首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   81篇
  免费   4篇
  国内免费   2篇
测绘学   1篇
大气科学   2篇
地球物理   34篇
地质学   15篇
海洋学   15篇
天文学   15篇
综合类   4篇
自然地理   1篇
  2020年   1篇
  2019年   1篇
  2018年   2篇
  2017年   5篇
  2016年   6篇
  2014年   2篇
  2013年   3篇
  2012年   2篇
  2011年   3篇
  2010年   4篇
  2009年   3篇
  2008年   3篇
  2007年   1篇
  2006年   6篇
  2005年   6篇
  2004年   4篇
  2003年   2篇
  2002年   3篇
  2000年   2篇
  1998年   5篇
  1994年   5篇
  1993年   1篇
  1992年   1篇
  1989年   1篇
  1985年   2篇
  1984年   4篇
  1983年   2篇
  1981年   1篇
  1979年   1篇
  1977年   2篇
  1976年   1篇
  1975年   1篇
  1970年   1篇
排序方式: 共有87条查询结果,搜索用时 31 毫秒
11.
A detailed analysis of rapid-run magnetograms from Guam (geomagnetic latitude = 4.2°) revealed that there are two kinds of geomagnetic sudden commencement (SC) observed in nighttime. One is the ordinary SC consisting of a main impulse only which has a smooth rise of the H-component. The other is a superposition by a small positive impulse on the very beginning part of the smooth rise of the main impulse and consequently the SC starts with a small stepwise increase of the H-component. The latter type of SC occurs between 20 and 08 h L.T. and its occurrence rate takes the maximum value of about 50% around 03 h L.T. Corresponding magnetograms from a dayside equatorial station (Huancayo, geomagnetic latitude = ?0.7°) were examined and a good correlation was found between the stepwise SC at the nightside (Guam) and SC1 with a preliminary reverse impulse (PRI) at the dayside (Huancayo). Since PRI observed at the dayside equator may be interpreted as an extension of an ionospheric current due to an dusk-to-dawn electric field impressed on the polar ionosphere, our results show that a polar originating ionospheric current can extend to the nightside equator and produce a small but observable magnetic effect in spite of much reduced nighttime ionospheric conductivity.  相似文献   
12.
This report investigates the suggestion that the pattern of plasma convection in the polar cleft region is directly determined by the interplanetary electric field (IEF). Owing to the geometrical properties of the magnetosphere, the East-West component of the IEF will drive field-aligned currents which connect to the ionosphere at points lying on either side of noon, while currents associated with the North-South component of the IEF will connect the two polar caps as sheet currents centered at noon. The effects of the hypothesized IEF driven cleft current systems on polar cap ionospheric plasma convection are investigated through a series of numerical simulations. The simulations demonstrate that this simple electrodynamic model can account for the narrow “throats” of strong dayside antisunward convection observed during periods of southward interplanetary magnetic field (IMF) as well as the sunward convection observed during periods of strongly northward IMF. Thedawn-dusk shift of polar cap convection which is related to the By component of the IMF is also accounted for by the model.  相似文献   
13.
The Sylhet Basin of Bangladesh is a sub-basin of the Bengal Basin. It contains a very thick (up to 22 km) Tertiary stratigraphic succession consisting mainly of sandstones and mudstones. The Sylhet succession is divided into the Jaintia (Paleocene–late Eocene), Barail (late Eocene–early Miocene), Surma (middle–late Miocene), Tipam (late Miocene–Pliocene) and Dupitila Groups (Pliocene–Pleistocene), in ascending order. The origin of the organic matter (OM) and paleoenvironment of deposition have been evaluated on the basis of C, N, S elemental analysis, Rock-Eval pyrolysis and gas chromatography–mass spectrometry (GC–MS) analysis of 60 mudstone samples collected from drill core and surface outcrops. Total organic carbon (TOC) content ranges from 0.11% to 1.56%. Sulfur content is low in most samples. TOC content in the Sylhet succession varies systematically with sedimentation rate, with low TOC caused by clastic dilution produced by high sedimentation rates arising from rapid uplift and erosion of the Himalaya.The OM in the succession is characterized by systematic variations in pristane/phytane (Pr/Ph), oleanane/C30 hopane, n-C29/n-C19 alkane, Tm/Ts [17α(H)-22,29,30-trisnorhopane/18α(H)-22,29,30-trisnorhopane] and sterane C29/(C27 + C28 + C29) ratios during the middle Eocene to Pleistocene. Based on biomarker proxies, the depositional environment of the Sylhet succession can be divided into three phases. In the first (middle Eocene to early Miocene), deposition occurred completely in seawater-dominated oxic conditions, with abundant input of terrestrial higher plants, including angiosperms. The second phase (middle to late Miocene) consisted of mainly freshwater anoxic conditions along with a small seawater influence according to eustasic sea level change, with diluted OM derived from phytoplankton and a lesser influence from terrestrial higher plants. Oxygen-poor freshwater conditions prevailed in the third phase (post-late Miocene). Planktonic OM was relatively abundant in this stage, while a high angiosperm influx prevailed at times. Tmax values of ca. 450 °C, vitrinite reflectance (Ro) of ca. 0.66% and methylphenanthrene index (MPI 3) of ca. 1 indicate the OM to be mature. The lower part (middle Eocene to early Miocene) of the succession with moderate TOC content and predominantly terrestrial OM could have generated some condensates and oils in and around the study area.  相似文献   
14.
To determine the effects of long-range transport of aerosols from an upwind area in East Asia to a downwind area in Japan, we chemically analyzed aerosols collected simultaneously on Tuoji Island (Shandong Province, China), Fukue Island (Nagasaki Prefecture, Japan), and Cape Hedo (Okinawa Prefecture, Japan). We focused on changes in the metallic composition of PM2.5 aerosols during long-range transport. The average mass concentrations of PM2.5 at the three sites decreased in the order Tuoji Island > Fukue Island ≈ Cape Hedo (48.3 ± 4.5, 13.9 ± 1.5, and 13.2 ± 0.9 μg/m3, respectively). The fraction of coarse particles in total suspended particles estimated by (1–PM2.5/TSP) was highest on Cape Hedo, indicating that the contribution of sea salts was increased by long-range transport of the aerosols over the ocean. Enrichment factor analysis revealed that at all three sites, Al, K, Ca, Mn, Fe, Co, Sr, and Ba originated from soil; whereas Cr, Ni, Cu, Zn, As, Mo, Ag, Cd, Sn, Sb, Tl, and P appeared to be of anthropogenic origin. Na was the most abundant element on Cape Hedo, indicating the addition of sea salts during aerosol transport. The V concentration was highest at Fukue Island, which was ascribed to V emission from ships. Sixty-one percent of the V on Fukue Island and 62% of the V on Cape Hedo were determined to have originated from ships, implicating of data obtained on dates during which backward trajectory analysis indicated that the same air mass passed over Tuoji Island, Fukue Island, and Cape Hedo in that order.  相似文献   
15.
This paper presents an experimental study on the performance of a shear-sliding stud-type damper composed of multiple friction units with high-tension bolts and disc springs. A numerical evaluation of the response reduction effects achieved by the stud-type damper is also presented. In dynamic loading tests, the behavior of stud-type multiunit friction damper specimens was investigated. Three different full-scale damper specimens, which were composed of five, six, or seven friction units with two or four sliding surfaces, were incorporated into loading devices for testing. The stud-type friction dampers demonstrated stable rigid-plastic hysteresis loops without any remarkable decrease in the sliding force even when subjected to repetitive loading, in addition to showing no unstable behavior such as lateral buckling. The damper produced a total sliding force approximately proportional to the number of sliding surfaces and friction units. The total sliding force of the stud-type damper can thus be estimated by summing the contributions of each friction unit. In an earthquake response simulation, the control effects achieved by stud-type dampers incorporated into an analytical high-rise building model under various input waves, including long-period, long-duration and pulse-like ground motions, were evaluated. A satisfactory response reduction was obtained by installing the developed stud-type dampers into the main frame without negatively impacting usability and convenience in terms of building planning.  相似文献   
16.
We calculate electric fields and currents in the ionosphere during the solar eclipse. Three dimensional anisotropic and inhomogeneous conductivity distribution is taken into consideration. Electrical conductivity is assumed to be decreased by the solar eclipse, but winds are not altered. It is shown that the additional currents and fields due to the eclipse are essentially understood as those generated by the obstruction to Sq currents. However, the currents flow preferably in the region nearer to the noon meridian and at the Equator and partly escape through the field-aligned currents to the opposite hemisphere, and therefore the effect of the solar eclipse is also seen in the opposite hemisphere in the deformed shape.  相似文献   
17.
18.
Three dimensional electric fields were measured at the altitude of about 27 km in the stratosphere over the Pacific Ocean about 200–400 km away from the Sanriku coast of Honsyu Island (L = 1·4) on 16–17 October 1973, which was magnetically disturbed. The average horizontal electric field thus measured is about 10 mV/m, and the electric field vectors made clockwise semidiurnal rotations rather than diurnal. Daily variation of this electric field was compared with data at L = 2·7–3·5 published by Mozer (1973) and was found to be very similar. This suggests that these electric fields are of common origin in the plasmasphere. From their mean daily variation it is estimated that the plasmaspheric convection is decreased in the night side and is increased in the day side by 200–300 m/sec, and there is an outward flow in the first half of the afternoon and an inward flow in the plasma bulge region of about 500 m/sec.  相似文献   
19.
Heat balance of the upper 200 m of the sea south of Japan is studied, by the use of marine meteorological and oceanographic data at Ocean Weather Station T (29°N, 135°E), intensively obtained from June 1950 to November 1953. Local time change of the heat content in the surface layer and the net heat flux through the air-sea interface are calculated directly from these data, and the heat convergence in the sea is estimated from their residuals. Regarding the relative importance of one- and three-dimensional processes, it is found that, on a time scale of a few days to one month, the variation of heat content depends on heat convergence in the sea, while on a seasonal time scale, the heat content is determined primarily by the heat flux through the sea surface in December through February, by heat convergence within the sea from March to May, and by both processes from June to November. It is inferred that the heat convergence in the sea is caused by advection of water masses which are bounded by sharp fronts. Spectral analysis of sea surface temperature indicates that they typically take 2 to 3 days to pass the station, and their typical size is estimated as around 20 km by assuming the typical advection velocity of water masses to be 10 cm s?1.  相似文献   
20.
Data of a comprehensive laboratory study on the coexistent system of wind waves and opposing swell (Mitsuyasu and Yoshida, 1989) have been reanalyzed to clarify the air-sea interaction phenomena under the coexistence of wind waves and swell. It is shown that the magnitude of the decay rate of swell due to an opposing wind is almost the same as that of the growth rate of swell caused by a following wind, as measured by Mitsuyasu and Honda (1982). The decay rate is much smaller than that obtained recently by Peirson et al. (2003), but the reason for the disagreement is not clear at present. The effect of an opposing swell on wind waves is very different from that of a following swell; wind waves are intensified by an opposing swell while they are attenuated by a following one. The phenomenon contradicts the model of Phillips and Banner (1974), but the reason for this is not clear at this time. The high-frequency spectrum of wind waves shows a small increase of the spectral density. Wind shear stress increases a little due to the effect of opposing swell. The intensification of wind waves by opposing swell and the small increase of the spectral density in a high-frequency region can be attributed to the increase of wind shear stress. Such organized phenomena lead to the conclusion that the hypothesis of local equilibrium for pure wind waves (Toba, 1972) can also be satisfied for wind waves that coexist with opposing swell. The recent finding of Hanson and Phillips (1999) can be explained by this mechanism.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号