全文获取类型
收费全文 | 5052篇 |
免费 | 170篇 |
国内免费 | 54篇 |
专业分类
测绘学 | 126篇 |
大气科学 | 452篇 |
地球物理 | 1094篇 |
地质学 | 1634篇 |
海洋学 | 473篇 |
天文学 | 949篇 |
综合类 | 12篇 |
自然地理 | 536篇 |
出版年
2021年 | 51篇 |
2020年 | 58篇 |
2019年 | 72篇 |
2018年 | 87篇 |
2017年 | 87篇 |
2016年 | 112篇 |
2015年 | 126篇 |
2014年 | 118篇 |
2013年 | 262篇 |
2012年 | 156篇 |
2011年 | 235篇 |
2010年 | 196篇 |
2009年 | 286篇 |
2008年 | 209篇 |
2007年 | 193篇 |
2006年 | 174篇 |
2005年 | 165篇 |
2004年 | 179篇 |
2003年 | 170篇 |
2002年 | 168篇 |
2001年 | 101篇 |
2000年 | 109篇 |
1999年 | 111篇 |
1998年 | 107篇 |
1997年 | 65篇 |
1996年 | 75篇 |
1995年 | 66篇 |
1994年 | 63篇 |
1993年 | 64篇 |
1992年 | 58篇 |
1991年 | 74篇 |
1990年 | 63篇 |
1989年 | 55篇 |
1988年 | 49篇 |
1987年 | 72篇 |
1986年 | 51篇 |
1985年 | 66篇 |
1984年 | 87篇 |
1983年 | 66篇 |
1982年 | 60篇 |
1981年 | 79篇 |
1980年 | 66篇 |
1979年 | 53篇 |
1978年 | 71篇 |
1977年 | 51篇 |
1976年 | 60篇 |
1975年 | 39篇 |
1974年 | 40篇 |
1973年 | 39篇 |
1971年 | 37篇 |
排序方式: 共有5276条查询结果,搜索用时 15 毫秒
111.
Measurements made when testing an interferometric system for imaging the sodium emission indicate that, between 3 and 7.5 arc min (9 and 23RJ) from Jupiter, the observed emissions can probably be attributed to terrestrial airglow. 相似文献
112.
Matthew J. Penn Donald L. Mickey Richard C. Canfield Barry J. Labonte 《Solar physics》1991,135(1):163-177
The Mees CCD (MCCD) instrument is an imaging spectroscopy device which uses the 25 cm coronagraph telescope and the 3.0 m Coudé spectrograph at Mees Solar Observatory (MSO) on Haleakala, Maui. The instrument works with resolving power up to R 200 000 with significant throughput from 3934 Å (Caii K) to 10 000 Å. A fast guiding active mirror stabilizes the image during observations. A rapidly writing magnetic tape storage system allows observations to be recorded at 256 kbytes s–1. Currently, the MCCD is used for imaging spectroscopy of solar flares at 6563 Å (H), and velocity measurements of umbral oscillations; future plans include emission line studies of active region coronae, and photospheric studies of solar oscillations. 相似文献
113.
A comparison is made between the flux-versus-time profile in the EUV band and the thick target electron flux profile as inferred from hard X-rays for a number of moderately large solar flares. This complements Kane and Donnelly's (1971) study of small flares. The hard X-ray data are from ESRO TD-1A and the EUV inferred from SFD observations.Use of a 2 minimising method shows that the best overall fit between the profile fine structures obtains for synchronism to 5 s which is within the timing accuracy. This suggests that neither conduction nor convection is fast enough as the primary mechanism of energy transport into the EUV flare and rather favours heating by the electrons themselves or by some MHD wave process much faster than acoustic waves.The electron power deposited, for a thick target model, is however far greater than the EUV luminosity for any reasonable assumptions about the area and depth over which EUV is emitted. This means that either most of the power deposited is conducted away to the optical flare or that only a fraction 1–10% of the X-ray emitting electrons are injected downwards. Recent work on H flare heating strongly favours the latter alternative - i.e. that electrons are mostly confined in the corona. 相似文献
114.
The stellar polarization data have been examined using a new catalogue containing accurate stellar distances. On the assumption of a magnetic alignment hypothesis, correlations on the larger distance scale indicate the existence of a dominant regular magnetic field, although its characteristics are difficult to determine. Within 500 pc its direction is towardsl45° and beyond this towardsl60°, though it is clear that such a longitudinal model is too simple. There is also some evidence for an inclination of this field to the galactic plane. The distribution of the polarization vectors away from the galactic plane has been examined and it is proposed that the two largest loop structures, previously identified as Supernova remnants, are linked by the regular field. Incremental polarization maps have been produced but they show little correlation with the spiral structure. The polarization appears to be saturated at about 1 kpc from the Sun, which is explained as the result of an observational selection effect. On the smaller distance scales an autocorrelation analysis in different directions has revealed no obvious coherence in the irregular component on scales greater than 50 pc. 相似文献
115.
Amino acids comprise from 15 to 36% by weight of humic substances from carbonate and non-carbonate sediments. Humic and fulvic acids extracted from carbonate sediments are characterized by an amino acid composition consisting primarily of the acidic amino acids, aspartic and glutamic acid. Humic substances from non-carbonate sediments have a distinctly different amino acid composition consisting primarily of glycine and alanine. Amino acid analyses of various molecular weight fractions of fulvic acids extracted from carbonates show that lower molecular weight fractions have appreciably higher relative abundances of the acidic amino acids compared to higher molecular weight fractions. Based on typical values for carboxyl group content in humic substances, acidic amino acids may be a significant contributor of these functional groups. Carbonate surfaces appear to selectively adsorb aspartic acid-enriched organic matter while non-carbonates do not have this property. 相似文献
116.
117.
118.
A high-resolution (0.6 cm?1) spectrum of Jupiter at 5 μm recorded at the Kuiper Airborne Observatory is used to determine upper limits to the column density of 19 molecules. The upper limits to the mixing ratios of SiH4, H2S, HCN, and simple hydrocarbons are discussed with respect to current models of Jupiter's atmosphere. These upper limits are compared to expectations based upon the solar abundance of the elements. This analysis permits upper limit measurements (SiH4), or actual detections (GeH4), of molecules with mixing ratios with hydrogen as low as 10?9. In future observations at 5 μm the sensitivity of remote spectroscopic analyses should permit the study of constituents with mixing ratios as low as 10?10, which would include the hydrides of such elements as Sn and As as well as numerous organic molecules. 相似文献
119.
120.
The effect of collisions of suprathermal electrons with a thermal background plasma is investigated and is shown to cause flattening of a monotonically descending velocity distribution of fast particles. As a result flare-produced energetic electrons that are trapped in a coronal magnetic arch and that are initially distributed in energy according to a power-law, can give rise to an instability of Langmuir waves in the background plasma and the subsequent emission of continuum radiation as observed in type IV dm bursts.To explain the appearance of sudden reductions observed in type IV dm outbursts the effects both of magnetic compressions of the source region and of renewed injections of fast particles on the continuum source are investigated. It is found that the latter process can explain the observed reductions. 相似文献