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71.
The latitudinal variation of Saturn’s tropospheric composition (NH3, PH3 and AsH3) and aerosol properties (cloud altitudes and opacities) are derived from Cassini/VIMS 4.6-5.1 μm thermal emission spectroscopy on the planet’s nightside (April 22, 2006). The gaseous and aerosol distributions are used to trace atmospheric circulation and chemistry within and below Saturn’s cloud decks (in the 1- to 4-bar region). Extensive testing of VIMS spectral models is used to assess and minimise the effects of degeneracies between retrieved variables and sensitivity to the choice of aerosol properties. Best fits indicate cloud opacity in two regimes: (a) a compact cloud deck centred in the 2.5-2.8 bar region, symmetric between the northern and southern hemispheres, with small-scale opacity variations responsible for numerous narrow light/dark axisymmetric lanes; and (b) a hemispherically asymmetric population of aerosols at pressures less than 1.4 bar (whose exact altitude and vertical structure is not constrained by nightside spectra) which is 1.5-2.0× more opaque in the summer hemisphere than in the north and shows an equatorial maximum between ±10° (planetocentric).Saturn’s NH3 spatial variability shows significant enhancement by vertical advection within ±5° of the equator and in axisymmetric bands at 23-25°S and 42-47°N. The latter is consistent with extratropical upwelling in a dark band on the poleward side of the prograde jet at 41°N (planetocentric). PH3 dominates the morphology of the VIMS spectrum, and high-altitude PH3 at p < 1.3 bar has an equatorial maximum and a mid-latitude asymmetry (elevated in the summer hemisphere), whereas deep PH3 is latitudinally-uniform with off-equatorial maxima near ±10°. The spatial distribution of AsH3 shows similar off-equatorial maxima at ±7° with a global abundance of 2-3 ppb. VIMS appears to be sensitive to both (i) an upper tropospheric circulation (sensed by NH3 and upper-tropospheric PH3 and hazes) and (ii) a lower tropospheric circulation (sensed by deep PH3, AsH3 and the lower cloud deck).  相似文献   
72.
Although artificial reefs are commonly used throughout the world as tools to mitigate for habitat alteration, their development is rarely subjected to a rigorous site selection process. We developed a simple site selection model using the following seven systematic steps: exclusion mapping, depth and slope verification, surficial substrate assessment, data weighting and the subsequent ranking analysis, visual transect surveys, benthic air‐lift sampling, and larval settlement collector deployment. American lobster (Homarus americanas) was selected as the target species for these investigations owing to the local commercial importance of the species. Results from each step in this process ultimately allowed us to select a site for an artificial reef at a target depth that received little wave action, had no slope, and possessed a surficial substrate type that could support the weight of a reef. The site also had the presence of a natural larval supply and low species diversity before reef installation. Each step in this site selection model was designed for easy adaptation to suit the needs of various artificial reef projects.  相似文献   
73.
This paper explores the issue of “trust” in the fisheries science community, a key corollary of effective risk communication. It presents the findings of a survey undertaken in Iceland, Greece, Spain, United Kingdom and Faroe Islands during 2008. The findings reveal differing levels of trust and mistrust in the fisheries science community between countries and between stakeholder groups, demonstrating areas for future attention in the interests of improving fisheries science and management. As this paper explores, unfortunately the “trust” necessary for effective stakeholder cooperation and participation within current fisheries science is currently somewhat lacking. The cited reasons behind this lack of trust include: a lack of soundness, credibility, responsiveness, flexibility and stakeholder involvement, flawed data and weak science, poor communications and political and lobby group interference. Notable from the results is a lack of consensus on the existence of a common language and vision. It is evident, however, that certain aspects of fisheries science are strong contributors to trust and that there are opportunities for improvement.  相似文献   
74.
We present the closed form of a general steady-state shape factor for a partially penetrating well in a uniform anisotropic aquifer. Our simple analytical expression for the shape factor has a uniform representation for full range of parameters and meets or exceeds the accuracy of previous results obtained through semiempirical methods (e.g., Bouwer and Rice [1976] equations). This general shape factor pertains to the flow of fluids (water or air) in subsurface formations when the upper formation boundary has constant potential and the lower boundary is impermeable. The results of our investigation are directly applicable to analyses of (1) slug tests with falling or rising head and (2) injection/extraction tests with constant head, essential techniques for the characterization of hydraulic conductivity of aquifers, streambeds, or lakebeds as well as the design of aquifer and soil remediation systems.  相似文献   
75.
Abstract

Two distributions of the α-effect in a sphere are considered. The inviscid limit is approached both by direct numerical solution and by solution of a simpler nonlinear eigenvalue problem deriving from asymptotic boundary layer analysis for the case of stress-free boundaries. The inviscid limit in both cases is dominated by the need to satisfy the Taylor constraint which states that the integral of the Lorentz force over cylindrical (geostrophic) contours in a homogeneous fluid must tend to zero. For a small supercritical range in α, this condition can only be met by magnetic fields which vanish as the viscosity goes to zero. In this range, the agreement of the two approaches is excellent. In a portion of this range, the method of finite amplitude perturbation expansion is useful, and serves as a guide for understanding the numerical results. For larger α, evidence from the nonlinear eigenvalue problem suggests both that the Taylor state exists, and that the transition from small to large amplitude can require a finite amplitude (oscillatory) instability in accord with the findings of Soward and Jones (1983). However, solutions of the full equations have not been found which are independent of viscosity at larger values of α.  相似文献   
76.
High‐precision bulk aluminum‐magnesium isotope measurements of calcium‐aluminum‐rich inclusions (CAIs) from CV carbonaceous chondrites in several laboratories define a bulk 26Al‐26Mg isochron with an inferred initial 26Al/27Al ratio of approximately 5.25 × 10?5, named the canonical ratio. Nonigneous CV CAIs yield well‐defined internal 26Al‐26Mg isochrons consistent with the canonical value. These observations indicate that the canonical 26Al/27Al ratio records initial Al/Mg fractionation by evaporation and condensation in the CV CAI‐forming region. The internal isochrons of igneous CV CAIs show a range of inferred initial 26Al/27Al ratios, (4.2–5.2) × 10?5, indicating that CAI melting continued for at least 0.2 Ma after formation of their precursors. A similar range of initial 26Al/27Al ratios is also obtained from the internal isochrons of many CAIs (igneous and nonigneous) in other groups of carbonaceous chondrites. Some CAIs and refractory grains (corundum and hibonite) from unmetamorphosed or weakly metamorphosed chondrites, including CVs, are significantly depleted in 26Al. At least some of these refractory objects may have formed prior to injection of 26Al into the protosolar molecular cloud and its subsequent homogenization in the protoplanetary disk. Bulk aluminum and magnesium‐isotope measurements of various types of chondrites plot along the bulk CV CAI isochron, suggesting homogeneous distribution of 26Al and magnesium isotopes in the protoplanetary disk after an epoch of CAI formation. The inferred initial 26Al/27Al ratios of chondrules indicate that most chondrules formed 1–3 Ma after CAIs with the canonical 26Al/27Al ratio.  相似文献   
77.
The extent of the Barents-Kara Sea ice sheet (northern Europe and Russia) during the Last Glacial Maximum (LGM), in Marine Isotope Stage (MIS) 2 is controversial, especially along the southern and northeastern (Russian High Arctic) margins. We conducted a multi-disciplinary study of various organic and mineral fractions, obtaining chronologies with 14C and luminescence dating methods on a 10.5 m long core from Changeable Lake (4 km from the Vavilov Ice Cap) on Severnaya Zemlya. The numeric ages indicate that the last glaciation at this site occurred during or prior to MIS 5d-4 (Early Middle Weichselian). Deglaciation was followed by a marine transgression which affected the Changeable Lake basin. After the regression the basin dried up. In late Middle Weichselian time (ca 25–40 ka), reworked marine sediments were deposited in a saline water body. During the Late Weichselian (MIS 2), the basin was not affected by glaciation, and lacustrine sediments were formed which reflect cold and arid climate conditions. During the termination of the Pleistocene and into the Holocene, warmer and wetter climate conditions than before led to a higher sediment input. Thus, our chronology demonstrates that the northeastern margin of the LGM Barents-Kara Sea ice sheet did not reach the Changeable Lake basin. This result supports a modest model of the LGM ice sheet in northern Europe determined from numeric ice sheet modelling and geological investigations.  相似文献   
78.
79.
Wind erosion of soil is an appreciable but unstudied event following fires in cold desert. We examined aeolian transport of sediment for 1 year following fire in semi-arid shrub steppe on loess soils in southern Idaho, USA. Sediment collectors were used to determine horizontal mass transport of soil and saltation sensors and anemometers were used to determine saltation activity (fraction of time having saltation) and threshold wind speed in an area burned in August and an unburned control site. Horizontal mass transport (per 30-day period) was negligible in the unburned area, but in the burned area was 5.40 kg m?1 in October and decreased to 2.80 kg m?1 in November and 0.32 kg m?1 in December. Saltation activity was high enough to determine threshold wind speeds only in the burn site during fall, when values ranged from 10.0 to 10.6 m s?1. Sediment flux and saltation activity in the burned site became much less pronounced following the emergence of herbaceous vegetation in the spring. Post-fire sediment flux in the shrub steppe we examined was of greater magnitude but shorter duration than post-fire fluxes in warm deserts or sandier regions that experience more frequent wind erosion.  相似文献   
80.
Several methods for characterizing the occurrence and rate of nitrate attenuation were tested at a field site near Monument Valley, Arizona. Spatial and temporal nitrate concentration data collected from a transect of monitoring wells located along the plume centerline were analyzed to evaluate the overall rates of natural attenuation. The occurrence and rate of denitrification was evaluated through microcosm experiments, nitrogen isotopic fractionation analysis, and solute-transport modeling. First-order denitrification-rate coefficients calculated with each method were comparable. In addition, the composite natural attenuation rate coefficient was similar to the denitrification-rate coefficients, which suggests that microbially induced decay primarily controls nitrate attenuation at the site. This research highlights the benefits associated with a multiple-method approach for the characterization of natural attenuation.  相似文献   
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