全文获取类型
收费全文 | 2369篇 |
免费 | 255篇 |
国内免费 | 491篇 |
专业分类
测绘学 | 730篇 |
大气科学 | 658篇 |
地球物理 | 193篇 |
地质学 | 520篇 |
海洋学 | 223篇 |
天文学 | 499篇 |
综合类 | 143篇 |
自然地理 | 149篇 |
出版年
2024年 | 7篇 |
2023年 | 15篇 |
2022年 | 44篇 |
2021年 | 85篇 |
2020年 | 85篇 |
2019年 | 118篇 |
2018年 | 70篇 |
2017年 | 124篇 |
2016年 | 113篇 |
2015年 | 108篇 |
2014年 | 131篇 |
2013年 | 123篇 |
2012年 | 160篇 |
2011年 | 149篇 |
2010年 | 134篇 |
2009年 | 140篇 |
2008年 | 130篇 |
2007年 | 172篇 |
2006年 | 154篇 |
2005年 | 127篇 |
2004年 | 116篇 |
2003年 | 125篇 |
2002年 | 102篇 |
2001年 | 98篇 |
2000年 | 86篇 |
1999年 | 77篇 |
1998年 | 81篇 |
1997年 | 46篇 |
1996年 | 36篇 |
1995年 | 32篇 |
1994年 | 30篇 |
1993年 | 18篇 |
1992年 | 18篇 |
1991年 | 20篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 2篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1954年 | 1篇 |
排序方式: 共有3115条查询结果,搜索用时 328 毫秒
981.
Justin Oelgoetz Anil K. Pradhan 《Monthly notices of the Royal Astronomical Society》2001,327(4):L42-L46
It is shown that the dielectronic satellites (DES) dominate X-ray spectral formation in the 6.7-keV K α complex of Fe xxv at temperatures below that of maximum abundance in collisional ionization equilibrium T m . Owing to their extreme temperature sensitivity, the DES are excellent spectral diagnostics for in photoionized, collisional or hybrid plasmas; whereas the forbidden, intercombination and resonance lines of Fe xxv are not. A diagnostic line ratio GD ( T ) is defined including the DES and the lines, with parameters from new relativistic atomic calculations. The DES absorption resonance strengths may be obtained from differential oscillator strengths, possibly to yield the column densities. The DES contribution to highly ionized Fe should be of interest for models of redward broadening of K α features, ionized accretion discs, accretion flows and K α temporal-temperature variability in AGN. 相似文献
982.
We model the reflected spectrum expected from localized magnetic flares above an ionized accretion disc. We concentrate on the case of very luminous magnetic flares above a standard accretion disc extending down to the last stable orbit, and use a simple parametrization to allow for an X-ray-driven wind. Full disc spectra including relativistic smearing are calculated. When fitted with the constant-density reflection models, these spectra give both a low reflected fraction and a small linewidth as seen in the hard spectra from galactic black hole binaries and active galactic nuclei. We fit our calculated spectra to real data from the low/hard state of Nova Muscae and Cyg X-1 and show that these models give comparable χ 2 to those obtained from the constant-density reflection models, which implied a truncated disc. This explicitly demonstrates that the data are consistent either with magnetic flares above an ionized disc extending down to the last stable orbit around a black hole, or with non-ionized, truncated discs. 相似文献
983.
Giorgio Matt 《Monthly notices of the Royal Astronomical Society》2002,337(1):147-150
We calculate the equivalent width of the core, and the centroid energy and relative flux of the first-order Compton shoulder of the iron Kα emission line from neutral matter. The calculations are performed with Monte Carlo simulations. We explore a large range of column densities for both transmitted and reflected spectra, and study the dependence on the iron abundance. The Compton shoulder is now becoming observable in many objects thanks to the improved sensitivity and/or energy resolution of the XMM – Newton and Chandra satellites, and the present work aims to provide a tool to derive information on the geometry and element abundances of the line-emitting matter from Compton shoulder measurements. 相似文献
984.
A comprehensive numerical study on the three-dimensional structure of a turbulent jet in crossflow is performed. Thejet-to-crossflow velocity ratio (R) varies in the range of 2 ~ 16; both vertical jets and inclined jets without excess streamwisemomentum are considered. The numerical results of the standard two-equation k-~ model show that the turbulent structurecan be broadly categorised according to the jet-to-crossflow velocity ratio. For strong to moderate jet discharges, i.e. R >4, the jet is characterized by a longitudinal transition through a bent-over phase during which the jet becomes almost parallelwith the main freestream, to a sectional vortex-pair flow with double concentration maxima; the computed flow details andscalar mixing characteristics can be described by self-similar relations beyond a dimensionless distance of around 20 ~ 60.The similarity coefficients are only weakly dependent on R. The cross-section scalar field is kidney-shaped and bifurcated,with distinct double concentration maxima; the aspect ratio is found to be around 1.2. A loss in vertical momentum is ob-served and the added mass coefficient of the jet motion is found to be approximately 1. On the other hand, for weak jets instrong crossflow, i.e. R≤ 2, the lee of the jet is characterized by a negative pressure region. Although the double vortexflow can still be noted, the scalar field becomes more symmetrical and no longer bifurcated. The similarity coeffcients are al-so noticeably different. The predicted jet flow characteristics and mixing rates are well supported by experimental and field data 相似文献
985.
986.
987.
L. A. Balona D. J. James W. A. Lawson R. R. Shobbrook 《Monthly notices of the Royal Astronomical Society》2001,324(4):1041-1053
We present new intensive photometric observations of the Be star μ Cen for several seasons which support a period close to 1 d. We also present high‐resolution spectroscopic data consisting of 302 spectra in 1999 and 864 spectra in 2000, all obtained within a two‐week observing run in each season. We use stacked grey-scale plots of spectra, from which the mean line profile has been removed, to examine the profile variations. We find that most nights show one residual absorption feature, moving from blue to red, visible in all helium and metal lines and also clearly visible in H α and other lines formed in the circumstellar environment. We therefore conclude that this feature is of circumstellar origin. In addition, a residual absorption feature moving from red to blue is sometimes seen at irregular intervals. We find that the residual absorption feature frequently strays outside the projected rotational velocity limit and that on occasions it remains well within this limit. The radial velocity data reproduce only two of the six frequencies previously found in the star. We point out that this by no means implies that the star is a multiperiodic, non-radial pulsator. Photometric data obtained over several seasons indicate a period very close to 1 d and not the 0.5-d period found from the radial velocities. We describe an outburst which occurred during the run and which resulted in increased H α emission two nights later. It is clear that outbursts in Be stars are localized events and that the gas released by outbursts is probably responsible for localized increased absorption, resulting in periodic light and line profile variations. 相似文献
988.
989.
990.
J.R. Barnes A. Collier Cameron D.J. James D. Steeghs 《Monthly notices of the Royal Astronomical Society》2001,326(3):1057-1066
We present two images of intermediate and low axial inclination G dwarfs (AP 149 and AP 193) in the young open cluster α Persei, and compare these with previous images of intermediate and high axial inclination objects in this cluster. All stars show starspots at high latitudes, with one star exhibiting a strong polar spot. Although low-latitude features are found on all stars to some degree, the detection of spots on AP 193 is marginal. The apparent difference in starspot morphology from one object to the next is probably the result of a stellar magnetic cycle, although the exact effect on the starspot distribution throughout a cycle is unknown.
Polar spots are found in many Doppler images of rapidly rotating cool stars. In the past, their existence has been called into question, and it has been suggested that they could be the manifestations of NLTE (e.g. chromospheric filling in of line profiles) effects rather than real photospheric features. We assume the polar spots to be real photospheric features, and conclude that the flat-bottomed nature of the profile shape can be attributed to photospheric polar spots. The degree of truncation of the profile depends not only on spot size and strength, but also on the effective foreshortening of the polar region, a function of axial inclination.
H α is in emission on AP 149 which shows a double peak at most phases. The time-series of the profile shows an s-wave pattern as the position of these peaks changes throughout the rotation cycle. We attribute this to coronal clouds located above the stellar surface in synchronous orbit. A maximum-entropy tomogram is derived revealing four distinct emission regions located near and above the corotation radius. 相似文献
Polar spots are found in many Doppler images of rapidly rotating cool stars. In the past, their existence has been called into question, and it has been suggested that they could be the manifestations of NLTE (e.g. chromospheric filling in of line profiles) effects rather than real photospheric features. We assume the polar spots to be real photospheric features, and conclude that the flat-bottomed nature of the profile shape can be attributed to photospheric polar spots. The degree of truncation of the profile depends not only on spot size and strength, but also on the effective foreshortening of the polar region, a function of axial inclination.
H α is in emission on AP 149 which shows a double peak at most phases. The time-series of the profile shows an s-wave pattern as the position of these peaks changes throughout the rotation cycle. We attribute this to coronal clouds located above the stellar surface in synchronous orbit. A maximum-entropy tomogram is derived revealing four distinct emission regions located near and above the corotation radius. 相似文献