全文获取类型
收费全文 | 869篇 |
免费 | 47篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 23篇 |
大气科学 | 46篇 |
地球物理 | 238篇 |
地质学 | 218篇 |
海洋学 | 61篇 |
天文学 | 164篇 |
自然地理 | 179篇 |
出版年
2021年 | 10篇 |
2020年 | 9篇 |
2019年 | 15篇 |
2018年 | 15篇 |
2017年 | 13篇 |
2016年 | 27篇 |
2015年 | 19篇 |
2014年 | 17篇 |
2013年 | 44篇 |
2012年 | 24篇 |
2011年 | 31篇 |
2010年 | 19篇 |
2009年 | 47篇 |
2008年 | 42篇 |
2007年 | 33篇 |
2006年 | 32篇 |
2005年 | 32篇 |
2004年 | 43篇 |
2003年 | 27篇 |
2002年 | 39篇 |
2001年 | 29篇 |
2000年 | 18篇 |
1999年 | 13篇 |
1998年 | 21篇 |
1997年 | 21篇 |
1996年 | 16篇 |
1995年 | 9篇 |
1994年 | 14篇 |
1993年 | 12篇 |
1992年 | 10篇 |
1991年 | 24篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 10篇 |
1986年 | 7篇 |
1985年 | 16篇 |
1984年 | 16篇 |
1983年 | 10篇 |
1982年 | 18篇 |
1981年 | 13篇 |
1980年 | 5篇 |
1979年 | 11篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1975年 | 7篇 |
1974年 | 6篇 |
1973年 | 9篇 |
1971年 | 5篇 |
1970年 | 4篇 |
排序方式: 共有929条查询结果,搜索用时 31 毫秒
911.
912.
David C. Booth Stuart Crampin Russ Evans Graham Roberts 《Geophysical Journal International》1985,83(1):61-73
Summary. The Turkish Dilatancy Projects (TDP1 in 1979 and TDP2 in 1980) recorded small earthquakes near the North Anatolian Fault with closely-spaced networks of three-component seismometers in order to investigate the possibility of diagnosing dilatancy from its effects of shear-wave propagation. This paper examines the polarizations of shear wavetrains recorded in the shear-wave window immediately above the earthquake foci. Abrupt changes in the orientation and/or ellipticity of the shear-wave polarizations are almost always observed during the first few cycles following the initial shear-wave arrival on each seismogram. The horizontal projections of the polarizations of the first shear-wave arrivals at any given station show nearly parallel alignments with approximately the same orientations at each of the recording sites (with one exception). It is difficult to explain this uniform alignment over a wide area in terms of scattering at the irregular surface topography or by earthquake focal mechanisms. We demonstrate that the shear-wave splitting is likely to be the result of anisotropy in the region above the earthquake foci, which could produce polarizations displaying the observed alignments. The temporal change of the azimuth of alignment, observed at one locality between 1979 and 1980, may be due to the release of a local stress anomaly by a very near earthquake. 相似文献
913.
914.
Donald R. Davis Clark R. Chapman Stuart J. Weidenschilling Richard Greenberg 《Icarus》1985,62(1):30-53
Collisional evolution studies of asteroids indicate that the initial asteroid population at the time mean collisional velocities were pumped up to ~5 km/sec was only modestly larger than it is today; i.e., the asteroid belt was already depleted relative to the mean surface density elsewhere in the planetary region. Numerical simulations of the collisional evolution of hypothetical initial asteroid populations have been run, subject to three constraints: they must (a) evolve to the present observed asteroid size distribution, (b) preserve Vesta's basaltic crust, and (c) produce at least the observed number of major Hirayama families. A “runaway growth” initial asteroid population distribution is found to best satisfy these constraints. A new model is presented for calculating the fragmental size distribution for the disruption of large, gravitationally bound bodies in which the material strength is increased by hydrostatic self-compression. This model predicts that large asteroid behave as intrinsically strong bodies, even if they have had a history of being collisionally fractured. This model, when applied to the breakup of the Themis and Eos family parent bodies, gives size distributions in reasonably good agreement with those observed. 相似文献
915.
A coupled neutral-ionic photochemical model has been used to interpret the ionic composition of the Venusian dayside ionosphere measured by the orbiter retarding potential analyser (ORPA) experiment on board the NASA Pioneer-Venus orbiter spacecraft. The electron and ion temperatures also measured by the ORPA are used for calculating the plasma-diffusion coefficients and scale heights for ions. The neutral temperature profiles and the densities of neutral constituents, particularly CO2 and O, play key roles in the determination of the height profiles of the ionic constituents. All these quantities vary substantially in the Venusian thermosphere near the terminator; the models presented are representatives of the solar zenith angle ~65°. The predicted O2+ densities below ~200km agree particularly well with observations by the ORPA, but the model values are significantly less than those measured by the orbiter ion mass spectrometer (OIMS). Models predict much smaller densities than observed values for all molecular ions above ~200km. The reason for the turn-up trend of the concentration gradient of molecular ions observed at these heights by both ORPA and OIMS is unknown. One of the models can predict O+ ion densities above ~200km compatible with observations, if an effective vertical escape flux (φ108cm?2sec?1) is assumed at the ionopause. The neutral air density required to explain the observed ion composition is about 1.4 times larger than the values measured by the orbiter neutral mass spectrometer (ONMS). 相似文献
916.
917.
918.
919.
Starting with the average actual distribution of ozone (Dütsch [15]) and temperature in the stratosphere, we have calculated the solar intensity as a function of wavelength and the instantaneous rates (molecules cm–3 sec–1) for each Chapman reaction and for each of several reactions of the oxides of nitrogen. The calculation is similar to that ofBrewer andWilson [5]. These reaction rates were calculated independently in each volume element in spherical polar coordinates defined by R=1 km from zero to 50, =5° latitude, and ø=15° longitude (thus including day and night conditions). Calculations were made for two times: summer-winter (January 15) and spring-fall (March 22). As input data we take observed solar intensities (Ackerman [1]) and observed, critically evaluated. constants for elementary chemical and photochemical reactions; no adjustable parameters are employed. (These are not photochemical equilibrium calculations.) According to the Chapman model, the instantaneous, integrated, world-wide rate of formation of ozone from sunlight is about five times faster than the rate of ozone destruction, and locally (lower tropical stratosphere) the rate of ozone formation exceeds the rate of destruction by a factors as great as 1000. The global rates of increase of ozone are more than 50 times faster thanBrewer andWilson's [5] estimate of the average annual transfer rate of ozone to the troposphere. The rate constants of the Chapman reactions are believed to be well-enough known that it is highly improbable that these discrepancies are, due to erroneous rate constants. It is concluded that something else besides neutral oxygen species is very important in stratospheric ozone photochemistry. The inclusion of a uniform concentration of the oxides of nitrogen (NOx as, NO and NO2) averaging 6.6×10–9 mole fraction gives a balance between global ozone formation and destruction rates. The inclusion of a uniform mole fraction of NOx at 28×10–9 also gives a global balance. These calculations support the hypethesis (Crutzen [10],Johnston [24]) that the oxides of nitrogen are the most important factor in the global, natural ozone balance. Several authors have recently evaluated the natural source strength of NOx in the stratosphere; the projected fleets of supersonic transports would constitute an artificial source of NOx about equal to the natural value, thus promising more or less to double an active natural stratospheric ingredient. 相似文献
920.
Summary An empirical investigation has been conducted on the practical methods of obtaining the spectra of micropulsation activity, and the interpretational limits which can be applied to the detailed use of spectra computed by the Power Spectral Density (PSD) method. Results indicate that sufficiently high resolution may be obtained reliably from the PSD method for it to be regarded as a useful improvement over handscaling methods. The limitation is shown to remain in the selection of events rather than in computational or instrumental properties. 相似文献