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NASA NTRS1994

Far-ultraviolet photometry of the globular cluster omega Cen

We present far-ultraviolet images of the globular cluster omega Centauri obtained with the Ultraviolet Imaging Telescope (UIT) during the 1990 December Astro-1 mission. A total of 1957 sources are detected at 1620 A to a limiting ultraviolet (UV) magnitude of 16.4 in the central 24 min diameter region of the field and a limit of 15.6 over the remainder of the 40 min diameter field. Over 1400 of these sources are matched with stars on a Stroemgren u band charge coupled devices (CCD) frame obtained with the Cerro Tololo Inter-American Observatory (CTIO) 0.9 m telescope to produce a (far-UV, u) color-magnitude diagram (CMD). Completeness of the sample and error estimates are determined by photometry of artificial stars added to the images. The horizontal branch (HB) of the CMD is heavily populated hotter than 9000 K. A large number of 'extreme HB' stars are found hotter than a conspicuous break in the HB at T(sub e) approximately 16000 K. There is also a significant population of stars above the HB, the brightest of which is 4 mag brighter than the HB. Most of the hotter of these appear to be 'AGB-manque' or 'Post-Early Asymptotic Giant Branch' stars. We compare the observations to recent theoretical evolutionary tracks for the zero-age HB and subsequent phases. The tracks match the data well, with the exception of the hotter HB stars, many of which fall below the zero-age horizontal branch. It is unclear as yet whether these are a special population or an artifact of errors in the models or photometry. We identify 33 stars with T(sub e) greater than or approximately = 50000 K, which are hotter than zero-age HB stars with envelope masses of 0.003 solar mass.

Whitney, Jonathan H.↗

NASA NTRS2016

Achieving Consistent Doppler Measurements from SDO/HMI Vector Field Inversions

NASA's Solar Dynamics Observatory is delivering vector magnetic field observations of the full solar disk with unprecedented temporal and spatial resolution; however, the satellite is in a highly inclined geosynchronous orbit. The relative spacecraft-Sun velocity varies by +/-3 kms−1 over a day, which introduces major orbital artifacts in the Helioseismic Magnetic Imager (HMI) data. We demonstrate that the orbital artifacts contaminate all spatial and temporal scales in the data. We describe a newly developed three-stage procedure for mitigating these artifacts in the Doppler data obtained from the Milne-Eddington inversions in the HMI pipeline. The procedure ultimately uses 32 velocity-dependent coefficients to adjust 10 million pixels-a remarkably sparse correction model given the complexity of the orbital artifacts. This procedure was applied to full-disk images of AR 11084 to produce consistent Dopplergrams. The data adjustments reduce the power in the orbital artifacts by 31 dB. Furthermore, we analyze in detail the corrected images and show that our procedure greatly improves the temporal and spectral properties of the data without adding any new artifacts. We conclude that this new procedure makes a dramatic improvement in the consistency of the HMI data and in its usefulness for precision scientific studies.

Schuck, Peter W.↗

NASA NTRS1999

The Photospheric Convection Spectrum

Spectra of the photospheric cellular flows are determined from Solar and Heliospheric Observatory/Michelson Doppler Imager (SOHO/MDI) observations. Spherical harmonic decompositions are obtained from the full-disc observations. Fourier decompositions are obtained from the high-resolution observations. The p-mode oscillation signals and instrumental artifacts are reduced by temporal filtering of the Doppler data. The resulting spectra give power (kinetic energy) per wavenumber for effective spherical harmonic degrees from 1 to about 3000. The spectra show distinct peaks representing granules and supergranules but no distinct features at wavenumbers representative of mesogranules or giant cells.

Hathaway, David H.↗

NASA NTRS1997

Absorption of Solar Radiation by the Cloudy Atmosphere Interpretations of Collocated Aircraft Measurements

As part of the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE), we have obtained and analyzed measurements made from collocated aircraft of the absorption of solar radiation within the atmospheric column between the two aircraft. The measurements were taken during October 1995 at the ARM site in Oklahoma. Relative to a theoretical radiative transfer model, we find no evidence for excess solar absorption in the clear atmosphere and significant evidence for its existence in the cloudy atmosphere. This excess cloud solar absorption appears to occur in both visible (0.224-0.68 microns) and near-infrared (0.68-3.30 microns) spectral regions, although not at 0.5 microns for the visible contribution, and it is shown to be true absorption rather than an artifact of sampling errors caused by measuring three-dimensional clouds.

Valero, Francisco P. J.↗

NASA NTRS1980

The magnetic field of Saturn - Further studies of the Pioneer 11 observations

Analysis of magnetic field observations by the Goddard Space Flight Center high-field flux gate magnetometer on the Pioneer 11 spacecraft during Saturn encounter yields estimates of the planetary field. The field is mainly dipolar but rather weaker than expected, with a moment equal to 0.20 G cubic Saturn radii or 4.3 x 10 to the 28th G cu cm, opposite in polarity to earth's. Surprisingly, the field appears to be axially symmetric but with a small (0.04 Saturn radii) offset to the north so that N (S) polar field intensities are 0.6 (0.4) G, respectively. The deduced polar offset appears not to be an artifact of the limited spatial extent of the observations or the presence of fields of external origin. The average stand-off distance of the magnetopause is expected to be 20 Saturn radii, i.e., at the orbit of Titan, so that this largest of solar system satellites is immersed not only in the Saturnian magnetosphere but also at times in its magnetosheath and sometimes even in the interplanetary medium.

Acuna, M. H.↗

NASA NTRS1986

Geologic periodicities and the Galaxy

New geologic and astronomical developments are reviewed that have recently led to the proposal of various Galactic theories to explain the temporal pattern of impact craters on earth. Linear and harmonic time series analyses have revealed that two dominant periodicities about equal to 33 + or - 3 Myr and 260 + or - 25 Myr underlie the geologic record of terrestrial impact cratering and global tectonic phenomena. It is argued that purely terrestrial mechanisms cannot account for the cratering cycles as being preservational artifacts; cratering rather appears to influence tectonism, possibly through perturbations of mantle convection. Galactic models proposed to explain the two long-term periodicities are reviewed critically. The most likely explanation for the 33 Myr cycle involves the comparatively stable half period of vertical oscillation of the solar system about the Galactic plane. The 260 Myr cycle may be related to rare encounters with Galactic spiral arms during the revolution of the solar system around the Galactic center.

Rampino, Michael R.↗

NASA NTRS1995

Fine-resolution imaging of solar features using Phase-Diverse Speckle

Phase-diverse speckle (PDS) is a novel imaging technique intended to overcome the degrading effects of atmospheric turbulence on fine-resolution imaging. As its name suggests, PDS is a blend of phase-diversity and speckle-imaging concepts. PDS reconstructions on solar data were validated by simulation, by demonstrating internal consistency of PDS estimates, and by comparing PDS reconstructions with those produced from well accepted speckle-imaging processing. Several sources of error in data collected with the Swedish Vacuum Solar Telescope (SVST) were simulated: CCD noise, quantization error, image misalignment, and defocus error, as well as atmospheric turbulence model error. The simulations demonstrate that fine-resolution information can be reliably recovered out to at least 70% of the diffraction limit without significant introduction of image artifacts. Additional confidence in the SVST restoration is obtained by comparing its spatial power spectrum with previously-published power spectra derived from both space-based images and earth-based images corrected with traditional speckle-imaging techniques; the shape of the spectrum is found to match well the previous measurements. In addition, the imagery is found to be consistent with, but slightly sharper than, imagery reconstructed with accepted speckle-imaging techniques.

Paxman, Richard G.↗

NASA NTRS2011

Properties of Supergranulation During the Solar Minima of Cycles 22/23 and 23/24

The solar minimum at the transition from cycle 23 to 24 was notable for its low level of activity and its extended duration. Among the various fields of study, the evolution of the solar convection zone may provide insight into the causes and consequences of this recent minimum. This study continues previous investigations of the characteristics of solar supergranulation, a convection component strongly linked to the structure of the magnetic field, namely the time-evolution of the global mean of supergranule cell size, determined from spectral analysis of MDI Dopplergrams from the two previous solar minima. Analyses of the global mean of supergranule sizes show a quasi-oscillatory nature to the evolution of this particular supergranule characteristic. Performing similar analyses on realistic, synthetic Doppler images show similar time-dependent characteristics. We conclude that the observed fluctuations are not observational artifacts, and that an underlying trend exists within the evolution of the supergranulation network.

Williams, Peter E.↗

NASA NTRS2011

A Novel Nonintrusive Method to Resolve the Thermal Dome Effect of Pyranometers: Radiometric Calibration and Implications

Traditionally the calibration equation for pyranometers assumes that the measured solar irradiance is solely proportional to the thermopile s output voltage; therefore, only a single calibration factor is derived. This causes additional measurement uncertainties because it does not capture sufficient information to correctly account for a pyranometer s thermal effect. In our updated calibration equation, temperatures from the pyranometer's dome and case are incorporated to describe the instrument's thermal behavior, and a new set of calibration constants are determined, thereby reducing measurement uncertainties. In this paper, we demonstrate why a pyranometer's uncertainty using the traditional calibration equation is always larger than a few percent, but with the new approach can become much less than 1% after the thermal issue is resolved. The highlighted calibration results are based on NIST traceable light sources under controlled laboratory conditions. The significance of the new approach lends itself to not only avoiding the uncertainty caused by a pyranometer's thermal effect but also the opportunity to better isolate and characterize other instrumental artifacts, such as angular response and nonlinearity of the thermopile, to further reduce additional uncertainties. We also discuss some of the implications, including an example of how the thermal issue can potentially impact climate studies by evaluating aerosol s direct radiative effect using field measurements with and without considering the pyranometer s thermal effect. The results of radiative transfer model simulation show that a pyranometer s thermal effect on solar irradiance measurements at the surface can be translated into a significant alteration of the calculated distribution of solar energy inside the column atmosphere.

Ji. Q.↗

NASA NTRS2011

A Novel Non-Intrusive Method to Resolve the Thermal-Dome-Effect of Pyranometers: Radiometric Calibration and Implications

Traditionally the calibration equation for pyranometers assumes that the measured solar irradiance is solely proportional to the thermopile's output voltage; therefore only a single calibration factor is derived. This causes additional measurement uncertainties because it does not capture sufficient information to correctly account for a pyranometer's thermal effect. In our updated calibration equation, temperatures from the pyranometer's dome and case are incorporated to describe the instrument's thermal behavior, and a new set of calibration constants are determined, thereby reducing measurement uncertainties. In this paper, we demonstrate why a pyranometer's uncertainty using the traditional calibration equation is always larger than a-few-percent, but with the new approach can become much less than 1% after the thermal issue is resolved. The highlighted calibration results are based on NIST-traceable light sources under controlled laboratory conditions. The significance of the new approach lends itself to not only avoiding the uncertainty caused by a pyranometer's thermal effect but also the opportunity to better isolate and characterize other instrumental artifacts, such as angular response and non-linearity of the thermopile, to further reduce additional uncertainties. We also discuss some of the implications, including an example of how the thermal issue can potentially impact climate studies by evaluating aerosol's direct-radiative effect using field measurements with and without considering the pyranometer's thermal effect. The results of radiative transfer model simulation show that a pyranometer's thermal effect on solar irradiance measurements at the surface can be translated into a significant alteration of the calculated distribution of solar energy inside the column atmosphere.

Ji, Qiang↗

NASA NTRS1985

Numerical Descriptions of Cosmic-Ray Transport

The behavior of energetic particles in the solar system is described by a well known Fokker-Planck equation. Although analytic methods yield insight into the nature of its solutions, especially in the diffusion regime, calculations that go beyond diffusion are very complicated. The reliability of these calculations is of concern, because numerical methods are notorious for their errors and artifacts. The well known Milne problem of classical transport theory was analyzed with the aid of three different numerical methods. These are: (1) The method of eigenfunctions in which the distribution function is approximated by a sum of eigenfunctions of the scattering operator, (2) Numerical solutions of a finite difference aquation; and (3) Direct simulation of the scattering and streaming of individual particles with the aid of Monte Carlo methods.

Earl, J. A.↗

NASA NTRS1988

Large molecular-cloud complexes

The properties of the large molecular cloud complexes in the solar vicinity and in the inner Galaxy are summarized. Particular attention is paid to the stability of the clouds against the tidal field of the Galaxy, associated clouds of warm atomic hydrogen gas and star formation in the clouds. The linewidth-size relation is discussed and the possibility that it is an artifact of the blending of unrelated clouds at different distances is raised.

Blitz, L.↗

NASA NTRS2007

Sampling Biases in MODIS and SeaWiFS Ocean Chlorophyll Data

Although modem ocean color sensors, such as MODIS and SeaWiFS are often considered global missions, in reality it takes many days, even months, to sample the ocean surface enough to provide complete global coverage. The irregular temporal sampling of ocean color sensors can produce biases in monthly and annual mean chlorophyll estimates. We quantified the biases due to sampling using data assimilation to create a "truth field", which we then sub-sampled using the observational patterns of MODIS and SeaWiFS. Monthly and annual mean chlorophyll estimates from these sub-sampled, incomplete daily fields were constructed and compared to monthly and annual means from the complete daily fields of the assimilation model, at a spatial resolution of 1.25deg longitude by 0.67deg latitude. The results showed that global annual mean biases were positive, reaching nearly 8% (MODIS) and >5% (SeaWiFS). For perspective the maximum interannual variability in the SeaWiFS chlorophyll record was about 3%. Annual mean sampling biases were low (<3%) in the midlatitudes (between -40deg and 40deg). Low interannual variability in the global annual mean sampling biases suggested that global scale trend analyses were valid. High latitude biases were much higher than the global annual means, up to 20% as a basin annual mean, and over 80% in some months. This was the result of the high solar zenith angle exclusion in the processing algorithms. Only data where the solar angle is <75deg are permitted, in contrast to the assimilation which samples regularly over the entire area and month. High solar zenith angles do not facilitate phytoplankton photosynthesis and consequently low chlorophyll concentrations occurring here are missed by the data sets. Ocean color sensors selectively sample in locations and times of favorable phytoplankton growth, producing overestimates of chlorophyll. The biases derived from lack of sampling in the high latitudes varied monthly, leading to artifacts in the apparent seasonal cycle from ocean color sensors. A false secondary peak in chlorophyll occurred in May-August, which resulted from lack of sampling in the Antarctic.

Gregg, Watson W.↗

NASA NTRS1990

A search for small solar-system bodies near the earth using a ground-based telescope - Technique and observations

A large, previously undetected flux of small bodies near earth is identified by employing the standard technique of detection of an individual object in two consecutive frames. The observational periods and viewing coordinates for the search for small bodies are presented. A null test is performed in order to further demonstrate that the signatures in the images are not due to instrumental artifacts. The observed fluxes, orbital motions, and radii of the small bodies detected are in agreement with those for the small cometlike objects previously reported. It is pointed out that the radii of the small bodies would be in the range of meters. Since an alternative interpretation of the small bodies is possible, it is suggested that the use of a telescope with larger aperture and/or array detectors with lesser noise levels is necessary to confirm the present observations.

Frank, L. A.↗

NASA NTRS2007

Curious Behavior of Sunspot Umbrae in the First Half of the 20th Century

We examined the behavior of the areas of sunspot umbrae and penumbrae as reported daily by the Royal Observatory, Greenwich (RGO) from May 1874 to December 1976. We calculated the ratio of the umbral area to the penumbral area (corrected for foreshortening as observed on the solar disc) for each sunspot group and for each day. We found: 1) that this ratio is about 0.2 on average, 2) that larger sunspot groups have slightly smaller ratios, 3) that there is a weak dependence on the phase of the solar cycle, 4) that there is no dependence on the latitude of the sunspot groups, and curiously 5) that for the smaller sunspot groups this ratio increased dramatically from about 1910 to 1930 and then returned to "normal" from 1930 to 1950. We examined other sunspot records to determine whether this behavior was an artifact of the RGO data and find evidence to indicate that the behavior was real. For the smaller sunspots (constituting the vast majority in both number and total area), the proportional size of the sunspot umbrae slowly increased by more than 50% and then returned to "normal" over this 40-year period.

Hathaway, D. H.↗

NASA NTRS2006

Thiophenes as Indicators of Aqueous Alteration in Carbonaceous Meteorites

A common class of organic compound in low petrographic type meteorites is the sulfur-containing thiophenes. The presence of this compound class in organic-rich meteorites which have experienced substantial levels of aqueous alteration is relatively unexplored. Early reports of these compounds attributed them to artifacts brought about by reactions between elemental sulfur and organic matter during high temperature extraction and analysis steps. Subsequent investigations confirmed their indigeneity, yet their environment of formation remained unconstrained. Here we present data which suggests that thiophenes are parent body alteration products that reflect the role of liquid water on asteroids in the early solar system.

Sephton, Mark A.↗

NASA NTRS1993

Sparking young minds with Moon rocks and meteorites

What could be more exciting than seeing pieces of other worlds? The Apollo program left a legacy of astounding accomplishments and precious samples. Part of the thrill of those lunar missions is brought to schools by the lunar sample educational disks, which contain artifacts of six piloted trips to the Moon. Johnson Space Center (JSC) is preparing 100 new educational disks containing pieces of meteorites collected in Antarctica. These represent chunks of several different asteroids, that were collected in one of the most remote, forbidding environments on Earth. These pieces of the Moon and asteroids represent the products of basic planetary processes (solar nebular processes, initial differentiation, volcanism, and impact), and, in turn, these processes are controlled by basic physical and chemical processes (energy, energy transfer, melting, buoyancy, etc.). Thus, the lunar and meteorite sample disks have enormous educational potential. New educational materials are being developed to accompany the disks. Present materials are not as effective as they could be, especially in relating samples to processes and to other types of data such as spectral studies and photogeology. Furthermore, the materials are out of date. New background materials will be produced for teachers, assembling slide sets with extensive captions, and devising numerous hands-on classroom activities to do while the disks are at a school and before and after they arrive. The classroom activities will be developed by teams of experienced teachers working with lunar and meteorite experts.

Taylor, G. Jeffrey↗

NASA NTRS2017

GOES-R L1b Readiness Implementation and Management Plan

A complement of Readiness, Implementation and Management Plans (RIMPs) to facilitate management of post-launch product test activities for the official Geostationary Operational Environmental Satellite (GOES-R) Level 1b (L1b) products have been developed and documented. Separate plans have been created for each of the GOES-R sensors including: the Advanced Baseline Imager (ABI), the Extreme ultraviolet and X-ray Irradiance Sensors (EXIS), Geostationary Lightning Mapper (GLM), GOES-R Magnetometer (MAG), the Space Environment In-Situ Suite (SEISS), and the Solar Ultraviolet Imager (SUVI). The GOES-R program has implemented these RIMPs in order to address the full scope of CalVal activities required for a successful demonstration of GOES-R L1b data product quality throughout the three validation stages: Beta, Provisional and Full Validation. For each product maturity level, the RIMPs include specific performance criteria and required artifacts that provide evidence a given validation stage has been reached, the timing when each stage will be complete, a description of every applicable Post-Launch Product Test (PLPT), roles and responsibilities of personnel, upstream dependencies, and analysis methods and tools to be employed during validation. Instrument level Post-Launch Tests (PLTs) are also referenced and apply primarily to functional check-out of the instruments.

Validation Readiness↗