全文获取类型
收费全文 | 2279篇 |
免费 | 327篇 |
国内免费 | 403篇 |
专业分类
测绘学 | 104篇 |
大气科学 | 816篇 |
地球物理 | 374篇 |
地质学 | 258篇 |
海洋学 | 216篇 |
天文学 | 923篇 |
综合类 | 83篇 |
自然地理 | 235篇 |
出版年
2024年 | 13篇 |
2023年 | 20篇 |
2022年 | 42篇 |
2021年 | 61篇 |
2020年 | 49篇 |
2019年 | 51篇 |
2018年 | 51篇 |
2017年 | 49篇 |
2016年 | 51篇 |
2015年 | 80篇 |
2014年 | 95篇 |
2013年 | 101篇 |
2012年 | 93篇 |
2011年 | 109篇 |
2010年 | 90篇 |
2009年 | 194篇 |
2008年 | 172篇 |
2007年 | 197篇 |
2006年 | 160篇 |
2005年 | 165篇 |
2004年 | 146篇 |
2003年 | 134篇 |
2002年 | 126篇 |
2001年 | 104篇 |
2000年 | 78篇 |
1999年 | 79篇 |
1998年 | 82篇 |
1997年 | 67篇 |
1996年 | 61篇 |
1995年 | 60篇 |
1994年 | 52篇 |
1993年 | 44篇 |
1992年 | 26篇 |
1991年 | 29篇 |
1990年 | 19篇 |
1989年 | 16篇 |
1988年 | 15篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有3009条查询结果,搜索用时 15 毫秒
901.
902.
E. Carretti S. Poppi W. Reich P. Reich E. Fürst G. Bernardi S. Cortiglioni C. Sbarra 《Monthly notices of the Royal Astronomical Society》2006,367(1):132-138
Polarized diffuse emission observations at 1.4 GHz in a high Galactic latitude area of the Northern celestial hemisphere are presented. The 3.2 × 3.2 deg2 field, centred at RA = 10h 58m , Dec. =+42°18' (B1950) , has Galactic coordinates l ∼ 172°, b ∼+63° and is located in the region selected as northern target of the Balloon-borne Radiometers for Sky Polarization Observations experiment. Observations have been performed with the Effelsberg 100-m telescope. We find that the angular power spectra of the E and B modes have slopes of β E =−1.79 ± 0.13 and β B =−1.74 ± 0.12 , respectively. Because of the very high Galactic latitude and the smooth emission, a weak Faraday rotation action is expected, which allows both a fair extrapolation to cosmic microwave background polarization (CMBP) frequencies and an estimate of the contamination by the Galactic synchrotron emission. We extrapolate the E -mode spectrum up to 32 GHz and confirm the possibility to safely detect the CMBP E -mode signal in the Ka band found in another low-emission region. Extrapolated up to 90 GHz, the Galactic synchrotron B mode looks to compete with the cosmic signal only for models with a tensor-to-scalar perturbation power ratio T / S < 0.001 , which is even lower than the T / S value of 0.01 found to be accessible in the only other high Galactic latitude area investigated to date. This suggests that values as low as T / S = 0.01 might be accessed at high Galactic latitudes. Such low-emission values can allow a significant redshift of the best frequency to detect the CMBP B mode, also reducing the contamination by Galactic dust, and opening interesting perspectives to investigate inflation models. 相似文献
903.
G. A. Galazutdinov G. Manicò J. Kreowski † 《Monthly notices of the Royal Astronomical Society》2006,366(3):1075-1080
In this paper, we report the very first observation of diffuse interstellar bands (DIBs) that, in the spectrum of HD 34078 (AE Aur), are blueshifted with respect to the normal position that they have in other objects, where the rest-wavelength velocity frame is determined using very sharp interstellar atomic lines or molecular features. Only reasonably narrow DIBs seemingly show this effect, which is absent in broader ones. The result is confirmed independently using three different spectrographs attached to two different telescopes. 相似文献
904.
R. J. Barber J. Tennyson G. J. Harris R. N. Tolchenov 《Monthly notices of the Royal Astronomical Society》2006,368(3):1087-1094
A computed list of H2 16 O infrared transition frequencies and intensities is presented. The list, BT2, was produced using a discrete variable representation two-step approach for solving the rotation–vibration nuclear motions. It is the most complete water line list in existence, comprising over 500 million transitions (65 per cent more than any other list) and it is also the most accurate (over 90 per cent of all known experimental energy levels are within 0.3 cm−1 of the BT2 values). Its accuracy has been confirmed by extensive testing against astronomical and laboratory data.
The line list has been used to identify individual water lines in a variety of objects including comets, sunspots, a brown dwarf and the nova-like object V838 Mon. Comparison of the observed intensities with those generated by BT2 enables water abundances and temperatures to be derived for these objects. The line list can also be used to provide an opacity for models of the atmospheres of M dwarf stars and assign previously unknown water lines in laboratory spectra. 相似文献
The line list has been used to identify individual water lines in a variety of objects including comets, sunspots, a brown dwarf and the nova-like object V838 Mon. Comparison of the observed intensities with those generated by BT2 enables water abundances and temperatures to be derived for these objects. The line list can also be used to provide an opacity for models of the atmospheres of M dwarf stars and assign previously unknown water lines in laboratory spectra. 相似文献
905.
906.
Ashish Asgekar Avinash A. Deshpande 《Monthly notices of the Royal Astronomical Society》2001,326(4):1249-1254
In the previous paper of this series, Deshpande & Rankin reported results regarding the sub-pulse drift phenomenon in pulsar B0943+10 at 430 and 111 MHz. This study has led to the identification of a stable system of sub-beams circulating around the magnetic axis of this star. Here, we present a single-pulse analysis of our observations of this pulsar at 35 MHz. The fluctuation properties seen at this low frequency, as well as our independent estimates of the number of sub-beams required and their circulation time, agree remarkably well with the reported behaviour at higher frequencies. We use the 'cartographic' transform mapping technique developed by Deshpande & Rankin in Paper I to study the emission pattern in the polar region of this pulsar. The significance of our results in the context of radio emission mechanisms is also discussed. 相似文献
907.
Anisotropic Beam Model for the Spectral Observations of Radio Burst Fine Structures on 1998 April 15
A fine structure consisting of three almost equidistant frequency bands was observed in the high frequency part of a solar burst on 1998 April 15 by the spectrometer of Beijing Astronomical Observatory in the range 2.6-3.8GHz. A model for this event based on beam-anisotropic instability in the solar corona is presented. Longitudinal plasma waves are excited at cyclotron resonance and then transformed into radio emission at their second harmonic.The model is in accordance with the observations if we suppose a magnetic field strength in the region of emission generation of about 200G. 相似文献
908.
Production of neutrons, neutrinos and gamma-rays by a very fast pulsar in the Galactic Centre region
W.Bednarek 《Monthly notices of the Royal Astronomical Society》2002,331(2):483-487
We consider the possibility that the excess of cosmic rays near ∼1018 eV, reported by the AGASA and SUGAR groups from the direction of the Galactic Centre, is caused by a young, very fast pulsar in the high-density medium. The pulsar accelerates iron nuclei to energies ∼1020 eV, as postulated by the Galactic models for the origin of the highest-energy cosmic rays. The iron nuclei, about 1 yr after pulsar formation, leave the supernova envelope without energy losses and diffuse through the dense central region of the Galaxy. Some of them collide with the background matter creating neutrons (from disintegration of Fe), neutrinos and gamma-rays (in inelastic collisions). We suggest that neutrons produced at a specific time after the pulsar formation are responsible for the observed excess of cosmic rays at ∼1018 eV. From normalization of the calculated neutron flux to the one observed in the cosmic ray excess, we predict the neutrino and gamma-ray fluxes. It has been found that the 1 km2 neutrino detector of the IceCube type should detect from a few up to several events per year from the Galactic Centre, depending on the parameters of the considered model. Moreover, future systems of Cherenkov telescopes (CANGAROO III, HESS, VERITAS) should be able to observe 1–10 TeV gamma-rays from the Galactic Centre if the pulsar was created inside a huge molecular cloud about 3–10×103 yr ago. 相似文献
909.
The contribution presents an analysis of the Leonids 1999 HDTV Leonid MAC data. The observed grouping of the Leonids over
a random level is explained by progressive fragmentation of meteoroids in space. The observed data are compared with a spatial
distribution model. Possible fragmentation processes of meteoroids in space are presented and discussed. 相似文献
910.
For decades, ground-based radio observations of Jovian synchrotron radiation have shown emission originating predominantly from the equatorial region and from high-latitude regions (lobes) near L∼2.5. The observations show a longitudinally asymmetric gap between the emission peaks of the lobes and the atmosphere of Jupiter. One possible explanation for these gaps is the loss of electrons through collisions with atmospheric neutrals as the electrons bounce along magnetic field lines and drift longitudinally in the presence of asymmetric magnetic fields. To assess this hypothesis, we applied the recently developed O6 and VIP4 magnetic field models to calculate the trajectories of electrons as they drift longitudinally in Jupiter's magnetic field, and derive the sizes of their equatorial drift loss cones. We then identified the shells on which electrons would be lost due to collisions with the atmosphere. The calculated drift loss cone sizes could be applied in future to the modeling of electron distribution functions in this region and could also be applied to the study of Jovian auroral zone. This method also allowed us to compute the shell-splitting effects for these drifting electrons and we find the shell-splitting to be small (?0.05RJ). This justifies a recent modeling assumption that particles drift on the same shells in a three-dimensional distribution model of electrons. We also compared the computed gaps with the observed gaps, and found that the atmospheric loss mechanism alone is not able to sufficiently explain the observed gap asymmetry. 相似文献