全文获取类型
收费全文 | 79071篇 |
免费 | 1073篇 |
国内免费 | 707篇 |
专业分类
测绘学 | 1909篇 |
大气科学 | 4992篇 |
地球物理 | 15138篇 |
地质学 | 28848篇 |
海洋学 | 7286篇 |
天文学 | 19017篇 |
综合类 | 267篇 |
自然地理 | 3394篇 |
出版年
2022年 | 666篇 |
2021年 | 1037篇 |
2020年 | 1123篇 |
2019年 | 1218篇 |
2018年 | 2628篇 |
2017年 | 2474篇 |
2016年 | 2895篇 |
2015年 | 1413篇 |
2014年 | 2679篇 |
2013年 | 4183篇 |
2012年 | 2926篇 |
2011年 | 3636篇 |
2010年 | 3298篇 |
2009年 | 4038篇 |
2008年 | 3527篇 |
2007年 | 3693篇 |
2006年 | 3375篇 |
2005年 | 2252篇 |
2004年 | 2222篇 |
2003年 | 2122篇 |
2002年 | 2014篇 |
2001年 | 1854篇 |
2000年 | 1696篇 |
1999年 | 1297篇 |
1998年 | 1340篇 |
1997年 | 1288篇 |
1996年 | 1064篇 |
1995年 | 1067篇 |
1994年 | 937篇 |
1993年 | 811篇 |
1992年 | 806篇 |
1991年 | 815篇 |
1990年 | 877篇 |
1989年 | 689篇 |
1988年 | 712篇 |
1987年 | 733篇 |
1986年 | 640篇 |
1985年 | 844篇 |
1984年 | 910篇 |
1983年 | 807篇 |
1982年 | 773篇 |
1981年 | 684篇 |
1980年 | 685篇 |
1979年 | 633篇 |
1978年 | 649篇 |
1977年 | 531篇 |
1976年 | 495篇 |
1975年 | 504篇 |
1974年 | 450篇 |
1973年 | 507篇 |
排序方式: 共有10000条查询结果,搜索用时 468 毫秒
281.
Seismic reflection profiles and long- and medium-range sidescan sonar were used to investigate a salt diapir complex and area of slope instability near the base of the Continental Slope off North Carolina. Within the area of investigation three diapirs are bounded on their upslope side by a scarp 60 m high and 50 km long. The slope above the scarp is characterized by a series of shallow rotational normal faults. The bottom below the scarp is furrowed by slide tracks, which were probably carved by large blocks that broke off the scarp face and slid downslope leaving rubble and scree lobes.Extensive slumping in this area appears to be a result of uplift and faulting associated with salt intrusion, which has fractured and oversteepened the slope leading to instability and failure. Sharply defined slide tracks suggest that slope failure above the breached diapir complex is a continuing process, in contrast to much of the surrounding slope area where few instability features were observed. 相似文献
282.
283.
An analysis previously developed for rough surface scattering with narrow-beam reception is extended to wide-beam or omnidirectional reception. The source is considered to be a pulsed electric dipole. The analysis includes multipathing effects on the received signal. The results are used to develop a model for the backscattered radar cross section of the ocean surface, up to a second-order approximation. 相似文献
284.
285.
S.I. Akasofu 《Planetary and Space Science》1985,33(1):81-87
We examine time variations of the total magnetospheric output UT and the two geomagnetic indices AE and Dst during the magnetospheric storm of 31 December 1967–1 January 1968. A unique feature of this particular storm is that the solar-wind magnetosphere dynamo power ε was nearly constant for about 24 h by maintaining a high value of ~1019 erg s?1. It is found that UT was also nearly constant during that period, indicating that the magnetosphere is primarily a directly driven system. However, during an early epoch of the storm, the electrojet intensity levelled off, while the ring current began to grow during the same epoch. Subsequently, there occurred a sudden surge of the electrojet intensity, while the growth of the ring-current levelled off. Later variations of both the AE and Dst indices were very complex. After the surge, the AE index continued to decline and became as low as ~250 nT during the maximum epoch of this major storm (when the Dst decrease attained the maximum values). This trend can also be seen in many other major storms, but is often masked by changes of ε. One possible cause for such features for ε ? 1019erg/s?1 is that the neutral wind is generated by the motion in the lower ionosphere, reducing the electrojet intensity and enhancing the ring-current particle-injection rate. 相似文献
286.
The accuracy of the AE index as a measure of the joule heat production rate is examined for a typical substorm event on 18 March 1978 by estimating the global joule heat production rate, as a function of time, using data obtained from the IMS six meridian chains. In spite of the fact that the AE index had had an initial slow growth which was followed by a rapid growth, the joule heat production rate attained a high level during the slow growth and thus their variations were considerably different from those of the AE index. Therefore, although the AE index is statistically linearly related to the global joule heat production rate, one should be cautious in assuming that details of time variations of the AE index during individual events represent those of the joule heat production rate. 相似文献
287.
The first year of sodium nightglow observations from Natal (6°S, 35°W) are examined. Time variations appear to follow a pattern of their own, different from low latitude results. The major seasonal peak occurs in September-October and the average variation during the night decreases from dusk to dawn. Statistics on cloud coverage show that Natal has roughly only about 3 clear hours per night. The best observing period is April with an average of 5 clear hours per night. 相似文献
288.
We investigate the evolution of rotation period and spindown age of a pulsar whose surface magnetic field undergoes a phase
of growth. Application of these results to the Crab pulsar strongly indicates that its parameters cannot be accounted for
by the field growth theories. 相似文献
289.
N. S. Chauhan 《Astrophysics and Space Science》1985,112(1):185-192
The treatment followed by Coxet al. (1973) for the estimation of the effects of thermal imbalance on the pulsational stability of stars is applied to Roche and point source (Chandrika Prashad) model. The calculations suggest that the eigenvalues obtained for the above model are correct to 0(t
ff/ts). The expressions for stability coefficients and corresponding eigenfunctions for the stars in thermal imbalance for the above models have also been derived. 相似文献
290.
P.S. Rainbow 《Estuarine, Coastal and Shelf Science》1985,21(5):669-686
The crab Carcinus maenas (L.) and the barnacle Elminius modestus Darwin were exposed to a range of dissolved concentrations of Zn, Cu and Cd for 21 days in artificial seawater. Accumulation of Zn and Cu by crabs has been interpreted in terms of the presence of a regulation mechanism to maintain constant body concentrations (83·2 ± 19·4 μg Zn g?1 dry wt.; 39·8 ± 9·8 μg Cu g?1 dry wt.) under varying external dissolved metal levels, until a threshold dissolved metal concentration (c. 400 μg Zn l?1; c. 170 μg Cu l?1) beyond which net accumulation of metal begins. Cadium appears to be accumulated by C. maenas at all exposures with no evidence for regulation of body cadmium concentrations. Exposure of E. modestus to Zn, Cu or Cd caused net accumulation of the respective metal in the bodies of the barnacles, with no evidence for regulation of body metal concentrations. 相似文献