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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.

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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

The sidereal semi-diurnal variation observed at high zenith angles at Mawson, 1968-1984, and the polarity of the solar main field

High zenith-angle North/South telescopes viewing equatorially and at midlatitudes through 40 MWE of atmosphere have been operating at Mawson since early 1968. It is evident that a sidereal semi-diurnal component of galactic origin has been observed, over and above a possible spurious component proposed by Nagashima, arising from a bi-directional component of the solar anisotropy. Although a very pronounced reduction in the semi-diurnal galactic response followed the reversal of polarity of the solar main field during 1969 to 1971, so far the observations indicate that there has been no recurrence of a larger galactic response following the reversal of polarity around 1981. The possible role of the latitudional extent lambda omicron of the wavy neutral sheet is discussed.

Jacklyn, R. M.↗
NASA NTRS2014

Weekly Gridded Aquarius L-band Radiometer-Scatterometer Observations and Salinity Retrievals over the Polar Regions - Part 2: Initial Product Analysis

Following the development and availability of Aquarius weekly polar-gridded products, this study presents the spatial and temporal radiometer and scatterometer observations at L band (frequency1.4 GHz) over the cryosphere including the Greenland and Antarctic ice sheets, sea ice in both hemispheres, and over sub-Arctic land for monitoring the soil freeze-thaw state. We provide multiple examples of scientific applications for the L-band data over the cryosphere. For example, we show that over the Greenland Ice Sheet, the unusual 2012 melt event lead to an L-band brightness temperature (TB) sustained decrease of 5 K at horizontal polarization. Over the Antarctic ice sheet, normalized radar cross section (NRCS) observations recorded during ascending and descending orbits are significantly different, highlighting the anisotropy of the ice cover. Over sub-Arctic land, both passive and active observations show distinct values depending on the soil physical state (freeze-thaw). Aquarius sea surface salinity (SSS) retrievals in the polar waters are also presented. SSS variations could serve as an indicator of fresh water input to the ocean from the cryosphere, however the presence of sea ice often contaminates the SSS retrievals, hindering the analysis. The weekly grided Aquarius L-band products used a redistributed by the US Snow and Ice Data Center at http:nsidc.orgdataaquariusindex.html, and show potential for cryospheric studies.

Aquarius↗
NASA NTRS1985

Isotropic intensity waves and features of their occurrence

Waves of daily average cosmic ray intensity, dependent on interplanetary magnetic fields (IMF) polarity, have been observed in 1982 (Jacklyn and Pomerantz, 1983) and again in 1983 and 1984. These waves at first appeared to be due to the North-South anisotropy. Further investigation revealed that the waves comprise two components, a large isotropic and a smaller anisotropic component. The anisotropic part is attributed to the North-South anisotropy and is in phase with the larger isotropic component in the Southern Hemisphere. Unlike the North-South anisotropy which is a permanent feature of cosmic ray modulation, the isotropic phenomenon appears to be episodic in character. When present, it is clearly dependent on IMF polarity but does not correlate well with IMF field strength. It is conjectured that the phenomenon might indicate some difference between the intensity regimes above and below the neutral sheet.

Duldig, M. L.↗
NASA NTRS1995

Anisotropic microturbulence near the Sun

Interplanetary scintillation observations which sample the spatial spectrum of electron density at scales between 10 and 100 km show power-law spectra which are flatter than the Kolmogorov spectra observed at larger scales by spacecraft and also, indirectly, by phase scintillation of coherent radio signals. Furthermore, angular broadening observations of compact radio sources have shown that the microscale density fluctuations are field-aligned and become more anisotropic as R decreases inside 10 solar radius. We present angular broadening observations taken in October of 1992 which were recorded nearly simultaneously on the VLA and VLBA arrays. The VLA samples structure at scales between 3 and 30 km, whereas the VLBA samples scales between 200 and 4000 km. The small scale VLA measurements of the south polar source 1246-075 showed lower turbulence than those of the equatorial source 1256-057, consistent with previous work showing that the density delta N(exp 2)(sub e) is a factor of 10-15 lower in coronal holes. The VLA observations inside of 10 solar radius were anisotropic, as expected. We were not able to measure the equatorial source with the VLBA inside of 10 solar radius because the scattering was too strong, however we did observe the polar source just inside this distance with both the VLA and the VLBA. Significant anisotropy was seen on the smaller scales, but the larger scales were essentially isotropic. This suggests that the process responsible for the anisotropic microturbulence is distinct from the larger scale isotropic turbulence.

Coles, William A.↗
NASA NTRS2014

Precision Epoch of Reionization Studies with Next-Generation CMB Experiments

Future arcminute resolution polarization data from ground-based Cosmic Microwave Background (CMB) observations can be used to estimate the contribution to the temperature power spectrum from the primary anisotropies and to uncover the signature of reionization near l = 1500 in the small angular-scale temperature measurements. Our projections are based on combining expected small-scale E-mode polarization measurements from Advanced ACTPol in the range 300 < l < 3000 with simulated temperature data from the full Planck mission in the low and intermediate l region, 2 < l < 2000. We show that the six basic cosmological parameters determined from this combination of data will predict the underlying primordial temperature spectrum at high multipoles to better than 1% accuracy. Assuming an efficient cleaning from multi-frequency channels of most foregrounds in the temperature data, we investigate the sensitivity to the only residual secondary component, the kinematic Sunyaev-Zel'dovich (kSZ) term. The CMB polarization is used to break degeneracies between primordial and secondary terms present in temperature and, in effect, to remove from the temperature data all but the residual kSZ term. We estimate a 15 sigma detection of the di use homogeneous kSZ signal from expected AdvACT temperature data at l > 1500, leading to a measurement of the amplitude of matter density fluctuations, sigma 8, at 1% precision.

Precision↗
NASA NTRS2006

The Variance of Solar Wind Magnetic Fluctuations: Solutions and Further Puzzles

We study the dependence of the variance directions of the magnetic field in the solar wind as a function of scale, radial distance, and Alfvenicity. The study resolves the question of why different studies have arrived at widely differing values for the maximum to minimum power (approximately equal to 3:1 up to approximately equal to 20:1). This is due to the decreasing anisotropy with increasing time interval chosen for the variance, and is a direct result of the "spherical polarization" of the waves which follows from the near constancy of |B|. The reason for the magnitude preserving evolution is still unresolved. Moreover, while the long-known tendency for the minimum variance to lie along the mean field also follows from this view (as shown by Barnes many years ago), there is no theory for why the minimum variance follows the field direction as the Parker angle changes. We show that this turning is quite generally true in Alfvenic regions over a wide range of heliocentric distances. The fact that nonAlfvenic regions, while still showing strong power anisotropies, tend to have a much broader range of angles between the minimum variance and the mean field makes it unlikely that the cause of the variance turning is to be found in a turbulence mechanism. There are no obvious alternative mechanisms, leaving us with another intriguing puzzle.

Roberts, D. A.↗
NASA NTRS1993

Review of rigorous coupled-wave analysis and of homogeneous effective medium approximations for high spatial-frequency surface-relief gratings

A review of the rigorous coupled-wave analysis as applied to the diffraction of electro-magnetic waves by gratings is presented. The analysis is valid for any polarization, angle of incidence, and conical diffraction. Cascaded and/or multiplexed gratings as well as material anisotropy can be incorporated under the same formalism. Small period rectangular groove gratings can also be modeled using approximately equivalent uniaxial homogeneous layers (effective media). The ordinary and extraordinary refractive indices of these layers depend on the gratings filling factor, the refractive indices of the substrate and superstrate, and the ratio of the freespace wavelength to grating period. Comparisons of the homogeneous effective medium approximations with the rigorous coupled-wave analysis are presented. Antireflection designs (single-layer or multilayer) using the effective medium models are presented and compared. These ultra-short period antireflection gratings can also be used to produce soft x-rays. Comparisons of the rigorous coupled-wave analysis with experimental results on soft x-ray generation by gratings are also included.

Glytsis, Elias N.↗
NASA NTRS1986

Studies of IRAS sources at high galactic latitudes. I - Source counts at /b/greater than 60 deg and evidence for a north-south anisotropy of cosmological significance

A study of the IRAS sky at b with an absolute value greater than 60 deg is conducted. Source counts at 12, 25, 60 and 100 microns are presented, and it is shown that emission from interstellar dust at 100 microns is localized to a few small areas of tathe galactic polar caps. At 12 and 25 microns, the sky is dominated by stars; at 60 and 100 microns, by galaxies. Comparison with the minisurvey source counts indicates the 12and 25-micron source denstiy is lower at the present latitude than at a latitude whereby the absolute value of b equals 10-40 deg. Due to the greatly reduced effects of emission from interstellar dust, the 100 micron survey reaches a factor 1.6 deeper in flux at the present latitude than the minisurvey. An anisotropy significant at the 4-sigma level was found between the north and south galactic polar caps at 60 and 100 microns, after exclusion of the Virgo cluster and of the few remaining areas significantly affected by interstellar-dust emission. It is suggested that this anisotropy represents a cosmologically significant anisotropy in the galaxy distribution. The scale of associated inhomogeneity is of the order of at least 100(50/H)Mpc.

Rowan-Robinson, M.↗
NASA NTRS1985

Penetration of solar protons into the Earth's magnetosphere on November 22, 1977

The low polar-orbiting Cosmos-900 satellite carried a large geometric factor Cerenkov counter to study the particle anisotropy and spectrum near the proton increase peak in solar cosmic rays. The data help understand different temporal behavior of the increases detected at the stations because the whole set of data showed that the angular distribution of solar cosmic ray particles in interplanetary space was narrow throughout the observation time, resulting in a rapid variation of particle intensity near the poles. The power-law index of the solar cosmic ray integral spectrum varied from -2.4 to -5.2 in the 1 to 4 GV rigidity range from 10.31 to 11.25 UT on November 22, 1977. The flare-time data from all orbits indicate an increased radiation intensity on L=3.5 to 4.0.

Gorchakov, E. V.↗
NASA NTRS1995

A Novel Application of Fourier Transform Spectroscopy with HEMT Amplifiers at Microwave Frequencies

The goal was to develop cryogenic high-electron-mobility transistor (HEMT) based radiometers and use them to measure the anisotropy in the cosmic microwave background (CMB). In particular, a novel Fourier transform spectrometer (FTS) built entirely of waveguide components would be developed. A dual-polarization Ka-band HEMT radiometer and a similar Q-band radiometer were built. In a series of measurements spanning three years made from a ground-based site in Saskatoon, SK, the amplitude, frequency spectrum, and spatial frequency spectrum of the anisotropy were measured. A prototype Ka-band FTS was built and tested, and a simplified version is proposed for the MAP satellite mission. The 1/f characteristics of HEMT amplifiers were quantified using correlation techniques.

Wilkinson, David T.↗
NASA NTRS1995

Variances of the components and magnitude of the polar heliospheric magnetic field

The heliolatitude dependences of the variances in the components and the magnitude of the heliospheric magnetic field have been analysed, using the Ulysses magnetic field observations from close to the ecliptic plane to 80 southern solar latitude. The normalized variances in the components of the field increased significantly (by a factor about 5) as Ulysses entered the purely polar flows from the southern coronal hole. At the same time, there was at most a small increase in the variance of the field magnitude. The analysis of the different components indicates that the power in the fluctuations is not isotropically distributed: most of the power is in the components of the field transverse to the radial direction. Examining the variances calculated over different time scales from minutes to hours shows that the anisotropy of the field variances is different on different scales, indicating the influence of the two distinct populations of fluctuations in the polar solar wind which have been previously identified. We discuss these results in terms of evolutionary, dynamic processes as a function of heliocentric distance and as a function of the large scale geometry of the magnetic field associated with the polar coronal hole.

Balogh, A.↗
NASA NTRS1985

The effect of the interplanetary magnetic field on sidereal variations observed at medium depth underground detectors

It has been known for some years that the intensity variations in sidereal time observed by muon detectors at moderate underground depths are sensitive to the polarity of the interplanetary magnetic field (ipmf) near the Earth. There are differences in the response to these anisotropies as observed in the Norhtern and southern hemispheres. When fully understood, the nature of the anisotropy seems likely to provide information on the 3-dimensional structure of the heliomagnetosphere, its time variations, and its linking with the local interstellar field. The summation harmonic dials for the sidereal diurnal variation during 1958 to 1982 show that there is a strong dependence on whether the ipmf near the Earth is directed outwards from the Sun or inwards it.

Humble, J. E.↗
NASA NTRS1994

Model atmospheres and radiation of magnetic neutron stars: Anisotropic thermal emission

We investigate the anisotropy of the thermal radiation emitted by a surface element of a neutron star atmosphere (e.g., by a polar cap of a radio pulsar). Angular dependences of the partial fluxes at various photon energies, and spectra at various angles are obtained for different values of the effective temperature T(sub eff) and magnetic field strength B, and for different directions of the magnetic field. It is shown that the local radiation of the magnetized neutron star atmospheres is highly anisotropic, with the maximum flux emitted in the magnetic field direction. At high B the angular dependences in the soft X-ray range have two maxima, a high narrow peak along B and a lower and broader maximum at intermediate angles. The radiation is strongly polarized, the modulation of the degree of polarization due to the rotation of the neurtron star may be much higher than that for the radiative flux. The results obtained are compared with recent ROSAT observations of the thermal-like radiation from the radio pulsars PSR 1929+10 and PSR J0437-4715.

Pavlov, G. G.↗
NASA NTRS1978

The effect of dilatancy on velocity anisotropy in Westerly granite

Jacketed samples of Westerly granite were fractured at confining pressures up to 1 kbar, and compressional and horizontally as well as vertically polarized shear velocities were measured in orthogonal directions perpendicular to the compression axis. Changes occurring with increased strain are described, and the velocity data were analyzed by application of the Anderson et al (1974) approach. Observed and calculated velocities are found to be in good agreement, and the degree of dilatancy was determined from the differences between the strains measured perpendicularly to the compression axis and the estimated elastic strains in those directions.

Soga, N.↗
NASA NTRS1976

Three-dimensional interplanetary stream magnetism and energetic particle motion

Cosmic rays interact with mesoscale configurations of the interplanetary magnetic field. A technique is presented for calculating such configurations in the inner solar system, which are due to streams and source conditions near the sun, and maps of magnetic field are constructed for some plausible stream and source conditions. One effect of these mesoscale configurations on galactic cosmic rays is shown to be an out-of-the-ecliptic gradient drift sufficient to explain Forbush decreases. The effects on solar energetic particles include small polar drifts due to the field gradients and a possibly large modification of the time-intensity profiles and anisotropy characteristics due to the formation of mirror configurations in space. If a diffusion model is applicable to solar particles, the true diffusion coefficient will be masked by the effects of streams. A conceptual model which incorporates these ideas and those of several other models is presented.

Barouch, E.↗
NASA NTRS1976

An interpretation of Jupiter's decametric radiation and the terrestrial kilometric radiation as direct amplified gyroemission

Direct amplified gyroemission due to an anisotropic distribution of suprathermal electrons is proposed as the most plausible emission mechanism for Jupiter's decametric radiation (DAM) and the terrestrial auroral kilometric radiation (AKR). It is suggested that the required electron distribution could be produced by electrons, initially with small pitch angles, precipitating from the magnetosphere. A quasi-linear treatment of the proposed mechanism is outlined, including satisfaction of the Doppler condition, calculation of the growth rate, conditions for quasi-linear relaxation, and generation of the anisotropy. The mechanism is applied to the Jovian DAM, emphasizing the growth rate, the power radiated, and the elliptical polarization of the radiation. It is found that the theory can account for the gross features of the DAM, provided the number density in the precipitating electron streams exceeds 20 per cu cm. Application of the theory to the AKR shows that the requirements concerning the properties of the precipitating electrons appear to be satisfied by the observed properties of those inverted V events which correlate with the emission of AKR.

Melrose, D. B.↗
NASA NTRS2006

Polarimeter Arrays for Cosmic Microwave Background Measurements

We discuss general system architectures and specific work towards precision measurements of Cosmic Microwave Background (CMB) polarization. The CMB and its polarization carry fundamental information on the origin, structure, and evolution of the universe. Detecting the imprint of primordial gravitational radiation on the faint polarization of the CMB will be difficult. The two primary challenges will be achieving both the required sensitivity and precise control over systematic errors. At anisotropy levels possibly as small as a few nanokelvin, the gravity-wave signal is faint compared to the fundamental sensitivity limit imposed by photon arrival statistics, and one must make simultaneous measurements with large numbers, hundreds to thousands, of independent background-limited direct detectors. Highly integrated focal plane architectures, and multiplexing of detector outputs, will be essential. Because the detectors, optics, and even the CMB itself are brighter than the faint gravity-wave signal by six to nine orders of magnitude, even a tiny leakage of polarized light reflected or diffracted from warm objects could overwhelm the primordial signal. Advanced methods of modulating only the polarized component of the incident radiation will play an essential role in measurements of CMB polarization. One promising general polarimeter concept that is under investigation by a number of institutions is to first use planar antennas to separate millimeter-wave radiation collected by a lens or horn into two polarization channels. Then the signals can be fed to a pair of direct detectors through a planar circuit consisting of superconducting niobium microstrip transmission lines, hybrid couplers, band-pass filters, and phase modulators to measure the Stokes parameters of the incoming radiation.

Stevenson, Thomas↗
NASA NTRS2011

Latest Progress on the Gamma-Ray Imager/Polarimeter for Solar Flares (GRIPS)

We present the latest progress on building the balloon-borne Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) instrument, including testing and calibration of the detectors and development of the imaging and aspect systems. A continental-US test flight is slated for fall 2012. GRIPS will provide a near-optimal combination of high-resolution imaging, spectroscopy, and polarimetry of solar-flare gamma-ray/hard X-ray emissions from approx.20. keV to > approx.10 MeV. The spectrometer/polarimeter consists of sixteen 3D position-sensitive germanium detectors (3D-GeDs), where each energy deposition is individually recorded with an energy resolution of a few keV FWHM and a spatial resolution to within <0.1 cu mm. Imaging is accomplished by a single multi-pitch rotating modulator (MPRM), a 2.5-cm thick tungsten-alloy grid with pitches that range quasi-continuously from 1 to 13 mm. With the MPRM situated 8 meters from the spectrometer, this instrument will provide excellent image quality and unparalleled angular resolution at gamma-ray energies (12.5 arcsec FWHM), sufficient to separate the 2.2 MeV footpoint sources for almost all flares. Polarimetry is accomplished by analyzing the anisotropy of reconstructed Compton scattering in the 3D-GeDs (i.e. as an active scatterer), with an estimated minimum detectable polarization of a few percent at 150-650 keV in an X-class flare. GRIPS will address questions relevant to particle acceleration and energy release that ha-.e been raised by recent solar flare observations, such as: What causes the spatial separation between energetic electron producing hard X-rays and energetic ions producing gamma-ray lines? How anisotropic are the accelerated electrons, and why do relativistic electron dominate in the corona? How does the composition of accelerated and ambient material vary with space and time, and why?

Shih, Albert Y.↗