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   181篇
  免费   8篇
  国内免费   3篇
测绘学   2篇
大气科学   6篇
地球物理   42篇
地质学   46篇
海洋学   27篇
天文学   59篇
自然地理   10篇
  2021年   2篇
  2019年   5篇
  2018年   2篇
  2017年   5篇
  2016年   4篇
  2015年   3篇
  2014年   6篇
  2013年   5篇
  2012年   3篇
  2011年   3篇
  2010年   5篇
  2009年   4篇
  2008年   6篇
  2007年   13篇
  2006年   9篇
  2005年   13篇
  2004年   7篇
  2003年   11篇
  2002年   5篇
  2001年   5篇
  2000年   1篇
  1999年   6篇
  1998年   5篇
  1997年   3篇
  1996年   6篇
  1994年   2篇
  1993年   5篇
  1992年   4篇
  1990年   2篇
  1988年   3篇
  1987年   2篇
  1986年   3篇
  1985年   4篇
  1984年   6篇
  1983年   3篇
  1982年   1篇
  1981年   4篇
  1980年   1篇
  1979年   6篇
  1978年   1篇
  1977年   1篇
  1976年   1篇
  1974年   1篇
  1972年   1篇
  1971年   3篇
  1970年   1篇
排序方式: 共有192条查询结果,搜索用时 125 毫秒
31.
In the present paper, the intensity of radiation emergent from the atmosphere bounded by a rough surface is discussed with the aid of the superposition method derived by Mukai (1973). The merit of this method is to express the laws of diffuse reflection and transmission for the planetary problem with a rough surface in terms of a scattering and a transmission function for the standard problem.Here the bottom surface is assumed to reflect light in accordance with the slope distribution given by Cox and Munk (1954a, b). The results are discussed in terms of the optical properties and roughness of the bottom surface.  相似文献   
32.
We present the first X-ray observations of the 4 Draconis system, consisting of an M3 III giant with a hot ultraviolet companion. It has been claimed that the companion is itself an AM Her-type binary system, an identification that places strong constraints on the evolution of cataclysmic variables. We find that the X-ray properties of 4 Draconis are consistent with the presence of an accreting white dwarf, but not consistent with the presence of an AM Her system. We conclude that 4 Draconis is therefore most likely a symbiotic binary containing a white dwarf accreting material from the wind of the red giant.
The X-ray spectrum of 4 Draconis is sometimes dominated by partially ionized photoelectric absorption, presumably due to the wind of the red giant. We note that X-ray monitoring of such systems would provide a powerful probe of the wind and mass-loss rate of the giant, and would allow a detailed test of wind accretion models.  相似文献   
33.
Quiescent prominences It is found that Heii 4686 is emitted in the same cold region of 10000 K as hydrogen, metal and neutral helium emission lines. This conclusion is based on the finding that the observed width of 4686 is the same as the calculated width of 4686. The calculated width is derived from the observed widths of hydrogen and metallic lines. The large intensity of Heii 4686 in 10000 K can be explained by the ionization of Heii due to the UV radiation below 228 Å that comes from the corona and the transition region.Loop prominences The very broad width (30 to 50 km s–1) of 4686 for two post-flare loop prominences shows that the Heii line is emitted in hot regions different from regions of hydrogen and metal emission. From the widths of the Balmer lines and many metallic lines the kinetic temperature for one loop is found to be 16000 K in one part and 7600 K in another part. The electron densities are 1012.0 cm–3 and less than 1011.0 cm–3 respectively.Chromosphere The intensity of 4686 in the chromosphere can be interpreted in terms of a temperature of 10000 K with the ionization due to UV radiation. But, since observations of the width of 4686 are not available, a definitive conclusion for the chromosphere cannot be reached.  相似文献   
34.
We have searched for the missing energy flux of the sunspot by measuring white-light photographs over the region around the sunspots exceeding ten times of the diameter of spots. It was found that the excess brightness amounts to 0.3–0.8% of the local continuum intensity immediately beyond the penumbra and smoothly decrease to zero at the edge of our inspected radii of 130 000–160000km. This result suggests that the flux deficit in sunspots of the order of 30% over the umbra-penumbra is roughly the same as the excess flux found in the very large surrounding region which includes photospheric facular contributions. Among nineteen spots fourteen showed this extended bright ring. Narrow band photoelectric scans revealed that the excess radiation originates in the continuum, not from weakened absorption lines.  相似文献   
35.
A new experimental setup for simultaneous P-wave velocity (VP) and density (ρ) measurements for liquid alloys is developed using ultrasonic and X-ray absorption methods combined with X-ray tomography at high pressures and high temperatures. The new setup allows us to directly determine adiabatic bulk moduli (KS) and to discuss the correlation between the VP and ρ of the liquid sample. We measured VP and ρ of liquid Ni68S32 up to 5.6 GPa and 1045 K using this technique. The effect of pressure on the VP and ρ values of liquid Ni68S32 is similar to that of liquid Fe57S43. (Both compositions correspond to near-eutectic ones.) The obtained KS values are well fitted to the finite strain equation with a KS0 value (KS at ambient pressure) of 31.1 GPa and a dKS/dP value of 8.44. The measured VP was found to increase linearly with increasing ρ, as approximated by the relationship: VP [m/s] = 1.29 ρ [kg/m3] – 5726, suggesting that liquid Ni–S follows an empirical linear relationship, Birch's law. The dVP/dρ slope is similar between Ni68S32 and Fe57S43 liquids, while the VPρ plot of liquid Ni–S is markedly different from that of liquid Fe–S, which indicates that the effect of Ni on Birch's law is important for understanding the VPρ relation of planetary and Moon's molten cores.  相似文献   
36.
Data from Equator-S and Geotail are used to study the dynamics of the plasma sheet observed during a substorm with multiple intensifications on 25 April 1998, when both spacecraft were located in the early morning sector (03–04 MLT) at a radial distance of 10–11 RE. In association with the onset of a poleward expansion of the aurora and the westward electrojet in the premidnight and midnight sector, both satellites in the morning sector observed plasma sheet thinning and changes toward a more tail-like field configuration. During the subsequent poleward expansion in a wider local time sector (20−04 MLT), on the other hand, the magnetic field configuration at both satellites changed into a more dipolar configuration and both satellites encountered again the hot plasma sheet. High-speed plasma flows with velocities of up to 600 km/s and lasting 2–5 min were observed in the plasma sheet and near its boundary during this plasma sheet expansion. These high-speed flows included significant dawn-dusk flows and had a shear structure. They may have been produced by an induced electric field at the local dipolarization region and/or by an enhanced pressure gradient associated with the injection in the midnight plasma sheet.  相似文献   
37.
A mammalian dentary discovered in the Coniacian Ashizawa Formation (Fukushima, northeastern Japan) is described. The specimen is a fragment of the horizontal ramus of a left edentulous dentary with five alveoli, the distal four of which are plugged with broken roots. Based on the morphologies of the dentary and the roots, it is considered to be of a therian mammal. This constitutes the first discovery of a Mesozoic mammal in northeastern Japan and highlights the potential for future mammal discoveries in the Cretaceous System in northeastern Japan, which will be significant for disclosure of the mammalian faunal evolution in East Asia during the Late Cretaceous.  相似文献   
38.
Drinking water supplies in Ulaanbaatar, the capital of Mongolia, are completely dependent on groundwater sourced from pumping wells located in an alluvial plain of the Tuul River which flows through Ulaanbaatar. The interaction between groundwater in the alluvial plain and river surface water was investigated using a hydrological and multi-tracers approach. The observed groundwater contour map clearly shows that the Tuul River recharges the floodplain groundwater and groundwater flows from east to west. The similarity of chemical and stable isotopic compositions suggests that groundwater is mainly recharged by Tuul River water in the vicinity of the river. In addition, considering groundwater contours and chemical composition, groundwater in the northern and southern mountain sides contribute to floodplain groundwater. Stable isotopic information suggests that winter season precipitation also contributes to the groundwater, because groundwater in a specific region has a considerably lower isotopic ratio. Using the End Member Mixing Analysis applying oxygen-18, SiO2 and HCO3 - as tracers, the contribution ratios of the Tuul River, groundwater in the northern and southern mountain regions, and winter season precipitation to floodplain groundwater are estimated to be 58% to 85%, 1% to 54%, 0% to 16%, and 0% to 12%, respectively.  相似文献   
39.
An analytical expansion of the disturbing function arising from direct planetary perturbations on the motion of satellites is derived. As a Fourier series, it allows the investigation of the secular effects of these direct perturbations, as well as of every argument present in the perturbation. In particular, we construct an analytical model describing the evection resonance between the longitude of pericenter of the satellite orbit and the longitude of a planet, and study briefly its dynamic. The expansion developed in this paper is valid in the case of planar and circular planetary orbits, but not limited in eccentricity or inclination of the satellite orbit.  相似文献   
40.
The seasonal intensive sampling of gases and particulate matter in ambient air was conducted at the site established in urban area of Japan to study the seasonal difference of the temporal variation of gases and particulate matter concentrations in urban atmosphere as well as to illustrate the different transport regimes that impacts air pollutants. The sample was collected by the four-stage filter-pack method with 6-h interval for one week in four seasons (spring, summer, autumn and winter). The trans-boundary transport of air pollutants with high concentration was characteristically observed in the spring sampling. On the other hand, we could successfully detect the in-country transports of air pollutants in the summer sampling. Four-season’s intensive survey considered, we could show the characteristic transport of air pollutants to provide the episodic high concentration for ambient air in the urban area of Japan, and successfully illustrate the seasonal-dependent transport regimes to impact on air pollutants.  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