全文获取类型
收费全文 | 6595篇 |
免费 | 983篇 |
国内免费 | 825篇 |
专业分类
测绘学 | 396篇 |
大气科学 | 268篇 |
地球物理 | 1608篇 |
地质学 | 2388篇 |
海洋学 | 515篇 |
天文学 | 2443篇 |
综合类 | 262篇 |
自然地理 | 523篇 |
出版年
2024年 | 20篇 |
2023年 | 59篇 |
2022年 | 120篇 |
2021年 | 157篇 |
2020年 | 176篇 |
2019年 | 195篇 |
2018年 | 157篇 |
2017年 | 181篇 |
2016年 | 223篇 |
2015年 | 248篇 |
2014年 | 343篇 |
2013年 | 344篇 |
2012年 | 312篇 |
2011年 | 308篇 |
2010年 | 289篇 |
2009年 | 500篇 |
2008年 | 466篇 |
2007年 | 596篇 |
2006年 | 477篇 |
2005年 | 401篇 |
2004年 | 372篇 |
2003年 | 350篇 |
2002年 | 287篇 |
2001年 | 264篇 |
2000年 | 251篇 |
1999年 | 243篇 |
1998年 | 241篇 |
1997年 | 122篇 |
1996年 | 141篇 |
1995年 | 89篇 |
1994年 | 82篇 |
1993年 | 74篇 |
1992年 | 67篇 |
1991年 | 42篇 |
1990年 | 63篇 |
1989年 | 41篇 |
1988年 | 33篇 |
1987年 | 16篇 |
1986年 | 15篇 |
1985年 | 12篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1978年 | 4篇 |
1977年 | 1篇 |
1954年 | 5篇 |
排序方式: 共有8403条查询结果,搜索用时 343 毫秒
971.
Thomas R. Metcalf Marc L. DeRosa Carolus J. Schrijver Graham Barnes Adriaan A. van Ballegooijen Thomas Wiegelmann Michael S. Wheatland Gherardo Valori James M. McTtiernan 《Solar physics》2008,247(2):269-299
We compare a variety of nonlinear force-free field (NLFFF) extrapolation algorithms, including optimization, magneto-frictional,
and Grad – Rubin-like codes, applied to a solar-like reference model. The model used to test the algorithms includes realistic
photospheric Lorentz forces and a complex field including a weakly twisted, right helical flux bundle. The codes were applied
to both forced “photospheric” and more force-free “chromospheric” vector magnetic field boundary data derived from the model.
When applied to the chromospheric boundary data, the codes are able to recover the presence of the flux bundle and the field’s
free energy, though some details of the field connectivity are lost. When the codes are applied to the forced photospheric
boundary data, the reference model field is not well recovered, indicating that the combination of Lorentz forces and small
spatial scale structure at the photosphere severely impact the extrapolation of the field. Preprocessing of the forced photospheric
boundary does improve the extrapolations considerably for the layers above the chromosphere, but the extrapolations are sensitive
to the details of the numerical codes and neither the field connectivity nor the free magnetic energy in the full volume are
well recovered. The magnetic virial theorem gives a rapid measure of the total magnetic energy without extrapolation though,
like the NLFFF codes, it is sensitive to the Lorentz forces in the coronal volume. Both the magnetic virial theorem and the
Wiegelmann extrapolation, when applied to the preprocessed photospheric boundary, give a magnetic energy which is nearly equivalent
to the value derived from the chromospheric boundary, but both underestimate the free energy above the photosphere by at least
a factor of two. We discuss the interpretation of the preprocessed field in this context. When applying the NLFFF codes to
solar data, the problems associated with Lorentz forces present in the low solar atmosphere must be recognized: the various
codes will not necessarily converge to the correct, or even the same, solution.
On 07/07/2007, the NLFFF team was saddened by the news that Tom Metcalf had died as the result of an accident. We remain grateful
for having had the opportunity to benefit from his unwavering dedication to the problems encountered in attempting to understand
the Sun’s magnetic field; Tom had completed this paper several months before his death, leading the team through the many
steps described above. 相似文献
972.
We demonstrate that the azimuthal ambiguity that is present in solar vector magnetogram data can be resolved with line-of-sight
and horizontal heliographic derivative information by using the divergence-free property of magnetic fields without additional
assumptions. We discuss the specific derivative information that is sufficient to resolve the ambiguity away from disk centre,
with particular emphasis on the line-of-sight derivative of the various components of the magnetic field. Conversely, we also
show cases where ambiguity resolution fails because sufficient line-of-sight derivative information is not available. For
example, knowledge of only the line-of-sight derivative of the line-of-sight component of the field is not sufficient to resolve the ambiguity away from disk centre. 相似文献
973.
J. Ireland C. A. Young R. T. J. McAteer C. Whelan R. J. Hewett P. T. Gallagher 《Solar physics》2008,252(1):121-137
Two different multiresolution analyses are used to decompose the structure of active-region magnetic flux into concentrations
of different size scales. Lines separating these opposite polarity regions of flux at each size scale are found. These lines
are used as a mask on a map of the magnetic field gradient to sample the local gradient between opposite polarity regions
of given scale sizes. It is shown that the maximum, average, and standard deviation of the magnetic flux gradient for α,β,β
γ, and β
γ
δ active-regions increase in the order listed, and that the order is maintained over all length scales. Since magnetic flux
gradient is strongly linked to active-region activity, such as flares, this study demonstrates that, on average, the Mt. Wilson
classification encodes the notion of activity over all length scales in the active-region, and not just those length scales
at which the strongest flux gradients are found. Further, it is also shown that the average gradients in the field, and the
average length-scale at which they occur, also increase in the same order. Finally, there are significant differences in the
gradient distribution, between flaring and non-flaring active regions, which are maintained over all length scales. It is
also shown that the average gradient content of active-regions that have large flares (GOES class “M” and above) is larger
than that for active regions containing flares of all flare sizes; this difference is also maintained at all length scales.
All of the reported results are independent of the multiresolution transform used. The implications for the Mt. Wilson classification
of active-regions in relation to the multiresolution gradient content and flaring activity are discussed. 相似文献
974.
We study the relationship between the brightness (I) and magnetic field (B) distributions of sunspots using 272 samples observed at the San Fernando Observatory and the National Solar Observatory,
Kitt Peak, whose characteristics varied widely. We find that the I – B relationship has a quadratic form for the spots with magnetic field less than about 2000 G. The slope of the linear part
of the I – B curve varies by about a factor of three for different types of spots. In general the slope increases as the spot approaches
disk center. The I – B slope does not have a clear dependency on the spot size but the lower limit appears to increase as a function of the ratio
of umbra and penumbra area. The I – B slope changes as a function of age of the sunspots. We discuss various sunspot models using these results. 相似文献
975.
976.
A. M. Finkelstein E. A. Skurikhina I. F. Surkis A. V. Ipatov I. A. Rakhimov S. G. Smolentsev 《Astronomy Letters》2008,34(1):59-68
Regular high-precision determinations of the Earth’s orientation parameters (EOPs) on the Quasar VLBI Network were begun in
August 2006. The observations are performed within the framework of two national programs: daily sessions at three observatories
of the Network to determine all five EOPs (the RU-E program) and 8-h sessions on the Zelenchukskaya-Badary and Svetloe-Badary
baselines to determine the Universal Time (the RU-U program). The observations from August 2006 through May 2007 are analyzed.
The rms deviations of the EOP values obtained in the RU-E program from the IERS C04 series are 1.1 mas for X
p and Y
p, 37 μs for UT1-UTC, and 0.7 and 0.6 mas for X
c and Y
c, respectively. These results closely match the prospective requirements of GLONASS. The rms deviations of the Universal Times
obtained in the RU-U program from the IERS C04 series are 146 μs. We consider the immediate prospects for improving the accuracy
of EOP determinations in daily sessions and for implementing the e-VLBI mode for an online determination of the Universal
Time.
Original Russian Text ? A.M. Finkelstein, E.A. Skurikhina, I.F. Surkis, A.V. Ipatov, I.A. Rakhimov, S.G. Smolentsev, 2008,
published in Pis’ma v Astronomicheskiĭ Zhurnal, 2008, Vol. 34, No. 1, pp. 66–76. 相似文献
977.
Observations in polarized emission reveal the existence of large‐scale coherent magnetic fields in a wide range of spiral galaxies. Radio‐polarization data show that these fields are strongly inclined towards the radial direction, with pitch angles up to 35° and thus cannot be explained by differential rotation alone. Global dynamo models describe the generation of the radial magnetic field from the underlying turbulence via the so called α ‐effect. However, these global models still rely on crude assumptions about the small‐scale turbulence. To overcome these restrictions we perform fully dynamical MHD simulations of interstellar turbulence driven by supernova explosions. From our simulations we extract profiles of the contributing diagonal elements of the dynamo α ‐tensor as functions of galactic height. We also measure the coefficients describing vertical pumping and find that the ratio between these two effects has been overestimated in earlier analytical work, where dynamo action seemed impossible. In contradiction to these models based on isolated remnants we always find the pumping to be directed inward. In addition we observe that depends on whether clustering in terms of superbubbles is taken into account. Finally, we apply a test field method to derive a quantitative measure of the turbulent magnetic diffusivity which we determine to be ∼2 kpckms–1. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
978.
Mechanisms of nonhelical large‐scale dynamos (shear‐current dynamo and effect of homogeneous kinetic helicity fluctuations with zero mean) in a homogeneous turbulence with large‐scale shear are discussed. We have found that the shearcurrent dynamo can act even in random flows with small Reynolds numbers. However, in this case mean‐field dynamo requires small magnetic Prandtl numbers (i.e., when Pm < Pmcr < 1). The threshold in the magnetic Prandtl number, Pmcr = 0.24, is determined using second order correlation approximation (or first‐order smoothing approximation) for a background random flow with a scale‐dependent viscous correlation time τc = (νk 2)–1 (where ν is the kinematic viscosity of the fluid and k is the wave number). For turbulent flows with large Reynolds numbers shear‐current dynamo occurs for arbitrary magnetic Prandtl numbers. This dynamo effect represents a very generic mechanism for generating large‐scale magnetic fields in a broad class of astrophysical turbulent systems with large‐scale shear. On the other hand, mean‐field dynamo due to homogeneous kinetic helicity fluctuations alone in a sheared turbulence is not realistic for a broad class of astrophysical systems because it requires a very specific random forcing of kinetic helicity fluctuations that contains, e.g., low‐frequency oscillations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
979.
The effect of meridional circulation on spherical shell dynamos is considered in the Parker approximation. We demonstrate that the type of the exited solution crucially depend on the intensity of the meridional circulation. If the circulation is equatorward or if it is polarward however do not exceed some critical value, an oscillating solution in form of an equatorward traveling wave is excited. If the polarward meridional circulation becomes too intensive the solution becomes steady growing. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
980.
环境磁学理论、方法和研究进展 总被引:32,自引:1,他引:31
环境磁学是一门介于地球科学、环境科学和磁学之间的边缘科学。环境磁学可提供大量有关区域或全球环境变化及人类活动对环境影响等的重要信息,其研究范围迅速扩大,已成为当今地学前沿学科之一。本文在简述了环境磁学的理论和方法的基础上,介绍了环境磁学中几个主要研究领域如古气候与古环境变化、土壤学和环境污染研究等方面所取得的进展,指出了环境磁学研究中存在的不足之处,并提出了近年来环境磁学的一些新的发展趋势:①加强磁信息与地质记录之间的定量研究;②利用磁信息加强应用研究;③环境磁学与地球化学方法密切相结合;④建立全球磁数据 相似文献