全文获取类型
收费全文 | 34766篇 |
免费 | 1345篇 |
国内免费 | 2385篇 |
专业分类
测绘学 | 1779篇 |
大气科学 | 3105篇 |
地球物理 | 5879篇 |
地质学 | 17500篇 |
海洋学 | 2817篇 |
天文学 | 4911篇 |
综合类 | 766篇 |
自然地理 | 1739篇 |
出版年
2024年 | 89篇 |
2023年 | 246篇 |
2022年 | 475篇 |
2021年 | 568篇 |
2020年 | 450篇 |
2019年 | 542篇 |
2018年 | 3837篇 |
2017年 | 3659篇 |
2016年 | 2319篇 |
2015年 | 612篇 |
2014年 | 723篇 |
2013年 | 752篇 |
2012年 | 1840篇 |
2011年 | 3602篇 |
2010年 | 3386篇 |
2009年 | 3421篇 |
2008年 | 2740篇 |
2007年 | 3394篇 |
2006年 | 588篇 |
2005年 | 931篇 |
2004年 | 741篇 |
2003年 | 758篇 |
2002年 | 510篇 |
2001年 | 202篇 |
2000年 | 206篇 |
1999年 | 168篇 |
1998年 | 161篇 |
1997年 | 168篇 |
1996年 | 134篇 |
1995年 | 101篇 |
1994年 | 83篇 |
1993年 | 94篇 |
1992年 | 85篇 |
1991年 | 66篇 |
1990年 | 55篇 |
1989年 | 51篇 |
1988年 | 51篇 |
1987年 | 51篇 |
1986年 | 57篇 |
1985年 | 44篇 |
1984年 | 48篇 |
1983年 | 48篇 |
1982年 | 48篇 |
1981年 | 53篇 |
1980年 | 59篇 |
1979年 | 42篇 |
1977年 | 21篇 |
1976年 | 25篇 |
1975年 | 23篇 |
1973年 | 23篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Yu. A. Fadeyev 《Astronomy Letters》2010,36(5):362-369
Hydrodynamic calculations of nonlinear radial oscillations of LBV stars with effective temperatures 1.5 × 104 K ⩽ T
eff ⩽ 3 × 104 K and luminosities 1.2 × 106
L
⊙ ⩽ L ⩽ 1.9 × 106
L
⊙ have been performed. Models for the evolutionary sequences of Population I stars (X = 0.7, Z = 0.02) with initial masses 70M
⊙ ⩽ M
ZAMS ⩽ 90M
⊙ at the initial helium burning stage have been used as the initial conditions. The radial oscillations develop on a dynamical
time scale and are nonlinear traveling waves propagating from the core boundary to the stellar surface. The amplitude of the
velocity variations for the outer layers is several hundred km s−1, while the bolometric magnitude variations are within ΔM
bol ⩽ 0·
m
2. The onset of oscillations is not related to the κ-mechanism and is attributable to the instability of a self-gravitating envelope gas whose adiabatic index is close to its
critical value of Γ1 = 4/3 due to the dominant contribution of radiation in the internal energy and pressure. The interval of magnitude variation
periods (6 days ≤ II ≤ 31 days) encompasses all currently available estimates of the microvariability periods for LBV stars,
suggesting that this type of nonstationarity is pulsational in origin. 相似文献
992.
A. Papaioannou O. Malandraki A. Belov R. Skoug H. Mavromichalaki E. Eroshenko A. Abunin S. Lepri 《Solar physics》2010,266(1):181-193
In this work an analysis of a series of complex cosmic ray events that occurred between 17 January 2005 and 23 January 2005
using solar, interplanetary and ground based cosmic ray data is being performed. The investigated period was characterized
both by significant galactic cosmic ray (GCR) and solar cosmic ray (SCR) variations with highlighted cases such as the noticeable
series of Forbush effects (FEs) from 17 January 2005 to 20 January 2005, the Forbush decrease (FD) on 21 January 2005 and
the ground level enhancement (GLE) of the cosmic ray counter measurements on 20 January 2005. The analysis is focusing on
the aforementioned FE cases, with special attention drawn on the 21 January 2005, FD event, which demonstrated several exceptional
features testifying its uniqueness. Data from the ACE spacecraft, together with GOES X-ray recordings and LASCO CME coronagraph
images were used in conjunction to the ground based recordings of the Worldwide Neutron Monitor Network, the interplanetary
data of OMNI database and the geomagnetic activity manifestations denoted by K
p and D
st indices. More than that, cosmic ray characteristics as density, anisotropy and density gradients were also calculated. The
results illustrate the state of the interplanetary space that cosmic rays crossed and their corresponding modulation with
respect to the multiple extreme solar events of this period. In addition, the western location of the 21 January 2005 solar
source indicates a new cosmic ray feature, which connects the position of the solar source to the cosmic ray anisotropy variations.
In the future, this feature could serve as an indicator of the solar source and can prove to be a valuable asset, especially
when satellite data are unavailable. 相似文献
993.
During studies of spectra of the star Θ1 Ori C with high photometric and spectral resolution we have discovered a system of spectral lines indicating a large negative
radial velocity (RV). We designate this star as Θ1 Ori C3. 相似文献
994.
The evolution of the 27-day recurrence in the series of two solar indices (Wolf number WN and 10.7 cm radio flux F) and two geomagnetic indices (Dst and ζ, variance of the geomagnetic field recorded at a magnetic observatory) have been studied over the 1957 – 2007 time
span. Spectral energies contained in two period domains (25 – 27.3 and 27.3 – 31 days), designated as E
1 and E
2, have been computed. Whereas the evolution of E
1 is the same for the four indices, that of E
2 is essentially different for WN and F on the one hand, Dst and ζ on the other hand. Some general conclusions on the dynamics of the solar outer layers are inferred from these results.
First the solar activity, as measured by WN, and when averaged over a few years, evolves in the same way whatever the latitude.
Second, two families of coronal holes (CHs) are identified; the rapidly and the slowly rotating CHs evolve quite differently. 相似文献
995.
G. A. Gontcharov 《Astronomy Letters》2010,36(8):584-595
The first results of the construction of a three-dimensional reddening map for stars within 1600 pc of the Sun are presented.
Analysis of the distribution of 70 million stars from the 2MASS catalog with the most accurate photometry on the (J-Ks)-Ks diagram supplemented with Monte Carlo simulations has shown that one of the maxima of this distribution corresponds to F-type
dwarfs and subgiants with a mean absolute magnitude M
Ks
= 2⊙
m
5. The shift of this maximum toward large J-Ks with increasing Ks reflects the reddening of these stars with increasing heliocentric distance. The distribution of the sample of stars over
Ks, l, and b cells with a statistically significant number of stars in each cell corresponds to their distribution over three-dimensional
spatial cells. As a result, the reddening E(J-Ks) has been determined with an accuracy of 0·
m
03 for spatial cells with a side of 100 pc. All of the known large absorbing clouds within 1600 pc of the Sun have manifested
themselves in the results obtained. The distances to the near and far edges of the clouds have been determined with a relative
accuracy of 15%. The cases where unknown clouds are hidden behind known ones on the same line of sight have been found. The
distance dependence of reddening is considered for various Galactic latitudes and longitudes. The absorbing matter of the
Gould Belt is shown to manifest itself at latitudes up to 40° and within 600 pc of the Sun. The size and influence of the
Gould Belt may have been underestimated thus far. The absorbing matter at latitudes up to 60° and within 1600 pc of the Sun
has been found to be distributed predominantly in the first and second quadrants in the southern hemisphere and in the third
and fourth quadrants in the northern hemisphere. The warping of the absorbing layer in the near Galaxy apparently manifests
itself in this way. A nonrandom orientation of the clouds relative to the Sun is possible. The mass of the baryonic dark matter
in solar neighborhoods can then be considerably larger than is generally believed. 相似文献
996.
Jayanta Roy Yashwant Gupta Ue-Li Pen Jeffrey B. Peterson Sanjay Kudale Jitendra Kodilkar 《Experimental Astronomy》2010,28(1):25-60
The new era of software signal processing has a large impact on radio astronomy instrumentation. Our design and implementation
of a 32 antennae, 33 MHz, dual polarization, fully real-time software backend for the GMRT, using only off-the-shelf components,
is an example of this. We have built a correlator and a beamformer, using PCI-based ADC cards and a Linux cluster of 48 nodes
with dual gigabit inter-node connectivity for real-time data transfer requirements. The highly optimized compute pipeline
uses cache efficient, multi-threaded parallel code, with the aid of vectorized processing. This backend allows flexibility
in final time and frequency resolutions, and the ability to implement algorithms for radio frequency interference rejection.
Our approach has allowed relatively rapid development of a fairly sophisticated and flexible backend receiver system for the
GMRT, which will greatly enhance the productivity of the telescope. In this paper we describe some of the first lights using
this software processing pipeline. We believe this is the first instance of such a real-time observatory backend for an intermediate
sized array like the GMRT. 相似文献
997.
A. F. Kholtygin S. N. Fabrika N. A. Drake V. D. Bychkov L. V. Bychkova G. A. Chountonov T. E. Burlakova G. G. Valyavin 《Astronomy Letters》2010,36(5):370-379
Based on an analysis of the catalog of magnetic fields, we have investigated the statistical properties of the mean magnetic
fields for OB stars. We show that the mean effective magnetic field B of a star can be used as a statistically significant characteristic of its magnetic field. No correlation has been found
between the mean magnetic field strength B and projected rotational velocity of OB stars, which is consistent with the hypothesis about a fossil origin of the magnetic
field. We have constructed the magnetic field distribution function for B stars, F(B), that has a power-law dependence on B with an exponent of ≈−1.82. We have found a sharp decrease in the function F(B) for B ⩽ 400 G that may be related to rapid dissipation of weak stellar surface magnetic fields. 相似文献
998.
O. V. Verkhodanov V. V. Sokolov M. L. Khabibullina S. V. Karpov 《Astrophysical Bulletin》2010,65(3):238-249
We analyze the randomness of the sky distribution of cosmic gamma-ray bursts. These events are associated with massive galaxies,
spiral or elliptical, and therefore their positions should trace the large-scale structure, which, in turn, could show up
in the sky distribution of fluctuations of the cosmicmicrowave background (CMB). We test this hypothesis by mosaic correlation
mapping of the distributions of CMB peaks and burst positions, find the distribution of these two signals to be correlated,
and interpret this correlation as a possible systematic effect. 相似文献
999.
The morphological content of Markarian galaxies (MG) in groups and in single galaxies is examined. It is found that MG do
not conform to the long established rule that the relative number of elliptical and lenticular galaxies in groups is higher
than in single galaxies. 相似文献
1000.
A time-dependent model for the energy of a flaring solar active region is presented based on an existing stochastic jump-transition
model (Wheatland and Glukhov in Astrophys. J.
494, 858, 1998; Wheatland in Astrophys. J.
679, 1621, 2008 and Solar Phys.
255, 211, 2009). The magnetic free energy of an active region is assumed to vary in time due to a prescribed (deterministic) rate of energy
input and prescribed (random) jumps downwards in energy due to flares. The existing model reproduces observed flare statistics,
in particular flare frequency – size and waiting-time distributions, but modeling presented to date has considered only the
time-independent choices of constant energy input and constant flare-transition rates with a power-law distribution in energy.
These choices may be appropriate for a solar active region producing a constant mean rate of flares. However, many solar active
regions exhibit time variation in their flare productivity, as exemplified by NOAA active region (AR) 11029, observed during
October – November 2009 (Wheatland in Astrophys. J.
710, 1324, 2010). Time variation is incorporated into the jump-transition model for two cases: (1) a step change in the rates of flare transitions,
and (2) a step change in the rate of energy supply to the system. Analytic arguments are presented describing the qualitative
behavior of the system in the two cases. In each case the system adjusts by shifting to a new stationary state over a relaxation
time which is estimated analytically. The model exhibits flare-like event statistics. In each case the frequency – energy
distribution is a power law for flare energies less than a time-dependent rollover set by the largest energy the system is
likely to attain at a given time. The rollover is not observed if the mean free energy of the system is sufficiently large.
For Case 1, the model exhibits a double exponential waiting-time distribution, corresponding to flaring at a constant mean
rate during two intervals (before and after the step change), if the average energy of the system is large. For Case 2 the
waiting-time distribution is a simple exponential, again provided the average energy of the system is large. Monte Carlo simulations
of Case 1 are presented which confirm the estimate for the relaxation time and the expected forms of the frequency – energy
and waiting-time distributions. The simulation results provide a qualitative model for observed flare statistics in AR 11029. 相似文献