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排序方式: 共有272条查询结果,搜索用时 31 毫秒
1.
Frdric Zagury 《New Astronomy》2001,6(8):345-486
This paper examines the relationship between spectra of stars of same spectral type with extremely low reddenings. According to the standard theory, the relationship between the spectrum of stars with same spectral type and small, but different reddenings should be different in the optical and in the UV. This difference is not observed: the ratio of the spectra of two stars in directions where the reddening is large enough to be detected and low enough not to give a noticeable 2200 Å bump is an exponential of 1/λ from the near-infrared to the far-UV. This result is in conformity with the ideas introduced in preceding papers: the exponential optical extinction extends to the UV, and the spectrum of stars with enough reddening is contaminated by light scattered at close angular distance from the stars. An application will be the determination of the spectrum of a non-reddened star from the spectrum of a star of same spectral type with little reddening. 相似文献
2.
The characteristics of Asian dust events in Northeast Asia during the springtime from 1993 to 2004 总被引:3,自引:1,他引:2
The characteristics of Asian dust events that occurred in Northeast Asia during the springtime from 1993 to 2004 are investigated using 3-hourly SYNOP reports (World Meteorological Organization). Occurrences of blowing sand and dust storm are low in 1997 and 1999, but have increased rapidly since 2000. The maximum occurrence was recorded in 2002. Wind velocity of 6.5 m s− 1 as a threshold wind velocity is not so exactly consistent with the occurrence of blowing sand. However, wind velocity of 14 m s− 1 as a strong wind causing dust storm had similar tendency to those of dust storm and Dust Storm Index.Source regions of Asian dust are divided into three regions (A: dry arid, B: semi-arid, and C: cultivated), based upon the occurrence of blowing sand and dust storm. Eight meteorological stations are selected in three regions, which have frequent occurrences of blowing sand. Source regions of Asian dust that affect the Korean peninsula are gradually extending eastward. Positive anomalies of NDVI occurred in 1994, 1995, and 1998 when temperature was high and precipitation was heavy. However, the frequent occurrence of the dust phenomena is not always consistent with lots of vegetation, high temperature, and much precipitation in this study. 相似文献
3.
Lightning discharge generated in the protoplanetary nebula is viewed as a temporally isolated surge in the flow of electrically charged particles, similar to that of terrestrial lightning. If the current is intense enough, a powerful circular impulse magnetic field is generated around the instantaneous virtual electric conductor. Such magnetic field is capable of magnetizing dust grains containing ferromagnetic components present in its vicinity to their saturation levels. As a result, dust grains attract one another, forming the aggregates. This magnetically driven attraction suggests an important process possibly operational at an early stage of the planetary accretion. Based on both a classical model for electric conductor, and the theory of Lienard–Wiechert electromagnetic potentials, our calculations show that the magnetic impulse due to a discharge channel of a few cm in diameter transferring a charge of about 104 electrons reaches as high as 10 T. At these magnetic fields, the ferromagnetic dust grains, and possibly the already-formed larger aggregates as well, are easily magnetized to the saturation levels, producing compact clusters exhibiting permanent magnetic moments. 相似文献
4.
We have used the ESO 10 m camera, TIMMI, to image with a very high angular resolution (PFoV: 0.3; FWHM:0.9) several main-sequence star disk candidates. Information on the -Pictoris dust disk has been obtained in a region largely inaccessible to previous observations: 0–80 AU, with a resolution of about 5 AU after deconvolution. Another promising target for 10 m imaging, 51 Ophiuchi, appears point-like.based on data collected at the European Southern Observatory (ESO), La Silla, Chile 相似文献
5.
The temporary capture of the dust grains in the exterior resonances with planets is studied in the frames of the planar circular three-body problem with Poynting-Robertson (PR) drag. For the Earth and particles ~ 10 m the resonances 4/5, 5/6, 6/7, 7/8 are shown to be most effective. The capture is only temporary (of order 105 years) and the position of resonance may be calculated from semi-analytical model using averaged disturbing function. These semi-analytical results are confirmed by numerical integration. For various planet this picture changes as with increasing planetary mass the more exterior resonances become more important. We showed that for Jupiter (at least in the space between Jupiter and Saturn) the resonance 1/2 plays the dominant role. The capture time is here several myr but again eccentricity is evolving to eccentricity e
0 ~ 0.48 of libration point for this resonance. 相似文献
6.
The degree to which dust emissions are controlled by geomorphic conditions, wind environments and land use was investigated using the dust storm frequency (DSF) and data from more than 300 meteorological stations throughout northern China. Our analysis showed that most dust emissions originated in gobi deserts that developed in piedmont alluvial fans of the Kunlun, Qilian and Helan mountains. Dust emissions are low from other gobi desert regions, such as the northern Gurbantunggut and eastern Taklimakan, where high vegetation coverage restrained dust emissions or where dust-size particles are not abundant after a long period of strong wind erosion. Sandy deserts with relatively high vegetation coverage or an extensive cover by mobile sands are not a major dust source. Although the highest dust emissions did not appear in regions with the highest wind energy, DSF trends in each region from 1960 to 2003 were closely related to local wind activity. DSF was low in regions with high levels of human activity, where the mean DSF from 1960 to 2003 did not exceed 4 days/year; even from the 1960s to the early 1970s, the period with the greatest DSF, frequency did not exceed 8 days/year, which indicates that extensive land use did not contribute to DSF. The low DSF in these areas might result from the fact that although land use could produce abundant fine soil fractions, vegetation coverage and soil moisture remained higher than in the gobi deserts of arid China, thereby decreasing dust-storm occurrence. 相似文献
7.
Georg Schettler Jens Mingram Jörg F. W. Negendank Liu Jiaqi 《Journal of Paleolimnology》2006,35(2):271-288
Palaeovariations of the atmospheric 210Pb flux in Northeast China (Long Gang area, Jilin province), quantified by high resolution 210Pb measurements on seasonally laminated sediments of Lake Sihailongwan are presented on decadal scale. The mean flux of unsupported
210Pb between 1790 and 1970 for the centre of the lake basin is 517 Bq m−2 yr−1 with maximum deviations between −23% and +27% of this value. Flux rates above this average were found between 1783 and 1813,
around 1836, and between 1860 and 1901. The mean 210Pbexc flux rates derived were clearly lower around 1821, 1908, 1930, and 1953. The 210Pbexc flux reached its minimum of 344 Bq m−2 yr−1 during the period 1977–1982. The atmospheric flux of unsupported 210Pb is correlated with the precipitation frequency during the summer monsoon and shows coinciding variability with geochemical
proxies that document the groundwater inflow into the lake. Al2O3-rich dust of remote provenance scavenged by wet-deposition in the rainy season is the major carrier of the atmospheric 210Pbexc flux. 相似文献
8.
Georg Schettler Qiang Liu Jens Mingram Jörg F. W. Negendank 《Journal of Paleolimnology》2006,35(2):239-270
The varved sediments of the dimictic Lake Sihailongwan (Long Gang mountain area, Jilin Province, Northeast China) represent
a palaeoclimatic archive which documents the local precipitation frequency during the summer monsoon, and variations in the
aeolian flux of dust with their remote sources in the arid and semi-arid regions of inner Asia. Based on a detailed discussion
of sediment genesis in Lake SHL, dust flux rates and palaeohydrological conditions were reconstructed on a decadal scale over
the past 220 years. The aeolian influx by dry and wet deposition was quantified and characterised in its chemical composition.
Photosynthetic production in the lake is positively correlated with the inflow of nutrient-rich groundwater. The groundwater
discharge largely reflects the strength of the summer monsoon. Net accumulation rates for biogenic silica were determined
for annually laminated sediments from the centre of the U-shaped lake basin based on sediment data. In a Si-balance model
of the modern lake, the depositional flux of biogenic silica could be independently quantified on the base of hydrochemical
monitoring data. Comparison of the both estimates allowed to asses the focussing of the particle flux in the lake. Though
water retention in Lake SHL is rather high (ca. 30 years), changes in the hydrological conditions are sensitively recorded
in the sediments because (i) nutrient-rich groundwater discharges into the productive zone of the lake, (ii) a substantial
proportion of the total dissolved Si-inventory of the mixed lake (ca. 30%) is annually consumed by diatom growth, and (3)
sediment accumulation is substantially focussed towards the flat bottom of the lake basin. The bulk siliciclastic sediment
fraction (ca. 75 wt.%) largely originates from influx of dust of remote provenance. In sediment thin-sections, the dry-deposited
dust fraction is microscopically identifiable as seasonal silt layer. Aeolian input by wet-deposition shows a distinctly higher
variability than the influx of dust by dry-deposition. As diatom production, wet-deposition of dust is positively correlated
with the rainfall during the summer monsoon. The inferred positive correlation between rainfall and dust flux during the summer
monsoon implies that dust deposition is determined by the out-wash efficiency of mineral particles for a permanent high atmospheric
dust concentration over Northeast China in the last 220 years. 相似文献
9.
10.
腾格里沙漠东南缘降尘粒度特征和沉积速率 总被引:15,自引:9,他引:15
从多年降尘实测资料入手,分析了粉尘粒度特征、沉积速率和时变过程。提出了气象要素相关估测模型.确定0.25mm为本区粉尘沉积粒径上限。周围沙漠是粉尘沉积细砂部分的尘源,粉砂和粘土部分沉积来自一个较大的时空范围,二者几乎各占降尘量的1/2。研究区年平均粉尘沉积速率4358kg/hm2,每年5月的季风转期具有年内最大沉积速率。 相似文献