全文获取类型
收费全文 | 75113篇 |
免费 | 1326篇 |
国内免费 | 636篇 |
专业分类
测绘学 | 2067篇 |
大气科学 | 5368篇 |
地球物理 | 15274篇 |
地质学 | 25435篇 |
海洋学 | 6704篇 |
天文学 | 17658篇 |
综合类 | 180篇 |
自然地理 | 4389篇 |
出版年
2021年 | 661篇 |
2020年 | 678篇 |
2019年 | 825篇 |
2018年 | 1716篇 |
2017年 | 1569篇 |
2016年 | 2043篇 |
2015年 | 1146篇 |
2014年 | 1896篇 |
2013年 | 3723篇 |
2012年 | 2110篇 |
2011年 | 2921篇 |
2010年 | 2679篇 |
2009年 | 3584篇 |
2008年 | 3051篇 |
2007年 | 3112篇 |
2006年 | 2902篇 |
2005年 | 2273篇 |
2004年 | 2298篇 |
2003年 | 2172篇 |
2002年 | 2174篇 |
2001年 | 1906篇 |
2000年 | 1780篇 |
1999年 | 1521篇 |
1998年 | 1521篇 |
1997年 | 1518篇 |
1996年 | 1258篇 |
1995年 | 1208篇 |
1994年 | 1117篇 |
1993年 | 1001篇 |
1992年 | 907篇 |
1991年 | 901篇 |
1990年 | 902篇 |
1989年 | 918篇 |
1988年 | 842篇 |
1987年 | 974篇 |
1986年 | 880篇 |
1985年 | 1050篇 |
1984年 | 1198篇 |
1983年 | 1071篇 |
1982年 | 1014篇 |
1981年 | 1008篇 |
1980年 | 853篇 |
1979年 | 841篇 |
1978年 | 824篇 |
1977年 | 765篇 |
1976年 | 688篇 |
1975年 | 681篇 |
1974年 | 650篇 |
1973年 | 711篇 |
1972年 | 448篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
361.
We have investigated the evolutionary behaviour of intermediate mass (2, 3, 4, 5, and 7M
) Population I stars, assuming two different rates of rotation at the threshold of stability.In the first part of the study, stars are assumed to start with a critical rotation (fast rotation model) and to progress to the point of rotational instability. The stars evolve by losing mass and become rotationally unstable before they reach the zero-age Main Sequence. It is argued that multiple star systems might be formed through the evolution of rapidly rotating stars. An expression for the rotational mass loss rate is derived as a function of the physical parameters of stars.In the second part of the study, stars are assumed to rotate at a rate below the critical value (slow rotation model). The evolution of slowly rotating stars is followed as far as zero-age Main Sequence on the theoretical Hertzsprung-Russell diagram and compared with that of normal stars. The evolutionary paths are found to be more or less similar to those of normal stars; but their positions on the Main Sequence are characterized by effective temperatures and luminosities lower than those of normal stars. The zero-age Main-Sequence times of these stars are longer than those of normal stars. The rotational rates obtained for the zero-age Main Sequence are in good agreement with observed values. 相似文献
362.
We have used observations of the rare isotopes of HCN and HNC to determine the relative abundance of these two chemical isomers along the central ridge of the Orion molecular cloud. The abundance ratio [HCN]/[HNC] decreases by more than an order of magnitude from the relatively warm plateau and hot core sources toward the KL nebula to the colder, more quiescent clouds to the north and south. Even in the cooler regions, however, the ratio is an order of magnitude larger than that found in previous investigations of cold dark clouds. We determine the kinetic temperature in the regions we have studied from new observations of methylacetylene (CH3CCH), together with other recent estimates of the gas temperature near KL. The results suggest that the warmer portions of the cloud are dominated by different chemical pathways than those in the general interstellar cloud material. 相似文献
363.
The assumption that the very low albedo determined for Halley's comet is typical of all short period comets, taken together with the assumption that the average sizes of long and short period comets are approximately equal, leads to an increase in the total mass of comets in the solar system by almost two orders of magnitude. If gravitational ejection from the Uranus - Neptune zone during the later phases of planet formation is indeed responsible for the classical Oort cloud between 104–1015 AU, then the mass of comets in this transplanetary region during cosmogonie times has to exceed the combined masses of Uranus and Neptune by over an order of magnitude. Furthermore, if the recent arguments for as many as 1014 comets in an inner Oort cloud between ~40– 104AU are valid, then the total mass of comets in the solar system approaches 2% of a solar mass. 相似文献
364.
Detailed comparisons of Culgoora 160 MHz radioheliograms of solar noise storms and Skylab EUV spectroheliograms of coronal loop structures are presented. It is concluded that: (1) there is a close association between changes in large-scale magnetic fields in the corona and the onset or cessation of noise storms; (2) these coronal changes result from the emergence of new magnetic flux at the photospheric level; (3) although new magnetic flux at the photospheric level is often accompanied by an increase in flare activity the latter is not directly responsible for noise storm activity; rather the new magnetic flux diffuses slowly outwards through the corona at rates 1–2 km s–1 and produces noise storms at 160 MHz 1–2 days later; (4) the coronal density above or in large-scale EUV loop systems is sufficiently dense to account for noise storm emission at the fundamental plasma frequency; (5) the scatter in noise storm positions can be accounted for by the appearance and disappearance of individual loops in a system. 相似文献
365.
Some attempts of polarimetric sounding of Comet Halley will be undertaken from the flyby probes. In order to facilitate the final planning and the future interpretation of these experiments we have done a thorough analysis of practically all available polarimetric observations. An emphasis is made on interpretation attempts and their discussion. The results of the phase dependence of polarization investigations are presented covering a wide range of phase angles . The chief peculiarities of this dependence are: maximum polarization at = 90, diminishing through zero at 20, negative values up to several per cent and a final growth to zero at zero . A division into gaseous and dusty comets on polarimetric basis is revealed. The wavelength dependence of polarization is discussed. The numerous results of detailed polarimetry are compared to the negative results of attempts to detect the elliptical polarization. New observational problems arising from the evidence given by the negative polarization at small phase angles and by the opposition effect recently discovered are discussed. 相似文献
366.
The data of Paper I (Doddet al., 1986), representing a catalogue of parameters measured for bright galaxies in field 444 of the Southern Sky Survey, are examined in a search for non-random effects. A global trend of bimodal alignment of galaxies in this field is found. This effect is present even for galaxies of large apparent ellipticity (e0.5) and is interpreted as revealing a real physical phenomenon on the large scale in this field. Analysis of nearest neighbouring pairs of galaxies reveals evidence also for small-scale interactions. The difference in apparent ellipticities between close pairs is larger than would be expected if the galaxies were randomly distributed. Nearest neighbouring galaxies also show a strong preference to be aligned with their major axes parallel. The results are discussed with regard to theories for galaxy formation. 相似文献
367.
The analytic construction of the direct periodic orbits in the circular 3-body problem is given in explicit form to the linear terms in . It is shown to be in good agreement with the numerically found orbits for large values of . 相似文献
368.
Regional variations in the incidence of premature deaths between 1971 and 1981 arc examined by mapping standardised mortality ratios for deaths under the age of 65. Premature mortality is also examined with regards to the number of years of normal life lost by those who die prematurely, using a specially defined unfulfilled life index. Unfulfilled life and premature deaths arc found to have a higher incidence in the major urban areas and a lower incidence in western counties, although there are some interesting differences in their spatial distributions. Differences with regard to sex. age at death and cause of death are also examined. 相似文献
369.
Energetic solar electrons in the interplanetary medium 总被引:3,自引:0,他引:3
R. P. Lin 《Solar physics》1985,100(1-2):537-561
ISEE-3 measurements extending down to 2 keV energy have provided a new perspective on energetic solar electrons in the interplanetary medium. Impulsive solar electron events are observed, on average, several times a day near solar maximum, with 40% detected only below 15 keV. The electron energy spectra have a nearly power-law shape extending smoothly down to 2 keV, indicating that the origin of these events is high in the corona. These coronal flare-like events often produced 3He-rich particle events.In large solar flares which accelerate electrons and ions to relativistic energies, the electron spectrum appears to be modified by a second acceleration which results in a double power-law shape above 10 keV with a break near 100 keV and flattening from 10–100 keV. Large flares result in long-lived (many days) streams of outflowing electrons which dominate the interplanetary fluxes at low energies. Even in the absence of solar activity, significant fluxes of low energy electrons flow out from the Sun.Solar type-III radio bursts are produced by the escaping 2–102 keV electrons through a beam-plasma instability. The detailed ISEE-3 measurements show that electron plasma waves are generated by the bump-on-tail distribution created by the faster electrons running ahead of the slower ones. These plasma waves appear to be converted into radio emission by nonlinear wave-wave interactions. 相似文献
370.
P. S. Goraya 《Astrophysics and Space Science》1985,112(2):325-336
A large sample of Be stars has been studied spectrophotometrically in the visible region. The continuum energy distribution data for 23 Be stars included in the list of Harmanecet al. (1983) are presented and discussed in the wavelength range 3200 Å–8000 Å. For 15 Be stars the observations reported in the present work are new. By comparing the observed continua with models, the effective temperatures of these stars have been estimated. It is found that, in general, Be stars have lower effective temperature than the corresponding normal B stars. The present study shows that the early-Be stars (B0–B5) possess near-ultraviolet and near-infrared excess emissions more frequently than the late-Be stars (B5–B9). The seven new Be stars are detected to show pole-on characteristics. 相似文献