全文获取类型
收费全文 | 36318篇 |
免费 | 6055篇 |
国内免费 | 7530篇 |
专业分类
测绘学 | 3539篇 |
大气科学 | 4466篇 |
地球物理 | 6078篇 |
地质学 | 19518篇 |
海洋学 | 3924篇 |
天文学 | 4415篇 |
综合类 | 2389篇 |
自然地理 | 5574篇 |
出版年
2024年 | 175篇 |
2023年 | 452篇 |
2022年 | 1187篇 |
2021年 | 1365篇 |
2020年 | 1254篇 |
2019年 | 1654篇 |
2018年 | 1140篇 |
2017年 | 1353篇 |
2016年 | 1376篇 |
2015年 | 1574篇 |
2014年 | 2025篇 |
2013年 | 1999篇 |
2012年 | 2125篇 |
2011年 | 2282篇 |
2010年 | 1988篇 |
2009年 | 2527篇 |
2008年 | 2467篇 |
2007年 | 2545篇 |
2006年 | 2506篇 |
2005年 | 2393篇 |
2004年 | 2083篇 |
2003年 | 1972篇 |
2002年 | 1685篇 |
2001年 | 1504篇 |
2000年 | 1482篇 |
1999年 | 1315篇 |
1998年 | 1144篇 |
1997年 | 793篇 |
1996年 | 668篇 |
1995年 | 556篇 |
1994年 | 530篇 |
1993年 | 449篇 |
1992年 | 311篇 |
1991年 | 278篇 |
1990年 | 192篇 |
1989年 | 152篇 |
1988年 | 117篇 |
1987年 | 73篇 |
1986年 | 41篇 |
1985年 | 35篇 |
1984年 | 19篇 |
1983年 | 18篇 |
1982年 | 16篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1978年 | 14篇 |
1977年 | 7篇 |
1976年 | 4篇 |
1954年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 640 毫秒
31.
O. Tamuz T. Mazeh S. Zucker † 《Monthly notices of the Royal Astronomical Society》2005,356(4):1466-1470
We suggest a new algorithm to remove systematic effects in a large set of light curves obtained by a photometric survey. The algorithm can remove systematic effects, such as those associated with atmospheric extinction, detector efficiency, or point spread function changes over the detector. The algorithm works without any prior knowledge of the effects, as long as they linearly appear in many stars of the sample. The approach, which was originally developed to remove atmospheric extinction effects, is based on a lower rank approximation of matrices, an approach which has already been suggested and used in chemometrics, for example. The proposed algorithm is especially useful in cases where the uncertainties of the measurements are unequal. For equal uncertainties, the algorithm reduces to the Principal Component Analysis (PCA) algorithm. We present a simulation to demonstrate the effectiveness of the proposed algorithm and we point out its potential, in the search for transit candidates in particular. 相似文献
32.
Sara L. Ellison 《Monthly notices of the Royal Astronomical Society》2006,368(1):335-340
The number of z ∼ 1 damped Lyα systems (DLAs, log N (H i ) ≥ 20.3) per unit redshift is approximately 0.1, making them relatively rare objects. Large, blind QSO surveys for low-redshift DLAs are therefore an expensive prospect for space-borne ultraviolet telescopes. Increasing the efficiency of these surveys by pre-selecting DLA candidates based on the equivalent widths (EWs) of metal absorption lines has previously been a successful strategy. However, the success rate of DLA identification is still only ∼35 per cent when simple EW cut-offs are applied, the majority of systems having 19.0 < log N (H i ) < 20.3. Here, we propose a new way to pre-select DLA candidates. Our technique requires high-to-moderate-resolution spectroscopy of the Mg ii λ2796 transition, which is easily accessible from the ground for 0.2 ≲ z ≲ 2.4. We define the D -index, the ratio of the line equivalent width to velocity spread, and measure this quantity for 19 DLAs and eight sub-DLAs in archival spectra obtained with echelle spectrographs. For the majority of absorbers, there is a clear distinction between the D -index of DLAs compared with sub-DLAs (Kolmogorov–Smirnov probability = 0.8 per cent). Based on this pilot data sample, we find that the D -index can select DLAs with a success rate of up to 90 per cent, an increase in selection efficiency by a factor of 2.5 compared with a simple EW cut. We test the applicability of the D -index at lower resolution and find that it remains a good discriminant of DLAs for full width at half-maximum (FWHM) ≲ 1.5 Å. However, the recommended D -index cut-off between DLAs and sub-DLAs decreases with poorer resolution and we tabulate the appropriate D -index values that should be used with spectra of different resolutions. 相似文献
33.
34.
Flavio Scappini Cesare Cecchi-Pestellini Harvey Smith William Klemperer Alexander Dalgarno 《Monthly notices of the Royal Astronomical Society》2003,341(2):657-661
We consider sulphur depletion in dense molecular clouds, and suggest hydrated sulphuric acid, H2 SO4 · n H2 O, as a component of interstellar dust in icy mantles. We discuss the formation of hydrated sulphuric acid in collapsing clouds and its instability in heated regions in terms of the existing hot core models and observations. We also show that some features of the infrared spectrum of hydrated sulphuric acid have correspondence in the observed spectra of young stellar objects. 相似文献
35.
36.
37.
We present a new general scheme for calculating the structure and dynamics of radiation-pressure-driven photoionized flows. The new method goes one step beyond the Sobolev approximation. It involves a numerical solution of the radiative transfer in absorption lines, including the effects of differential expansion and line interactions such as line locking and blanketing. We also present a new scheme for calculating the radiation pressure due to trapped line photons in finite, differentially expanding flows. We compare our results for the radiation pressure force with those obtained using the Sobolev approximation and show the limitations of the latter. In particular, we demonstrate that the Sobolev method gives a poor approximation near discontinuity surfaces and its neglect of line blanketing can lead to erroneous results in high-velocity flows. We combine the newly calculated radiation pressure force with self-consistent photoionization and thermal calculations to study the dynamics and spectral features of broad absorption-line flows and highly ionized gas flows in active galactic nuclei (AGN). A comparison with Sobolev-type calculations shows that the latter overestimates the terminal velocity of the flow and, conversely, underestimates its opacity. We also show that line locking on broad emission lines can have a significant effect on the dynamics and spectral features of AGN flows. 相似文献
38.
The time evolution behaviour of the superhumps of the dwarf nova1RXS J232953.9+ 062814 is investigated with the wavelet analysis
method. On the basis of two nights CCD photometry performed during its first superoutburst as well as other published brightness
data, we reveal the superhump's time-dependence as a function of periods and time. Our light curves, which phased in the rapid
decay ending portion of the superoutburst and in the dawn of a following normal outburst, are important to help trace the
superhump evolution for the star. Both the superhump period and the orbital period of the binary system are detected in the
present data. We obtain P
sh=0.0458±0.0002d and P
orb=0.0450±0.0002d. They agree with those existing values. The two periods exchanged their roles during the superhump evolution.
The general profile of brightness fading over the outbursts roughly followed an exponential decay law or a form of a five-order
polynomial.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
39.
MENGLi-li 《中国地理科学(英文版)》2003,13(2):190-192
The exploitation and utilization of the tour resources of tournament athletics, including skiling, boat sail-ing, archery, ice engraving, snow engraving, has become a new trend of the development of Chinese tourism. Due tothe unique cold climate and superior geographic location, Harbin is a promising city for developing tour resources of tourna-ment athletics. Based on the analysis of the superiority and peculiarity of Harbin, the speculation on development of tour resources of tournament athletics in Harbin is proposed as follows : 1 ) Harbin should develop its special tour resources of tournament athletics associated with needs of market; 2) Harbin should take the advantages of rich resources and devel-op ice and snow entertainment in winter and travel for sight-seeing and spending summer;, 3) the adjustment of the layoutof ice and snow resources should be based on the idea of taking Harbin as the center and all-side opening at the largescale in the way of radiation; 4) tourism should be developed by the combined efforts of various departments to make feasi-ble plan, and the organizers should pay much attention to ensuring the safetv of tourists. 相似文献
40.