全文获取类型
收费全文 | 2010篇 |
免费 | 71篇 |
国内免费 | 12篇 |
专业分类
测绘学 | 48篇 |
大气科学 | 175篇 |
地球物理 | 438篇 |
地质学 | 641篇 |
海洋学 | 202篇 |
天文学 | 400篇 |
综合类 | 1篇 |
自然地理 | 188篇 |
出版年
2020年 | 26篇 |
2019年 | 38篇 |
2018年 | 51篇 |
2017年 | 28篇 |
2016年 | 46篇 |
2015年 | 37篇 |
2014年 | 77篇 |
2013年 | 126篇 |
2012年 | 67篇 |
2011年 | 95篇 |
2010年 | 71篇 |
2009年 | 92篇 |
2008年 | 83篇 |
2007年 | 76篇 |
2006年 | 64篇 |
2005年 | 77篇 |
2004年 | 85篇 |
2003年 | 57篇 |
2002年 | 54篇 |
2001年 | 36篇 |
2000年 | 39篇 |
1999年 | 28篇 |
1998年 | 26篇 |
1997年 | 23篇 |
1996年 | 24篇 |
1995年 | 22篇 |
1994年 | 22篇 |
1993年 | 19篇 |
1992年 | 20篇 |
1991年 | 21篇 |
1990年 | 17篇 |
1989年 | 23篇 |
1988年 | 23篇 |
1987年 | 25篇 |
1986年 | 29篇 |
1985年 | 38篇 |
1984年 | 34篇 |
1983年 | 40篇 |
1982年 | 22篇 |
1981年 | 29篇 |
1980年 | 28篇 |
1979年 | 31篇 |
1978年 | 34篇 |
1977年 | 29篇 |
1976年 | 20篇 |
1975年 | 13篇 |
1974年 | 12篇 |
1973年 | 23篇 |
1972年 | 18篇 |
1971年 | 21篇 |
排序方式: 共有2093条查询结果,搜索用时 31 毫秒
991.
Paula Lindgren John Parnell Craig Norman Darren F. Mark Martin Baron Jens Orm Erik Sturkell James Conliffe Wesley Fraser 《Meteoritics & planetary science》2007,42(11):1961-1969
Abstract— The Ordovician Lockne impact structure is located in central Sweden. The target lithology consisted of limestone and black unconsolidated shale overlaying a Precambrian crystalline basement. The Precambrian basement is uranium‐rich, and the black shale is both uranium‐ and organic‐rich. This circumstance makes Lockne a good candidate for testing the occurrence of U‐Th‐rich bitumen nodules in an impact structure setting. U‐Th‐rich bitumen nodules are formed through irradiation; hence the increase in the complexity of organic matter by a radioactive (uranium‐ and thorium‐rich) mineral phase. U‐Th‐rich bitumen nodules were detected in crystalline impact breccia and resurge deposits from the impact structure, but samples of non‐impact‐affected rocks from outside the impact structure do not contain any U‐Th‐rich bitumen nodules. This implies that in the Lockne impact structure, the nodules are associated with impact‐related processes. U‐Th‐rich bitumen nodules occur throughout the geological record and are not restricted to an impact structure setting, but our studies at Lockne show that this process of irradiation can readily occur in impact structures where fracturing of rocks and a post‐impact hydrothermal system enhances fluid circulation. The irradiation of organic matter by radioactive minerals has previously been proposed as a process for concentration of carbon on the early Earth. Impact structures are suggested as sites for prebiotic chemistry and primitive evolution, and irradiation by radioactive minerals could be an important mechanism for carbon concentration at impact sites. 相似文献
992.
993.
994.
X-ray bright points are small dynamic loop structures that are observed all over the solar corona. The high spatial and temporal
resolution of the TRACE instrument allows bright points to be studied in much greater detail than previously possible. This
paper focuses on a specific bright point which occurred for about 20 hours on 13–14 June 1998 and examines its dynamic structure
in detail. This example suggests that the mechanisms that cause bright points to form and evolve are more complex than previously
thought. In this case, reconnection probably plays a major part during the formation and brightening of the loop structure.
However, later on the foot points rotate injecting twist into the bright point which may cause an instability to occur with
dynamic results.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1017907406350 相似文献
995.
996.
We present the results of a study of meteoroid bulk densities determined from meteor head echoes observed by radar. Meteor
observations were made using the Advanced Research Projects Agency Long-Range Tracking And Instrumentation Radar (ALTAIR).
ALTAIR is particularly well suited to the detection of meteor head echoes, being capable of detecting upwards of 1000 meteor
head echoes per hour. Data were collected for 19 beam pointings and are comprised of approximately 70 min. of VHF observations.
During these observations the ALTAIR beam was directed largely at the north apex sporadic source. Densities are calculated
using the classical physical theory of meteors. Meteoroid masses are determined by applying a full wave scattering theory
to the observed radar cross-section. Observed meteoroids are predominantly in the 10−10 to 10−6 kg mass range. We find that the vast majority of meteoroid densities are consistent with low density, highly porous objects
as would be expected from cometary sources. The median calculated bulk density was found to be 900 kg/m3. The orbital distribution of this population of meteoroids was found to be highly inclined. 相似文献
997.
Past analyses of flare hard X-ray (HXR) spectra have largely ignored the effect of nonuniform ionization along the electron
paths in the thick-target model, though it is very significant for well-resolved spectra. The inverse problem (photon spectrum
to electron injection spectrum F
0(E
0)) is disturbingly non-unique. However, we show that it is relatively simple to allow for the effect in forward fitting of
parametric models of F
0(E
0)) and provide an expression to evaluate it for the usual single power-law form of F
0(E
0)).The expression involves the column depth N
* of the transition region in the flare loop as one of the parameters so data fitting can enable derivation of N
* (and its evaporative evolution) as part of the fitting procedure. The fit to RHESSI data on four flares for a single power
law F
0(E
0)) is much improved when ionization structure is included compared to when the usual fully ionized approximation is used.
This removes the need, in these events at least, to invoke broken power laws, or other forms, of the acceleration spectrum
F
0(E
0)) to explain the observed photon spectrum 相似文献
998.
Efforts to link minor meteor showers to their parent bodies have been hampered both by the lack of high-accuracy orbits for
weak showers and the incompleteness of our sample of potential parent bodies. The Canadian Meteor Orbital Radar (CMOR) has
accumulated over one million meteor orbits. From this large data set, the existence of weak showers and the accuracy of the
mean orbits of these showers can be improved. The ever-growing catalogue of near-Earth asteroids (NEAs) provides the complimentary
data set for the linking procedure. By combining a detailed examination of the background of sporadic meteors near the orbit
in question (which the radar data makes possible) and by computing the statistical significance of any shower association
(which the improved NEA sample allows) any proposed shower–parent link can be tested much more thoroughly than in the past.
Additional evidence for the links is provided by a single-station meteor radar at the CMOR site which can be used to dispel
confusion between very weak showers and statistical fluctuations in the sporadic background. The use of these techniques and
data sets in concert will allow us to confidently link some weak streams to their parent bodies on a statistical basis, while
at the same time showing that previously identified minor showers have little or no activity and that some previously suggested
linkages may simply be chance alignments. 相似文献
999.
The velocity distribution of meteoroids at the Earth is measured using a time-of-flight measurement technique applied to data
collected by the CMOR radar (29.85 MHz). Comparison to earlier velocity measurements from the Harvard Radio Meteor Project
suggests that HRMP suffered from biases which underestimated the number of fragmenting meteoroids. This bias results in a
systematic underestimation of the numbers of higher velocity meteoroids. Other works (cf. Taylor and Elford, 1998) have also
found additional biases in the HRMP which suggest the original HRMP meteoroid velocity analysis may have underestimated the
fraction of high velocity meteors by factors up to 104. 相似文献
1000.
The ability of magnetic reconnection solutions to explain statistical flare data is discussed. It is assumed that flares occur at well-defined, isolated sites within an active region, determined by the null points and separators of the coronal magnetic field (Craig, 2001). Statistical flare observations then derive from a multiplicity of independent sites, flaring in parallel, that produce events of widely varying output (Wheatland, 2002). Given that the `separator length' at an individual site controls the event frequency and the mean energy release, it is shown that the observed frequency-energy spectrum N(E)can be inverted to yield a source function that relates directly to the distribution of separator lengths. It is also pointed out that, under the parallel flaring model, inferred waiting-time distributions are naturally interpreted as a superposition of individual point processes. Only a modest number of flaring separators is required to mimic a Poisson process. 相似文献