全文获取类型
收费全文 | 77176篇 |
免费 | 1253篇 |
国内免费 | 578篇 |
专业分类
测绘学 | 1870篇 |
大气科学 | 5211篇 |
地球物理 | 15402篇 |
地质学 | 26876篇 |
海洋学 | 6823篇 |
天文学 | 18276篇 |
综合类 | 163篇 |
自然地理 | 4386篇 |
出版年
2021年 | 708篇 |
2020年 | 781篇 |
2019年 | 822篇 |
2018年 | 1803篇 |
2017年 | 1628篇 |
2016年 | 2056篇 |
2015年 | 1150篇 |
2014年 | 1939篇 |
2013年 | 3854篇 |
2012年 | 2158篇 |
2011年 | 2982篇 |
2010年 | 2719篇 |
2009年 | 3708篇 |
2008年 | 3092篇 |
2007年 | 3188篇 |
2006年 | 2926篇 |
2005年 | 2303篇 |
2004年 | 2273篇 |
2003年 | 2165篇 |
2002年 | 2109篇 |
2001年 | 1887篇 |
2000年 | 1726篇 |
1999年 | 1489篇 |
1998年 | 1494篇 |
1997年 | 1482篇 |
1996年 | 1246篇 |
1995年 | 1220篇 |
1994年 | 1109篇 |
1993年 | 991篇 |
1992年 | 928篇 |
1991年 | 913篇 |
1990年 | 999篇 |
1989年 | 921篇 |
1988年 | 856篇 |
1987年 | 1050篇 |
1986年 | 897篇 |
1985年 | 1113篇 |
1984年 | 1232篇 |
1983年 | 1146篇 |
1982年 | 1042篇 |
1981年 | 1085篇 |
1980年 | 896篇 |
1979年 | 862篇 |
1978年 | 835篇 |
1977年 | 810篇 |
1976年 | 742篇 |
1975年 | 720篇 |
1974年 | 723篇 |
1973年 | 752篇 |
1971年 | 455篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
441.
At present, long-term (over 30 years) multicolor photometric observations give the possibility to determine general properties
of spotted areas on late-type stars. Star-spot modeling from broadband photometric data has been carried out by Alekseev and
Gershberg since 1996 under the assumption that spots are situated in two latitudinal zones. Here we propose a new analysis
of their results for several G and K dwarf stars with high irregular activity. On these stars, EK Dra, VY Ari, V775 Her, and
V833 Tau, two spot belts exist separately and do not merge into a single equatorial active region, as occurs on cooler red-dwarf
stars. The zonal spottedness models allow us to fit simultaneously both rotational modulation and long-term variability of
stellar brightness. These models give evidence for an equatorward drift of the lower latitude boundary of the spotted region,
φ0, during the rising phase of activity, beyond any possible errors concerned with our methodology. In order to evaluate the
drift rate we introduce the concept of `effective' spot belt, whose width is independent of longitude. This permits us to
construct butterfly diagrams for stellar spots. The equatorward drift rates of the lower boundary of the spotted region D=dφlow/dt are (− 1)–(− 2) deg year−1 in the years of increasing spottedness. These values are less than the analogous solar one D≈−4 deg year−1 for the rising phase of the cycle. Thus, cyclic activity can be revealed from butterfly diagrams and derived drifts of starspots
prior to a possible detection from the spectral analysis of photometric variability. Finally, we briefly discuss a possible
explanation of high-latitude activity and surface drifts of starspots in the framework of the current state of dynamo theory. 相似文献
442.
The effect of particle-particle interaction on the adiabatic index γ for an electron-positron plasma is considered. An improved method for numerically calculating the Hartree-Fock exchange integral is presented and its relativistic asymptotics is determined. An approximation formula is derived for the correlation part of the interaction in the low-density limit. This formula includes degeneracy and the positron component. 相似文献
443.
Abstract— Eucrite meteorites are igneous rocks that derived from a large asteroid, probably 4 Vesta. Past studies have shown that after most eucrites formed, they underwent metamorphism in temperatures up to ≥800°C. Much later, many were brecciated and heated by large impacts into the parent body surface. The less common basaltic, unbrecciated eucrites also formed near the surface but, presumably, escaped later brecciation, while the cumulate eucrites formed at depths where metamorphism may have persisted for a considerable period. To further understand the complex HED parent body thermal history, we determined new 39Ar‐40Ar ages for 9 eucrites classified as basaltic but unbrecciated, 6 eucrites classified as cumulate, and several basaltic‐brecciated eucrites. Precise Ar‐Ar ages of 2 cumulate eucrites (Moama and EET 87520) and 4 unbrecciated eucrites give a tight cluster at 4.48 ± 0.02 Gyr (not including any uncertainties in the flux monitor age). Ar‐Ar ages of 6 additional unbrecciated eucrites are consistent with this age within their relatively larger age uncertainties. By contrast, available literature data on Pb‐Pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4.4–4.515 Gyr, and 147Sm‐143Nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4.41–4.55 Gyr. Similar Ar‐Ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites, as was previously proposed. We suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the K‐Ar and some other chronometers to remain as open diffusion systems. From the strong clustering of Ar‐Ar ages at ?4.48 Gyr, we propose that these meteorites were excavated from depth in a single large impact event ?4.48 Gyr ago, which quickly cooled the samples and started the K‐Ar chronometer. A large (?460 km) crater postulated to exist on Vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with Vesta. Some Pb‐Pb and Sm‐Nd ages of cumulate and unbrecciated eucrites are consistent with the Ar‐Ar age of 4.48 Gyr, and the few older Pb‐Pb and Sm‐Nd ages may reflect an isotopic closure before the large cratering event. One cumulate eucrite gives an Ar‐Ar age of 4.25 Gyr; 3 additional cumulate eucrites give Ar‐Ar ages of 3.4–3.7 Gyr; and 2 unbrecciated eucrites give Ar‐Ar ages of ?3.55 Gyr. We attribute these younger ages to a later impact heating. Furthermore, the Ar‐Ar impact‐reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3.5–4.1 Gyr. Among these, Piplia Kalan, the first eucrite to show evidence for extinct 26Al, was strongly impact heated ?3.5 Gyr ago. When these data are combined with eucrite Ar‐Ar ages in the literature, they confirm that several large impact heating events occurred on Vesta between ?4.1–3.4 Gyr ago. The onset of major impact heating may have occurred at similar times for both Vesta and the moon, but impact heating appears to have persisted for a somewhat later time on Vesta. 相似文献
444.
R. U. Claudi M. Cancian M. Barbieri R. Gratton S. Desidera M. Montalto G. P. Piotto S. Scuderi 《Astrophysics and Space Science》2006,304(1-4):251-251
RATS is an Italian project devoted to Hot Jupiter search with the transit method. A planet transiting in front of a host star can be mimed by several, and well defined, astrophysical phenomena (Brown, 2003). In order to recognize these false alarms we can utilize a preventive strategy to limit false alarm rates and a spectroscopic follow up to refuse no transit candidates. As preventive strategy it is important to develop an accurate target field selection, with well defined requisites, in order to maximize the solar type star numbers and to minimize the risk of possible astrophysical false alarms. 相似文献
445.
M.L. Litvak I.G. Mitrofanov A.B. Sanin W.V. Boynton D. Hamara R.S. Saunders 《Icarus》2006,180(1):23-37
In this paper, we have analyzed neutron spectroscopy data gathered by the High Energy Neutron Detector (HEND) instrument onboard Mars Odyssey for comparison of polar regions. It is known that observation of the neutron albedo of Mars provides important information about the distribution of water-ice in subsurface layers and about peculiarities of the CO2 seasonal cycle. It was found that there are large water-rich permafrost areas with contents of up to ∼50% water by mass fraction at both the north and south Mars polar regions. The water-ice layers at high northern latitudes are placed close to the surface, but in the south they are covered by a dry and relatively thick (10-20 cm) layer of soil. Analysis of temporal variations of neutron flux between summer and winter seasons allowed the estimation of the masses of the CO2 deposits which seasonally condense at the polar regions. The total mass of the southern seasonal deposition was estimated as 6.3×1015 kg, which is larger than the total mass of the seasonal deposition at the north by 40-50%. These results are in good agreement with predictions from the NASA Ames Research Center General Circulation Model (GCM). But, the dynamics of the condensation and sublimation processes are not quite as consistent with these models: the peak accumulation of the condensed mass of CO2 occurred 10-15 degrees of Ls later than is predicted by the GCM. 相似文献
446.
A. D. Biggs D. Rusin I. W. A. Browne A. G. de Bruyn N. J. Jackson L. V. E. Koopmans J. P. McKean S. T. Myers R. D. Blandford K.-H. Chae C. D. Fassnacht M. A. Norbury T. J. Pearson P. M. Phillips A. C. S. Readhead P. N. Wilkinson 《Monthly notices of the Royal Astronomical Society》2003,338(4):1084-1088
447.
448.
The collisionless interaction of an expanding plasma cloud with a magnetized background plasma is examined in the framework of a 3D kinetic-hydrodynamic model. The slowing down of a hydrogen cloud is studied for high Alfven-Mach numbers and magneto-laminar interaction parameters. A particle-in-cell method is used to study the dynamics of the magnetic field, plasma cloud, background plasma, and collisionless shock wave generated by the intense particle flux. A numerical simulation is consistent with the nonstationary interactions between the plasma shells formed during nova and supernova explosions and the interstellar plasma medium. 相似文献
449.
E. García L. De Haro C. O'Sullivan G. Cahill J. A. López Fernández F. Tercero B. Galocha J. L. Besada 《Experimental Astronomy》2003,15(3):173-193
A classical radioastronomy receiver is fed with a corrugated horn and an independent lens, both placed in a cryostat to lower
the noise temperature. The beam is focused and directed using a combination of elliptical and plane mirrors. This paper proposes
modifying the initial feeding system by placing the lens onto the horn aperture, thereby allowing a size reduction of the
horn and lens, and a simplification of their mechanical design. The profiled lens is shaped to correct the phase error on
the horn aperture. A quasi-optical model of the horn-plus-lens system has been developed using a Beam Mode Expansion (BME).
Results using both a hyperbolic-planar lens and a spherical-elliptical lens, as well as results obtained by using Geometrical
Optics (GO) with a Kirchoff–Huygens integration to get the far-field pattern, have been compared with measurements. As a direct
application, a full focusing system for the new 40-m radiotelescope at the “Centro Astronómico de Yebes” is presented for
the 22, 30 and 45 GHz bands. This paper has developed a QO model for a corrugated conical horn with a phase-correcting lens.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
450.