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31.
Experimental and theoretical aspects of the fundamental process of magnetic flux transfer ('field line reconnection') are reviewed. Explosively rapid events are observed in a laboratory magnetoplasma device designed to test certain aspects of neutral point theories of the solar flare mechanism. In these events, ultra-fast release of magnetic energy through reconnection of field lines is effected by a conduction mode instability in which an abrupt upward transition to anomalous resistivity takes place in the neighbourhood of the X-point. The events are triggered when the X-point current density exceeds the conduction mode instability threshold. The energy that is released is that which is associated with the induced current system that supports field line reconnection under ordinary quiescent conditions. Many characteristics of the laboratory events and their associated phenomena, when appropriately scaled, show remarkable agreement with corresponding flare observational data. Some new directions for further development of flare theory are therefore recommended. These experiments indicate that a number of assumptions commonly made in theoretical analysis of the reconnection process are inappropriate. The flux transfer process can be well understood theoretically in terms of a simple circuit analogue which models the laboratory process. 相似文献
32.
Andy Thean Alan Pedlar Marek J. Kukula Stefi A. Baum Christopher P. O'Dea 《Monthly notices of the Royal Astronomical Society》2000,314(3):573-588
We present 8.4-GHz VLA A-configuration observations of 87 sources from the mid-infrared-selected AGN sample of Rush et al. These 0.25-arcsec-resolution observations allow elongated radio structures tens of pc in size to be resolved, and enable radio components smaller than 3.5 arcsec to be isolated from diffuse galactic disc emission. When combined with previous data, matched radio observations covering 90 per cent of the sample have been collected, and these represent the largest subarcsecond–resolution radio imaging survey of a homogeneously selected sample of Seyfert galaxies to date.
We use our observations to identify five radio-loud AGN in the sample. The nature of the radio emission from Seyfert nuclei will be discussed in subsequent papers. 相似文献
We use our observations to identify five radio-loud AGN in the sample. The nature of the radio emission from Seyfert nuclei will be discussed in subsequent papers. 相似文献
33.
Presented is a review of the radiative properties of ice clouds from three perspectives: light scattering simulations, remote sensing applications, and broadband radiation parameterizations appropriate for numerical models. On the subject of light scattering simulations, several classical computational approaches are reviewed, including the conventional geometric-optics method and its improved forms, the finite-difference time domain technique, the pseudo-spectral time domain technique, the discrete dipole approximation method, and the T-matrix method, with specific applications to the computation of the singlescattering properties of individual ice crystals. The strengths and weaknesses associated with each approach are discussed.With reference to remote sensing, operational retrieval algorithms are reviewed for retrieving cloud optical depth and effective particle size based on solar or thermal infrared(IR) bands. To illustrate the performance of the current solar- and IR-based retrievals, two case studies are presented based on spaceborne observations. The need for a more realistic ice cloud optical model to obtain spectrally consistent retrievals is demonstrated. Furthermore, to complement ice cloud property studies based on passive radiometric measurements, the advantage of incorporating lidar and/or polarimetric measurements is discussed.The performance of ice cloud models based on the use of different ice habits to represent ice particles is illustrated by comparing model results with satellite observations. A summary is provided of a number of parameterization schemes for ice cloud radiative properties that were developed for application to broadband radiative transfer submodels within general circulation models(GCMs). The availability of the single-scattering properties of complex ice habits has led to more accurate radiation parameterizations. In conclusion, the importance of using nonspherical ice particle models in GCM simulations for climate studies is proven. 相似文献
34.
Andy Thean Alan Pedlar Marek J. Kukula Stefi A. Baum Christopher P. O'Dea 《Monthly notices of the Royal Astronomical Society》2001,325(2):737-760
We discuss the properties of compact nuclear radio components in Seyfert galaxies from the extended 12-μm AGN sample of Rush et al. Our main results can be summarized as follows.
Type 1 and type 2 Seyferts produce compact radio components which are indistinguishable in strength and aspect, indicating that their central engines are alike, as proposed by the unification model. Infrared IRAS fluxes are more closely correlated with low-resolution radio fluxes than high-resolution radio fluxes, suggesting that they are dominated by kiloparsec-scale, extranuclear emission regions; extranuclear emission may be stronger in type 2 Seyferts. Early-type Seyfert galaxies tend to have stronger nuclear radio emission than late-type Seyfert galaxies. V-shaped extended emission-line regions, indicative of 'ionization cones', are usually found in sources with large, collimated radio outflows. Hidden broad lines are most likely to be found in sources with powerful nuclear radio sources. Type 1 and type 2 Seyferts selected by their IRAS 12-μm flux densities have well-matched properties. 相似文献
Type 1 and type 2 Seyferts produce compact radio components which are indistinguishable in strength and aspect, indicating that their central engines are alike, as proposed by the unification model. Infrared IRAS fluxes are more closely correlated with low-resolution radio fluxes than high-resolution radio fluxes, suggesting that they are dominated by kiloparsec-scale, extranuclear emission regions; extranuclear emission may be stronger in type 2 Seyferts. Early-type Seyfert galaxies tend to have stronger nuclear radio emission than late-type Seyfert galaxies. V-shaped extended emission-line regions, indicative of 'ionization cones', are usually found in sources with large, collimated radio outflows. Hidden broad lines are most likely to be found in sources with powerful nuclear radio sources. Type 1 and type 2 Seyferts selected by their IRAS 12-μm flux densities have well-matched properties. 相似文献
35.
Marek J. Kukula Alan Pedlar S. W. Unger S. Baum C. O'Dea 《Astrophysics and Space Science》1994,216(1-2):371-372
We present the results of an 8.4 GHz VLA survey of the CfA Seyferts. We find that the luminosity functions for Seyfert 1s and 2s are essentially identical, but that the type 2 objects are more likely to contain extended radio structures. This seems to be consistent with unified models for Seyferts, in which the differences between the two classes are largely due to orientation effects. 相似文献
36.
Using separation of variables in space and time we find two-dimensional solutions consistent with simple forms of the inertial Ohm's law, Maxwell's equations, and the equation of momentum transport. conduction mode variation (a temporal change in electrical conductivity due, e.g., to onset of a plasma instability) is frequently found necessary. The result for the effective conductivity shows that either the conduction mode variation or inertial effects may dominate. In the inertial limit and in the absence of conduction mode variation the results are similar to the limiting results of Speiser. However, the important time constant is the inverse growth rate of the current density rather than the lifetime of a particle in the system. Several solutions have magnetic field structures with neutral points. 相似文献
37.
It has been widely conjectured that solar flares are energized by the magnetic energy stored in complex active regions. Paradoxically, however, in attempting to show that magnetic changes cause or characterize flares, solar magnetic observations have produced equivocal results.In previous attempts at resolving the paradox, it has been contended that magnetic measurements are simply imprecise or that magnetic theories of flares are incorrect. We present an alternative explanation: the present use of magnetograms to examine active region structure through numerical integration of miscellaneous field lines (under various force-free assumptions) provides qualitative information only and does not utilize the quantitative information available. Therefore, we propose a new approach to the analysis of magnetograms which is illustrated with a highly symmetrized example that permits integration in closed form. The proposed approach exploits the cellular structure of the flux of field lines present in a complex active region. The various topological connectivities distinguish parent and daughter flux cells. A function F is developed expressing the flux partitioned into the daughter cell of interconnected field lines in a potential field. This F is a function of the location, strength, and relative motions of the photospheric sources. Then dF/dt is used as an EMF in the direct calculation of the stored magnetic energy available for flare production. In carrying out this program the flux partitioning surface (separatrix) is calculated along with its line of self-intersection (separator). The separator is the location of the principal energy release site. 相似文献
38.
Shallow landsliding in the Seattle, Washington, area, has caused the occasional loss of human life and millions of dollars in damage to property. The effective management of the hazard requires an understanding of the rainfall conditions that result in landslides. We present an empirical approach to quantify the antecedent moisture conditions and rainstorm intensity and duration that have triggered shallow landsliding using 25 years of hourly rainfall data and a complementary record of landslide occurrence. Our approach combines a simple water balance to estimate the antecedent moisture conditions of hillslope materials and a rainfall intensity–duration threshold to identify periods when shallow landsliding can be expected. The water balance is calibrated with field‐monitoring data and combined with the rainfall intensity–duration threshold using a decision tree. Results are cast in terms of a hypothetical landslide warning system. Two widespread landslide events are correctly identified by the warning scheme; however, it is less accurate for more isolated landsliding. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
39.
Regional landslide-hazard assessment for Seattle, Washington, USA 总被引:13,自引:6,他引:13
Rex L. Baum Jeffery A. Coe Jonathan W. Godt Edwin L. Harp Mark E. Reid William Z. Savage William H. Schulz Dianne L. Brien Alan F. Chleborad Jonathan P. McKenna John A. Michael 《Landslides》2005,2(4):266-279
Landslides are a widespread, frequent, and costly hazard in Seattle and the Puget Sound area of Washington State, USA. Shallow
earth slides triggered by heavy rainfall are the most common type of landslide in the area; many transform into debris flows
and cause significant property damage or disrupt transportation. Large rotational and translational slides, though less common,
also cause serious property damage. The hundreds of landslides that occurred during the winters of 1995–96 and 1996–97 stimulated
renewed interest by Puget Sound communities in identifying landslide-prone areas and taking actions to reduce future landslide
losses. Informal partnerships between the U.S. Geological Survey (USGS), the City of Seattle, and private consultants are
focusing on the problem of identifying and mapping areas of landslide hazard as well as characterizing temporal aspects of
the hazard. We have developed GIS-based methods to map the probability of landslide occurrence as well as empirical rainfall
thresholds and physically based methods to forecast times of landslide occurrence. Our methods for mapping landslide hazard
zones began with field studies and physically based models to assess relative slope stability, including the effects of material
properties, seasonal groundwater levels, and rainfall infiltration. We have analyzed the correlation between historic landslide
occurrence and relative slope stability to map the degree of landslide hazard. The City of Seattle is using results of the
USGS studies in storm preparedness planning for emergency access and response, planning for development or redevelopment of
hillsides, and municipal facility planning and prioritization. Methods we have developed could be applied elsewhere to suit
local needs and available data. 相似文献
40.
Analyses of the patterns of national population change have been influenced by the US experience where major differences between a older north and a newer south are prominent in many studies. The current paper argues that these perspectives overlook some inertia in the pattern of population and jobs. Drawing upon US and European experience the paper explores the pattern of change in Australia. There it finds that population has favoured newer locations but that gains in share of national population have not been large. At the same time some of the older regions have maintained population share, or at worst recorded small declines. That broad insight is enriched by sectoral analysis where it is clear that only the retail sector has experienced the large gains in new regions; employment change in other sectors showed the links with population change are weak. The paper concludes that simple notions of new region population and employment gains are oversimplified and overlook the substantial development around established centres. In particular it is possible that the old regions are re-structuring their spatial form and spilling over statistical boundaries and so actually maintaining established role in national networks. 相似文献