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851.
We report the three-dimensional fluorescence spectra of Titan tholins in water and acetonitrile, and separate some of the fluorescent components of tholins using two-dimensional thin layer chromatography. In acetonitrile, tholins exhibit a broad fluorescence with a maximum at 471 nm, and an excitation maximum at 410 nm. The fluorescence spectrum of a water extraction displays a more complicated spectrum with multiple peaks. TLC results indicate the presence of at least three distinct fluorescent species. In addition, we obtained the two-dimensional fluorescence spectrum of tholins in water ice at 77 K, close to the surface temperature of Titan. The fluorescence of tholins in a 77 K ice matrix is red-shifted in comparison to a water solution, and undergoes a further red-shift when the water solution is heated prior to freezing. These results suggest that a simple fluorescence probe could be used on the surface of Titan to identify sites where tholins have been mixed with water, and possibly reveal information about the extent of heating the tholin-water mixture has undergone. This would be useful for a future mission with the goal of examining the organic chemistry of Titan. 相似文献
852.
Stable carbon and oxygen isotope analyses of ungulate grazers from four archaeological sites located in different environs within the Caledon River Valley have provided a relatively well‐dated proxy palaeoenvironmental and palaeoclimatic sequence for the period between 16 000 and 6000 calendar (cal.) yr BP. Within the overall trend towards hot mid‐Holocene temperatures and a summer rainfall pattern, stable carbon isotope results show that there were three periods when growth season temperatures were cool enough for C3 grasses to be present: 16 000–14 000; 10 200–9600, and 8400–8000 cal. yr BP. Similar trends were recorded in stable oxygen isotope values, reflecting shifts in either temperature or available moisture. Although having a similar pattern to that of the lower altitude site, sites situated in foothills and montane portions of the valley consistently maintained lower temperatures until the mid‐Holocene altithermal. At this time growth season temperatures warmed sufficiently for a 100% C4 grassland to expand in altitude from the warmer low lying localities. In relation to present understanding of synoptic and global climatic patterning, these findings suggest that the early to middle Holocene transition was not a gradual warming trend, but rather it was marked by a series of climatic fluctuations. Of particular note is the possible global, rather than regional, occurrence of the 8200 cal. yr BP ‘event’. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
853.
854.
A real-time colour display of the Culgoora acousto-optical radio spectrograph is described and illustrated. 相似文献
855.
Summary. The quality of marine magnetic anomaly sources is described with the power-density representation of a stochastic model of random temporal and spatial emplacement of the marine magnetic anomaly source in the oceanic crust. Typical values of sea-floor spreading and emplacement parameters define a high-fidelity process of recording and sea-surface detection of the palaeomagnetic field reversals for spreading rates over 20 mm yr-1 . An analogous stochastic model is developed for the formation of sea-floor topography by normal faulting. It is shown that the random process of normal faulting observed in the inner walls of the FAMOUS rift valley can account for the quality of the adjacent West Rift Mountains topography. 相似文献
856.
Walker O. Smith 《Estuarine, Coastal and Shelf Science》1982,15(4):459-465
The attenuation of light by chlorophyll, particulate and dissolved material, and seawater off the coast of north-west Africa showed distinct cross-shelf variations. Attenuation was most rapid nearshore because of absorption by non-biological particles. This particulate matter was probably aeolian material from the nearby Sahara Desert of resuspended sand from the sediment. Regardless of the source, the marked light extinction resulted in decreased phytoplankton growth nearshore. Light attenuation characteristics of the region were anomalous when compared to those of other ocenic regimes. 相似文献
857.
The absolute electrical conductivity at15deg C of several lots of standard seawater has been measured with great precision as a function of chlorinity. Potassium chloride (KCI) solutions of known concentration and having almost the same conductivity were also measured and the concentration giving the same conductivity at15deg C as35.0000permil standard seawater (Chl =19.37394permil ) was found to be 32.4352 g/kg. 相似文献
858.
859.
McConnell Mark L. Ryan James M. Smith David M. Lin Robert P. Emslie A. Gordon 《Solar physics》2002,210(1-2):125-142
Although designed primarily as a hard X-ray imager and spectrometer, the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)
is also capable of measuring the polarization of hard X-rays (20–100 keV) from solar flares. This capability arises from the
inclusion of a small unobstructed Be scattering element that is strategically located within the cryostat that houses the
array of nine germanium detectors. The Ge detectors are segmented, with both a front and rear active volume. Low-energy photons
(below about 100 keV) can reach a rear segment of a Ge detector only indirectly, by scattering. Low-energy photons from the
Sun have a direct path to the Be and have a high probability of Compton scattering into a rear segment of a Ge detector. The
azimuthal distribution of these scattered photons carries with it a signature of the linear polarization of the incident flux.
Sensitivity estimates, based on Monte Carlo simulations and in-flight background measurements, indicate that a 20–100 keV
polarization sensitivity of less than a few percent can be achieved for X-class flares. 相似文献
860.
P. A. Sturrock P. J. Baum J. M. Beckers C. E. Newman E. R. Priest H. Rosenberg D. F. Smith D. G. Wentzel 《Solar physics》1976,46(2):411-431
This report summarizes the proceedings of a meeting held on 17–20 September 1974, at Stanford University. The purpose was to explore plasma physics problems which arise in the study of solar physics. Sessions were concerned with specific questions including the following: Is the solar plasma thermal or non-thermal? What spectroscopic data are required? What types of magnetic field structures exist? Do MHD instabilities occur? Do resistive or non-MHD instabilities occur? What mechanisms of particle acceleration have been proposed? What information do we have concerning shock waves? Very few questions were answered categorically but, for each question, there was discussion concerning the observational evidence, theoretical analyses, and existing or potential laboratory and numerical experiments. 相似文献