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481.
Brian Mason 《Meteoritics & planetary science》1983,18(3):245-245
Delaney et al. (1983) propose to redefine howardites as basaltic achondrites containing more than 10% of magnesian orthopyroxenite (diogenite) component. Since the 10% requirement is arbitrary and of no genetic significance this redefinition should be rejected and the earlier definition of Score et al. (1982) “Howardites are polymict pyroxene-plagioclase achondrites containing magnesian orthopyroxene” — in any amount — should be retained. 相似文献
482.
The Marine Physical Laboratory has designed, fabricated, and taken to sea self-contained, freely drifting acoustic sensors which can measure signal propagation and ambient ocean noise in the 1-20-Hz band for up to 25-hour periods. The deployment of several freely drifting floats forms an array of sensors whose outputs can be combined after the experiment with a beamformer. A Kalman filter and a least-squares estimator have been developed to estimate float positions from travel-time measurements. Computer simulation is used to compare filter performance-under several deployment scenarios. Results show that the Kalman filter performs better than the least-squares filter when the floats are subjected to small-magnitude accelerations between measurements. Neither filter was sensitive to relatively major changes in deployment geometry as long as the sound-speed profile is known exactly 相似文献
483.
We show that the rotation of coronal holes can be understood in terms of a current-free model of the coronal magnetic field, in which holes are the footpoint locations of open field lines. The coronal field is determined as a function of time by matching its radial component to the photospheric flux distribution, whose evolution is simulated including differential rotation, supergranular diffusion, and meridional flow. We find that ongoing field-line reconnection allows the holes to rotate quasi-rigidly with their outer-coronal extensions, until their boundaries become constrained by the neutral line of the photospheric field as it winds up to form stripes of alternating magnetic polarity. This wind-up may be significantly retarded by a strong axisymmetric field component which forces the neutral line to low latitudes; it is also gradually halted by the cross-latitudinal transport of flux via supergranular diffusion and a poleward bulk flow. We conclude that a strong axisymmetric field component is responsible for the prolonged rigid rotation of large meridional holes during the declining phase of the sunspot cycle, but that diffusion and flow determine the less rigid rotation observed near sunspot maximum, when the holes corotate with their confining polarity stripes. 相似文献
484.
A logarithmic transformation may be used to improve the efficiency of estimates of the mean when observations follow the lognormal distribution. But if this transformation is applied to observations that follow another distribution, bias may be introduced. We consider some consequences of erroneously applying lognormal estimation theory and demonstrate that biased estimates may be obtained for certain classes of distributions. Illustrations of bias obtained in gold sampling are given. 相似文献
485.
486.
Robert C. Fleck Jr. 《Astrophysics and Space Science》1984,100(1-2):417-419
Two comments are prompted by the recent paper of Quiroga (1983). First, it is pointed out that Quiroga's identification of two distinct types of motion (hydrodynamic and turbulent) within the galactic disk supports the suggestion that turbulent motions in the Galaxy are generated by the shearig action of differential galactic rotation. Secondly, as a result of these turbulent motions dominating the systematic hydrodynamics of galactic rotation at scale sizes below 100–300 pc, it appears that some process(es) associated with interstellar turbulence rather than with galactic rotation should play a dominant role in the establishment of the angular momentum of interstellar material. 相似文献
487.
Suburban growth is a complex process attributable partly to decentralization and partly to deconcentration. Data on source of migrants and population density are used to operationalize these concepts in an empirical study of the Cleveland, Ohio SMSA for the period 1950 to 1970. Statistical analysis of census tract data for the suburban ring reveals a piling up of densities at the inner edge and suggests a complex pattern with respect to sources of the migrants contributing to suburban growth. 相似文献
488.
Austin T.C. Edson J.B. McGillis W.R. Purcell M. Petitt R.A. Jr. McElroy M.K. Grant C.W. Ware J. Hurst S.K. 《Oceanic Engineering, IEEE Journal of》2002,27(2):228-234
Underwater observatories with real-time data and virtually unlimited power transmission capabilities compared to traditional oceanographic moorings are beginning to provide scientists with continuous access to the coastal and open ocean. However, for any coastal observatory to serve as a cost-effective system for the collection of long-term scientific and environmental data, it must have a simple, upgradeable power and telemetry system and an instrument interface that is compatible with existing standards. It must be designed for extended environmental exposure and ease of service to avoid high maintenance costs. Most importantly, the observatory must be accessible to all potential users, including students, scientists, engineers, and policy makers. This strategy was applied to the design of the Martha's Vineyard Coastal Observatory on the south shore of the island of Martha's Vineyard. The new facility, and in particular its system architecture, as developed by the Woods Hole Oceanographic Institution with support from the National Science Foundation, are described 相似文献
489.
490.
As part of our study of the larger-scale remanent magnetic field of the Moon, we have examined the effects of cratering in
an otherwise spherically symmetrical shell magnetized by a concentric dipolar magnetic fieldH
o to an intensity of magnetizationc
H
o, wherec is a constant. In our initial model, we assume that the material excavated from the craters is distributed with random orientation
and thus does not contribute to the remanent dipole momentM
g
. We further assume that the mare fill does not contribute significantly toM
g
. We choose the magnetizing dipole momentM
o and the constantc such that the magnitude of the productcH
o ≃ 3 × 10−4Г at the outer surface of the shell in the equatorial plane of the dipole. This value of the intensity of remanent magnetization
was chosen to be within the range 10−7−10−3Г’; the intensities of thermo-remanent magnetization exhibited by Apollo samples. Finally, we use the locations and diameters
of the 10 largest craters on the Moon and the depth-to-diameter ratios of Pike’s formulation to model approximately the excavation
of the magnetized shell.
The remanent dipole momentM
g
was calculated for each of three orthogonal orientations of the magnetizing dipoleM
o. The three magnitudes ofM
g
fall in the range 4 × 1018−1 × 1019Г cm3 which is close to the upper limit of 1019Г cm3 estimated forM
g
from the field measurements obtained with the Apollo subsatellites. Further, the distribution of the craters is such as to
produce a significant transverse component ofM
g
with acute angles between the spin axis andM
g
in the range 51°–77°. 相似文献