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41.
Kazuho Horiuchi Tomoko Uchida Yuko Sakamoto Aoi Ohta Hiroyuki Matsuzaki Yasuyuki Shibata Hideaki Motoyama 《Quaternary Geochronology》2008,3(3):253-261
The cosmogenic nuclide 10Be was analyzed by using accelerator mass spectrometry on an ice core drilled at the Dome Fuji station, inland Antarctica, for 700–1900 yr CE. The measured concentration of 10Be in the Dome Fuji ice core and the derived 10Be flux show similar fluctuations, with both increasing at known solar-activity minima over the last millennium in agreement with earlier observations of 10Be and 14C. Based on the similar nature of the 10Be flux to the reconstructed 14C production rate patterns, a 10Be–14C correlation age model for the Dome Fuji ice core was successfully constructed. This age model agrees well with the initial version of the tephrochronology of the core. The 10Be-flux record contains information on variability in the amount of cosmic radiation incident on the atmosphere, which is mainly attributable to high-frequency change in solar activity and low-frequency background intensity adjustment of the geomagnetic field. High-resolution 10Be analyses of the Dome Fuji ice cores promise to provide potentially important information on the history of cosmic radiation intensity over the past several hundred thousand years. 相似文献
42.
J. E. J. Lovell H. Hirabayashi H. Kobayashi Y. Murata P. G. Edwards G. A. Moellenbrock S. Horiuchi E. B. Fomalont S. Kameno K. M. Shibata 《New Astronomy Reviews》1999,43(8-10)
An overview and status report of the VLBI Space Observatory Programme is presented. 相似文献
43.
We study the structure of a stationary and axisymmetric charge-deficient region (or potential gap) in the outer magnetosphere of a spinning neutron star. Assuming the existence of global current flow patterns in the magnetosphere, the charge depletion causes a large electric field along the magnetic field lines. This longitudinal electric field accelerates migratory electrons and/or positrons to ultrarelativistic energies. These relativistic electrons/positrons radiate γ -ray photons by curvature radiation. These γ -rays, in turn, produce yet more radiating particles by colliding with ambient X-ray photons, leading to a pair production cascade in the gap. The replenished charges partially screen the longitudinal electric field, which is self-consistently solved together with the distribution of e± and γ -ray photons. We find the voltage drop in the gap as a function of the soft photon luminosity. It is demonstrated that the voltage drop is less than 3×1013 V when the background X-ray radiation is as luminous as Vela . However, this value increases with decreasing X-ray luminosity and attains 3×1015 V when the X-ray luminosity is as low as L X =1031 erg s−1 . 相似文献
44.
Phenocryst-bulk rock composition relations of abyssal tholeiites and their petrogenetic significance
Tsugio Shibata 《Geochimica et cosmochimica acta》1976,40(11):1407-1417
In order to infer equilibrium phase relations of abyssal tholeiites, olivine, plagioclase, augite, and pigeonite tholeiites from the ocean floor are plotted in terms of the CIPW norm proportions in the tetrahedron olivine-plagioclase-diopside-quartz. The phase relations of abyssal tholeiites have a general similarity in form to those of the experimentally studied relevant systems. Experimental studies on natural basalts allow the pressure of crystallization for abyssal tholeiitic magmas to be evaluated approximately. It appears that the pressure at which the phenocryst-stage crystallization of abyssal tholeiites takes place is as high as 2 or 3 kbar, provided that abyssal tholeiitic magmas are ‘dry’.Abyssal tholeiites could be derived from liquids that are in equilibrium with Ca-poor pyroxene in the pressure range of about 5–8 kbar. Major element chemistry of abyssal tholeiites is incompatible with the view that these tholeiitic basalts are derived from picritic magma by olivine fractionation. 相似文献
45.
46.
Richard S. Fiske Katharine V. Cashman Atsushi Shibata Kazuki Watanabe 《Bulletin of Volcanology》1998,59(4):262-275
A new and detailed bathymetric map of the Myojinsho shallow submarine volcano provides a framework to interpret the physical
volcanology of its 1952–1953 eruption, especially how the silicic pyroclasts, both primary and reworked, enlarged the volcano
and were dispersed into the surrounding marine environment. Myojinsho, 420 km south of Tokyo along the Izu–Ogasawara arc,
was the site of approximately 1000 phreatomagmatic explosions during the 12.5-month eruption. These explosions shattered growing
dacite domes, producing dense clasts that immediately sank into the sea; minor amounts of pumice floated on the sea surface
after some of these events. The Myojinsho cone has slopes of almost precisely 21° in the depth range 300–700 m.We interpret
this to be the result of angle-of-repose deposition of submarine pyroclastic gravity flows that traveled downslope in all
directions. Many of these gravity flows resulted from explosions and associated dome collapse, but others were likely triggered
by the remobilization of debris temporarily deposited on the summit and steep upper slopes of the cone. Tephra was repeatedly
carried into air in subaerial eruption columns and fell into the sea within 1–2 km of the volcano's summit, entering water
as deep as 400 m. Because the fall velocity of single particles decreased by a factor of ∼30 in passing from air into the
sea, we expect that the upper part of the water column was repeatedly choked with hyperconcentrations of fallout tephra. Gravitational
instabilities within these tephra-choked regions could have formed vertical density currents that descended at velocities
greater than those of the individual particles they contained. Upon reaching the sea floor, many of these currents probably
continued to move downslope along Myojinsho's submarine slopes. Fine tephra was elutriated from the rubbly summit of the volcano
by upwelling plumes of heated seawater that persisted for the entire duration of the eruption. Ocean currents carried this
tephra to distal areas, where it presumably forms a pyroclastic component of deep-sea sediment.
Received: 5 December 1996 / Accepted: 17 September 1997 相似文献
47.
S. Shibata J. Miyazaki & F. Takahara 《Monthly notices of the Royal Astronomical Society》1998,295(4):L53-L58
We present a steady one-dimensional model for a pulsar polar cap accelerator, where the field-aligned electric field and flow are solved self-consistently with a given current density. It is assumed that no particles return to the star. It is known that the space-charge-limited flow is accelerated to energies high enough to create electron–positron pairs if the assumed current density is high enough. We find that when pairs are created in such a space-charge-limited flow, the accelerating electric field is screened out within a short distance after pair creation, if the pair particle flux is larger than a critical value. We also find that a space charge density wave is excited in the screening region.
We find that a pair flux larger than the critical value Mc =103 –105 must be reached in a layer with thickness equal to the braking distance for the decelerating component. Therefore, the required multiplicity – the number of pairs created by one primary particle – is too large to be realized in the actual pulsar magnetosphere. We suggest that in order to obtain a localized potential drop along the polar cap magnetic flux, one needs to take into account additional effects such as wave–particle interaction or quasi-periodic pair creation. 相似文献
We find that a pair flux larger than the critical value M
48.
Kazunari Shibata 《Solar physics》1980,66(1):61-70
During the emerging phase of sunspot pores strong downdrafts are observed in photospheric lines, whereas the upward flow is not detected within the observational accuracy. For the study of the origin of these downdrafts, we present the numerical solutions of non-steady hydrodynamic equations in one dimension along the rising magnetic flux tubes. In these solutions, it is assumed that the solar gas inside the tube is initially in the hydrostatic HSRA and then the tube rises with a given velocity and configuration. The results reproduce well the observed radial velocity distributions, hence it is concluded that the observed strong downdrafts originate from the sliding matter along the rising magnetic flux tubes. 相似文献
49.
A recurrent H surge was observed on 7 October, 1991 on the western solar limb with the Meudon MSDP spectrograph. The GOES satellite recorded X-ray subflares coincident with all three events. During two of the surges high-resolutionYohkoh Soft X-ray Telescope (SXT) images have been taken. Low X-ray loops overlying the active region where the surges occurred were continuously restructuring. A flare loop appeared at the onset of each surge event and somewhat separated from the footpoint of the surge. The loops are interpreted as causally related to the surges. It is suggested that surges are due to magnetic reconnection between a twisted cool loop and open field lines. Cold plasma bubbles or jets squeezed among untwisting magnetic field lines could correspond to the surge material. No detection was made of either X-ray emission along the path of the surges or X-ray jets, possibly because of the finite detection threshold of theYohkoh SXT. 相似文献
50.
The sea surface wind speed (SSWS) derived by a microwave radiometer can be contaminated by changes of the brightness temperature
owing to the angle between the sensor azimuth and the wind direction (Relative Wind Direction effect: RWD effect). We attempt
to apply the method proposed by Konda and Shibata (2004) to the SSWS derived by Advanced Microwave Scanning Radiometer (AMSR)
on Advanced Earth Observing Satellite II (ADEOS-II), in order to correct for the RWD effect. The improvement of accuracy of
the SSWS estimation amounts to roughly 60% of the error caused by the RWD effect. Comparison with in situ observation at the
Tropical Atmosphere Ocean (TAO) array shows that the root mean square error of the corrected SSWS is 1.1 ms−1. It is found that the impact of the RWD effect on the estimation of the latent heat flux can amount to about 30 Wm−2 on average. We applied the method to the SSWS derived by AMSR for Earth Observing System (AMSR-E) and obtained a similar
result. 相似文献