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At least 469 records · Page 26

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NASA NTRS1994

Correlative analysis of hard and soft x ray observations of solar flares

We have developed a promising new technique for jointly analyzing BATSE hard X-ray observations of solar flares with simultaneous soft X-ray observations. The technique is based upon a model in which electric currents and associated electric fields are responsible for the respective heating and particle acceleration that occur in solar flares. A useful by-product of this technique is the strength and evolution of the coronal electric field. The latter permits one to derive important flare parameters such as the current density, the number of current filaments composing the loop, and ultimately the hard X-ray spectrum produced by the runaway electrons. We are continuing to explore the technique by applying it to additional flares for which we have joint BATSE/Yohkoh observations. A central assumption of our analysis is the constant of proportionality alpha relating the hard X-ray flux above 50 keV and the rate of electron acceleration. For a thick-target model of hard X-ray production, it can be shown that cv is in fact related to the spectral index and low-energy cutoff of precipitating electrons. The next step in our analysis is to place observational constraints on the latter parameters using the joint BATSE/Yohkoh data.

Zarro, Dominic M.↗

NASA NTRS1991

Near-Earth asteroids: Observer alert network and database analysis

The Planetary Science Institute (PSI) was funded by SERCulpr to develop a communication network to alert observers of newly discovered near-earth asteroids (NEA's). This network is intended to encourage observers to obtain physical observations of NEA's, which are needed in order to characterize and assess the resource potential of these bodies. This network was declared operational in October 1990 via an announcement to the asteroid observing community. The PSI is also supported to develop the Near-Earth Asteroid Database (NEAD), a comprehensive database of physical and dynamical data on NEA's. In the past year, the database was updated on newly discovered NEA's during 1990, and new data on radar observations and dynamical classifications were added.

Davis, Donald R.↗

DOE OSTI2022

Behavior and mechanisms of Doppler wind lidar error in varying stability regimes

Abstract. Wind lidars are widespread and important tools in atmospheric observations. An intrinsic part of lidar measurement error is due to atmospheric variability in the remote-sensing scan volume. This study describes and quantifies the distribution of measurement error due to turbulence in varying atmospheric stability. While the lidar error model is general, we demonstrate the approach using large ensembles of virtual WindCube V2 lidar performing a profiling Doppler-beam-swinging scan in quasi-stationary large-eddy simulations (LESs) of convective and stable boundary layers. Error trends vary with the stability regime, time averaging of results, and observation height. A systematic analysis of the observation error explains dominant mechanisms and supports the findings of the empirical results. Treating the error under a random variable framework allows for informed predictions about the effect of different configurations or conditions on lidar performance. Convective conditions are most prone to large errors (up to 1.5 m s−1 in 1 Hz wind speed in strong convection), driven by the large vertical velocity variances in convective conditions and the high elevation angle of the scanning beams (62∘). Range-gate weighting induces a negative bias into the horizontal wind speeds near the surface shear layer (−0.2 m s−1 in the stable test case). Errors in the horizontal wind speed and direction computed from the wind components are sensitive to the background wind speed but have negligible dependence on the relative orientation of the instrument. Especially during low winds and in the presence of large errors in the horizontal velocity estimates, the reported wind speed is subject to a systematic positive bias (up to 0.4 m s−1 in 1 Hz measurements in strong convection). Vector time-averaged measurements can improve the behavior of the error distributions (reducing the 10 min wind speed error standard deviation to <0.3 m s−1 and the bias to <0.1 m s−1 in strong convection) with a predictable effectiveness related to the number of decorrelated samples in the time window. Hybrid schemes weighting the 10 min scalar- and vector-averaged lidar measurements are shown to be effective at reducing the wind speed biases compared to cup measurements in most of the simulated conditions, with time averages longer than 10 min recommended for best use in some unstable conditions. The approach in decomposing the error mechanisms with the help of the LES flow field could be extended to more complex measurement scenarios and scans.

17 WIND ENERGY↗

NASA NTRS1992

The presence of field geologists in Mars-like terrain

Methods of ethnographic observation and analysis have been coupled with object-oriented analysis and design concepts to begin the development of a clear path from observations in the field to the design of virtual presence systems. The existence of redundancies in field geology and presence allowed for the application of methods for understanding complex systems. As a result of this study, some of these redundancies have been characterized. Those described are all classes of continuity relations, including the continuities of continuous existence, context-constituent continuities, and state-process continuities. The discussion of each includes statements of general relationships, logical consequences of these, and hypothetical situations in which the relationships would apply. These are meant to aid in the development of a theory of presence. The discussion also includes design considerations, providing guidance for the design of virtual planetary exploration systems and other virtual presence systems. Converging evidence regarding continuity in presence is found in the nature of psychological dissociation. Specific methodological refinements should enhance ecological validity in subsequent field studies, which are in progress.

Mcgreevy, Michael W.↗

NASA NTRS1995

A long-term space astrophysics research program: An x-ray perspective of the components and structure of galaxies

X-ray studies of galaxies by the Smithsonian Astrophysical Observatory (SAO) and MIT are described. Activities at SAO include ROSAT PSPC x-ray data reduction and analysis pipeline; x-ray sources in nearby Sc galaxies; optical, x-ray, and radio study of ongoing galactic merger; a radio, far infrared, optical, and x-ray study of the Sc galaxy NGC247; and a multiparametric analysis of the Einstein sample of early-type galaxies. Activities at MIT included continued analysis of observations with ROSAT and ASCA, and continued development of new approaches to spectral analysis with ASCA and AXAF. Also, a new method for characterizing structure in galactic clusters was developed and applied to ROSAT images of a large sample of clusters. An appendix contains preprints generated by the research.

Fabbiano, G.↗

NASA NTRS2020

Study of Antarctic Blowing Snow Storms Using MODIS and CALIOP Observations With a Machine Learning Model

As a common phenomenon over Antarctica, blowing snow (BLSN), especially the large BLSN storms, play an important role in the Antarctic surface mass balance, radiation budget, and planetary boundary layer processes. This study presents the work on BLSN storm identification and analysis with observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite. Spectral analysis shows that BLSN identification is feasible with MODIS daytime data. A random forest machine learning model is developed and observations from the Cloud‐Aerosol Lidar with Orthogonal Polarization are used for training. Model performance results show that machine‐learning based classification can achieve over 90% overall accuracy when classifying MODIS pixels into cloud, clear, and BLSN categories. The machine learning model is applied to MODIS observations during the month of October 2009 for BLSN storm analysis. Results show that the size of BLSN storms has a large spectrum and can reach hundreds of thousands km2. The MODIS based BLSN storm frequency map extends the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations coverage limit from 82°S to the South Pole. A BLSN storm belt, which extends from the South Pole region to the coastal area between 130°E and 160°E along the Transantarctic Mountains, provides a potential pathway of snow transport. These results are important in improving the understanding of BLSN impact on Antarctic surface mass balance and boundary layer processes.

Antarctic↗

NASA NTRS1978

Analysis of extreme-ultraviolet observations of a polar coronal hole

Emission gradient curves for extreme-ultraviolet resonance lines of lithiumlike ions have been constructed from spectroheliograms of a northern polar coronal hole observed on August 14, 1973, with the Harvard experiment on Skylab. An emission-measure analysis indicates both reduced density and coronal temperature in the coronal hole. The boundary geometry of the coronal hole is determined, and a temperature-density model that is consistent with the observed intensities is constructed. The model gives a conductive flux of 60,000 erg/sq cm per sec at 1.03 solar radii. The boundary geometry and density distribution are combined with typical solar-wind parameters at the earth to determine an outflow velocity of 15 km/s at 1.08 solar radii.

Mariska, J. T.↗

NASA NTRS1980

Ultraviolet observations of the Cygnus Loop

The analysis of IUE observations of a filament on the western edge of the Cygnus Loop yields a shock velocity of about 130 km/sec, significantly higher than the velocity estimated from the optical spectrum. This velocity together with the high forbidden O III/H-beta ratio reported by Miller (1974) indicates that much of the hydrogen recombination region predicted by steady flow shock models is absent. This situation may occur because this part of the supernova remnant is in the shell-formation phase that accompanies the transition from the adiabatic to the snowplow phase or because the observed filament is a recently shocked cloud.

Raymond, J. C.↗

NASA NTRS1985

Severe Storms Branch research report (April 1984 April 1985)

The Mesoscale Atmospheric Processes Research Program is a program of integrated studies which are to achieve an improved understanding of the basic behavior of the atmosphere through the use of remotely sensed data and space technology. The program consist of four elements: (1) special observations and analysis of mesoscale systems; (20 the development of quanitative algorithms to use remotely sensed observations; (3) the development of new observing systems; and (4) numerical modeling. The Severe Storms Branch objectives are the improvement of the understanding, diagnosis, and prediction of a wide range of atmospheric storms, which includes severe thunderstorms, tornadoes, flash floods, tropical cyclones, and winter snowstorms. The research often shed light upon various aspects of local weather, such as fog, sea breezes, air pollution, showers, and other products of nonsevere cumulus cloud clusters. The part of the program devoted to boundary layer processes, gust front interactions, and soil moisture detection from satellites gives insights into storm growth and behavior.

Dubach, L.↗

NASA NTRS1991

Report of the panel on earth rotation and reference frames, section 7

Objectives and requirements for Earth rotation and reference frame studies in the 1990s are discussed. The objectives are to observe and understand interactions of air and water with the rotational dynamics of the Earth, the effects of the Earth's crust and mantle on the dynamics and excitation of Earth rotation variations over time scales of hours to centuries, and the effects of the Earth's core on the rotational dynamics and the excitation of Earth rotation variations over time scales of a year or longer. Another objective is to establish, refine and maintain terrestrial and celestrial reference frames. Requirements include improvements in observations and analysis, improvements in celestial and terrestrial reference frames and reference frame connections, and improved observations of crustal motion and mass redistribution on the Earth.

Dickey, Jean O.↗

NASA NTRS1993

Electron distributions in the inner Jovian magnetosphere: Voyager 1 observations

Using several improvements in the analysis of the observations of the Low Energy Charged Particle (LECP) experiment on Voyager 1, electron phase space densities in the inner Jovian magnetosphere (5 - 10 R(sub J) were first calculated at constant first and second invariants (represented by mu and K, respectively), based on the LECP measurements. The calculated electron phase space density profiles show that in the inner Jovian magnetosphere there exist evident time and longitude variations, energetic electron injections, and present radial transport and distributed losses. To study the radial and pitch angle diffusions of Jovian electrons, we have calculated the phase space densities in the K-L space. It is found that the electron population in the inner Jovian magnetosphere seems to consist of two components: electrons radially diffusing from a main external source and electrons generated from local sources. The radially diffusing electrons have a relatively time stationary and isotropic distribution, while the locally created electrons mainly concentrate around the equatorial plane and have relatively lower energies, in comparison with the inward diffusing electrons. Consequently, the sources of precipitation losses to the ionosphere must be primarily electrons transported from outer sources, and the major precipitations occur in the inner magnetosphere (L less than 7.5 R(sub J). In the inner Jovian magnetosphere (L = 5 to approximately 10 R(sub J)) it is estimated that for electrons with magnetic moment mu = 300 MeV/G, the diffusion coefficient D is roughly 10(exp -8) to approximately 10(exp -6) R(exp 2)(sub J)/s, and the lifetime against the diffusion losses is of the order of 10(exp 4) to approximately 10(exp 6) s.

Ye, G.↗

NASA NTRS1999

Further Evaluation of an Emperical Equation for Annual Total Evaporation

An empirical equation for annual total evaporation based on annual precipitation and net radiation was found to provide evaporation within 10% of the observed values at seven locations within temperate and tropical regions, but it overestimated evaporation by 90% at one location within the tundra region. A synthesis of observations at two other locations within the tundra region gives overestimates of about 65%. A general analysis of observed precipitation, net radiation, and runoff within the tundra region shows that the empirical equation is generally biased to overestimate annual evaporation within the tundra region. A theoretical analysis is being done to understand the reason behind this bias.

Choudhury, Bhaskar J.↗

NASA NTRS1985

Evidence for collisionless conduction fronts in impulsive solar flares

The present paper is concerned with an analysis of new observations of impulsive hard X-ray and microwave bursts. The results are compared with predictions of a thermal flare model. A description is presented of the thermal model with conduction-front confinement, taking into account the development and previous applications of the model, the high-energy limit of confinement and its implications for microwave emission, the formation of a test based on the revised predictions of the model, and derived parameters. The observations and the test of the model are discussed, giving attention to instrumentation, event selection, selection of a homogeneous sample of impulsive rises,. observed and derived parameters for each rise, a correlation analysis of observed and predicted rise times, uncertainties in the measurements, consistency checks, and a comparison with results of Crannell et al. (1978).

Batchelor, D. A.↗

NASA NTRS1994

The derived population of luminous supersoft X-ray sources

The existence of a new class of astrophysical object, luminous supersoft X-ray sources, has been established through ROSAT satellite observations and analysis during the past approximately 3 yr. Because most of the radiation emitted by supersoft sources spans a range of wavelengths readily absorbed by interstellar gas, a substantial fraction of these sources may not be detectable with present satellite instrumentation. It is therefore important to derive a reliable estimate of the underlying population, based on the approximately 30 sources that have been observed to date. The work reported here combines the observational results with a theoretical analysis, to obtain an estimate of the total number of sources likely to be present in M31, the Magellanic Clouds, and in our own Galaxy. We find that in M31, where approximately 15 supersoft sources have been identified and roughly an equal number of sources are being investigated as supersoft candidates, there are likely to be approximately 2500 active supersoft sources at the present time. In our own Galaxy, where about four supersoft sources have been detected in the Galactic plane, there are likely to be approximately 1000 active sources. Similarly, with about six and about four (nonforeground) sources observed in the Large (LMC) and Small Magellanic Clouds (SMC), respectively, there should be approximately 30 supersoft sources in the LMC, and approximately 20 in the SMC. The likely uncertainties in the numbers quoted above, and the properties of observable sources relative to those of the total underlying population, are also derived in detail. These results can be scaled to estimate the numbers of supersoft sources likely to be present in other galaxies. The results reported here on the underlying population of supersoft X-ray sources are in good agreement with the results of a prior population synthesis study of the white dwarf accretor model for luminous supersoft X-ray sources. It should be emphasized, however, that the questions asked in these two investigations are distinct, that the approaches taken to answer these questions are largely independent and that the findings of these two studies could in principle have been quite different.

Di Stefano, R.STEFANO↗

NASA NTRS1986

Cosmic rays near the heliospheric current sheet. II - An ensemble approach to comparing theory and observation

A quantitative comparison is carried out between theoretical predictions and observations of the intensity of galactic cosmic rays near the interplanetary current sheet. Model calculations are compared with a statistical analysis of observations of galactic cosmic rays at the earth and the simultaneous position of the current sheet. Since the observations were made over a period of several years, the inclination of the current sheet varied considerably, and comparison with any one model calculation referring to a given inclination would not be appropriate. An ensemble of different current sheet inclinations are used to generate expected values with the model in order to make the analysis of the computations approximate the method used to analyze the data. Agreement is found between theory and observation at energies of the order of a few GeV.

Jokipii, J. R.↗

DOE OSTI2026

Creating high-precision reference gas standards of 85Kr for groundwater age-dating

Absolute gas counting (AGC) was applied to two gas blends of 85Kr in argon-methane (P10) counting gas to establish a high-precision specific activity (Bq/cm3) reference value for characterizing 85Kr detection efficiency for groundwater age dating measurements. The AGC or length-compensated technique has been utilized by the metrology community for decades and is an accepted method for developing radioactive gas standards. The AGC capability at Pacific Northwest National Laboratory (PNNL) uses a set of nine unequal-length proportional counters with precisely-measured internal volumes, and a gas loading system with high-precision pressure and temperature sensors. A series of AGC measurements were collected at multiple pressures to determine the inverse pressure relationship (1/P) for 85Kr and define a wall-effect correction that accounts for events decaying into the detector wall and not depositing sufficient energy in the gas to be detected. In addition to the wall-effect, two additional corrections were evaluated and are discussed in detail. Specifically, the threshold effect which accounts for events deposited below the analysis threshold and a detection efficiency as a function of detector volume effect that was observed during analysis. A robust uncertainty model was developed using the Guide to the expression of Uncertainty in Measurements (GUM) approach. The combination of carefully scrutinized correction factors, precise measurements of pressure, temperature and detector volume, and robust counting statistics resulted in the determination of high-precision specific activity values with 0.50% or less total combined uncertainty for two Kr-in-P10 reference gas standards (KP10) that will enable new groundwater age-dating measurements at PNNL.

Kr-85↗

NASA NTRS1975

EUV emission, filament activation and magnetic fields in a slow-rise flare

Results are reported for observations and analysis of synoptic data on a 1B flare that occurred on January 19, 1972. The observations include large-scale H-alpha movies of the flare and pre-flare developments, OSO-7 satellite data on soft X-ray and EUV developments, magnetograms, and hard X-ray observations. Theoretical force-free magnetic field configurations are compared with structures seen in the soft X-ray, EUV, and H-alpha images, and the evolution of the flare is described. The energy available for the flare is estimated from the change of magnetic field inferred from the H-alpha filtergrams and from force-free field calculations. It is suggested that the flare originated in a twisted filament where it was compressed by emerging fields, and it is shown that the flare started below the corona and appeared to derive its energy from the magnetic fields in or near the filament.

Rust, D. M.↗

NASA NTRS1991

Studies of extended planetary atmospheres

The following topics are covered: (1) telescopic observations and analysis of planetary atmospheres (including the Moon and Mars) and the Io torus; (2) occultation observations; and (3) supporting laboratory studies.

Hunten, D. M.↗