全文获取类型
收费全文 | 970篇 |
免费 | 40篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 29篇 |
大气科学 | 88篇 |
地球物理 | 211篇 |
地质学 | 292篇 |
海洋学 | 108篇 |
天文学 | 180篇 |
综合类 | 7篇 |
自然地理 | 101篇 |
出版年
2021年 | 12篇 |
2020年 | 10篇 |
2019年 | 20篇 |
2018年 | 13篇 |
2017年 | 16篇 |
2016年 | 26篇 |
2015年 | 18篇 |
2014年 | 22篇 |
2013年 | 51篇 |
2012年 | 28篇 |
2011年 | 48篇 |
2010年 | 40篇 |
2009年 | 51篇 |
2008年 | 31篇 |
2007年 | 44篇 |
2006年 | 42篇 |
2005年 | 24篇 |
2004年 | 19篇 |
2003年 | 29篇 |
2002年 | 20篇 |
2001年 | 20篇 |
2000年 | 21篇 |
1999年 | 21篇 |
1998年 | 18篇 |
1996年 | 10篇 |
1995年 | 18篇 |
1994年 | 10篇 |
1993年 | 19篇 |
1992年 | 11篇 |
1991年 | 7篇 |
1990年 | 9篇 |
1989年 | 12篇 |
1988年 | 15篇 |
1987年 | 15篇 |
1986年 | 10篇 |
1985年 | 20篇 |
1984年 | 19篇 |
1983年 | 22篇 |
1982年 | 13篇 |
1981年 | 18篇 |
1980年 | 9篇 |
1979年 | 6篇 |
1978年 | 15篇 |
1977年 | 13篇 |
1976年 | 12篇 |
1975年 | 16篇 |
1974年 | 8篇 |
1973年 | 16篇 |
1972年 | 7篇 |
1970年 | 6篇 |
排序方式: 共有1016条查询结果,搜索用时 31 毫秒
171.
In contrast to earth, the atmosphere of the moon is exceedingly tenuous and appears to consist mainly of noble gases. The solar wind impinges on the lunar surface, supplying detectable amounts of helium, neon and 36Ar. Influxes of solar wind protons and carbon and nitrogen ions are significant, but atmospheric gases containing these elements have not been positively identified. Radiogenic 40Ar and 222Rn produced within the moon have been detected. The present rate of effusion of argon from the moon accounts for about 0.4% of the total production of 40Ar due to decay of 40K if the average abundance of potassium in the moon is 1000 ppm. Lack of weathering processes in the regolith suggests that most of the atmospheric 40Ar originates deep in the lunar interior, perhaps in a partially molten core. If so, other gases may be vented along with the argon. 相似文献
172.
173.
174.
Harvey Karen L. Jones Harrison P. Schrijver Carolus J. Penn Matthew J. 《Solar physics》1999,190(1-2):35-44
Simultaneous measurements of the magnetic fields in the photosphere and chromosphere were used to investigate if magnetic
flux is submerging at sites between adjacent opposite polarity magnetic network elements in which the flux is observed to
decrease or `cancel'. These data were compared with chromospheric and coronal intensity images to establish the timing of
the emission structures associated with these magnetic structures as a function of height. We found that most of the cancelation
sites show either that the bipole is observed longer in the photosphere than in the chromosphere and corona (44%) or that
the timing difference of the disappearance of the bipole between these levels of the atmosphere is unresolved. The magnetic
axis lengths of the structures associated with the cancelation sites are on average slightly smaller in the chromosphere than
the photosphere. These observations suggest that magnetic flux is retracting below the surface for most, if not all, of the
cancelation sites studied. 相似文献
175.
Accumulation in Svalbard glaciers deduced from ice cores with nuclear tests and Chernobyl reference layers 总被引:1,自引:0,他引:1
Jean Francis Pinglot Michel Pourchet Bernard Lefauconnier Jon Ove Hagen Elisabeth Isaksson Rein Vaikmäe Kokichi Kamiyama 《Polar research》1999,18(2):315-321
Mean net annual balance and the related spatio-temporal variations have been determined on the basis of well-dated artificial layers in shallow ice cores (Chernobyl, 1986, and atmospheric thermonuclear tests, mainly in 1961-62 in Novaya Zemlya). Seventy ice cores from 13 Svalbard glaciers have been analysed. On each glacier, in its accumulation area and at the highest elevation, one ice core was recovered down to about 40 m and sampled for radioactivity measurements to determine the 1986 and 1962-63 layer (1954 was the initial date of the nuclear tests). For each glacier, at least five complementary ice cores from the accumulation area were analysed to determine the Chernobyl reference layer. Six ice cores exhibit both the Chernobyl and nuclear tests layers and are of special interest in this study.
This work provides new data on the deposition rates of natural and artificial radioisotopes. Using ice cores samples from the Arctic glaciers, even with superimposed ice accumulation, it is possible to distinguish between the Chernobyl and the nuclear tests fallouts. This work also shows that the mean annual net balance did not significantly change for at least five ice core locations in the Svalbard glaciers for the two periods extending from 1963 to 1986 to the recent date of drilling. 相似文献
This work provides new data on the deposition rates of natural and artificial radioisotopes. Using ice cores samples from the Arctic glaciers, even with superimposed ice accumulation, it is possible to distinguish between the Chernobyl and the nuclear tests fallouts. This work also shows that the mean annual net balance did not significantly change for at least five ice core locations in the Svalbard glaciers for the two periods extending from 1963 to 1986 to the recent date of drilling. 相似文献
176.
As with many rivers of the Pyrenees Mountains, reservoirs on the River Ariège (SW Francc) are used for hydroelectricity generation, irrigation and to maintain summer flows in the River Garonne. Two river reservoirs on the River Ariège were simultaneously emptied in April 1991 to remove accumulated sediment. Fish and ten environmental variables were sampled using Point Abundance Sampling by electrofishing before (February 1991) and one year later (February 1992) at three study sites downstream of the dams. We examined the short-term impact of reservoir cleaning on the relative abundance, habitat availability and habitat use of three species of small fish: minnow Phoxinus phoxinus (L.), gudgeon Gobio gobio (L.) and stone loach Barbatula barbatula (L.).¶After the reservoir cleaning, areas covered by gravel, sand and mud increased significantly as the result of the sediment release. The three species did not react in the same manner to these environmental changes. Stone loach appeared to be the most sensitive species, with its relative abundance decreasing at all sites. Conversely, minnow seemed to be the most resistant species and as such is a leading pioneer species for colonizing new or impacted stretches of rivers channels. Finally, the number of gudgeon decreased particularly at the most downstream site where large deposits of mud were recorded. 相似文献
177.
178.
Dtissibe Francis Yongwa Ari Ado Adamou Abba Titouna Chafiq Thiare Ousmane Gueroui Abdelhak Mourad 《Natural Hazards》2020,104(2):1211-1237
Natural Hazards - Nowadays, floods have become the widest global environmental and economic hazard in many countries, causing huge loss of lives and materials damages. It is, therefore, necessary... 相似文献
179.
AbstractUsing observations from 688 debris flows, we analyse the hydrologic and landscape characteristics that influenced debris-flow initiation mechanisms and locations in a watershed that had been partially burned by the 2012 Whitewater-Baldy Complex Fire in the Gila Mountains, southern New Mexico. Debris flows can initiate due to different processes. Slopes can fail as discrete landslides and then become fluidized and move downstream as debris flows (landslide initiated) or progressive bulking of sediment from a distributed area can become channelized and concentrated as it moves downslope (runoff generated). In this study, we have an unusual opportunity to investigate both types of debris-flow initiation mechanisms in our observations of debris flows, triggered by an exceptional rainstorm in the autumn of 2013. Additionally, we compare our observations with those of a dataset of 1138 debris flows in the Colorado Front Range, triggered during the same weather system. We found that runoff-generated debris flows dominated in burn areas, and runoff required to start these flows could be well characterized by the Shields stress. Landslide-initiated debris flows were dominant in unburned areas. Debris-flow densities were tied to total rainfall and precipitation intensities. Like the observations in the Colorado Front Range, debris-flow initiation locations were found primarily in areas of relatively sparse vegetation on south-facing slopes between 25 and 40°, and with upslope contributing areas less than 1000 m2. In terms of preferential locations for debris-flow initiations, 2013 vegetation coverage, approximated by Green–Red Vegetation Index metrics, proved to be more influential than the 2012 burn-severity designation. The uniformity of observations between our study area and those in the Colorado Front Range indicate that the underlying hydrologic and landscape patterns of the debris-flow initiation locations documented in these studies could be applicable to the wider southwest and Rocky Mountain regions. © 2019 John Wiley & Sons, Ltd. 相似文献
180.
Elizabeth A. McHuron James T. Harvey J. Margaret Castellini Craig A. Stricker Todd M. O’Hara 《Marine pollution bulletin》2014
We measured total selenium and total mercury concentrations ([TSe] and [THg]) in hair (n = 138) and blood (n = 73) of harbor seals (Phoca vitulina) from California to assess variation by geography and sex, and inferred feeding relationships based on carbon, nitrogen, and sulfur stable isotopes. Harbor seals from Hg-contaminated sites had significantly greater [THg], and lesser [TSe] and TSe:THg molar ratios than seals from a relatively uncontaminated site. Males had significantly greater [THg] than females at all locations. Sulfur stable isotope values explained approximately 25% of the variability in [THg], indicating increased Hg exposure for seals with a greater use of estuarine prey species. Decreased [TSe] in harbor seals from Hg-contaminated regions may indicate a relative Se deficiency to mitigate the toxic effects of Hg. Further investigation into the Se status and the potential negative impact of Hg on harbor seals from Hg-contaminated sites is warranted. 相似文献