天王星环的自引力势     

周济林  孙义燧 《天文学报》1996,37(2):140-146

本文用Ｎ流线方法（Ｎ→∞）导出了天王星具非零偏心率环中任一点处自引力势的解析表达式，并由数值计算给出了表达式的系数，据此解释并验证了二流线的自引力模型将会过高估计环质量的结论，并给出了定量的结果：对ε、α、β环，Ｎ流线方法（Ｎ→∞）得到的环质量将约为二流线方法相应结果的２／５．  相似文献   

Occultations by possible material in Saturn’s outer magnetosphere—2     

R. Vasundhara 《Journal of Astrophysics and Astronomy》1989,10(1):85-88

Results of a search for occultations of stars in the SAO catalogue by Saturn’s outer magnetosphere during 1989-1990 are presented. A total of nine events are predicted to occur during this period. The most favourable event will be the occultation of 28 Sgr (SAO 187255) during July 2–3, 1989. Occultations of SAO 187036, SAO 188348 and SAO 188120 occur near opposition and therefore can be observed over a wide longitude range  相似文献   

A paleoclimatic context for the drought of 1999–2001 in the northern Great Plains of North America     

David J Sauchyn  Jennifer Stroich  Antoine Beriault 《The Geographical journal》2003,169(2):158-167

The least annual precipitation in the western interior of North America occurs in the northern Great Plains, including an area that encompasses parts of south-eastern Alberta, south-western Saskatchewan and eastern Montana. During 1999–2001, most climate stations in this region had record low precipitation. This paper examines this three-year drought in the context of historical climate records from Medicine Hat, Alberta and Havre, Montana and summer (June–July) and annual (August–July) precipitation reconstructed from standardized tree-ring widths (residual chronologies) from Pinus contorta (lodgepole pine) sampled in the Cypress Hills of Alberta and Saskatchewan and the Bears Paw Mountains of north-central Montana. Drought is operationally defined as precipitation in the lower 10th and 20th percentiles. Plots of reconstructed precipitation and cumulative departure from median values indicate a shift in climate variability prior to the twentieth century, when EuroCanadians settled in this region. The eighteenth and nineteenth centuries are characterized by sustained periods of progressively wetter and drier conditions, including prolonged drought. Various archival sources document the significant impacts of these prolonged droughts. While drought was frequent in the twentieth century, it tended to be of short duration and the impacts also were ameliorated by intervening periods of relatively high precipitation. Increasing aridity in response to global warming could expose a larger area of the northern Great Plains to the impacts of drought.  相似文献   

August–September Runoff Variation in the Kara Darya River Determined from Juniper (Juniperus turkestanica) Tree Rings in the Pamirs-Alai Mountains, Kyrgyzstan, Back to 1411 CE     

ZHANG Heli  HE Qing  CHEN Feng  Dogdurbek CHONTOEV  Rysbek SATYLKANOV  Bakytbek ERMENBAEV  CHEN Youping 《《地质学报》英文版》2020,94(3):682-689

A 606 year runoff reconstruction of the Kara Darya River was developed, based on the tree-ring width chronology of Turkestan juniper (Juniperus turkestanica) from the Pamir-Alai Mountains of Kyrgyzstan. Preliminary comparison between the snow cover variation and these climate/runoff reconstructions found that Central Asian snow cover may have strong associations with large-scale ocean-atmosphere-land circulations. The runoff reconstruction demonstrated that instrumental runoff was not representative of runoff over the past 606 years. The drought of the 1960s-1990s resulted in low runoff levels during the past 50 years; however, this probably does not represent a worst-case scenario for the Kara Darya because the runoff reconstruction showed additional extremely low runoff prior to the 20th century. The reconstruction will provide a long-term perspective on runoff variation in the Kara Darya River basin, aid sustainable water resource management and be useful in guiding expectations of future variations and water resource planning.  相似文献   

Saturn's F Ring: Kinematics and Particle Sizes from Stellar Occultation Studies     

A.S. Bosh  C.B. OlkinR.G. French  P.D. Nicholson 《Icarus》2002,157(1):57-75

The occultation of GSC5249-01240 by Saturn's rings was observed in a spectrally resolved mode using the Faint Object Spectrograph on the Hubble Space Telescope. By combining these data with other occultation data, we have determined the inclination of the F ring to be 0.0065±0.0014 deg. Our inclined F ring orbit model explains an abrupt decrease in flux at the west ansa of the ring that was observed during the November 1995 ring-plane crossing as well as the nondetection of 1995 S5 during one set of observations in November 1995. The F ring's equivalent depth is found to have no significant dependence on wavelength between 0.27 and 0.74 μm, indicating the presence of a population of ring particles larger in size than ∼10 μm. This contrasts with the results from a previous analysis of Voyager images at various phase angles, in which the particles were determined to be predominantly submicron in size. The difference may be due to temporal or longitudinal variability within the ring.  相似文献   

The Composition of Saturn's Rings     

F. PouletJ.N. Cuzzi 《Icarus》2002,160(2):350-358

A composite spectrum between 0.30 and 4.05 μm of Saturn's rings is analyzed using the Shkuratov scattering theory (Shkuratov et al. 1999, Icarus137, 235-246). Several types of surface and composition are discussed. We demonstrate that both the strong reddening over the interval 0.3-0.7 μm and the water ice absorption features are well reproduced by an intimate (“salt-and-pepper”) mixture of four coarse particles of two different materials: 93% are grains (typical sizes of 10, 200, and 2000 μm) of water ice containing a few percent of refractory organic solid (tholin) impurities within their bulk, and 7% are coarse grains of a dark material (amorphous carbon). The cosmogenic implications of the inferred composition are discussed.  相似文献   

More Microwave Observations of Saturn: Modeling the Ring with a Monte Carlo Radiative Transfer Code     

David E. DunnLawrence A. Molnar  John D. Fix 《Icarus》2002,160(1):132-160

We present a second epoch of Very Large Array Saturn observations taken in February 1997 spanning wavelengths 1.3-21 cm. These observations complement earlier observations at Saturn's autumnal equinox in November 1995. In this epoch, however, we generally have better signal-to-noise ratios and the ring inclination of the present observations was −5.0°, whereas the previous observations were made with ring inclination +2.7°.Our observations confirm the latitudinal structure on the saturnian disk as seen at 2.0, 3.6, and 6.1 cm. We also see some latitudinal structure at 1.3 cm for the first time. The details of this structure have changed dramatically from those reported by I. de Pater and J. R. Dickel (1991, Icarus94, 474-492) for the 1980s and are consistent with those seen in F. van der Tak et al. (1999, Icarus142, 125-147). The most prominent features are a pair of brightness enhancements just inside the edges of the Equatorial Zone.The rings do not show the east-west asymmetry seen in our previous epoch, perhaps indicative of a viewing angle effect on the scattering properties of the rings. The radial trend in brightness in the ansae is generally consistent with that expected from optical depth variations and increasing distance from the source of scattered light. In particular the increased optical depth towards the center of the C ring is evident. Azimuthal variation in brightness in the C ring shows the forward scattering expected of Mie scattering. By contrast, the A and B rings show little or no azimuthal variation.We present Monte Carlo simulations of the ring brightness under the assumptions of isotropic and Mie scattering. These are the first synthetic maps of Saturn which can be directly compared to the images we obtained. Neither model fits all the data well. However, a hybrid model combining isotropic and Mie scattering does fit well. We interpret the consistency with isotropic scattering in the outer rings as an indication that near-field effects may be important. This in turn implies geometrically thin rings, as predicted by dynamical simulations of these rings.  相似文献   

喜马拉雅冷杉和林芝云杉年轮稳定碳同位素气候意义比较   总被引：3，自引：0，他引：3       下载免费PDF全文

陈克龙  刘晓宏 《盐湖研究》2003,11(2):29-34

对采自西藏东南部的喜马拉雅冷杉和林芝云杉交叉定年后 ,进行了过去 1 0 0年的年轮δ13 C分析。结果表明 :冷杉和云杉年轮δ13 C序列记录了大气CO2 浓度升高引起大气CO2 的δ13 C下降的全球性趋势 ,但两者的响应程度不同。冷杉和云杉年轮δ13 C高频变化包含共同的气候环境变化信息 ,但在细节变化上存在差异。在对气候变化的响应上 ,两者记录的信息可以进行相互比较 ,以便利用不同地区不同树种的δ13 C进行大区域气候重建。  相似文献   

Dealing with Uncertainty in Flood Risk Assessment of Dike Rings in the Netherlands   总被引：1，自引：0，他引：1  

Herman Van Der Most  Mark Wehrung 《Natural Hazards》2005,36(1-2):191-206

Current flood protection policies in the Netherlands are based on design water levels. This concept does not allow for a proper evaluation of costs and benefits of flood protection. Hence, research is being carried out on the introduction of a flood risk approach, which looks into both the probability of flooding and the consequences of flooding. This research is being carried out within the framework of a major project called the Floris project (FLOod RISk in the Netherlands). To assess the probability of flooding the Floris project distinguishes different failure modes for dikes and structures within the dike ring. Based on a probabilistic analysis of both loads and resistance the probability of failure is determined for each failure mode. Subsequently the probabilities of failure for different failure modes and dike sections are integrated into an estimate of the probability of flooding of the dike ring as a whole. In addition the Floris project looks into the different consequences of flooding, specifically the economic damages and the number of casualties to be expected in case of flooding of a particular dike ring. The paper describes the approach in the Floris project to assess the flood risk of dike rings in the Netherlands. One of the characteristics of the Floris project is the explicit attention to different types of uncertainties in assessing the probability of flooding. The paper discusses the different starting-points adopted and presents an outline on how the Floris project will deal with uncertainties in the analysis of weak spots in a dike ring as well as in the cost benefit analysis of flood alleviation measures.  相似文献   

树木年轮资料的可靠性分析——以陕西华山松为例   总被引：8，自引：0，他引：8  

吴祥定  邵雪梅 《地理科学进展》1997,16(1):51-56

依据陕西华山的华山松树木年轮宽度、最大密度与最小密度年表，以及对树木生长模拟结果，本文分析了树木径向生长对气候因子（气温和降水）的响应状况。结果表明树木年轮生长的多种指标均能相互印证，并具有生理学意义，从而证实了它们作为气候变化的代用资料是可靠的。这不仅强调了树木年轮多途径分析的必要性，而且为重建过去气候变化提供了依据。  相似文献