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排序方式: 共有263条查询结果,搜索用时 31 毫秒
101.
Mark E. Hervig Larry L. Gordley Michael H. Stevens James M. Russell Scott M. Bailey Gerd Baumgarten 《Journal of Atmospheric and Solar》2009,71(3-4):316-330
The Solar Occultation For Ice Experiment (SOFIE) was launched onboard the Aeronomy of Ice in the Mesosphere (AIM) spacecraft to measure polar mesospheric clouds (PMCs) and their environment. This work describes methods for identifying PMCs in SOFIE observations and determining mass density, particle shape, particle effective radius, and the parameters of a Gaussian size distribution. Results using SOFIE measurements from the northern summer of 2007 are compared with concurrent observations by the ALOMAR lidar in northern Norway. Ice particle properties determined from SOFIE are in good agreement with the lidar results, considering the differences in instrument characteristics. 相似文献
102.
Michael H. Stevens Christoph R. Englert Mark Hervig Svetlana V. Petelina Werner Singer Kim Nielsen 《Journal of Atmospheric and Solar》2009,71(3-4):401-407
We present the first measurement of polar mesospheric cloud (PMC) occurrence frequency over the diurnal cycle from a satellite. The observations are made during the 2007 northern hemisphere PMC season by the Spatial Heterodyne IMager for MEsospheric Radicals (SHIMMER), which views the limb near 309 nm typically between 34 and 98 km. The PMC diurnal variation is derived between 50 and 58°N, where local times at the tangent point precess by ~30 min/day allowing for observations between 0330 and 2130 local time during the PMC season. We find that the occurrence frequencies exhibit a strong semidiurnal behavior with peaks near 0600 and 1800 local time and a minimum between 0900 and 1600 during which they are on average an order of magnitude less. The semidiurnal dependence is strongly correlated with concurrent ground-based measurements of meridional winds and temperatures measured at the same latitude. Our results for PMC frequency over the diurnal cycle can be used to help reconcile observations from other satellites that only permit cloud measurements at discrete local times. 相似文献
103.
High-altitude data assimilation system experiments for the northern summer mesosphere season of 2007 总被引:1,自引:0,他引:1
Stephen D. Eckermann Karl W. Hoppel Lawrence Coy John P. McCormack David E. Siskind Kim Nielsen Andrew Kochenash Michael H. Stevens Christoph R. Englert Werner Singer Mark Hervig 《Journal of Atmospheric and Solar》2009,71(3-4):531-551
A global numerical weather prediction system is extended to the mesosphere and lower thermosphere (MLT) and used to assimilate high-altitude satellite measurements of temperature, water vapor and ozone from MLS and SABER during May–July 2007. Assimilated temperature and humidity from 100 to 0.001 hPa show minimal biases compared to satellite data and existing analysis fields. Saturation ratios derived diagnostically from these assimilated temperature and water vapor fields at PMC altitudes and latitudes compare well with seasonal variations in PMC frequency measured from the aeronomy of ice in the mesosphere (AIM) satellite. Synoptic maps of these diagnostic saturation ratios correlate geographically with three independent transient mesospheric cloud events observed at midlatitudes by SHIMMER on STPSat-1 and by ground observers during June 2007. Assimilated temperatures and winds reveal broadly realistic amplitudes of the quasi 5-day wave and migrating tides as a function of latitude and height. For example, analyzed winds capture the dominant semidiurnal MLT wind patterns at 55°N in June 2007 measured independently by a meteor radar. The 5-day wave and migrating diurnal tide also modulate water vapor mixing ratios in the polar summer MLT. Possible origins of this variability are discussed. 相似文献
104.
R. Machado C. Lana G. Stevens C.R.S. Filho W.U. Reimold Iain McDonald 《Geochimica et cosmochimica acta》2009,73(23):7183-7201
This paper provides important insights into the generation, extraction and crystallization of clast-laden impact melt rocks from the Araguainha impact structure, central Brazil. Despite the mixed nature of the Araguainha target rocks (comprising a 2 km thick sequence of sedimentary rocks and underlying granitic basement), the exposed melt bodies are characterised by an alkali-rich granitic matrix embedding mineral and rock fragments derived only from the target granite. The melt rocks occur in the form of a massive impact melt sheet overlying the eroded central uplift structure, and as melt veins in the granite of the core of the central uplift. Bulk-rock major and trace element data (including platinum group elements) indicate that the precursor melts were generated locally, principally by partial melting of the target granite, without any contribution from the sedimentary sequence or the projectile. The dense network of melt veins was formed in isolation, by selective melting of plagioclase and alkali feldspar within the granite target. Plagioclase and alkali feldspar melted discretely and congruently, producing domains in the matrix of the melt veins, which closely match the stoichiometry of these minerals. The compositionally discrete initial melt phases migrated through a dense network of microfractures before being assembled into larger melt veins. Freezing of the melt veins was substantially fast, and the melt components were quenched in the form of alkali-feldspar and plagioclase schlieren in the matrix of the melt veins. The overlying impact melt rock is, in contrast, characterised by a granophyric matrix consisting of albite, sanidine, quartz, biotite and chlorite. In this case, melt components appear to have been more mobile and to have mixed completely to form a granitic parental melt. We relate the melting of the minerals to post-shock temperatures that exceeded the melting point of feldspars. 相似文献
105.
Lagrangian drifters, moored acoustic Doppler current meters and hydrographic observations are combined with wind observations to describe the mean and variable nature of flow around Kapiti Island, New Zealand. Thirteen day-long deployments of up to six Lagrangian drifters show the mean flow is to the southwest, with evidence of stronger flows in the channel separating the island from the mainland, and an island wake in the lee of the island. Vortices in this island wake may be tidally driven. Scaling considerations suggest the flow is strong enough that tidal-generated vortices are shed on each tidal cycle. Both the drifters and mooring data suggest that the d’Urville Current around Kapiti Island has a significant wind-driven component. During north-westerlies, the drifters tend to hug the coast, and south-eastwards flows in the Rauoterangi Channel are accelerated. We suggest the observed correlation is the local expression of a South Taranaki basin scale response to the winds. 相似文献
106.
The trace element compositions of S-type granites: evidence for disequilibrium melting and accessory phase entrainment in the source 总被引:4,自引:2,他引:2
Arnaud Villaros Gary Stevens Jean-François Moyen Ian S. Buick 《Contributions to Mineralogy and Petrology》2009,158(4):543-561
Within individual plutons, the trace element concentrations in S-type granites generally increase with maficity (total iron
and magnesium content and expressed as atomic Fe + Mg in this study); the degree of variability in trace element concentration
also expands markedly with the same parameter. The strongly peraluminous, high-level S-type granites of the Peninsular Pluton
(Cape Granite Suite, South Africa) are the product of biotite incongruent melting of a metasedimentary source near the base
of the crust. Leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties
represent mixtures of melt and peritectic garnet and ilmenite. Trace elements such as Rb, Ba, Sr and Eu, that are concentrated
in reactant minerals in the melting process, show considerable scatter within the granites. This is interpreted to reflect
compositional variation in the source. In contrast, elements such as LREE, Zr and Hf, which are concentrated within refractory
accessory phases (zircon and monazite), show well-defined negative correlations with increasing SiO2 and increase linearly with increasing maficity. This is interpreted to reflect coupled co-entrainment of accessory minerals
and peritectic phases to the melt: leucocratic rocks cannot have evolved from the more mafic compositions in the suite by
a process of fractional crystallisation because in this case they would have inherited the zircon-saturated character of this
hypothetical earlier magma. Trace element behaviour of granites from the Peninsular Pluton has been modelled via both equilibrium
and disequilibrium trace element melting. In the disequilibrium case, melts are modelled as leaving the source with variable
proportions of entrained peritectic phases and accessory minerals, but before the melt has dissolved any accessory minerals.
Thus, the trace element signature of the melt is largely inherited from the reactants in the melting reaction, with no contribution
from zircon and monazite dissolution. In the equilibrium case, melt leaves the source with entrained crystals, after reaching
zircon and monazite saturation. A significant proportion of the rocks of the Peninsular Pluton have trace element concentrations
below those predicted by zircon and monazite saturation. In the case of the most leucocratic rocks all compositions are zircon
undersaturated; whilst the majority of the most mafic compositions are zircon oversaturated. However, in both cases, zircon
is commonly xenocrystic. Thus, the leucocratic rocks represent close to pure melts, which escaped their sources rapidly enough
that some very closely match the trace element disequilibrium melting model applied in this study. Zircon dissolution rates
allow the residency time for the melt in the source to be conservatively estimated at less than 500 years. 相似文献
107.
J. B. Stevens M. J. Coe D. A. H. Buckley 《Monthly notices of the Royal Astronomical Society》1999,309(2):421-429
The fields of eight X-ray sources in the Magellanic Clouds believed to be Be/X-ray binaries have been searched for possible Be-star counterparts. BVR c and H α CCD imaging was employed to identify early-type emission stars through colour indices and H α fluxes. Spectroscopy of five sources confirms the presence of H α emission in each case. Based on the positional coincidence of emission-line objects with the X-ray sources, we identify Be-star counterparts to the ROSAT sources RX J0032.9-7348, RX J0049.1-7250, RX J0054.9-7226 and RX J0101.0-7206, and to the recently discovered ASCA source AX J0051-722. We confirm the Be star nature of the counterpart to the HEAO1 source H0544-66. In the field of the ROSAT source RX J0051.8-7231 we find that there are three possible counterparts, each showing evidence for H α emission. We find a close double in the error circle of the EXOSAT source EXO 0531.1-6609, each component of which could be a Be star associated with the X-ray source. 相似文献
108.
109.
Observations at millimetre wavelengths are presented for a representative sample of 22 X-ray-selected BL Lac objects (XBLs). This sample comprises 19 high-energy cut-off BL Lac objects (HBLs), 1 low-energy cut-off BL Lac object (LBL) and 2 'intermediate' sources. Data for LBLs, which are mostly radio-selected BL Lac objects (RBLs), are taken from the literature. It is shown that the radio–millimetre spectral indices of HBLs are slightly steeper than those of the LBLs . A correlation exists between α 5–230 and 230 GHz luminosity. While this correlation could be an artefact of comparing two populations of BL Lac objects with intrinsically different radio properties, it is also consistent with the predictions of existing unified schemes that relate BL Lac objects to Fanaroff–Riley class I radio galaxies.
The HBLs have significantly flatter submillimetre–X-ray spectral indices than the LBLs although the two intermediate sources also have intermediate values of α230–X ∼−0.9. It is argued that this difference cannot be explained entirely by the viewing-angle hypothesis and requires a difference in physical-source parameters. The α 230–X values for the HBLs are close to the canonical value found for large samples of radio sources and thus suggest that synchrotron radiation is the mechanism that produces the X-ray emission. As suggested by Padovani & Giommi, the inverse-Compton mechanism is likely to dominate in the LBLs requiring the synchrotron spectra of these sources to steepen or cut off at lower frequencies than those of the HBLs. 相似文献
The HBLs have significantly flatter submillimetre–X-ray spectral indices than the LBLs although the two intermediate sources also have intermediate values of α
110.