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591.
We present new ultraviolet spectra of the hottest known, peculiar white dwarf H1504+65, obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. H1504+65 is the hottest known white dwarf (T
eff=200 000 K) and has an atmosphere mainly composed by carbon and oxygen, augmented with high amounts of neon and magnesium.
This object is unique and the origin of its surface chemistry is completely unclear. We probably see the naked core of either
a C–O white dwarf or even a O–Ne–Mg white dwarf. In the latter case, this would be the first proof that such white dwarfs
can be the outcome of single-star evolution. The new observations were performed to shed light on the origin of this mysterious
object. 相似文献
592.
Boris Shustov Mikhail Sachkov Ana I. Gómez?de?Castro Klaus Werner Norbert Kappelmann Alexander Moisheev 《Astrophysics and Space Science》2011,335(1):273-282
Continuous access to the UV domain has been considered of importance to astrophysicists and planetary scientists since the
mid-sixties. However, the future of UV missions for the post-HST era is believed by a significant part of astronomical community
to be less encouraging. We argue that key science problems of the coming years will require further development of UV observational
technologies. Among these hot astrophysical issues are: the search for missing baryons, revealing the nature of astronomical
engines, properties of atmospheres of exoplanets as well as of the planets of the Solar System etc. We give a brief review
of UV-missions both in the past and in the future. We conclude that UV astronomy has a great future but the epoch of very
large and efficient space UV facilities seems to be a prospect for the next decades. As to the current state of the UV instrumentation
we think that this decade will be dominated by the HST and coming World Space Observatory-Ultraviolet (WSO-UV) with a 1.7 m
UV-telescope onboard. The international WSO-UV mission is briefly described. It will allow high resolution/high sensitivity
imaging and high/low resolution spectroscopy from the middle of the decade. 相似文献
593.
594.
Eddy-Covariance Flux Measurements in the Complex Terrain of an Alpine Valley in Switzerland 总被引:1,自引:1,他引:0
We measured the surface energy budget of an Alpine grassland in highly complex terrain to explore possibilities and limitations
for application of the eddy-covariance technique, also for CO2 flux measurements, at such non-ideal locations. This paper focuses on the influence of complex terrain on the turbulent energy
measurements of a characteristic high Alpine grassland on Crap Alv (Alp Weissenstein) in the Swiss Alps during the growing
season 2006. Measurements were carried out on a topographic terrace with a slope of 25◦ inclination. Flux data quality is assessed via the closure of the energy budget and the quality flag method used within the
CarboEurope project. During 93% of the time the wind direction was along the main valley axis (43% upvalley and 50% downvalley
directions). During the transition times of the typical twice daily wind direction changes in a mountain valley the fraction
of high and good quality flux data reached a minimum of ≈50%, whereas during the early afternoon ≈70% of all records yielded
good to highest quality (CarboEurope flags 0 and 1). The overall energy budget closure was 74 ± 2%. An angular correction
for the shortwave energy input to the slope improved the energy budget closure slightly to 82 ± 2% for afternoon conditions.
In the daily total, the measured turbulent energy fluxes are only underestimated by around 8% of net radiation. In summary,
our results suggest that it is possible to yield realistic energy flux measurements under such conditions. We thus argue that
the Crap Alv site and similar topographically complex locations with short-statured vegetation should be well suited also
for CO2 flux measurements. 相似文献
595.
A traditional mulching technique used in Lanzarote, Canary Islands, allows dry farming as well as pronounced water savings in irrigation. It is known to reduce evaporational losses, but is also supposed to enhance the nocturnal condensation of water vapour from the atmosphere. The mulch layer consists of porous volcanic rock fragments abundantly available on the island. The mulched surface is believed to cool rapidly and to be more hygroscopic than a bare soil surface. This was investigated during a field experiment conducted over 68 nights during different seasons in 2001 and 2002, as well as some simple laboratory measurements. It was found that nocturnal condensation on the mulch surface (max 0.33 mm) was lower than on the bare soil surface (max 0.57 mm) or any one of three alternative mulch substrates. However, a slightly stronger nocturnal cooling of the mulched as compared to the bare surface was present. It is shown that these contrary findings can be explained by the higher hygroscopicity of the dry loam soil, resulting in condensation gains beyond the strict definition of dew. Differences in plant-availability of non-hygroscopic dew water and hygroscopic water uptakes are discussed, and conditions under which mulching would show positive condensation effects are defined. This includes a theoretical section demonstrating that non-hygroscopic mulch layers of a proper thickness can provide small amounts of dew to plant roots at the mulch–soil interface. This condensation could also happen during the day and would be favoured by a high amplitude of the diurnal atmospheric moisture cycle. 相似文献
596.
M. A. Barstow M. P. Kowalski R. G. Cruddace K. S. Wood F. Auchere N. J. Bannister M. F. Bode G. E. Bromage M. R. Burleigh A. C. Cameron A. Cassatella F. Delmotte J. G. Doyle B. Gaensicke B. Gibson C. S. Jeffery C. Jordan N. Kappelmann R. Lallement J. S. Lapington D. de Martino S. A. Matthews M. Orio E. Pace I. Pagano K. J. H. Phillips M.-F. Ravet J. H. M. M. Schmitt B. Y. Welsh K. Werner G. Del Zanna 《Experimental Astronomy》2009,23(1):169-191
This paper describes a proposed high resolution soft X-ray and Extreme Ultraviolet spectroscopy mission to carry out a survey
of Stellar and Galactic Environments (SAGE). The payload is based on novel diffraction grating technology which has already
been proven in a sub-orbital space mission and which is ready to fly on a satellite platform with minimal development. We
discuss the goals of a SAGE base-line mission and demonstrate the scientific importance of high resolution spectroscopy in
the Extreme Ultraviolet for the study of stars and the local interstellar medium. 相似文献
597.
M. A. Barstow M. R. Burleigh N. J. Bannister J. S. Lapington M. P. Kowalski R. G. Cruddace K. S. Wood F. Auchere M. F. Bode G. E. Bromage B. Gibson A. C. Cameron A. Cassatella F. Delmotte M.-F. Ravet J. G. Doyle C. S. Jeffery B. Gaensicke C. Jordan N. Kappelmann K. Werner R. Lallement D. de Martino S. A. Matthews K. J. H. Phillips G. Del Zanna M. Orio E. Pace I. Pagano J. H. M. M. Schmitt B. Y. Welsh 《Astrophysics and Space Science》2009,320(1-3):231-238
This paper describes a proposed high resolution soft X-ray and Extreme Ultraviolet (EUV) spectroscopy mission to carry out a survey of Stellar and Galactic Environments (SAGE). The payload is based on novel diffraction grating technology which has already been proven in a sub-orbital space mission and which is ready to fly on a satellite platform with minimal development. Much of the technical detail of the instrumentation has been reported elsewhere and we concentrate our discussion here on the scientific goals of a SAGE base-line mission, demonstrating the scientific importance of high resolution spectroscopy in the Extreme Ultraviolet for the study of stars and the local interstellar medium. 相似文献
598.
Massimo Vellante Matthias Förster Michael Pezzopane Norbert Jakowski Tie Long Zhang Umberto Villante Marcello De Lauretis Bruno Zolesi Werner Magnes 《Earth, Moon, and Planets》2009,104(1-4):25-27
Cross-spectral analysis of ULF wave measurements recorded at ground magnetometer stations closely spaced in latitude allows accurate determinations of magnetospheric field line resonance (FLR) frequencies. This is a useful tool for remote sensing temporal and spatial variations of the magnetospheric plasma mass density. The spatial configuration of the South European GeoMagnetic Array (SEGMA, 1.56 < L < 1.89) offers the possibility to perform such studies at low latitudes allowing to monitor the dynamical coupling between the ionosphere and the inner plasmasphere. As an example of this capability we present the results of a cross-correlation analysis between FLR frequencies and solar EUV irradiance (as monitored by the 10.7-cm solar radio flux F10.7) suggesting that changes in the inner plasmasphere density follow the short-term (27-day) variations of the solar irradiance with a time delay of 1–2 days. As an additional example we present the results of a comparative analysis of FLR measurements, ionospheric vertical soundings and vertical TEC measurements during the development of a geomagnetic storm. 相似文献
599.
N. Werner † I. Zhuravleva E. Churazov A. Simionescu S. W. Allen W. Forman C. Jones J. S. Kaastra 《Monthly notices of the Royal Astronomical Society》2009,398(1):23-32
The dense cores of X-ray emitting gaseous haloes of large elliptical galaxies with temperatures kT ≲ 0.8 keV show two prominent Fe xvii emission features, which provide a sensitive diagnostic tool to measure the effects of resonant scattering. We present here high-resolution spectra of five bright nearby elliptical galaxies, obtained with the reflection grating spectrometers (RGS) on the XMM-Newton satellite. The spectra for the cores of four of the galaxies show the Fe xvii line at 15.01 Å being suppressed by resonant scattering. The data for NGC 4636 in particular allow the effects of resonant scattering to be studied in detail and to prove that the 15.01 Å line is suppressed only in the dense core and not in the surrounding regions. Using deprojected density and temperature profiles for this galaxy obtained with the Chandra satellite, we model the radial intensity profiles of the strongest resonance lines, accounting for the effects of resonant scattering, for different values of the characteristic turbulent velocity. Comparing the model to the data, we find that the isotropic turbulent velocities on spatial scales smaller than ≈1 kpc are less than 100 km s−1 and the turbulent pressure support in the galaxy core is smaller than 5 per cent of the thermal pressure at the 90 per cent confidence level, and less than 20 per cent at 95 per cent confidence. Neglecting the effects of resonant scattering in spectral fitting of the inner 2 kpc core of NGC 4636 will lead to underestimates of the chemical abundances of Fe and O by ∼10–20 per cent. 相似文献
600.
Werner Wassenberg 《Solar physics》1971,20(1):130-135
The polarization distribution of 17 GHz bursts is studied observed within a period of 1 yr after maximum solar activity. The typical variation of polarization with time of impulsive bursts leads to the conclusion of a thermalization of the emission region in the post-burst phase. The fine structure of the polarization curve of complex bursts is shown and two possible interpretations of the observed inversion of the polarization at 17 GHz during a complex event are given. 相似文献