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931.
932.
The Spaceborne Laser Ranging System is a proposed short pulse laser on board an orbiting spacecraft.1,2,3,4 It measures the distance between the spacecraft and many laser retroreflectors (targets) deployed on the Earth’s surface.
The precision of these range measurements is assumed to be about ±2 cm (M. W. Fitzmaurice, private communication). These measurements are then used together with the orbital dynamics of the spacecraft, to derive
the relative position of the laser ground targets. Assuming a six day observing period with 50% cloud cover, uncertainties
in the baseline for target separations of 50 km to 1200 km were estimated to be on the order of 1 to 3 cm and the corresponding
values in the vertical direction, ranged from 1 cm to 12 cm. By redetermining the measurements of the relative target positions,
the estimated precision in the baseline for a target separation of 50 km is less than 0.3 cm and for a separation of 1200
km is less than 1 cm. In the vertical direction, the estimated precision ranged from 0.4 cm to 4.0 cm respectively. As a result
of the repeated estimation of the relative laser target positions, most of the non-temporal effects of error sources as exemplified
by the errors in geopotential are reduced.
The Spaceborne Laser Ranging System’s capability of determining baselines to a high degree of precision provides a measure
of strain and strain rate as shown byCohen, 1979. These quantities are essential for crustal dynamic studies which include determination and monitoring of strain near
seismic zones, land subsidence, and edifice building preceding volcanic eruptions. It is evident that such a system can also
be used for geodetic surveys where such precisions are more than adquate. 相似文献
933.
Youri Lammers Charlotte L. Clarke Christer Ersus Antony G. Brown Mary E. Edwards Ludovic Gielly Haflidi Haflidason Jan Mangerud Emilia Rota John Inge Svendsen Inger Greve Alsos 《Boreas: An International Journal of Quaternary Research》2019,48(2):317-329
While there are extensive macro‐ and microfossil records of a range of plants and animals from the Quaternary, earthworms and their close relatives amongst annelids are not preserved as fossils and therefore the knowledge of their past distributions is limited. This lack of fossils means that clitellate worms (Annelida) are currently underused in palaeoecological research, even though they can provide valuable information about terrestrial and aquatic environmental conditions. Their DNA might be preserved in sediments, which offers an alternative method for detection. Here we analyse lacustrine sediments from lakes in the Polar Urals, Arctic Russia, covering the period 24 000–1300 cal. a BP, and NE Norway, covering 10 700–3300 cal. a BP, using a universal mammal 16S rDNA marker. While mammals were recorded using the marker (reindeer was detected twice in the Polar Urals core at 23 000 and 14 000 cal. a BP, and four times in the Norwegian core at 11 000 cal. a BP and between 3600–3300 cal. a BP), worm extracellular DNA ‘bycatch’ was rather high. In this paper we present the first reported worm detection from ancient DNA. Our results demonstrate that both aquatic and terrestrial clitellates can be identified in late‐Quaternary lacustrine sediments, and the ecological information retrievable from this group warrants further research with a more targeted approach. 相似文献
934.
M. A. K. Glasmacher M. A. Catanese M. C. Chantell C. E. Covault J. W. Cronin B. E. Fick L. F. Fortson J. W. Fowler K. D Green D. B. Kieda J. Matthews B. J. Newport D. F. Nitz R. A. Ong S. Oser D. Sinclair J. C. van der Velde 《Astroparticle Physics》1999,10(4):183-302
The energy spectrum of cosmic rays with primary energies between 1014 eV and 1016 eV has been studied with the CASA-MIA air shower array. The measured differential energy spectrum is a power law (dj/dE ∝ E−y) with spectral indices γ of 2.66±0.02 below approximately 1015 eV and 3.00±0.05 above. A new method is used for measuring primary energy derived from ground-based data in a compositionally insensitive way. In contrast with some previous reports, the “knee” of the energy spectrum does not appear sharp, but rather a smooth transition over energies from 1015 eV to 3.0 × 1015 eV. 相似文献
935.
We have undertaken a study of coronal features observed at meter-decameter wavelengths using the Clark Lake radioheliograph.
Among the coronal structures we have studied are the radio manifestations of coronal streamers on the solar disk and above
the solar limb. We have analyzed the radio data quantitatively, using ray-tracing models for comparison with the maps. Our
study provides information about the streamers' three-dimensional shapes, scales, and density profiles, for comparison with
related observations using white-light coronagraphs. 相似文献
936.
E. V. Derishev F. A. Aharonian V. V. Kocharovsky Vl. V. Kocharovsky 《Astrophysics and Space Science》2005,297(1-4):21-30
We discuss the properties of gamma-ray radiation accompanying the acceleration of cosmic rays via the converter mechanism.
The mechanism exploits multiple photon-induced conversions of high-energy particles from charged into neutral state (namely,
protons to neutrons and electrons to photons) and back. Because a particle in the neutral state can freely cross the magnetic
field lines, this allows to avoid both particle losses downstream and reduction in the energy gain factor, which normally
takes place due to highly collimated distribution of accelerated particles. The converter mechanism efficiently operates in
relativistic outflows under the conditions typical for Active Galactic Nuclei, Gamma-Ray Bursts, and microquasars, where it
outperforms the standard diffusive shock acceleration.
The accompanying radiation has a number of distinctive features, such as an increase of the maximum energy of synchrotron
photons and peculiar radiation beam-pattern, whose opening angle is much wider at larger photon energies. This provides an
opportunity to observe off-axis relativistic jets in GeV–TeV energy range. One of the implications is the possibility to explain
high-latitude unidentified EGRET sources as off-axis but otherwise typical relativistic-jet sources, such as blazars. 相似文献
937.
E. Juvigné 《Quaternary Research》1975,5(1):121-123
The following note on a new method for extracting pollen from fossil pollen grains summarizes two articles in French, published in Annales de la Société Géologique de Belgique (Juvigné, 1973a; 1973b).The new method proposed has three important parts: (1) Elutriation of the sediment in water; (2) the specific gravity of the heavy liquid must be between 2.25 and 2.5; and (3) the dispersion of the sediment in the heavy liquid is made with a high-speed mixer. 相似文献
938.
A. Määttänen F. Montmessin F. Scholten F. González-Galindo A. Spiga E. Hauber J.-P. Bibring 《Icarus》2010,209(2):452-469
This study presents the latest results on the mesospheric CO2 clouds in the martian atmosphere based on observations by OMEGA and HRSC onboard Mars Express. We have mapped the mesospheric CO2 clouds during nearly three martian years of OMEGA data yielding a cloud dataset of ∼60 occurrences. The global mapping shows that the equatorial clouds are mainly observed in a distinct longitudinal corridor, at seasons Ls = 0-60° and again at and after Ls = 90°. A recent observation shows that the equatorial CO2 cloud season may start as early as at Ls = 330°. Three cases of mesospheric midlatitude autumn clouds have been observed. Two cloud shadow observations enabled the mapping of the cloud optical depth (τ = 0.01-0.6 with median values of 0.13-0.2 at λ = 1 μm) and the effective radii (mainly 1-3 μm with median values of 2.0-2.3 μm) of the cloud crystals. The HRSC dataset of 28 high-altitude cloud observations shows that the observed clouds reside mainly in the altitude range ∼60-85 km and their east-west speeds range from 15 to 107 m/s. Two clouds at southern midlatitudes were observed at an altitude range of 53-62 km. The speed of one of these southern midlatitude clouds was measured, and it exhibited west-east oriented speeds between 5 and 42 m/s. The seasonal and geographical distribution as well as the observed altitudes are mostly in line with previous work. The LMD Mars Global Climate Model shows that at the cloud altitude range (65-85 km) the temperatures exhibit significant daily variability (caused by the thermal tides) with the coldest temperatures towards the end of the afternoon. The GCM predicts the coldest temperatures of this altitude range and the season Ls = 0-30° in the longitudinal corridor where most of the cloud observations have been made. However, the model does not predict supersaturation, but the GCM-predicted winds are in fair agreement with the HRSC-measured cloud speeds. The clouds exhibit variable morphologies, but mainly cirrus-type, filamented clouds are observed (nearly all HRSC observations and most of OMEGA observations). In ∼15% of OMEGA observations, clumpy, round cloud structures are observed, but very few clouds in the HRSC dataset show similar morphology. These observations of clumpy, cumuliform-type clouds raise questions on the possibility of mesospheric convection on Mars, and we discuss this hypothesis based on Convective Available Potential Energy calculations. 相似文献
939.
940.
Michael E. McCormick 《Ocean Engineering》1976,3(3):133-144
The analysis of a pneumatic-type wave-energy conversion buoy is developed assuming independence of the buoy heaving motion and the motion of the water column within the center pipe. Results of the analysis are then compared with experimental data in a study of the relative air velocity within the turbine passage. The results compare very well. The effect of the variation of the center pipe length is found to be significant for periods about the surge chamber resonance but is negligible in the neighborhood of the heaving resonance period. Further, the theory is applied to a prototype buoy study of the U.S. Coast Guard, and a dimensionless design curve is developed from the results of the prototype analysis. 相似文献