全文获取类型
收费全文 | 111688篇 |
免费 | 1731篇 |
国内免费 | 1127篇 |
专业分类
测绘学 | 2915篇 |
大气科学 | 7943篇 |
地球物理 | 22784篇 |
地质学 | 39670篇 |
海洋学 | 9701篇 |
天文学 | 25019篇 |
综合类 | 376篇 |
自然地理 | 6138篇 |
出版年
2022年 | 660篇 |
2021年 | 1188篇 |
2020年 | 1280篇 |
2019年 | 1331篇 |
2018年 | 3061篇 |
2017年 | 2879篇 |
2016年 | 3681篇 |
2015年 | 2105篇 |
2014年 | 3456篇 |
2013年 | 5995篇 |
2012年 | 3676篇 |
2011年 | 4851篇 |
2010年 | 4135篇 |
2009年 | 5384篇 |
2008年 | 4829篇 |
2007年 | 4629篇 |
2006年 | 4385篇 |
2005年 | 3423篇 |
2004年 | 3371篇 |
2003年 | 3191篇 |
2002年 | 2978篇 |
2001年 | 2693篇 |
2000年 | 2588篇 |
1999年 | 2062篇 |
1998年 | 2204篇 |
1997年 | 2049篇 |
1996年 | 1749篇 |
1995年 | 1738篇 |
1994年 | 1516篇 |
1993年 | 1354篇 |
1992年 | 1326篇 |
1991年 | 1221篇 |
1990年 | 1361篇 |
1989年 | 1123篇 |
1988年 | 1081篇 |
1987年 | 1241篇 |
1986年 | 1090篇 |
1985年 | 1341篇 |
1984年 | 1486篇 |
1983年 | 1352篇 |
1982年 | 1317篇 |
1981年 | 1155篇 |
1980年 | 1068篇 |
1979年 | 1012篇 |
1978年 | 1012篇 |
1977年 | 884篇 |
1976年 | 841篇 |
1975年 | 830篇 |
1974年 | 792篇 |
1973年 | 785篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The reconstruction of solar activity during the late 18th century is a puzzle for researchers due to the scarcity of sunspot
observations in that epoch. In this work, we analyse some details of the solar activity during the years 1781–1788, inferred
from geomagnetic measurements and visual observations of aurorae performed by the Portuguese scientist Bento Sanches Dorta
from Rio de Janeiro. We describe in greater detail four large solar storms that induced large changes in daily values of geomagnetic
declination and, simultaneously, correspond to visual observations of aurorae described by Sanches Dorta. 相似文献
992.
We examine the temporal variation of power in low-ℓ modes using GONG data for the period of May 1995–October 2005 and compare
this with disk-integrated flare and CME indices. A poor correlation between the running means of Flare Index and mode power
is found. A similar result is found for CME Index also. Variations in the running mean mode power corresponding to ℓ = 0 modes
with different radial orders are generally stochastic in nature. This behaviour is also reflected in the distribution of mode
power. 相似文献
993.
994.
995.
996.
The seasonal variation of the nocturnal average intensity, and the typical variation of the sodium airglow intensity during the night have been deduced, using a data set that has been accumulated over a period of 5 years. The maximum intensity, which occurs at the equinoxes, is about 2–3 times as large as the intensity during winter, when the intensity of the D2 line is about 30 R. The nocturnal variation is symmetrical about midnight, where the intensity is some 30% less than just after (before) dusk (dawn) values. 相似文献
997.
We describe the use of a package of subroutines for general-relativistic algebraic computations written in the LISP-based algebraic programming system REDUCE developed by Anthony Hearn. The first group of routines calculates concomitants of the metric tensor such as the Riemann tensor, Ricci tensor, Ricci scalar, Einstein tensor and Weyl tensor from given covariant or covariant and contravariant metric tensor components. One of these procedures includes the evaluation of the Maxwell equations and invariants as well as the Maxwell energy momentum tensor. A second group of routines takes the components of a null tetrad as input and evaluates the null frame projections of the Riemann tensor, the Einstein tensor as well as – if required – the 14 local invariants of the Riemann tensor. It also includes a determination of the Petrov type. Perturbation calculations may be performed and run effectively. The output can be presented in a flexible format chosen according to the needs of the user. A number of further special-purpose programs are available on request. The article should enable a reader familar with General Relativity but unfamilar with formula manipulating system, to employ the package. 相似文献
998.
999.
Falconer David A. Jordan Stuart D. Brosius Jeffrey W. Davila Joseph M. Thomas Roger J. Andreatta Vicenzo Hara Hirohisa 《Solar physics》1998,180(1-2):179-191
We investigate the possibility that strong EUV lines observed with the Goddard Solar EUV Rocket Telescope and Spectrograph (SERTS) provide good proxies for estimating the total coronal flux over shorter wavelength ranges. We use coordinated SERTS and Yohkoh observations to obtain both polynomial and power-law fits relating the broad-band soft X-ray fluxes to the intensities of Fexvi 335 Ú and 361 Ú, Fexv 284 Ú and 417 Ú, and Mgix 368 Ú measured with SERTS. We found that the power-law fits best cover the full range of solar conditions from quiet Sun through active region, though not surprisingly the cooler Mgix 368 Ú line proves to be a poor proxy. The quadratic polynomial fits yield fair agreement over a large range for all but the Mgix line. However, the linear fits fail conspicuously when extrapolated into the quiet-Sun regime. The implications of this work for the Heii 304 Ú line formation problem are also briefly considered. 相似文献
1000.
Bianchi type V cosmological models are studied that contain a relativistic ideal Boltzmann gas. The effect of a cosmological constant upon the space-time geometry is also considered. In both high and low temperature limit the general solution of the Einstein gravitational field equations can be expressed in an exact closed parametric form. At final stages, depending on the presence or absence of the cosmological constant, cosmologies are driven to an isotropic inflationary open de Sitter type Universe or to an isotropic open Friedmann era. 相似文献