全文获取类型
收费全文 | 5982篇 |
免费 | 147篇 |
国内免费 | 62篇 |
专业分类
测绘学 | 123篇 |
大气科学 | 503篇 |
地球物理 | 1373篇 |
地质学 | 1985篇 |
海洋学 | 592篇 |
天文学 | 957篇 |
综合类 | 14篇 |
自然地理 | 644篇 |
出版年
2021年 | 47篇 |
2020年 | 65篇 |
2019年 | 73篇 |
2018年 | 111篇 |
2017年 | 116篇 |
2016年 | 132篇 |
2015年 | 103篇 |
2014年 | 129篇 |
2013年 | 315篇 |
2012年 | 177篇 |
2011年 | 260篇 |
2010年 | 238篇 |
2009年 | 236篇 |
2008年 | 241篇 |
2007年 | 209篇 |
2006年 | 228篇 |
2005年 | 182篇 |
2004年 | 184篇 |
2003年 | 171篇 |
2002年 | 165篇 |
2001年 | 117篇 |
2000年 | 108篇 |
1999年 | 103篇 |
1998年 | 95篇 |
1997年 | 85篇 |
1996年 | 88篇 |
1995年 | 92篇 |
1994年 | 86篇 |
1993年 | 76篇 |
1992年 | 86篇 |
1991年 | 70篇 |
1990年 | 98篇 |
1989年 | 85篇 |
1988年 | 75篇 |
1987年 | 100篇 |
1986年 | 75篇 |
1985年 | 96篇 |
1984年 | 131篇 |
1983年 | 108篇 |
1982年 | 97篇 |
1981年 | 100篇 |
1980年 | 88篇 |
1979年 | 100篇 |
1978年 | 71篇 |
1977年 | 86篇 |
1976年 | 71篇 |
1975年 | 70篇 |
1974年 | 55篇 |
1973年 | 61篇 |
1972年 | 35篇 |
排序方式: 共有6191条查询结果,搜索用时 15 毫秒
61.
Robert Gilbert Sarah Crookshanks Kyle R. Hodder John Spagnol Roland B. Stull 《Journal of Paleolimnology》2006,35(4):737-745
Severe rainfall in mid October, 2003 produced the largest floods in almost a century of record on rivers in the Cordillera
of southwestern British Columbia. Sediment deposited in Lillooet Lake as a result of this event is clearly distinguished by
stratigraphy, colour, texture, magnetic properties, and organic content. Each of these physical properties is related to the
lacustrine processes, especially turbid underflow, that distributed the sediment through the lake. The flood, which lasted
less than a week, delivered 8–12 times the amount of sediment that accumulates in most entire years in the deepest, central
parts of the lake. Recognition of events of this type in the stratigraphic record offers a means of assessing the changing
nature of extreme hydroclimatic events, and their relation to more ubiquitous, lower-energy processes. 相似文献
62.
Robert F. Howard 《Journal of Astrophysics and Astronomy》2000,21(3-4):119-125
The tilt angles of sunspot groups are defined, using the Mount Wilson data set. It is shown that groups with tilt angles greater
than or less than the average value (≈ 5 deg) show different latitude dependences. This effect is also seen in
synoptic magnetic field data defining plages. The fraction of the total sunspot group area that is found in the leading spots
is discussed as a parameter that can be useful in studying the dynamics of sunspot groups. This parameter is larger for low
tilt angles, and small for extreme tilt angles in either direction. The daily variations of sunspot group tilt angles are
discussed. The result that sunspot tilt angles tend to rotate toward the average value is reviewed. It is suggested that at
some depth, perhaps 50 Mm, there is a flow relative to the surface that results from a rotation rate faster than the surface
rate by about 60 m/sec and a meridional drift that is slower than the surface rate by about 5 m/sec. This results in a slanted
relative flow at that depth that is in the direction of the average tilt angle and may be responsible for the tendency for
sunspot groups (and plages) to rotate their magnetic axes in the direction of the average tilt angle. 相似文献
63.
Josef Gochermann Walter F. Wargau Claus Tappert Theodor Schmidt-Kaler Robert S. Stobie Fred Marang Greg Roberts Francois G. van Wyk Peter Rucks 《Experimental Astronomy》1999,9(1):1-15
In order to decide whether the seeing conditions at SAAO/Sutherland justify the erection of a 3.5 m telescope and also to compare Sutherland with the Gamsberg/Namibia site, a seeing campaign covering 15 months has been carried out. For direct comparison with the results of the seeing campaign at Gamsberg twenty years before the same QUESTAR telescope was employed. The seeing is determined by the scattering of the star-trail exposed on a film in the focal plane of the telescope. The campaign commenced in February 1992. Up to May 1993, data for 204 nights, that is 47.3% of the total number of nights, were collected. Due to wind speeds above 30 km h-1, 25 out of the 204 nights were not considered in the final reduction. The useful 179 nights are evenly distributed over the campaign period. The median seeing value for the whole period is = 0.52. There are differences during the year: the best season gives = 0.42, the worst = 0.67. Each night was divided into three intervals, although data for each of the three intervals were not always available. Generally, there is an improvement in the seeing during the course of a night. The results are compared to the seeing values of Gamsberg/Namibia and ESO/La Silla. 相似文献
64.
A two-dimensional nonlinear hydrodynamic model has been developed for studying the global scale winds, temperature, and compositional structure of the mesosphere and thermosphere of Venus. The model is driven by absorption of solar radiation. Ultraviolet radiation produces both heating and photodissociation. Infrared solar heating and thermal cooling are also included with an accurate NLTE treatment. The most crucial uncertainty in determining the solar drive is the efficiency by which solar radiation is converted to heat. This question was analyzed in Part I, where it was concluded that essentially all hot atom and O(1D) energy may be transferred to vibrational-rotational energy of CO2 molecules. If this is so, the minimum possible euv heating occurs and is determined by the quenching of the resulting excess rotational energy. The hydrodynamic model is integrated with this minimum heating and neglecting any small-scale vertical eddy mixing. The results are compared with predictions of another model with the same physics except that it assumes that 30% of radiation goes into heat and that the heating from longer-wavelength radiation includes the O(1D) energy. For the low-efficiency model, exospheric temperatures are ?300°K on the dayside and drop to < 180°K at the antisolar point. For the higher-efficiency model, the day-to-night temperature variation is from ?600°K to ?250°K. Both versions of the model predict a wind of several hundred meters per second blowing across the terminator and abruptly weakening to small values on the nightside with the mass flow consequently going into a strong tongue of downward motion on the nightside of the terminator. The presence of this circulation could be tested observationally by seeing if its signature can be found in temperature measurements. Both versions of the model indicate that a self-consistent large-scale circulation would maintain oxygen concentrations with ?5% mixing ratios near the dayside F-1 ionospheric peak but ?40% at the antisolar point at the same pressure level. 相似文献
65.
Barbara J. Thompson Sarah E. Gibson Peter C. Schroeder David F. Webb Charles N. Arge Mario M. Bisi Giuliana de Toma Barbara A. Emery Antoinette B. Galvin Deborah A. Haber Bernard V. Jackson Elizabeth A. Jensen Robert J. Leamon Jiuhou Lei Periasamy K. Manoharan M. Leila Mays Patrick S. McIntosh Gordon J. D. Petrie Simon P. Plunkett Liying Qian Peter Riley Steven T. Suess Munetoshi Tokumaru Brian T. Welsch Thomas N. Woods 《Solar physics》2011,274(1-2):29-56
We present an overview of the data and models collected for the Whole Heliosphere Interval, an international campaign to study the three-dimensional solar?Cheliospheric?Cplanetary connected system near solar minimum. The data and models correspond to solar Carrington Rotation 2068 (20 March??C?16 April 2008) extending from below the solar photosphere, through interplanetary space, and down to Earth??s mesosphere. Nearly 200 people participated in aspects of WHI studies, analyzing and interpreting data from nearly 100 instruments and models in order to elucidate the physics of fundamental heliophysical processes. The solar and inner heliospheric data showed structure consistent with the declining phase of the solar cycle. A closely spaced cluster of low-latitude active regions was responsible for an increased level of magnetic activity, while a highly warped current sheet dominated heliospheric structure. The geospace data revealed an unusually high level of activity, driven primarily by the periodic impingement of high-speed streams. The WHI studies traced the solar activity and structure into the heliosphere and geospace, and provided new insight into the nature of the interconnected heliophysical system near solar minimum. 相似文献
66.
Jeffrey Robert Thompson 《Geological Journal》2015,50(4):550-551
67.
68.
Fluorometric Determination of Chlorophyll 总被引:32,自引:0,他引:32
69.
Some approaches for location of centroids of quartz grain outlines to increase homology between Fourier amplitude spectra 总被引:2,自引:0,他引:2
The ability to test for similarities and differences among families of shapes by closed-form Fourier expansion is greatly enhanced by the concept of homology. Underlying this concept is the assumption that each term of a Fourier series, when compared to the same term in another series, represents the same thing. A method that ensures homology is one which minimizes the centering error, as reflected in the first harmonic term of the Fourier expansion. The problem is to chose a set of edge points derived from a much larger, but variable, number of edge points such that a valid homologous Fourier series can be calculated. Methods are reviewed and criteria given to define a proper solution. An algorithm is presented which takes advantage of the fact that minimization of the error term can be accomplished by minimizing the distance between the origin of the polar coordinate system in the calculation of the Fourier series and the shape centroid. The use of this algorithm has produced higher quality solutions for quartz grain provenance studies. 相似文献
70.
N. Brice Orange Hakeem M. Oluseyi David L. Chesny Maulik Patel Patrick Champey Katie Hesterly Dylan Anthony Robert Treen 《Solar physics》2014,289(5):1901-1915
Achieving subarcsecond co-registration across varying time-lines of multi-wavelength and instrument images is difficult and requires an accurate characterization of the instrument pointing jitter. We investigated the internal pointing errors on daily and yearly time-scales that occur across the Solar Dynamics Observatory’s (SDO) Atmospheric Imaging Assembly (AIA) and Helioseismic Magnetic Imager (HMI). Using cross-correlation techniques on the AIA 1700 Å passband and the HMI line-of-sight magnetograms from three years of observational image pairs at approximately three-day intervals, internal pointing errors were quantified. Pointing variations of ±?0.26″ (jitter-limited) and ±?0.50″ in the solar East–West (x) and North–South (y) directions, respectively, were measured. AIA observations of the Venus transit in June 2012 were used to measure existing coalignment offsets in all passbands. We found that the AIA passband pointing variations are 〈ΔX CO〉=1.10″±1.41″ and 〈ΔY CO〉=1.25″±1.24″ when aligned to the HMI nominal image center, referred to here as the CutOut technique. Minimal long-term pointing variations found between limb and correlation derived pointings provide evidence that the image-center positions provided by the instrument teams achieve single-pixel accuracy on time scales shorter than their characterization. However, daily AIA passband pointing variations of ??1.18″ indicate that autonomous subarcsecond co-registration is not fully achieved yet. 相似文献