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291.
292.
The Total Irradiance Monitor (TIM) instrument is designed to measure total solar irradiance with an absolute accuracy of 100
parts per million. Four electrical substitution radiometers behind precision apertures measure input radiant power while providing
redundancy. Duty cycling the use of the radiometers tracks degradation of the nickel-phosphorous absorptive black radiometer
interiors caused by solar exposure. Phase sensitive detection at the shutter frequency reduces noise and simplifies the estimate
of the radiometer's equivalence ratio. An as-designed uncertainty budget estimates the instrument's accuracy goal. The TIM
measurement equation defines the conversion from measured signal to solar irradiance. 相似文献
293.
Fossil hyrax dung and evidence of Late Pleistocene and Holocene vegetation types in the Namib Desert
Pollen was derived from fossil dung of herbivorous hyraxes, deposited in a rock shelter on the highest mountain in Namibia, Dâures or Brandberg, situated on the Namib Desert margin. Radiocarbon dating ranging in age between modern times and 30 000 yr BP showed it represents the first empirical pollen evidence of continental palaeovegetation during the Late Pleistocene along the western escarpment of southern Africa. The initial results indicate Last Glacial Maximum vegetation differed totally from the current pattern as vegetation types were dominated by small Asteraceae shrubs, in contrast to those of the Holocene and modern times which show more succulents, grass and woody elements (arboreal pollen). The results suggest that Cape floral communities did not reach into the tropics along the western escarpment of Africa, despite such pollen types occurring in marine cores. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
294.
295.
William S. Burgett Michael M. Vick David S. Davis Matthew Colless Roberto De Propris Ivan Baldry Carlton Baugh Joss Bland-Hawthorn Terry Bridges Russell Cannon Shaun Cole Chris Collins Warrick Couch Nicholas Cross Gavin Dalton Simon Driver George Efstathiou Richard Ellis Carlos S. Frenk Karl Glazebrook Edward Hawkins Carole Jackson Ofer Lahav Ian Lewis Stuart Lumsden Steve Maddox Darren Madgwick Peder Norberg John A. Peacock Will Percival Bruce Peterson Will Sutherland Keith Taylor 《Monthly notices of the Royal Astronomical Society》2004,352(2):605-654
296.
George Chatzigeorgiou Vincent Picandet Abdelhafid Khelidj Gilles Pijaudier‐Cabot 《国际地质力学数值与分析法杂志》2005,29(10):1005-1018
A significant increase of the permeability of concrete upon micro‐cracking and a good correlation between the evolution of damage (material stiffness) and permeability are observed experimentally. The present contribution investigates this correlation theoretically, with the help of lattice analyses. Scaling analysis of lattices which contain elastic brittle bonds has shown that the material degradation should be described by the evolution of the material stiffness, or compliance, in a continuum setting (damage models). This result is reviewed and further documented in the first part of the paper. In the second part, hydro‐mechanical problems are considered with the construction of a hydraulic lattice, dual to the mechanical one. We observe that the average permeability upon micro‐cracking is the lattice scale‐independent controlling variable in the hydraulic problem. Additionally, results show that in a continuum poro‐mechanical approach, the evolution of the material permeability ought to be related to the elastic unloading stiffness, described e.g. with the help of continuum damage variables. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
297.
George J. Flynn 《Earth, Moon, and Planets》1995,71(3):469-474
Carbon delivered to the Earth by interplanetary dust particles may have been an important source of pre-biotic organic matter (Anders, 1989). Interplanetary dust is shown to deliver an order-of-magnitude higher surface concentration of carbon onto Mars than onto Earth, suggesting interplanetary dust may be an important source of carbon on Mars as well. 相似文献
298.
Radiative transfer 总被引:1,自引:0,他引:1
George B. Rybicki 《Journal of Astrophysics and Astronomy》1996,17(3-4):95-112
Chandrasekhar’s work in radiative transfer theory began in 1944 and culminated with the publication of his influential treatiseRadiative Transfer in 1950. In this review his major contributions to radiative transfer will be recounted and evaluated. These include his
development of the discrete ordinates method, the invariance principles, and his formulation and solution of the transfer
equation for polarized light. 相似文献
299.
We present the UV spectroscopy and timing of three nearby pulsars (Vela, B0656+14 and Geminga) recently observed with the
Space Telescope Imaging Spectrograph. We also review the optical and X-ray properties of these pulsars and establish their
connection with the UV properties. We show that the multiwavelengths properties of neutron stars (NSs) vary significantly
within the sample of middle-aged pulsars. Even larger differences are found between the thermal components of Geminga and
B0656+14 as compared to those of radio-quiet isolated NSs. These differences could be attributed to different properties of
the NS surface layers.
This work was supported by STScI grants GO-9182 and GO-9797 and NASA grant NAG5-10865. 相似文献
300.
Peter M. Woods Vyacheslav E. Zavlin George G. Pavlov 《Astrophysics and Space Science》2007,308(1-4):239-246
Unique among neutron stars, 1E 1207.4-5209 is an X-ray pulsar with a spin period of 424 ms that contains at least two strong absorption features in its energy spectrum. This neutron star is positionally coincident with the supernova remnant PKS 1209-51/52 and has been identified as a member of the growing class of radio-quiet compact central objects in supernova remnants. From previous observations with Chandra and XMM-Newton, it has been found that the 1E 1207.4-5209 is not spinning down monotonically as is common for young, isolated pulsars. The spin frequency history requires either strong, frequent glitches, the presence of a fall-back disk, or a binary companion. Here, we report on a sequence of seven XMM-Newton observations of 1E 1207.4-5209 performed during a 40 day window between 2005 June 22 and July 31. Due to unanticipated variance in the phase measurements during the observation period that was beyond the statistical uncertainties, we could not identify a unique phase-coherent timing solution. The three most probable timing solutions give frequency time derivatives of +0.9, ?2.6, and +1.6×10?12 Hz s?1 (listed in descending order of significance). We conclude that the local frequency derivative during our XMM-Newton observing campaign differs from the long-term spin-down rate by more than an order of magnitude. This measurement effectively rules out glitch models for 1E 1207.4-5209. If the long-term spin frequency variations are caused by timing noise, the strength of the timing noise in 1E 1207.4-5209 is much stronger than in other pulsars with similar period derivatives. Therefore, it is highly unlikely that the spin variations are caused by the same physical process that causes timing noise in other isolated pulsars. The most plausible scenario for the observed spin irregularities is the presence of a binary companion to 1E 1207.4-5209. We identified a family of orbital solutions that are consistent with our phase-connected timing solution, archival frequency measurements, and constraints on the companions mass imposed by deep IR and optical observations. 相似文献