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431.
王晓静 《测绘与空间地理信息》2015,(4):82-83
介绍了在线路测量中,当中线方向受阻时确定方向转点的简便方法——中线支距法,并对其原理、计算、操作方法、精度评定进行了分析与实践。 相似文献
432.
Yue Shen Xin Liu Yu-Qing Lou 《Monthly notices of the Royal Astronomical Society》2005,356(4):1333-1356
We construct analytically stationary global configurations for both aligned and logarithmic spiral coplanar magnetohydrodynamics (MHD) perturbations in an axisymmetric background MHD disc with a power-law surface mass density Σ0 ∝ r −α , a coplanar azimuthal magnetic field B 0 ∝ r −γ , a consistent self-gravity and a power-law rotation curve v 0 ∝ r −β , where v 0 is the linear azimuthal gas rotation speed. The barotropic equation of state Π∝Σ n is adopted for both MHD background equilibrium and coplanar MHD perturbations where Π is the vertically integrated pressure and n is the barotropic index. For a scale-free background MHD equilibrium, a relation exists among α, β, γ and n such that only one parameter (e.g. β) is independent. For a linear axisymmetric stability analysis, we provide global criteria in various parameter regimes. For non-axisymmetric aligned and logarithmic spiral cases, two branches of perturbation modes (i.e. fast and slow MHD density waves) can be derived once β is specified. To complement the magnetized singular isothermal disc analysis of Lou, we extend the analysis to a wider range of −1/4 < β < 1/2 . As an illustrative example, we discuss specifically the β= 1/4 case when the background magnetic field is force-free. Angular momentum conservation for coplanar MHD perturbations and other relevant aspects of our approach are discussed. 相似文献
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436.
Global lopsided instability in a purely stellar galactic disc 总被引:1,自引:0,他引:1
Kanak Saha Francoise Combes Chanda J. Jog 《Monthly notices of the Royal Astronomical Society》2007,382(1):419-432
It is shown that pure exponential discs in spiral galaxies are capable of supporting slowly varying discrete global lopsided modes, which can explain the observed features of lopsidedness in the stellar discs. Using linearized fluid dynamical equations with the softened self-gravity and pressure of the perturbation as the collective effect, we derive self-consistently a quadratic eigenvalue equation for the lopsided perturbation in the galactic disc. On solving this, we find that the ground-state mode shows the observed characteristics of the lopsidedness in a galactic disc, namely the fractional Fourier amplitude A 1 , increases smoothly with the radius. These lopsided patterns precess in the disc with a very slow pattern speed with no preferred sense of precession. We show that the lopsided modes in the stellar disc are long-lived because of a substantial reduction (approximately a factor of 10 compared to the local free precession rate) in the differential precession. The numerical solution of the equations shows that the ground-state lopsided modes are either very slowly precessing stationary normal mode oscillations of the disc or growing modes with a slow growth rate depending on the relative importance of the collective effect of the self-gravity. N -body simulations are performed to test the spontaneous growth of lopsidedness in a pure stellar disc. Both approaches are then compared and interpreted in terms of long-lived global m = 1 instabilities, with almost zero pattern speed. 相似文献
437.
Noam I. Libeskind Shaun Cole Carlos S. Frenk Takashi Okamoto Adrian Jenkins 《Monthly notices of the Royal Astronomical Society》2007,374(1):16-28
We investigate the properties of satellite galaxies formed in N -body/SPH simulations of galaxy formation in the ΛCDM cosmology. The simulations include the main physical effects thought to be important in galaxy formation and, in several cases, produce realistic spiral discs. In total, a sample of nine galaxies of luminosity comparable to the Milky Way was obtained. At magnitudes brighter than the resolution limit, MV =−12 , the luminosity function of the satellite galaxies in the simulations is in excellent agreement with data for the Local Group. The radial number density profile of the model satellites, as well as their gas fractions also match observations very well. In agreement with previous N -body studies, we find that the satellites tend to be distributed in highly flattened configurations whose major axis is aligned with the major axis of the (generally triaxial) dark halo. In two out of three systems with sufficiently large satellite populations, the satellite system is nearly perpendicular to the plane of the galactic disc, a configuration analogous to that observed in the Milk Way. The discs themselves are perpendicular to the minor axis of their host haloes in the inner parts, and the correlation between the orientation of the galaxy and the shape of the halo persists even out to the virial radius. However, in one case the disc's minor axis ends up, at the virial radius, perpendicular to the minor axis of the halo. The angular momenta of the galaxies and their host halo tend to be well aligned. 相似文献
438.
R. A. Angiras † C. J. Jog † A. Omar † K. S. Dwarakanath † 《Monthly notices of the Royal Astronomical Society》2006,369(4):1849-1857
The H i surface density maps for a sample of 18 galaxies in the Eridanus group are Fourier analysed. This analysis gives the radial variation of the lopsidedness in the H i spatial distribution. The lopsidedness is quantified by the Fourier amplitude A 1 of the m = 1 component normalized to the average value. It is also shown that in the radial region where the stellar disc and H i overlap, their A 1 coefficients are comparable. All the galaxies studied show significant lopsidedness in H i . The mean value of A 1 in the inner regions of the galaxies (1.5–2.5 scalelengths) is ≥ 0.2. This value of A 1 is twice the average value seen in the field galaxies. Also, the lopsidedness is found to be smaller for late-type galaxies; this is opposite to the trend seen in the field galaxies. These two results indicate a different physical origin for disc lopsidedness in galaxies in a group environment compared to the field galaxies. Further, a large fraction (∼30 per cent) shows a higher degree of lopsidedness ( A 1 ≥ 0.3). It is also seen that the disc lopsidedness increases with the radius as demonstrated in earlier studies, but over a radial range that is two times larger than done in the previous studies. The average lopsidedness of the halo potential is estimated to be ∼10 per cent, assuming that the lopsidedness in H i disc is due to its response to the halo asymmetry. 相似文献
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440.
Katia Ganda Jesús Falcón-Barroso Reynier F. Peletier Michele Cappellari Eric Emsellem Richard M. McDermid P. Tim de Zeeuw C. Marcella Carollo 《Monthly notices of the Royal Astronomical Society》2006,367(1):46-78
We present the stellar and gas kinematics of a sample of 18 nearby late-type spiral galaxies (Hubble types ranging from Sb to Sd), observed with the integral-field spectrograph SAURON at the 4.2-m William Herschel Telescope. SAURON covers the spectral range 4800–5380 Å, allowing us to measure the Hβ, Fe, Mg b absorption features and the emission in the Hβ line and the [O iii ]λλ4959, 5007 Å and [N i ]λλ5198, 5200 Å doublets over a 33 × 41-arcsec2 field of view. The maps cover the nuclear region of these late-type galaxies and in all cases include the entire bulge. In many cases the stellar kinematics suggests the presence of a cold inner region, as visible from a central drop in the stellar velocity dispersion. The ionized gas is almost ubiquitous and behaves in a complicated fashion: the gas velocity fields often display more features than the stellar ones, including wiggles in the zero-velocity lines, irregular distributions, ring-like structures. The line ratio [O iii ]/Hβ often takes on low values over most of the field, probably indicating a wide-spread star formation. 相似文献