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排序方式: 共有840条查询结果,搜索用时 31 毫秒
721.
722.
723.
Stephen Serjeant † Alberto Carramiñana † Eduardo Gonzáles-Solares Phillipe Héraudeau Raúl Mújica † Ismael Perez-Fournon Nicola Sedgwick † Michael Rowan-Robinson Alberto Franceschini Thomas Babbedge Carlos del Burgo Paolo Ciliegi reas Efstathiou Fabio La Franca Carlotta Gruppioni David Hughes Carlo Lari Seb Oliver Francesca Pozzi Manfred Stickel Mattia Vaccari 《Monthly notices of the Royal Astronomical Society》2004,355(3):813-818
724.
Andrea Tiengo Paolo Esposito Sandro Mereghetti Lara Sidoli Diego Götz Marco Feroci Roberto Turolla Silvia Zane Gian Luca Israel Luigi Stella Peter Woods 《Astrophysics and Space Science》2007,308(1-4):33-37
We present a systematic analysis of all the BeppoSAX data of SGR1900+14. The observations spanning five years show that the source was brighter than usual on two occasions: ~20 days after the August 1998 giant flare and during the 105?s long X-ray afterglow following the April 2001 intermediate flare. In the latter case, we explore the possibility of describing the observed short term spectral evolution only with a change of the temperature of the blackbody component. In the only BeppoSAX observation performed before the giant flare, the spectrum of the SGR1900+14 persistent emission was significantly harder and detected also above 10 keV with the PDS instrument. In the last BeppoSAX observation (April 2002) the flux was at least a factor 1.2 below the historical level, suggesting that the source was entering a quiescent period. 相似文献
725.
Open-shutter RHESSI observations of 3–15 keV X-rays are found to exhibit active-region transient brightenings and microflares
at a rate of a least 10 per hour occurring even during the periods of lowest solar activity so far in the mission. A thermal
component fitted by temperatures of 6–14 MK dominates from 3 keV to about 9 keV, but can be traced up to 14 keV in some cases,
and has an average duration of 131(±103) s at 7–8 keV. The duration increases with decreasing photon energy. The peak count
rate defined by cross-correlation is delayed at low energies. The temperature peaks early in the event and then decreases,
whereas the emission measure increases throughout the event. The properties are consistent with thermal conduction dominating
the evolution. In some of the bigger events, a second component was found in the 11–14 keV range extending down to 8 keV in
some cases. The duration is typically 3 times shorter and ends near the peak time of the thermal component consistent with
the Neupert effect of regular flares. Therefore the second component is suggested to be of non-thermal origin, presumably
causing the beam-driven evaporation of the first component. The two components can be separated and analyzed in detail for
the first time. Low-keV measurements allow a reliable estimate of the energy input by microflares necessary to assess their
relevance for coronal heating.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022496515506 相似文献
726.
Seismology of the solar atmosphere 总被引:1,自引:0,他引:1
Wolfgang Finsterle Stuart M. Jefferies Alessandro Cacciani Paolo Rapex Cynthia Giebink Allister Knox Vincenzo DiMartino 《Solar physics》2004,220(2):317-331
We describe a new instrument for seismically probing the properties of the Sun's lower atmosphere, and present some first
results from an observational campaign carried out at the geographic South Pole during the austral summer of 2002/2003. A
preliminary analysis of the data (simultaneous, high-cadence observations of the velocity signals from the photosphere and
low chromosphere) shows that the well-known suppression of acoustic power in regions of strong magnetic field, and enhancement
of high-frequency power around active regions (acoustic halos), are both consistent with a spreading out of the magnetic field
lines with increasing height in the atmosphere. The data have also revealed some unexpected wave behavior. First, evanescent-like
waves are found at frequencies substantially above the acoustic cut-off frequency in regions of intermediate magnetic field.
Second, upward- and downward-propagating waves are detected in areas of strong magnetic field such as sunspots and plage:
even at frequencies below the acoustic cut-off frequency. Third, the wave behavior in regions of strong magnetic field can
change over periods of a few hours from propagating to evanescent. While we have no concrete explanation for the first two
results, the latter result opens up the question of whether sound waves are involved in short-term events such as flares or
CME's. 相似文献
727.
Piercarlo Bonifacio Elisabetta Caffau Miriam Centurión Paolo Molaro Giovanni Vladilo 《Monthly notices of the Royal Astronomical Society》2001,325(2):767-771
By using UV spectra for the O star HD 93521 taken with the ORFEUS II echelle spectrograph, we determine an 'astrophysical' f value for the S ii λ 94.7-nm line: f =0.00498−0.00138 +0.00172 , error at 1 σ level. This is almost a factor of 30 smaller than the guessed value found in the Kurucz data base ( f =0.1472) , which was until now the only one available for this transition. We use our 'astrophysical' f to investigate the S abundance in two damped Ly α absorption systems (DLAs) observed with the UV–Visual Echelle Spectrograph (UVES) at the European Southern Observatory's 8.2-m Kueyen telescope. In the case of the absorber at z abs =3.02486 towards QSO 0347-3819, we find a sulphur column density which is consistent, within errors, with that determined by Centurión et al. by means of the λ 125.9-nm line, thus providing an external check on the accuracy of our f value. For the damped absorber at z abs =4.4680 towards BR J0307-4945, we determine a high value of the S abundance, which, however, is probably the result of blending with Ly α forest lines. 相似文献
728.
Paolo Padovani 《Astrophysics and Space Science》2007,309(1-4):63-71
The “blazar sequence” posits that the most powerful BL Lacertae objects and flat-spectrum radio quasars should have relatively
small synchrotron peak frequencies, ν
peak, and that the least powerful such objects should have the highest ν
peak values. This would have strong implications for our understanding of jet formation and physics and the possible detection
of powerful, moderately high-redshift TeV blazars. I review the validity of the blazar sequence by using the results of very
recent surveys and compare its detailed predictions against observational data. I find that the blazar sequence in its simplest
form is ruled out. However, powerful flat-spectrum radio quasars appear not to reach the ν
peak typical of BL Lacs. This could indeed be related to some sort of sequence, although it cannot be excluded that it is instead
due to a selection effect.
相似文献
729.
730.
Bringing GEOSS Services into Practice: A Capacity Building Resource on Spatial Data Infrastructures (SDI)
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Gregory Giuliani Pierre Lacroix Yaniss Guigoz Roberto Roncella Lorenzo Bigagli Mattia Santoro Paolo Mazzetti Stefano Nativi Nicolas Ray Anthony Lehmann 《Transactions in GIS》2017,21(4):811-824
Data discoverability, accessibility, and integration are frequent barriers for scientists and a major obstacle for favorable results on environmental research. To tackle this issue, the Group on Earth Observations (GEO) is leading the development of the Global Earth Observation System of Systems (GEOSS), a voluntary effort that connects Earth Observation resources world‐wide, acting as a gateway between producers and users of environmental data. GEO recognizes the importance of capacity building and education to reach large adoption, acceptance and commitment on data sharing principles to increase the capacity to access and use Earth Observations data. This article presents “Bringing GEOSS services into practice” (BGSIP), an integrated set of teaching material and software to facilitate the publication and use of environmental data through standardized discovery, view, download, and processing services, further facilitating the registration of data into GEOSS. So far, 520 participants in 10 countries have been trained using this material, leading to numerous Spatial Data Infrastructure implementations and 1,000 tutorial downloads. This workshop lowers the entry barriers for both data providers and users, facilitates the development of technical skills, and empowers people. 相似文献