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  • Wiley  (7)
  • Oxford University Press  (6)
  • American Physical Society  (4)
  • Society of Exploration Geophysicists (SEG)  (2)
  • American Institute of Physics
  • Copernicus
  • 2010-2014  (19)
  • 1
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    45 years of rotation of the Crab pulsar (2014)
    Oxford University Press
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    Publikationsdatum: 2014-11-15
    Beschreibung: The 30-Hz rotation rate of the Crab pulsar has been monitored at Jodrell Bank Observatory since 1984 and by other observatories before then. Since 1968, the rotation rate has decreased by about 0.5 Hz, interrupted only by sporadic and small spin-up events (glitches). 24 of these events have been observed, including a significant concentration of 15 occurring over an interval of 11 yr following MJD 50000. The monotonic decrease of the slowdown rate is partially reversed at glitches. This reversal comprises a step and an asymptotic exponential with a 320-d time constant, as determined in the three best-isolated glitches. The cumulative effect of all glitches is to reduce the decrease in slowdown rate by about 6 per cent. Overall, a low mean braking index of 2.342(1) is measured for the whole period, compared with values close to 2.5 in intervals between glitches. Removing the effects of individual glitches reveals an underlying power-law slowdown with the same braking index of 2.5. We interpret this value in terms of a braking torque due to a dipolar magnetic field in which the inclination angle between the dipole and rotation axes is increasing. There may also be further effects due to a monopolar particle wind or infalling supernova debris.
    Print ISSN: 0035-8711
    Digitale ISSN: 1365-2966
    Thema: Physik
    Publiziert von Oxford University Press
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  • 2
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    Accuracy and precision of photoacoustic spectroscopy not guaranteed (2013)
    Wiley
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    Publikationsdatum: 2013-07-20
    Beschreibung: Because of their accuracy and precision for measuring gas concentrations, gas chromatographs (GC) are standard analytical instruments used in investigations of nitrous oxide (N 2 O) and carbon dioxide (CO 2 ) exchange between the soil and the atmosphere. Iqbal et al. (2012) indicate that photoacoustic spectroscopy (PAS) performs similar to GCs for this purpose. We welcome this addition to the literature, given the increasing number of studies using PAS (e.g., Predotova et al. 2009; Leytem et al. 2011) and the few comparative analyses available (Ambus & Robertson 1998; Yamulki & Jarvis 1999). However, poor performance of PAS in some assessments (Akdeniz et al ., 2009) and data from our own tests (reported below) raise questions about whether Iqbal et al.'s (2012) results are generally applicable to PAS instruments or unique to the experimental conditions and calibration of their instruments. This article is protected by copyright. All rights reserved.
    Print ISSN: 1354-1013
    Digitale ISSN: 1365-2486
    Thema: Biologie , Energietechnik , Geographie
    Publiziert von Wiley
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  • 3
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    The mass-radius relationship for very low mass stars: four new discoveries from the HATSouth Survey (2013)
    Oxford University Press
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    Publikationsdatum: 2013-12-23
    Beschreibung: We report the discovery of four transiting F-M binary systems with companions between 0.1 and 0.2 M in mass by the HATSouth survey. These systems have been characterized via a global analysis of the HATSouth discovery data, combined with high-resolution radial velocities and accurate transit photometry observations. We determined the masses and radii of the component stars using a combination of two methods: isochrone fitting of spectroscopic primary star parameters and equating spectroscopic primary star rotation velocity with spin–orbit synchronization. These new very low mass companions are HATS550-016B ( $0.110_{-0.006}^{+0.005}\,\mathrm{M}_{\odot }$ , $0.147_{-0.004}^{+0.003}\,\mathrm{R}_{\odot }$ ), HATS551-019B ( $0.17_{-0.01}^{+0.01}\,\mathrm{M}_{\odot }$ , $0.18_{-0.01}^{+0.01}\,\mathrm{R}_{\odot }$ ), HATS551-021B ( $0.132_{-0.005}^{+0.014}\,\mathrm{M}_{\odot }$ , $0.154_{-0.008}^{+0.006}\,\mathrm{R}_{\odot }$ ) and HATS553-001B ( $0.20_{-0.02}^{+0.01}\,\mathrm{M}_{\odot }$ , $0.22_{-0.01}^{+0.01}\,\mathrm{R}_{\odot }$ ). We examine our sample in the context of the radius anomaly for fully convective low-mass stars. Combining our sample with the 13 other well-studied very low mass stars, we find a tentative 5 per cent systematic deviation between the measured radii and theoretical isochrone models.
    Print ISSN: 0035-8711
    Digitale ISSN: 1365-2966
    Thema: Physik
    Publiziert von Oxford University Press
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  • 4
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    Gedanken experiments with Casimir forces and vacuum energy (2010)
    American Physical Society
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    Publikationsdatum: 2010-09-10
    Print ISSN: 1050-2947
    Digitale ISSN: 1094-1622
    Thema: Elektrotechnik, Elektronik, Nachrichtentechnik , Physik
    Publiziert von American Physical Society
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  • 5
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    Radiative forcing of natural forest disturbances (2011)
    Wiley
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    Publikationsdatum: 2011-10-11
    Beschreibung: Forest disturbances are major sources of carbon dioxide to the atmosphere, and therefore impact global climate. Biogeophysical attributes, such as surface albedo (reflectivity), further control the climate-regulating properties of forests. Using both tower-based and remotely-sensed data sets, we show that natural disturbances from wildfire, beetle outbreaks, and hurricane wind throw can significantly alter surface albedo, and the associated radiative forcing either offsets or enhances the CO 2 forcing caused by reducing ecosystem carbon sequestration over multiple years. In the examined cases, the radiative forcing from albedo change is on the same order of magnitude as the CO 2 forcing. The net radiative forcing resulting from these two factors leads to a local heating effect in a hurricane-damaged mangrove forest in the sub-tropics, and a cooling effect following wildfire and mountain pine beetle attack in boreal forests with winter snow. Although natural forest disturbances currently represent less than half of gross forest cover loss, that area will likely increase in the future under climate change, making it imperative to represent these processes accurately in global climate models.
    Print ISSN: 1354-1013
    Digitale ISSN: 1365-2486
    Thema: Biologie , Energietechnik , Geographie
    Publiziert von Wiley
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  • 6
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    The Effects of Alignment Error and Alignment Filtering on the Sitewise Detection of Positive Selection (2012)
    Oxford University Press
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    Publikationsdatum: 2012-04-16
    Beschreibung: When detecting positive selection in proteins, the prevalence of errors resulting from misalignment and the ability of alignment filters to mitigate such errors are not well understood, but filters are commonly applied to try to avoid false positive results. Focusing on the sitewise detection of positive selection across a wide range of divergence levels and indel rates, we performed simulation experiments to quantify the false positives and false negatives introduced by alignment error and the ability of alignment filters to improve performance. We found that some aligners led to many false positives, whereas others resulted in very few. False negatives were a problem for all aligners, increasing with sequence divergence. Of the aligners tested, PRANK's codon-based alignments consistently performed the best and ClustalW performed the worst. Of the filters tested, GUIDANCE performed the best and Gblocks performed the worst. Although some filters showed good ability to reduce the error rates from ClustalW and MAFFT alignments, none were found to substantially improve the performance of PRANK alignments under most conditions. Our results revealed distinct trends in error rates and power levels for aligners and filters within a biologically plausible parameter space. With the best aligner, a low false positive rate was maintained even with extremely divergent indel-prone sequences. Controls using the true alignment and an optimal filtering method suggested that performance improvements could be gained by improving aligners or filters to reduce the prevalence of false negatives, especially at higher divergence levels and indel rates.
    Print ISSN: 0737-4038
    Digitale ISSN: 1537-1719
    Thema: Biologie
    Publiziert von Oxford University Press
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  • 7
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    Air-water gas exchange and CO2 flux in a mangrove-dominated estuary (2013)
    Wiley
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    Publikationsdatum: 2013-12-11
    Beschreibung: [1]  Mangrove forests are highly productive ecosystems, but the fate of mangrove-derived carbon remains uncertain. Part of that uncertainty stems from the fact that gas transfer velocities in mangrove-surrounded waters are not well determined, leading to uncertainty in air-water CO 2 fluxes. Two SF 6 tracer release experiments were conducted to determine gas transfer velocities ( k (600) = 8.3 ± 0.4 and 8.1 ± 0.6 cm h -1 ), along with simultaneous measurements of pCO 2 to determine the air-water CO 2 fluxes from Shark River, FL (232.11 ± 23.69 and 171.13 ± 20.28 mmol C m -2 d -1 ), an estuary within the largest contiguous mangrove forest in North America. The gas transfer velocity results are consistent with turbulent kinetic energy dissipation measurements, indicating a higher rate of turbulence and gas exchange than predicted by commonly used wind speed/gas exchange parameterizations. The results have important implications for carbon fluxes in mangrove ecosystems.
    Print ISSN: 0094-8276
    Digitale ISSN: 1944-8007
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 8
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    Seasonal evapotranspiration patterns in mangrove forests (2014)
    Wiley
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    Publikationsdatum: 2014-03-14
    Beschreibung: Diurnal and seasonal controls on water vapor fluxes were investigated in a subtropical mangrove forest in Everglades National Park, Florida. Energy partitioning between sensible and latent heat fluxes was highly variable during the 2004-2005 study period. During the dry season, the mangrove forest behaved akin to a semiarid ecosystem as most of the available energy was partitioned into sensible heat, which gave Bowen ratio values exceeding 1.0 and minimum latent heat fluxes of 5 MJ day -1 . In contrast, during the wet season the mangrove forest acted as a well-watered, broadleaved deciduous forest, with Bowen ratio values of 0.25 and latent heat fluxes reaching 18 MJ day -1 . During the dry season, high salinity levels (〉 30 parts per thousand, ppt) caused evapotranspiration to decline and correspondingly resulted in reduced canopy conductance. From multiple linear regression, daily average canopy conductance to water vapor declined with increasing salinity, vapor pressure deficit, and daily sums of solar irradiance but increased with air temperature and friction velocity. Using these relationships, appropriately modified Penman-Monteith and Priestley-Taylor models reliably reproduced seasonal trends in daily evapotranspiration. Such numerical models, using site-specific parameters, are crucial for constructing seasonal water budgets, constraining hydrological models, and driving regional climate models over mangrove forests.
    Print ISSN: 0148-0227
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 9
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    Quantitative interpretation of pulsed neutron capture logs: Part 1 — Fast numerical simulation (2011)
    Society of Exploration Geophysicists (SEG)
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    Publikationsdatum: 2011-05-01
    Beschreibung: Pulsed neutron capture (PNC) logs are commonly used for formation evaluation behind casing and to assess time-lapse variations of hydrocarbon pore volume. Because conventional interpretation methods for S logs assume homogeneous formations, errors may arise, especially in thinly bedded formations, when appraising petrophysical properties of hydrocarbon-bearing beds. There exist no quantitative interpretation methods to account for shoulder-bed effects on S logs acquired in sand-shale laminated reservoirs. Because of diffusion effects between dissimilar beds, S logs acquired in such formations do not obey mixing laws between the S responses of pure-sand and pure-shale end members of the sedimentary sequence. We have developed a new numerical method to simulate PNC rapidly and accurately logs. The method makes use of late-time, thermal-neutron flux sensitivity functions (FSFs) to describe the contribution of multilayer formations toward the measured capture cross section. It includes a correction procedure based on 1D neutron diffusion theory that adapts the transport-equation-derived, base-case FSF of a homogeneous formation to simulate the response of vertically heterogeneous formations. Benchmarking exercises indicate that our simulation method yields average differences smaller than two capture units within seconds of computer central processing unit time with respect to PNC logs simulated with rigorous Monte Carlo methods for a wide range of geometrical, petrophysical, and fluid properties.
    Print ISSN: 0016-8033
    Digitale ISSN: 1942-2156
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Society of Exploration Geophysicists (SEG)
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  • 10
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    Quantitative interpretation of pulsed neutron capture logs: Part 2 — Inversion of measurements in thinly bedded formations (2011)
    Society of Exploration Geophysicists (SEG)
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    Publikationsdatum: 2011-05-01
    Beschreibung: We have developed an inversion method to reduce shoulder-bed effects on pulsed neutron capture (PNC) logs for estimating layer-by-layer-capture cross sections S. The method is based on a previously developed rapid approximation of PNC logs. Tests performed on synthetic examples that include a variety of lithology, saturating-fluid, and bed-thickness configurations confirm the efficiency, reliability, and stability of the inversion procedure. Inversion consistently improves the vertical resolution and S definition of PNC logs across beds thinner than 45 cm. Our fast, iterative algorithm inverts S logs in seconds of CPU time and is therefore suitable for joint petrophysical interpretation with other open- and cased-hole logs.
    Print ISSN: 0016-8033
    Digitale ISSN: 1942-2156
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Society of Exploration Geophysicists (SEG)
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