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941.
Ballooning disturbances in a finite-pressure plasma in a curvilinear magnetic field are described by the system of coupled equations for the Alfvén and slow magnetosonic modes. In contrast to most previous works that locally analyzed the stability of small-scale disturbances using the dispersion relationship, a global analysis outside a WKB approximation but within a simple cylindrical geometry, when magnetic field lines are circles with constant curvature, is performed in the present work. This model is relatively simple; nevertheless, it has the singularities necessary for the formation of the ballooning mode: field curvature and non-uniform thermal plasma pressure. If the disturbance finite radial extent is taken into account, the instability threshold increases as compared to a WKB approximation. The simplified model used in this work made it possible to consider the pattern of unstable disturbances at arbitrary values of the azimuthal wavenumber (k y ). Azimuthally large-scale disturbances can also be unstable, although the increment increases with decreasing azimuthal scale and reaches saturation when the scales are of the order of the pressure nonuniformity dimension. 相似文献
942.
943.
N. G. Kleimenova O. V. Kozyreva M. Kubicki S. Michnowski 《Geomagnetism and Aeronomy》2011,51(3):394-401
This paper studies time variations in the near-ground atmospheric electric field (Ez) at the geomagnetic latitude of 74° (Hornsund observatory) during polar substorms. Ez variations are compared with those in the potential drop across the polar cap (Up), according to SuperDARN radar observations. It is found that in the morning sector, time variations in Ez are strongly driven by time variations in the electrojet and almost do not depend on time variations in Up, which is presumably due to the penetration of the electric field of the electrojet into tropospheric altitudes. 相似文献
944.
The energy spectrum of electrons with energies of 0.8–6.0 MeV has been analyzed based on the data of the Express-A2 geostationary
satellite and time variations in the fluxes of electrons with energies higher than 0.6 and 2 MeV (according to the GOES-10
satellite data) before and after a weak geomagnetic storm on April 9–10, 2002, which developed during the prolonged (about
ten days) recovery phase of a strong magnetic storm on April 6, 2000. The effect of the secondary injection and acceleration
caused by an intensification of substorm activity during a weak storm on the electron flux dynamics has been studied for the
first time. The energy spectra and time variations in the electron flux dynamics before and after a weak storm have been described
based on analytical solutions to the kinetic equation for the electron distribution function with regard to the stochastic
acceleration and loss rates. It has been established that there were different acceleration and loss rates before and after
the weak storm of April 9–10, 2000. 相似文献
945.
S. Demberel G. Batarsuren V. S. Imaev A. L. Strom O. P. Smekalin A. V. Chipizubov N. N. Grib A. A. Syasko A. V. Kachaev 《Seismic Instruments》2011,47(4):314-320
The Gunzhin system of NE-trending active faults is described on the basis of results of special seis-motectonic studies carried
out for the first time around Ulan Bator, Mongolia. This system crosses watershed parts of stream valley. It is named after
one of them. The total length of the fault segment traced on aerial photos is 15–20 km. In valleys of some temporary stream
flows there are considerable visible horizontal displacements attaining 20–25 m, which testify to the right lateral slip (Khundullun
River). Revealed structural parageneses of thrusts and overthrusts, divergent as a fan-shaped system to the both sides from
the axial sub-vertical shift zone, are reliably confirmed by the data of geophysical investigations. Taking into account the
known correlation relationships between seismodislocation parameters (length and maximum displacement amplitude) and earthquake
magnitudes, it is possible to suggest that the Gunzhin Fault generated two paleoearthquakes with the magnitude of about 7.0
in the Late Holocene. It means that displacements along that fault could attain the intensity of 9–10 degrees in the Ulan
Bator territory according to the MSK-64 scale. This result must be taken into account in estimation of seismic hazard in the
territory discussed. 相似文献
946.
Seismotectonic regionalization of the Kamchatka subduction zone was carried out by retrospective analysis of the temporal
sequence and locations of earthquake occurrence and an examination of relationships between the earthquake hypocenters and
morphostructures in the continental slope of eastern Kamchatka. Ten segments separated with earthquake-generating strike-slip
faults have been identified in the overthrusting (overhanging) margin of the Sea-of-Okhotsk plate in the zone where the Pacific
and the Sea-of-Okhotsk plates interact orthogonally. Two to three earthquake-generating thrust blocks have been identified
within these segments. This type of subduction is consistent with the keyboard-block model of L.I. Lobkovskii and B.V. Baranov.
We put forward a model involving segmentation and generation of thrust blocks due to nonuniform coupling between the subducted
Pacific plate and the overhanging Sea-of-Okhotsk plate. According to this model, both segmentation and the formation of thrust
blocks are caused by nonuniform plate coupling due to unevenness in the relief of the plunging plate. The thrusts have relief
expression as underwater highs and terraces, which indicate that a tsunami-generating earthquake can occur at this location.
The highest rate of occurrence for magnitude 7 or greater earthquakes is found at the sharp bend of the Pacific plate, where
the subduction angle is 10°–12° instead of 50°–51°, corresponding to a frontal (tectonic) arc, which can be traced by a positive
free-air gravity anomaly and by an isostatic anomaly. 相似文献
947.
Luca M. Abbühl Kevin P. Norton John D. Jansen Fritz Schlunegger Ala Aldahan Göran Possnert 《地球表面变化过程与地形》2011,36(11):1464-1473
A steep escarpment edge, deep gorges and distinct knickzones in river profiles characterize the landscape on the Western Escarpment of the Andes between ~5°S and ~18°S (northern Peru to northern Chile). Strong north–south and east–west precipitation gradients are exploited in order to determine how climate affects denudation rates in three river basins spanning an otherwise relatively uniform geologic and geomorphologic setting. Late Miocene tectonics uplifted the Meseta/Altiplano plateau (~3000 m a.s.l.), which is underlain by a series of Tertiary volcanic‐volcanoclastic rocks. Streams on this plateau remain graded to the Late Miocene base level. Below the rim of the Meseta, streams have responded to this ramp uplift by incising deeply into fractured Mesozoic rocks via a series of steep, headward retreating knickzones that grade to the present‐day base level defined by the Pacific Ocean. It is found that the Tertiary units on the plateau function as cap‐rocks, which aid in the parallel retreat of the sharp escarpment edge and upper knickzone tips. 10Be‐derived catchment denudation rates of the Rio Piura (5°S), Rio Pisco (13°S) and Rio Lluta (18°S) average ~10 mm ky?1 on the Meseta/Altiplano, irrespective of precipitation rates; whereas, downstream of the escarpment edge, denudation rates range from 10 mm ky?1 to 250 mm ky?1 and correlate positively with precipitation rates, but show no strong correlation with hillslope angles or channel steepness. These relationships are explained by the presence of a cap‐rock and climate‐driven fluvial incision that steepens hillslopes to near‐threshold conditions. Since escarpment retreat and the precipitation pattern were established at least in the Miocene, it is speculated that the present‐day distribution of morphology and denudation rates has probably remained largely unchanged during the past several millions of years as the knickzones have propagated headward into the plateau. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
948.
949.
C. Damiani J.P. Rozelot S. Lefebvre A. Kilcik A.G. Kosovichev 《Journal of Atmospheric and Solar》2011,73(2-3):241-250
We hereby present a review on solar oblateness measurements. By emphasizing historical data, we illustrate how the discordance between experimental results can lead to substantial improvements in the building of new technical apparatus as well as to the emergence of new ideas to develop new theories. We stress out the need to get accurate data from space to enhance our knowledge of the solar core in order to develop more precise ephemerids and ultimately build possible new gravitational theories. 相似文献
950.
G. Lanzo M. Tallini G. Milana G. Di Capua F. Del Monaco A. Pagliaroli S. Peppoloni 《Bulletin of Earthquake Engineering》2011,9(6):1855-1875
The paper focuses on the strong motion array deployed in the upper Aterno River Valley, in the immediate outskirts north-west
of the town of L’Aquila, which is part of the Italian Strong Motion Network operated by the Department of Civil Protection.
The array is composed of six accelerometric stations located along a cross section of the valley. The importance of this array
relies on the fact that a large amount of high-quality records were obtained during the 2009 L’Aquila seismic sequence, from
both the mainshock and several aftershocks. These data are especially important to investigate site effects in sediment-filled
valleys during moderate earthquakes in epicentral area because well-documented observational studies are very limited in the
literature. However, the main drawback for the study of site effects in the Aterno valley is the lack of a detailed knowledge
of the geometry of the valley, soil layering and dynamic properties of materials. The main motivation for this study stems
from the need to provide a reliable subsoil model of the valley coupled with high-quality strong motion data. Based on the
above, in the framework of S4 project, a major effort was undertaken to get a trustworthy cross section of the valley by an
ad hoc investigation, comprising geological and geotechnical surveys as well as an extensive geophysical campaign, characterized
by both active and passive measurements. These results were complemented by additional geological and geotechnical data available
in the literature. By merging all the information acquired, a 2D subsoil model of the transversal section of the upper Aterno
valley has been produced. The valley is characterised by an asymmetric shape with a shallower rock basement at the western
edge of the valley that deepens at the valley centre. Moreover, based on the results of geophysical tests, representative
Vs values were assigned to the different lithologic units forming the alluvial deposits filling the valley. Shear wave velocity
is a fundamental parameter for ground response studies and it is also effective in classifying the accelerometric station
from a seismic point of view. The 2D model may be therefore, considered a benchmark model for future studies of site effects.
It will offer the possibility to examine site effects with a complex underlying geology and to validate the results of numerical
simulations of site response analyses with the numerous observations from earthquake recordings, both for weak and strong
ground motion conditions. 相似文献