collection of millennial-length (1000+ year) simulations of complex coupled climate models with a representation of ocean, atmosphere, sea ice, and land surface, and their interactions. Standard model simulations are generally only a few hundred years long. However, modeling the long-term equilibration in response to radiative forcing perturbation is important for understanding many climate phenomena, such as the evolution of ocean circulation, time-and temperature-dependent feedbacks, and the differentiation of forced signal and internal variability. The aim of LongRunMIP is to facilitate research into these questions by serving as an archive for simulations that capture as much of this equilibration as possible. The only requirement to participate in LongRunMIP is to contribute a simulation with elevated, constant CO2 forcing that lasts at least 1000 years. LongRunMIP is a MIP of opportunity in that the simulations were mostly performed prior to the conception of the archive without an agreed-upon set of experiments. For most models, the archive contains a preindustrial control simulation and simulations with an idealized (typically abrupt) CO2 forcing. We collect 2D surface and top-of-atmosphere fields, and 3D ocean temperature and salinity fields. Here, we document the collection of simulations and discuss initial results, including the evolution of surface and deep ocean temperature and cloud radiative effects. As of summer 2019, the collection includes 50 simulations of 15 models by 10 modeling centers. The data of LongRunMIP are publicly available. We encourage submission of more simulations in the future.

Maria Rugenstein↗
NASA NTRS2018

The SPASE Data Model: A Metadata Standard for Registering, Finding, Accessing, and Using Heliophysics Data Obtained from Observations and Modeling

The Space Physics Archive Search and Extract Consortium has developed and implemented the SPASE Data Model that provides a common language for registering a wide range of Heliophysics data and other products. The Data Model enables discovery and access tools such that any researcher can obtain data easily, thereby facilitating research, including on space weather. The Data Model includes descriptions of Simulation Models and Numerical Output, pioneered by the Integrated Medium for Planetary Exploration (IMPEx) group in Europe, and subsequently adopted by the Community Coordinated Modeling Center (CCMC). The SPASE group intends to register all relevant Heliophysics data resources, including space-, ground-, and model-based. Substantial progress has been made, especially for space-based observational data and associated observatories, instruments, and display data. Legacy product registrations and access go back more than 50 years. Real-time data will be included. The National Aeronautics and Space Administration (NASA) portion of the SPASE group has funding that assures continuity in the upkeep of the Data Model and aids with adding new products. Tools are being developed for making and editing data descriptions. Digital Object Identifiers (DOIs) for Data Products can now be included in the descriptions. The data access that SPASE facilitates is becoming more uniform, and work is progressing on Web Service access via a standard Application Programming Interface. The SPASE Data Model is stable; changes over the past 9 years were additions of terms and capabilities that are backward compatible. This paper provides a summary of the history, structure, use, and future of the SPASE Data Model.

Roberts, D. Aaron↗
NASA NTRS2015

Random Predictor Models for Rigorous Uncertainty Quantification: Part 2

This and a companion paper propose techniques for constructing parametric mathematical models describing key features of the distribution of an output variable given input-output data. By contrast to standard models, which yield a single output value at each value of the input, Random Predictors Models (RPMs) yield a random variable at each value of the input. Optimization-based strategies for calculating RPMs having a polynomial dependency on the input and a linear dependency on the parameters are proposed. These formulations yield RPMs having various levels of fidelity in which the mean, the variance, and the range of the model's parameter, thus of the output, are prescribed. As such they encompass all RPMs conforming to these prescriptions. The RPMs are optimal in the sense that they yield the tightest predictions for which all (or, depending on the formulation, most) of the observations are less than a fixed number of standard deviations from the mean prediction. When the data satisfies mild stochastic assumptions, and the optimization problem(s) used to calculate the RPM is convex (or, when its solution coincides with the solution to an auxiliary convex problem), the model's reliability, which is the probability that a future observation would be within the predicted ranges, is bounded rigorously.

Crespo, Luis G.↗
NASA NTRS2015

Random Predictor Models for Rigorous Uncertainty Quantification: Part 1

This and a companion paper propose techniques for constructing parametric mathematical models describing key features of the distribution of an output variable given input-output data. By contrast to standard models, which yield a single output value at each value of the input, Random Predictors Models (RPMs) yield a random variable at each value of the input. Optimization-based strategies for calculating RPMs having a polynomial dependency on the input and a linear dependency on the parameters are proposed. These formulations yield RPMs having various levels of fidelity in which the mean and the variance of the model's parameters, thus of the predicted output, are prescribed. As such they encompass all RPMs conforming to these prescriptions. The RPMs are optimal in the sense that they yield the tightest predictions for which all (or, depending on the formulation, most) of the observations are less than a fixed number of standard deviations from the mean prediction. When the data satisfies mild stochastic assumptions, and the optimization problem(s) used to calculate the RPM is convex (or, when its solution coincides with the solution to an auxiliary convex problem), the model's reliability, which is the probability that a future observation would be within the predicted ranges, can be bounded tightly and rigorously.

Crespo, Luis G.↗
NASA NTRS1991

Ginga observation of an AM Herculis type source H0538 + 608

Results from a Ginga observation of the AM Her type object H0538 + 608 are presented. During the first half of the observation (pulsing state) the source showed periodic modulations caused by the white dwarf rotation, and the modulation profile was characterized by a flat top and a flat bottom. During the second half, the source was in a remarkable flaring state and the periodic modulation disappeared. The dramatic change of the X-ray light curve is interpreted in terms of a change of accretion pattern between an eclipsing spot and a noneclipsing spot. The spectrum is well described throughout by a thin thermal model including an iron K-line with equivalent width of some hundred eV, supporting the standard model for the hard X-ray emission region. The temperature in the pulsing state was found to change with the spin phase of the white dwarf. In the flaring state, the temperature showed a positive correlation with the source luminosity.

Ishida, M.↗
NASA NTRS2000

Climatology of the Finite-Volume CCM and Sensitivity Studies

A new model has been developed in the Data Assimilation Office at NASA Goddard Space Flight Center. It is intended for data assimilation and general circulation modeling, including representations of radiatively active trace gases such as stratospheric ozone. The model is based on the flux-form semi-Lagrangian' dynamical core, developed by Lin and Rood. In collaboration with the National Center for Atmospheric Research, the physical parametrization package from the Community Climate model, Version 3, has been included. This paper will discuss the climatology of a multi-year integration of the model, with observed boundary conditions (sea-surface temperature, sea ice, and incoming solar radiation). The standard model version has a resolution of 2.5 degrees longitude, 2 degrees latitude and has 55 levels between the surface and 80km. The mean climate and interannual variability of this model will be discussed, with emphasis on the middle atmosphere. The sensitivity to horizontal resolution will be examined, as will the impact of increasing and decreasing the vertical resolution. Changes which differ significantly from the interannual variations of the control run will be highlighted. Impacts of using long-term mean boundary conditions or values which vary from year to year will be discussed. Changes in the tropospheric circulation induced by lowering the upper boundary of the model will be discussed. The emphasis of the paper is on the climatology of the middle atmosphere and its impacts on the troposphere, with particular attention given to the climatologically important region near the tropopause.

Pawson, Steven↗
NASA NTRS1989

The ages of globular cluster stars - Effects of rotation on pre-main-sequence, main-sequence, and turnoff evolution

Evolutionary sequences for low-metallicity stars (Z ranging from 0.001 to 0.0001) to study the effects of internal stellar rotation on the evolutionary time scales in the pre-main sequence, the main sequence (MS), and around the MS turnoff. Although a substantial amount of angular momentum remains in the interior, rotation is only a minor perturbation on the structure and ages of globular cluster stars. Even models with large initial angular momenta have MS lifetimes that are within 1 percent of those of standard models of the same mass and composition. Therefore, rotation does not affect age estimates of globular clusters from isochrone fitting. Furthermore, the models suggest that because rotation is not likely to affect horizontal-branch (HB) morphology, it does not affect significantly age estimates from the Delta-V method. Nevertheless, the internal angular momentum in the models is consistent with observations of surface rotational velocities on the HB, which require the preservation of a large reservoir of internal angular momentum.

Deliyannis, Constantine P.↗
NASA NTRS1995

Development of a two-equation turbulence model for hypersonic flows: Evaluation of a low Reynolds number correction to the Kappa - epsilon two equation compressible turbulence model - Volume 1

The objective of the current research is the development of an improved k-epsilon two-equation compressible turbulence model for turbulent boundary layer flows experiencing strong viscous-inviscid interactions. The development of an improved model is important in the design of hypersonic vehicles such as the National Aerospace Plane (NASP) and the High Speed Civil Transport (HSCT). Improvements have been made to the low Reynolds number functions in the eddy viscosity and dissipation of solenoidal dissipation of the k-epsilon turbulence mode. These corrections offer easily applicable modifications that may be utilized for more complex geometries. The low Reynolds number corrections are functions of the turbulent Reynolds number and are therefore independent of the coordinate system. The proposed model offers advantages over some current models which are based upon the physical distance from the wall, that modify the constants of the standard model, or that make more corrections than are necessary to the governing equations. The code has been developed to solve the Favre averaged, boundary layer equations for mass, momentum, energy, turbulence kinetic energy, and dissipation of solenoidal dissipation using Keller's box scheme and the Newton spatial marching method. The code has been validated by removing the turbulent terms and comparing the solution with the Blasius solution, and by comparing the turbulent solution with an existing k-epsilon model code using wall function boundary conditions. Excellent agreement is seen between the computed solution and the Blasius solution, and between the two codes. The model has been tested for both subsonic and supersonic flat-plate turbulent boundary layer flow by comparing the computed skin friction with the Van Driest II theory and the experimental data of Weighardt; by comparing the transformed velocity profile with the data of Weighardt, and the Law of the Wall and the Law of the Wake; and by comparing the computed results of an adverse pressure gradient with the experimental data of Fernando and Smits. Good agreement is obtained with the experimental correlations for all flow conditions.

Knight, Doyle D.↗
NASA NTRS1990

Evidence against a simple two-component model for the far-infrared emission from galaxies

Two of the first Infrared Astronomy Satellite (IRAS) results were that galaxies have a wide range of values for the ratio of 60 micron to 100 micron flux density (0.2 less than or equal to S sub 60/S sub 100 less than or equal to 1.0) and that this ratio is correlated with L sub fir, L sub b, L sub fir being the total far-infrared luminosity and L sub b being the luminosity at visible wavelengths (de Jong et al. 1984; Soifer et al. 1984). From these results arose the following simple model for the far-infrared emission from galaxies (de Jong et al. 1984), which has remained the standard model ever since. In this model, the far-infrared emission comes from two dust components: warm dust (T approx. equals 50 K) intermingled with, and heated by, young massive OB stars in molecular clouds and HII regions, and colder dust (T approx. equals 20 K) associated with the diffuse atomic hydrogen in the interstellar medium and heated by the general interstellar radiation field. As the number of young stars in a galaxy increases, S sub 60/S sub 100 increases, because there is a greater proportion of warm dust, and so does L sub fir/L sub b, because most of the radiation from the young stars is absorbed by the dust, leading to a swifter increase in far-infrared emission than in visible light. Although this model explains the basic IRAS results, it is inelegant. It uses two free parameters to fit two data (the 60 and 100 micron flux densities)-and there are now several observations that contradict it. Despite these major problems with the two-component model, it is not clear what should be put in its place. When considering possible models for the far-infrared emission from galaxies, the observational evidence for our own galaxy must be considered. Researchers suspect that the study by Boulanger and Perault (1988) of the far-infrared properties of the local interstellar medium may be particularly relevant. They showed that molecular clouds are leaky - that most of the light from OB stars in molecular clouds does not heat the dust in the clouds, but instead leaks out. The consequence of this is that that while most of the far-infrared emission from the solar neighborhood is from dust associated with diffuse HI, this dust is mostly heated by young stars.

Eales, Stephen A.↗
NASA NTRS1991

Accretion and evolution of solar system bodies

We use a combination of analytical and numerical methods to study dynamical processes involved in the formation of planets and smaller bodies in the solar system. Our goal was to identify and understand critical processes and to link them in a numerical model of planetesimal accretion. We study effects of these processes by applying them in the context of the standard model of solar system formation, which involves accretion of the terrestrial planets and cores of the giant planet from small planetesimals. The principal focus of our research effort is the numerical simulation of accretion of a swarm of planetesimals into bodies of planetary size. Our computer code uses a Monte Carlo method to determine collisional interactions within the swarm. These interactions are not determined simply by a relative velocity, but rather by explicit distributions of keplerian orbital elements. The planetesimal swarm is divided into a number of zones in semimajor axis, which are allowed to interact. The present version of our code has the capability of following detailed distributions of size, eccentricity, and inclination in each zone.

Weidenschilling, S. J.↗
NASA NTRS1985

Response of Winter Forecasts Made with the GLA Fourth Order GCM to Changes in the Horizontal Grid Resolution

The GLA Fourth Order General Circulation Model was modified in 1984 to allow it to be run at a finer horizontal grid resolution. Previously the standard model versions integrated the primitive equations on a 4 degree latitude by 5 degree longitude grid. Forecasts made at this resolution are compared to others which were made using the finer 2 degree latitude by 2 1/2 degree longitude grid resolution. Three winter cases are included with initial conditions from 15 December 1978, 5 January 1979 and 21 January 1979. The 21 January 1979 initial conditions were taken from a GLAS analysis (experiment 2728) which incorporated the GLAS temperature retrievals. The other two cases used European Center Analysis for initial conditions. The forecasts were all verified against European Center Analysis.

Pfaendtner, J.↗
NASA NTRS1985

The determination of the helium abundance in main-sequence B stars

It is pointed out that the abundances of deuterium, helium, and lithium provide fundamental constraints on cosmological models. The central question is related to the compatibility of the observed abundances with big bang models of primordial nucleosynthesis, taking into account a modification by subsequent nuclear processing in stars or a modification of standard models. The present paper has the objective to assess critically the feasibility of deriving accurate helium abundances from measurements of the photospheric lines in main-sequence B-type stars. A method is established for assigning atmospheric parameters to main-sequence stars with spectral types in the range (approximately) B1-B5. It is found that an analysis of stars in distant anticenter H II regions and clusters offer an alternative method which seems capable of determining relative abundances with more than the requisite accuracy.

Wolff, S. C.↗
NASA NTRS1994

The luminosity function for stars with minus 0.6 less than or equal to M (sub nu) less than or equal to 6.5 in the globular cluster M30: Is there a problem with stellar evolution models?

The V band luminosity function (LF) for stars ranging from the tip of the giant branch to approximately 2.5 mag fainter than the main-sequence turnoff (MSTO) in M30 (=NGC 7099, GC 2137-234) is derived from CCD observations. Comparison with the theoretical LFs of Bergbusch & VandenBerg (1992) for (M/H) = -2, 16 Gyr models shows discrepancies which can be described either of two ways: (1) If the observations are normalized to the models over a few magnitudes fainter than the MSTO, then there is an excess (N (sub obs) greater than N(sub predicted)) of subgiant stars within 0.5 mag of the MSTO at the level of 4.8 sigma and an excess of stars (approximately 20%) compared to the models all along the giant branch. (2) If the normalization is made to match predicted and observed numbers of stars on the giant branch, then there is a discrepancy between models and observations in the sense that there is a broad depression in the observed LF fainter than the MSTO that is not present in the models. Consideration of LFs generated from 12 Gyr models or models adjusted to a shorter (m - M)(sub nu) = 14.5) distance modulus diminishes the magnitude of some of these discrepancies but introduces other mismatches between the observations and models. LFs constructed from stellar models that have 10% of the stellar mass in an isothermal core qualitatively match the observations better than the 'standard' models.

Bolte, Michael↗
NASA NTRS1999

Uncertainty Analysis of Inertial Model Attitude Sensor Calibration and Application with a Recommended New Calibration Method

Statistical tools, previously developed for nonlinear least-squares estimation of multivariate sensor calibration parameters and the associated calibration uncertainty analysis, have been applied to single- and multiple-axis inertial model attitude sensors used in wind tunnel testing to measure angle of attack and roll angle. The analysis provides confidence and prediction intervals of calibrated sensor measurement uncertainty as functions of applied input pitch and roll angles. A comparative performance study of various experimental designs for inertial sensor calibration is presented along with corroborating experimental data. The importance of replicated calibrations over extended time periods has been emphasized; replication provides independent estimates of calibration precision and bias uncertainties, statistical tests for calibration or modeling bias uncertainty, and statistical tests for sensor parameter drift over time. A set of recommendations for a new standardized model attitude sensor calibration method and usage procedures is included. The statistical information provided by these procedures is necessary for the uncertainty analysis of aerospace test results now required by users of industrial wind tunnel test facilities.

Tripp, John S.↗
NASA NTRS2019

Influence of Strain-Gage Balance Characteristics on the Precision Error of the Drag Coefficient

Improvements and results of a new method are presented that computes a pre-test estimate of the precision error of the drag coefficient of a wind tunnel model. The error estimate is defined as the part of the drag coefficient's precision error that is primarily associated with the precision error of the angle of attack measurement and physical characteristics of the chosen strain-gage balance. The method indirectly describes the precision error of the angle of attack measurement by using an assumed balance gage output variation of one microV/V. The physical characteristics of the balance, on the other hand, are described by partial derivatives of the axial and normal forces with respect to the strain-gage outputs. These derivatives can directly be obtained from the data reduction matrix of the balance. The precision error estimate itself is calculated by applying a simple explicit equation that uses the model reference area, the dynamic pressure, the angle of attack, the coefficients of the linear terms of the data reduction matrix, and the electrical output variation of one microvolt per volt as input. Precision errors at constant angle of attack may be visualized as contour plots by plotting them, for example, versus the Mach number and the total pressure. Characteristics of NASA's MC60E balance are used in combination with the reference area of a generic wind tunnel model in order to demonstrate that error estimates are independent of both the balance load format and the units chosen for the description of balance loads, model reference area, and the dynamic pressure. Finally, experimental data from a wind tunnel test of the Ames Check Standard Model in the NASA Ames 11-foot Transonic Wind Tunnel illustrates the application of the method to real-world test data.

precision error↗
NASA NTRS1994

Motion of the heliospheric termination shock. 2: Energy loss due to energetic particle acceleration

We present a generalization of earlier analysis of the motion of the heliospheric termination shock in response to heliospheric disturbances (Barnes, 1993) (paper 1), to allow jump conditions that include an energy sink at the shock front. The motivation for this study is that acceleration of the anomalous cosmic ray component may in fact represent such a sink. We have idealized the situation by assuming an infinitely thin shock parameterized by a quantity lambda(0 less than or equals lambda less than or equals 1), defined as the fraction of solar wind energy that is lost due to acceleration of the energetic particle component. If the sink is strong (consuming, say, 50% or more of the incident solar wind energy), the model leads to the following principal conclusions: (1) the shocked plasma would be much denser and cooler than in the standard gasdynamic case, thereby leading to more favorable conditions for direct observation of the shocked plasma; (2) the equilibrium shock position would be slightly farther (less than 10%) from the Sun than in the standard model; (3) as in the gasdynamic case, the shock would normally be in motion, so that the instantaneous position of the termination shock is not determined by interstellar conditions but by the recent history of the wind that has passed through the termination shock; and (4) the response of the shock to upstream disturbances would be similar to the response in the gasdynamic case, but the speed of the new termination shock would be somewhat smaller (probably by a f actor of 4 or less). We estimate that this speed is typically, approximately a few tens of kilometers per second corresponding to an inward or outward excursion of order of less than 1 to several AU, rather less than conventional estimates of several tens of AU.

Barnes, Aaron↗