全文获取类型
收费全文 | 160篇 |
免费 | 4篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 22篇 |
地球物理 | 42篇 |
地质学 | 42篇 |
海洋学 | 8篇 |
天文学 | 35篇 |
自然地理 | 13篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 2篇 |
2021年 | 8篇 |
2020年 | 6篇 |
2019年 | 2篇 |
2018年 | 4篇 |
2017年 | 2篇 |
2016年 | 9篇 |
2014年 | 12篇 |
2013年 | 8篇 |
2012年 | 4篇 |
2011年 | 16篇 |
2010年 | 6篇 |
2009年 | 10篇 |
2008年 | 11篇 |
2007年 | 15篇 |
2006年 | 6篇 |
2005年 | 7篇 |
2004年 | 8篇 |
2003年 | 3篇 |
2002年 | 5篇 |
2001年 | 2篇 |
2000年 | 5篇 |
1998年 | 1篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1953年 | 1篇 |
1952年 | 1篇 |
1951年 | 1篇 |
排序方式: 共有165条查询结果,搜索用时 15 毫秒
111.
Elizabeth Bass Meers Oppenheim Jorge Chau Alice Olmstead 《Earth, Moon, and Planets》2008,102(1-4):379-382
This paper examines current techniques used to determine meteoroid mass from high-power, large aperture (HPLA) radar observations.
We demonstrate why the standard approach of fitting a polynomial to velocity measurements gives inaccurate results by applying
this technique to artificial datasets. We then suggest an alternate approach, fitting velocity data to an ablation model.
Using data taken at the Jicamarca Radio Observatory in July 2005, we compare the results of both methods and demonstrate that
fitting velocity data to an ablation model yields a reasonable result in some instances where alternate methods produce physically
unrealistic mass estimates. 相似文献
112.
113.
Allison E. Ray John R. Bargar Alice C. Dohnalkova Brent M. Peyton Timothy S. Magnuson 《Geochimica et cosmochimica acta》2011,75(10):2684-6510
Microbial reduction of hexavalent uranium has been studied widely for its potential role in bioremediation and immobilization of soluble U(VI) in contaminated groundwater. More recently, some microorganisms have been examined for their role in immobilization of U(VI) via precipitation of uranyl phosphate minerals mediated by microbial phosphate release, alleviating the requirement for long-term redox control. Here, we investigated the mechanism of U(VI) removal mediated by an environmental isolate, strain UFO1, that is indigenous to the Field Research Center (FRC) in Oak Ridge, TN and has been detected in U(VI)-contaminated sediments. Changes in U(VI) speciation were examined in the presence and absence of the electron-shuttling moiety, anthraquinone-2,6-disulfonate (AQDS). Cell suspensions were capable of nearly complete removal of 100 μM U(VI) from solution within 48 h; U(VI) removal was not dependent on the presence of an exogenous electron donor or AQDS, although AQDS increased the rate of U(VI) removal. X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic measurements indicated that U(IV) was the predominant oxidation state of uranium in cell suspensions in both the absence and presence of 100 μM AQDS. Interestingly, 17% of the cell-associated precipitates in a U(VI)-treated suspension that lacked AQDS had spectral characteristics consistent with a uranyl phosphate solid phase. The potential involvement of phosphate was consistent with observed increases in soluble phosphate concentrations over time in UFO1 cell suspensions, which suggested phosphate liberation from the cells. TEM-EDS confirmed the presence of uranyl phosphate with a U:P ratio consistent with autunite (1:1). EXAFS analyses further suggested that U(IV) was bound to low-Z neighbors such as C or P, inferred to be present as functional groups on biomass. These results suggest that strain UFO1 has the ability to facilitate U(VI) removal from solution via reductive and phosphate precipitation mechanisms. Both mechanisms offer potential for the remediation of U-contaminated sediments at the FRC or elsewhere. 相似文献
114.
Alice K. Harding Isabelle A. Grenier Peter L. Gonthier 《Astrophysics and Space Science》2007,309(1-4):221-230
Radio-quiet γ-ray pulsars like Geminga may account for a number of the unidentified EGRET sources in the Galaxy. The number of Geminga-like
pulsars is very sensitive to the geometry of both the γ-ray and radio beams. Recent studies of the shape and polarization of pulse profiles of young radio pulsars have provided
evidence that their radio emission originates in wide cone beams at altitudes that are a significant fraction (1–10%) of their
light cylinder radius. Such wide radio emission beams will be visible at a much larger range of observer angles than the narrow
core components thought to originate at lower altitude. Using 3D geometrical modeling that includes relativistic effects from
pulsar rotation, we study the visibility of such radio cone beams as well as that of the γ-ray beams predicted by slot gap and outer gap models. From the results of this study, one can obtain revised predictions
for the fraction of Geminga-like, radio quiet pulsars present in the γ-ray pulsar population.
相似文献
115.
A. Pasha Hosseinbor Richard G. Edgar Alice C. Quillen Amanda LaPage 《Monthly notices of the Royal Astronomical Society》2007,378(3):966-972
We investigate the effect of a planet on an eccentric orbit on a two-dimensional low-mass gaseous disc. At a planet eccentricity above the planet's Hill radius divided by its semimajor axis, we find that the disc morphology differs from that exhibited by a disc containing a planet in a circular orbit. An eccentric gap is created with eccentricity that can exceed the planet's eccentricity and precesses with respect to the planet's orbit. We find that a more massive planet is required to open a gap when the planet is on an eccentric orbit. We attribute this behaviour to spiral density waves excited at corotation resonances by the eccentric planet. These act to increase the disc's eccentricity and exert a torque opposite in sign to that exerted by the Lindblad resonances. The reduced torque makes it more difficult for waves driven by the planet to overcome viscous inflow in the disc. 相似文献
116.
A new picture of pulsar high-energy emission is proposed that is different from both the traditional polar cap and outer gap
models, but combines elements of each. The slot gap model is based on electron acceleration along the edge of the open field
region from the neutron star surface to near the light cylinder and thus could form a physical basis for the two-pole caustic
model of Dyks and Rudak (2003). Along the last open field line, the pair formation front rises to very high altitude forming
a slot gap, where the accelerating electric field is unscreened by pairs. The resulting radiation features both hollow cones
from the lower-altitude pair cascades, seen at small viewing angles, as well as caustic emission on the trailing-edge field
lines at high altitude, seen from both poles at large viewing angle. The combination of the small solid angle of slot gap
emission (≪ 1 sr) with a high probability of viewing the emission predicts that more gamma-ray pulsars could be detected at
larger distances. In this picture, many of the positional coincidences of radio pulsars with unidentified EGRET sources become
plausible as real associations, as the flux predicted by the slot gap model for many of the pulsars would provide the observed
EGRET source flux. The expected probability of seeing radio-quiet gamma-ray pulsars in this model will also be discussed. 相似文献
117.
Marine reservoir age is reported for Foxe Basin, Canada, during deglaciation of the Laurentide Ice Sheet. Radiocarbon (14C) measurements were made on pairs of contemporaneous molluscs and Salix (willow) macrofossils, yielding a mean marine reservoir age of 985 ± 10 14C a. Mean regional ΔR is calculated to be 615 ± 20 14C a relative to a mean global reservoir age of ca. 400 14C a. Previous studies in the Canadian Arctic that have been conducted on modern pre‐bomb molluscs give ΔR values of ca. 100–300 14C a. The difference between modern and deglacial reservoir ages indicates that ΔR in Foxe Basin has changed over time, which has implications for the reconstruction of past events based on marine 14C dates. We recalculate the timing of deglaciation of Hudson Strait with this new reservoir age and relate this to the 8.2 cal. ka event. We recommend that local ΔR be determined for the time period being examined whenever possible, and that in cases where this is impossible a ΔR value of 615 14C a be applied when calibrating marine samples from Foxe Basin and vicinity during deglaciation. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
118.
The crystallographic preferred orientations of a series of experimentally deformed fine-grained albite aggregates were measured by synchrotron source X-ray diffraction. Most samples were deformed and extensively recrystallized by low-temperature recrystallization-accommodated dislocation creep. In axial compression as well as simple shear these samples developed weak but distinct crystallographic preferred orientations consistent with intracrystalline slip on {001}<100>; the sheared samples have a marked asymmetry of the <100> maxima with respect to the shear zone boundaries. One sample was axially compressed by solution precipitation creep; it developed a somewhat different but equally strong preferred orientation, perhaps reflecting crystallographic anisotropy in rates of dissolution and growth. 相似文献
119.
We describe a parallel hybrid symplectic integrator for planetary system integration that runs on a graphics processing unit (GPU). The integrator identifies close approaches between particles and switches from symplectic to Hermite algorithms for particles that require higher resolution integrations. The integrator is approximately as accurate as other hybrid symplectic integrators but is GPU accelerated. 相似文献
120.