全文获取类型
收费全文 | 30633篇 |
免费 | 564篇 |
国内免费 | 221篇 |
专业分类
测绘学 | 792篇 |
大气科学 | 2460篇 |
地球物理 | 6038篇 |
地质学 | 11228篇 |
海洋学 | 2590篇 |
天文学 | 6402篇 |
综合类 | 67篇 |
自然地理 | 1841篇 |
出版年
2021年 | 287篇 |
2020年 | 319篇 |
2019年 | 317篇 |
2018年 | 784篇 |
2017年 | 771篇 |
2016年 | 908篇 |
2015年 | 526篇 |
2014年 | 865篇 |
2013年 | 1665篇 |
2012年 | 960篇 |
2011年 | 1277篇 |
2010年 | 1110篇 |
2009年 | 1420篇 |
2008年 | 1225篇 |
2007年 | 1218篇 |
2006年 | 1211篇 |
2005年 | 864篇 |
2004年 | 860篇 |
2003年 | 794篇 |
2002年 | 793篇 |
2001年 | 726篇 |
2000年 | 697篇 |
1999年 | 537篇 |
1998年 | 510篇 |
1997年 | 597篇 |
1996年 | 460篇 |
1995年 | 493篇 |
1994年 | 485篇 |
1993年 | 387篇 |
1992年 | 392篇 |
1991年 | 369篇 |
1990年 | 375篇 |
1989年 | 333篇 |
1988年 | 344篇 |
1987年 | 370篇 |
1986年 | 326篇 |
1985年 | 422篇 |
1984年 | 423篇 |
1983年 | 458篇 |
1982年 | 423篇 |
1981年 | 366篇 |
1980年 | 406篇 |
1979年 | 332篇 |
1978年 | 315篇 |
1977年 | 304篇 |
1976年 | 271篇 |
1975年 | 271篇 |
1974年 | 279篇 |
1973年 | 271篇 |
1971年 | 182篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
L. L. Sokolov N. A. Petrov A. A. Vasil’ev G. A. Kuteeva A. S. Shmyrov B. B. Eskin 《Solar System Research》2018,52(4):338-346
The possibilities of deflecting an asteroid from its collision course with the Earth by changing its velocity with an impact are considered. Using the asteroid Apophis as an example, the time dependence of the positions and sizes of the keyholes leading to collision is studied. It has been found that the possibility of deflecting this asteroid usually exists, and the impact can be accomplished in principle, given the capabilities of modern space technology. A change in the velocity should be performed before the encounter of 2029 in order to use the gravitational maneuver effect. The possible accuracy of determining Apophis’ orbit and the keyholes that lead to collision and are associated with the resonance returns are considered. 相似文献
33.
34.
35.
In this paper, we have constructed the cosmological model of the universe in f(R, T) theory of gravity in a Bianchi type \(\mathrm{VI}_h\) universe for the functional f(R, T) in the form \(f(R,T)=\mu R+\mu T\), where R and T are respectively Ricci scalar and trace of energy momentum tensor and \(\mu \) is a constant. We have made use of the hyperbolic scale factor to find the physical parameters and metric potentials defined in the space-time. The physical parameters are constrained from different representative values to build up a realistic cosmological model aligned with the observational behaviour. The state finder diagnostic pair is found to be in the acceptable range. The energy conditions of the model are also studied. 相似文献
36.
N. P. Danilkin S. V. Zhuravlev N. G. Kotonaeva V. B. Lapshin I. V. Romanov M. Y. Filippov E. N. Khotenko G. A. Zhbankov 《Solar System Research》2018,52(7):684-690
The satellite ionosondes in highly elliptical orbits are proposed to be used for the task of continuous monitoring of the Arctic ionosphere. The monitoring scheme with the ionosonde location aboard the Arktika-M satellites is presented. The calculations of the vertical topside and transionospheric sounding using the SIMP1 ionospheric model are performed, which show the feasibility of continuous monitoring. 相似文献
37.
Elisa Maria Alessi Giulia Schettino Alessandro Rossi Giovanni B. Valsecchi 《Celestial Mechanics and Dynamical Astronomy》2018,130(5):34
We present the main findings of a dynamical mapping performed in the Low Earth Orbit region. The results were obtained by propagating an extended grid of initial conditions, considering two different epochs and area-to-mass ratios, by means of a singly averaged numerical propagator. It turns out that dynamical resonances associated with high-degree geopotential harmonics, lunisolar perturbations and Solar radiation pressure can open natural deorbiting highways. For area-to-mass ratios typical of the orbiting intact objects, these corridors can be exploited only in combination with the action exerted by the atmospheric drag. For satellites equipped with an area augmentation device, we show the boundary of application of the drag, and where the Solar radiation pressure can be exploited. 相似文献
38.
A. Kilcik V. Yurchyshyn B. Donmez V. N. Obridko A. Ozguc J. P. Rozelot 《Solar physics》2018,293(4):63
We analyzed temporal and periodic variations of sunspot counts (SSCs) in flaring (C-, M-, or X-class flares), and non-flaring active regions (ARs) for nearly three solar cycles (1986 through 2016). Our main findings are as follows: i) temporal variations of monthly means of the daily total SSCs in flaring and non-flaring ARs behave differently during a solar cycle and the behavior varies from one cycle to another; during Solar Cycle 23 temporal SSC profiles of non-flaring ARs are wider than those of flaring ARs, while they are almost the same during Solar Cycle 22 and the current Cycle 24. The SSC profiles show a multi-peak structure and the second peak of flaring ARs dominates the current Cycle 24, while the difference between peaks is less pronounced during Solar Cycles 22 and 23. The first and second SSC peaks of non-flaring ARs have comparable magnitude in the current solar cycle, while the first peak is nearly absent in the case of the flaring ARs of the same cycle. ii) Periodic variations observed in the SSCs profiles of flaring and non-flaring ARs derived from the multi-taper method (MTM) spectrum and wavelet scalograms are quite different as well, and they vary from one solar cycle to another. The largest detected period in flaring ARs is \(113\pm 1.6~\mbox{days}\) while we detected much longer periodicities (\(327\pm 13\), \(312 \pm 11\), and \(256\pm 8~\mbox{days}\)) in the non-flaring AR profiles. No meaningful periodicities were detected in the MTM spectrum of flaring ARs exceeding \(55\pm 0.7~\mbox{days}\) during Solar Cycles 22 and 24, while a \(113\pm 1.3~\mbox{days}\) period was detected in flaring ARs of Solar Cycle 23. For the non-flaring ARs the largest detected period was only \(31\pm 0.2~\mbox{days}\) for Cycle 22 and \(72\pm 1.3~\mbox{days}\) for the current Cycle 24, while the largest measured period was \(327\pm 13~\mbox{days}\) during Solar Cycle 23. 相似文献
39.
O.?V.?YushkovaEmail author A.?L.?Gavrik V.?N.?Marchuk V.?V.?Yushkov V.?M.?Smirnov M.?A.?Laptev B.?V.?Chernyshev I.?N.?Dutyshev V.?P.?Lebedev A.?V.?Medvedev A.?A.?Petrukovich 《Solar System Research》2018,52(4):287-300
An overview of radiophysical investigations of the lunar soil and plasma shell by active radar detection with the use of spacecraft is presented. The possibility is analyzed of conducting bistatic measurements using the Irkutsk Incoherent Scattering Radar and the onboard radar system RLK-L which is being developed for the orbital station of the Luna-Resurs mission. 相似文献
40.
Alteration of hydrological processes and streamflow with juniper (Juniperus virginiana) encroachment in a mesic grassland catchment 下载免费PDF全文
Chris B. Zou Donald J. Turton Rodney E. Will David M. Engle Sam D. Fuhlendorf 《水文研究》2014,28(26):6173-6182
To understand the effect of woody plant encroachment on hydrological processes of mesic grasslands, we quantified infiltration capacity in situ, the temporal changes in soil water storage, and streamflow of a grassland catchment and a catchment heavily encroached by juniper (Juniperus virginiana, eastern redcedar) in previously cultivated, non‐karst substrate grasslands in north‐central Oklahoma for 3 years. The initial and steady‐state infiltration rates under the juniper canopy were nearly triple to that of the grassland catchment and were intermediate in the intercanopy spaces within the encroached catchment. Soil water content and soil water storage on the encroached catchment were generally lower than on the grassland catchment, especially when preceding the seasons of peak rainfall in spring and fall. Frequency and magnitude of streamflow events were reduced in the encroached catchment. Annual runoff coefficients for the encroached catchment averaged 2.1%, in contrast to 10.6% for the grassland catchment. Annual streamflow duration ranged from 80 to 250 h for the encroached catchment compared with 600 to 800 h for the grassland catchment. Our results showed that the encroachment of juniper into previously cultivated mesic grasslands fundamentally alters catchment hydrological function. Rapid transformation of mesic grassland to a woodland state with juniper encroachment, if not confined, has the potential to drastically reduce soil water, streamflow and flow duration of ephemeral streams in the Southern Great Plains. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献