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91.
P. JenniskensE. Lyytinen M.C. de LignieC. Johannink K. JobseR. Schievink M. LangbroekM. Koop P. GuralM.A. Wilson I. YrjöläK. Suzuki H. OgawaP. de Groote 《Icarus》2002,159(1):197-209
We calculate the position of dust trails from comet 8P/Tuttle, in an effort to explain unusual Ursid meteor shower outbursts that were seen when the comet was near aphelion. Comet 8P/Tuttle is a Halley-type comet in a 13.6-year orbit, passing just outside of Earth's orbit. We find that the meteoroids tend to be trapped in the 12:14 mean motion resonance with Jupiter, while the comet librates in a slightly shorter period orbit around the 13:15 resonance. It takes 6 centuries to decrease the perihelion of the meteoroid orbits enough to intersect Earth's orbit, during which time the meteoroids and comet separate in mean anomaly by 6 years, thus explaining the 6-year lag between the comet's return and Ursid outbursts. The resonances also prevent dispersion along the comet orbit and limit viewing to only one year in each return. We identified past dust trail encounters with dust trails from 1392 (Dec. 1945) and 1378 (Dec. 1986) and predicted another outburst on 2000 December 22 at around 7:29 and 8:35 UT, respectively, from dust trails dating to the 1405 and 1392 returns. This event was observed from California using video and photographic techniques. At the same time, five Global-MS-Net stations in Finland, Japan, and Belgium counted meteors using forward meteor scatter. The outburst peaked at 8:06±07 UT, December 22, at zenith hourly rate ∼90 per hour, and the Ursid rates were above half peak intensity during 4.2 h. We find that most Ursid orbits do scatter around the anticipated positions, confirming the link with comet 8P/Tuttle and the epoch of ejection. The 1405 and 1392 dust trails appear to have contributed similar amounts to the activity profile. Some orbits provide a hint of much older debris being present as well. This work is the strongest evidence yet for the relevance of mean motion resonances in Halley-type comet dust trail evolution. 相似文献
92.
High-resolution mosaics of the solar polar magnetic fields have been constructed using individual magnetograms obtained with the video magnetograph of the Big Bear Solar Observatory, and the properties of these mosaics are demonstrated in this paper. The mosaics show selected regions of the polar fields on several days during the rising phase of Cycle 23, and are related to the global polar fields (i) by superposing the mosaic for a given day on to a full-disk SOHO-MDI magnetogram obtained on the same day, (ii) by plotting the mosaics in polar projection and using these to identify the approximate regions reported by the mosaics on the NSOKP polar synoptic plots, and (iii) by imposing the locations of the H filaments on to the mosaics in order to infer the neutral lines of the large-scale fields. We have studied the fine structure of the large-scale unipolar fields near the poles and, in particular, have constructed histograms of the magnetic field intensities within particular regions of the mosaics and, in this way, have estimated the ratios of the number of magnetic knots of opposite polarities within the unipolar plumes. We have also generated enlargements of the polar regions of the NSOKP daily magnetograms. These and statistical studies have shown that on days for which the BBSO mosaics are not available, the NSOKP enlargements may be used to study the high-resolution polar fields. Time-series of mosaics obtained over four-hour periods on September 6 and November 18 show that considerable evolution in the structure of existing flux knots and the formation of several new knots has taken place during these periods. 相似文献
93.
The Mount Wilson synoptic magnetic data for the period September 1987 through March 1996 are completely revised and used to provide polar plots of the solar magnetic fields for both hemispheres. This period, from Carrington rotations 1793 to 1906, covers the reversals of the polar magnetic fields in cycle 22. Comparison of our plots with the presently available H filtergrams for this period shows that the polarity boundaries are consistent in these two data sets where they overlap. The Mount Wilson plots show that the polar field reversals involve a complex sequence of events. Although the details differ slightly, the basic patterns are similar in each hemisphere. First the old polarity becomes isolated at the pole, then shortly thereafter, the isolation is broken, and the polar field includes unipolar regions of both polarities. The old polarity then reclaims the polar region, but when the isolation of this field is established for a second time, it declines in both area and strength. We take the reversal to be complete when the old polarity field is no longer observed in the Mount Wilson plots. With this criterion we find that the polar field reversal is completed in the north by CR 1836, i.e., by December 1990, and in the south by CR 1853, i.e., March 1992. 相似文献
94.
This paper focuses on the attribute weight issue and advocates use of modi?able attribute weights in terrain‐based environmental analysis and classi?cation. A question was asked: ‘How much will the result of a terrain‐based environmental analysis be affected if the weights of used terrain attributes are changed?’ The literature on landform classi?cation and the fuzzy k‐means method was reviewed in particular to help clarify the background and importance of this weight assignment issue. As an example, the effects of modifying attribute weights were evaluated for fuzzy k‐means landform classi?cation in a case study area. A total of 102 classi?cations were compared with each other and with a soil map, and comparison methods were speci?cally designed to evaluate the differences between these classi?cations. The results show that fuzzy k‐means landform classi?cation is sensitive to weight adjustments of adopted terrain attributes. The sensitivity is particularly high when the attribute weights started to be tuned away from the standard (i.e. uniform) weight of one. Better matching between landform classi?cation and a soil map may be produced when attribute weights are tuned. In all, we advocate the widespread adoption of an exploratory attitude in assigning attribute weights for environmental analysis and classi?cation. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
95.
John Wilson Stewart Fotheringham Gary Hunter Jochen Albrecht 《Transactions in GIS》2006,10(2):153-153
96.
An approximate analytical solution for the deposition of heavy particles released from an elevated line source 总被引:1,自引:1,他引:1
This work re-examines and further develops an analytical solution for the deposition swath of heavy particles released in
the atmosphere from an elevated source over uniform terrain, correcting the particle diffusivity for the crossing trajectory
effect. The revised (approximate) analytical solution proves to be accurate within 20% over a wide range of micrometeorological
conditions and particle size, despite its neglect of the turbulence component of the deposition flux. It compares very satisfactorily
with experimental data and with the simulations of a Lagrangian stochastic model, provided the variable U(H)/w
g
≤7 (ratio of the mean horizontal wind speed at source height to the particle settling velocity). In this domain of validity,
simple formulae relating the statistics of the deposition swath to U(H)/w
g
are derived. 相似文献
97.
A numerical simulation of residual circulation in Tampa Bay. Part I: Low-frequency temporal variations 总被引:1,自引:0,他引:1
Steven D. Meyers Mark E. Luther Monica Wilson Heather Havens Amanda Linville Kristin Sopkin 《Estuaries and Coasts》2007,30(4):679-697
The residual (time-average) salinity and circulation in a numerical ocean model of the Tampa Bay estuary are shown to experience
significant temporal variation under realistic forcing conditions. A version of the Estuarine Coastal Ocean Model developed
for Tampa Bay with 70 by 100 horizontal grid points and 11 sigma levels is examined for the years 2001–2003. Model output
variables are averaged over the entire time of the simulation to generate long-term residual fields. The residual axial current
is found to be dominated by the buoyancy-driven baroclinic circulation with an outflow (southwestward) at the surface and
to the sides of the shipping channel, and an inflow (northeastward) usually occurring subsurface within or above the shipping
channel. Averages over 30 d are used to examine variations in the residual fields. During the simulation the average surface
salinity near the head of Tampa Bay varies with the freshwater inflow, from 12‰ to 33%. At the bay mouth salinity varies from
30%. to 36%.. A localized measure of the baroclinic circulation in the shipping channel indicates the residual circulation
can vary strongly, attaining a magnitude triple the long-term mean value. The baroclinic circulation can be disrupted, going
to near zero or even reversing, when the buoyancy-driven flow is weak and the surface winds are to the northeast. Three time
periods, representing different environmental conditions, are chosen to examine these results in detail. A scaling argument
indicates the relative strength of buoyancy versus wind as ΔρgH2(LC
Dω2)−1, where δρ is head-to-mouth density difference across the bay,g is gravitational acceleration,H is depth,L is bay length,C
D is the surface wind drag coefficient, andw is wind speed. Tampa Bay is usually in the buoyancy dominated regime. The importance of winds in the weak-buoyancy case is
demonstrated in an additional simulation without wind stress. 相似文献
98.
Aloísio J.B. Cotta Jacinta Enzweiler Stephen A. Wilson Carlos A. Pérez Antonio J.R. Nardy João H. Larizzatti 《Geostandards and Geoanalytical Research》2007,31(4):379-393
Reference materials (RM) are required for quantitative analyses and their successful use is associated with the degree of homogeneity, and the traceability and confidence limits of the values established by characterisation. During the production of a RM, the chemical characterisation can only commence after it has been demonstrated that the material has the required level of homogeneity. Here we describe the preparation of BRP-1, a proposed geochemical reference material, and the results of the tests to evaluate its degree of homogeneity between and within bottles. BRP-1 is the first of two geochemical RM being produced by Brazilian institutions in collaboration with the United States Geological Survey (USGS) and the International Association of Geoanalysts (IAG). Two test portions of twenty bottles of BRP-1 were analysed by wavelength dispersive-XRF spectrometry and major, minor and eighteen trace elements were determined. The results show that for most of the investigated elements, the units of BRP-1 were homogeneous at conditions approximately three times more rigorous than those strived for by the test of "sufficient homogeneity". Furthermore, the within bottle homogeneity of BRP-1 was evaluated using small beam (1 mm2 ) synchrotron radiation XRF spectrometry and, for comparison, the USGS reference materials BCR-2 and GSP-2 were also evaluated. From our data, it has been possible to assign representative minimum masses for some major constituents (1 mg) and for some trace elements (1-13 mg), except Zr in GSP-2, for which test portions of 74 mg are recommended. 相似文献
99.
A procedure for simultaneous analysis of multiple kinds of observations is developed for binaries that contain X-ray pulsars. A wide variety of observation types might be included, although we consider only velocity: pulse and light: velocity: pulse cases at present. The model operates with equipotentials and can accommodate non-synchronous rotation and eccentric orbits. The duration condition imposed by observed X-ray eclipses is incorporated as an embedded constraint, so that only solutions consistent with the eclipse duration are found. Relations needed for the method of differential corrections in least-squares solutions are specified, and we apply the Marquardt scheme for improvement of solution convergence. Parameters of Vela X1: GP Velorum are obtained from seven combinations of pulse and radial velocity data. The estimated masses are thereby put into some perspective, especially for the neutron star. The mass of the supergiant ( m sin3 i ) now ranges only ±5 per cent from the mean of seven results, although the uncertainty in i makes the actual mass range larger. We discuss the determinacy of certain parameters and their historical consistency. For example, the systemic radial velocity differs among spectroscopic data sets by more than 20 km s−1 . We show that, contrary to published assertions, the orbital inclinations of high-mass X-ray binaries are essentially indeterminable by the usual methods, although lower limits to inclination may be meaningful for some examples. 相似文献
100.
Samuel Heinrich Schwabe, the discoverer of the sunspot cycle, observed the Sun routinely from Dessau, Germany during the interval
of 1826–1868, averaging about 290 observing days per year. His yearly counts of ‘clusters of spots’ (or, more correctly, the
yearly number of newly appearing sunspot groups) provided a simple means for describing the overt features of the sunspot
cycle (i.e., the timing and relative strengths of cycle minimum and maximum). In 1848, Rudolf Wolf, a Swiss astronomer, having
become aware of Schwabe's discovery, introduced his now familiar ‘relative sunspot number’ and established an international
cadre of observers for monitoring the future behavior of the sunspot cycle and for reconstructing its past behavior (backwards
in time to 1818, based on daily sunspot number estimates). While Wolf's reconstruction is complete (without gaps) only from
1849 (hence, the beginning of the modern era), the immediately preceding interval of 1818–1848 is incomplete, being based
on an average of 260 observing days per year. In this investigation, Wolf's reconstructed record of annual sunspot number
is compared against Schwabe's actual observing record of yearly counts of clusters of spots. The comparison suggests that
Wolf may have misplaced (by about 1–2 yr) and underestimated (by about 16 units of sunspot number) the maximum amplitude for
cycle 7. If true, then, cycle 7's ascent and descent durations should measure about 5 years each instead of 7 and 3 years,
respectively, the extremes of the distributions, and its maximum amplitude should measure about 86 instead of 70. This study
also indicates that cycle 9's maximum amplitude is more reliably determined than cycle 8's and that both appear to be of comparable
size (about 130 units of sunspot number) rather than being significantly different. Therefore, caution is urged against the
indiscriminate use of the pre-modern era sunspot numbers in long-term studies of the sunspot cycle, since such use may lead
to specious results. 相似文献