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51.
H. C. Bates A. J. King K. S. Shirley E. Bonsall C. Schröder F. Wombacher T. Fockenberg R. J. Curtis N. E. Bowles 《Meteoritics & planetary science》2024,59(5):1006-1028
On the microscale, the Winchcombe CM carbonaceous chondrite contains a number of lithological units with a variety of degrees of aqueous alteration. However, an understanding of the average hydration state is useful when comparing to other meteorites and remote observations of airless bodies. We report correlated bulk analyses on multiple subsamples of the Winchcombe meteorite, determining an average phyllosilicate fraction petrologic type of 1.2 and an average water content of 11.9 wt%. We show the elemental composition and distribution of iron and iron oxidation state are consistent with measurements from other CM chondrites; however, Winchcombe shows a low Hg concentration of 58.1 ± 0.5 ng g−1. We demonstrate that infrared reflectance spectra of Winchcombe are consistent with its bulk modal mineralogy, and comparable to other CM chondrites with similar average petrologic types. Finally, we also evaluate whether spectral parameters can estimate H/Si ratios and water abundances, finding generally spectral parameters underestimate water abundance compared to measured values. 相似文献
52.
本文提出一种新的航电设备减振控制策略,采用半主动磁流变阻尼器与小阻尼柔性橡胶支座来取代目前采用高阻尼刚性橡胶支座的方法,以降低对航电设备的耐振要求,增强飞机飞行的安全性。本文建立了航电设备减振控制的两自由度刚体平-扭耦联模型,荷载激励考虑了基底平动与转动加速度。磁流变阻尼器的半主动控制算法选用限幅最优控制算法,主控制器为H2/LQG控制器,并采用一种加速度反馈的控制策略。针对目前的被动控制方案,本文研究了支座阻尼对减振控制效果的影响。为了得到最优的半主动控制效果,本文对控制器的权矩阵进行了参数优化分析。文中针对一系列工况详细比较了本文所提出的半主动控制策略与目前被动控制策略的减振控制效果。仿真分析结果表明,磁流变阻尼器可以非常有效地减小航电设备的动力反应,新的减振控制策略远优于现有的被动控制方案。 相似文献
53.
The tephrochronology of Iceland and the North Atlantic region is reviewed in order to construct a unified framework for the last 400 kyr BP. Nearly all of the tephra layers described are also characterised geochemically. A number of new tephra layers are analysed for the first time for their geochemical signature and a number of pre‐Holocene tephra layers have been given an informal denotation. The tephrostratigraphy of Ash Zone II is highlighted. Where possible the rhyolitic tephra layers found outside Iceland have been correlated to known Icelandic tephra layers or to the volcanic source area. The application of tephra fallout in various depositional environments is described and discussed. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
54.
Derk C. Bergquist Jason A. Hale Patrick Baker Shirley M. Baker 《Estuaries and Coasts》2006,29(3):353-360
The Suwannee River watershed is one of the least developed in the eastern United States, but with increasing urbanization
it is facing potential long-term alterations in freshwater flow to its estuary in the Gulf of Mexico. The purpose of this
study was to develop biological indicators of oyster reef state along a natural salinity gradient in the Suwannee River estuary
in order to allow the rapid assessment of the effect of changing freshwater input to this system. Percent cover and density
of three size classes of living oysters, as well as the abundance of several predominant reef-associated invertebrates, were
measured along a broad salinity gradient in the estuary and were correlated with salinity estimates from a long-term database
for the preceding 12–24 mo. All eastern oyster,Crassostrea virginica, parameters (percent cover and density of three size classes) were significantly and negatively related to salinity. Data
from samples collected near the lower intertidal were more closely dependent upon salinity than were samples from the higher
intertidal at the same sites. Salinity differences were most closely reflected in differences in total oyster cover. This
relationship corresponded with a general decline in oyster habitat with increasing distance from the mouth of the Suwannee
River. Species richness was significantly and positively correlated with allC. virginica parameters (percent cover and density of three size classes), but the relationship explained only about half the variability.
Density data of the hooked mussel,Ischadium recurvum, and a mud crab,Eurypanopeus depressus, were positively and strongly correlated withC. virginica parameters, likely reflecting the abundance of habitat provided byC. virginica shells. All of the biological indicators measured responded similarly along the salinity gradient, indicating they provide
reliable indices of the effect of changing salinities in the Suwannee River estuary over the previous 1 or 2 yr. Some areas
of positive relief defined as reefs 30 years ago are no longer oyster habitat, suggesting an ongoing decline, but nearshoreC. virginica were abundant. *** DIRECT SUPPORT *** A02BY003 00002 相似文献
55.
56.
Elia M. Leibowitz Shirley Hemar & Marina Orio 《Monthly notices of the Royal Astronomical Society》1998,300(2):463-468
We have monitored the R I magnitudes of the black hole candidate system A0620 − 00 (V616 Mon) in the years 1991–1995 at the Wise Observatory. Combining our data with some additional measurements, we analyse a sparsely covered 7-yr light curve of the star. We find that the average R -band magnitude varies on a time-scale of a few hundred days, with a peak-to-peak amplitude of 0.3 mag. The two maxima in the well-known double hump binary cycle, as well as one of the minima between them, vary by a few per cent relative to one another, in a seemingly random way. One maximum is, on average, higher by 0.05 mag than the other. The depth of the second minimum varies with significantly higher amplitude, in clear correlation with the long-term variability of the mean magnitude of the system. It is shallower than the other minimum at times of maximum light. It deepens when the system brightness declines, and it becomes the deeper of the two minima at times of minimum system light. According to the commonly acceptable phasing of the binary cycle, the systematically varying minimum corresponds to the inferior conjunction of the red dwarf. We cannot suggest any simple geometrical model for explaining the regularities that we find in the long-term photometric behaviour of the V616 Mon binary system. 相似文献
57.
The principal polar-crown coronal helmet structures were selected from nearly three years (May, 1965–January, 1968) of K-coronameter observations made at Haleakala and Mauna Loa, Hawaii. Six isolated and long-lived helmet systems were found at latitudes of 45° and above. Their developments are compared with underlying chromospheric and photospheric activity and a simple phenomenological model is presented showing that a coronal system is formed over an active region. Thereafter the center of gravity of the system gradually drifts poleward with the trailing unipolar magnetic region (UMR), and it becomes a high latitude coronal helmet, arched over a polar crown filament.By comparison of these coronal helmets with observations of the outer corona (to circa 4 R
) made at solar eclipse, lunar sunset, and with balloon and rocket-borne externally occulted corona-graphs, it appears that ground-based K-coronameter measurements to a distance of 1.5–2.0 R
are sufficient to detect the coronal streamers.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
58.
Richard T. Hansen Shirley F. Hansen Charles J. Garcia Dorothy E. Trotter 《Solar physics》1971,18(2):271-275
A systematic investigation was made of the K-corona immediately overlying the positions of the brightest and most isolated chromospheric plages during the years 1964–1967. In all cases, the corona was found to be enhanced with peak brightness proportional to the plage area. In the absence of plages, the K-coronal brightness remained at a quiet level which was essentially the same thoughout this part of the solar cycle.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
59.
Brendan Hermalyn Peter H. Schultz Mark Shirley Kimberly Ennico Anthony Colaprete 《Icarus》2012,218(1):654-665
The Lunar CRater Observation and Sensing Satellite mission (LCROSS) impacted the moon in a permanently shadowed region of Cabeus crater on October 9th 2009, excavating material rich in water ice and volatiles. The thermal and spatial evolution of LCROSS ejecta is essential to interpretation of regolith properties and sources of released volatiles. The unique conditions of the impact, however, made analysis of the data based on canonical ejecta models impossible. Here we present the results of a series of impact experiments performed at the NASA Ames Vertical Gun Range designed to explore the LCROSS event using both high-speed cameras and LCROSS flight backup instruments. The LCROSS impact created a two-component ejecta plume: the usual inverted lampshade “low-angle” curtain, and a high speed, high-angle component. These separate components excavated to different depths in the regolith. Extrapolations from experiments match the visible data and the light curves in the spectrometers. The hollow geometry of the Centaur led to the formation of the high-angle plume, as was evident in the LCROSS visible and infrared measurements of the ejecta. Subsequent ballistic return of the sunlight-warmed ejecta curtain could scour the surface out to many crater radii, possibly liberating loosely bonded surface volatiles (e.g., H2). Thermal imaging reveals a complex, heterogeneous distribution of heated material after crater formation that is present but unresolved in LCROSS data. This material could potentially serve as an additional source of energy for volatile release. 相似文献
60.
Fast Fourier analysis of the detrended record of solar irradiance obtained by the Nimbus-7 cavity pyrheliometer shows a rich spectrum of significant frequencies between about 30 and 850 nHz (periods between 13 and 400 days). Wolff and Hickey (1987a, b), elaborating on a model developed by Wolff (1974a, b, 1976, 1983, 1984), suggest that many of these peaks arise due to interference of rigidly rotating global solar oscillations (r- and g-modes). Their model fit is quite good in the region above about 135 nHz, but less satisfactory below this threshold. We note that the FFT spectrum of d2
L/d2
t, the second derivative of angular momentum of the solar inertial motion, contains peaks matching the large peaks in the irradiance spectrum below 400 nHz with periods near 0.08, 0.24, 0.65, and about 1 yr. We discuss the origins of the peaks in the d2
L/d2t spectra and review some previous studies bearing on the question of a possible relationship of solar motion and solar activity. The future persistence of the observed spectral peaks of irradiance with periods near 0.24 and 0.65 yr will provide a key test for this hypothesis. 相似文献