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排序方式: 共有236条查询结果,搜索用时 15 毫秒
81.
T.T. Koskinen R.V. Yelle D.S. Snowden P. Lavvas B.R. Sandel F.J. Capalbo Y. Benilan R.A. West 《Icarus》2011,216(2):507-534
Stellar occultations observed by the Cassini/UVIS instrument provide unique data that probe the mesosphere and lower thermosphere of Titan at altitudes between 400 and 1400 km. This region is a site of complex photochemistry that forms hydrocarbon and nitrile species, and plays a crucial role in the formation of the organic hazes observed in the stratosphere, but has yet to be adequately characterized. We analyzed publicly available data obtained between flybys Tb in December 2004 and T58 in July 2009, with an emphasis on two stable occultations obtained during flybys T41 and T53. We derived detailed density profiles for CH4, C2H2, C2H4, C4H2, HCN, HC3N and C6H6 between ∼400 and 1200 km and extinction coefficients for aerosols between 400 and 900 km. Our analysis reveals the presence of extinction layers in the occultation data that are associated with large perturbations in the density profiles of the gaseous species and extinction profiles of the aerosols. These relatively stable features vary in appearance with location and change slowly over time. In particular, we identify a sharp extinction layer between 450 and 550 km that coincides with the detached haze layer. In line with recent images obtained by Cassini/ISS, the altitude of this layer changes rapidly around the equinox in 2009. Our results point to unexpected complexity that may have significant consequences for the dynamics and physical processes taking place in the upper atmosphere of Titan. 相似文献
82.
Michael Solontoi Željko Ivezić Mario Jurić Andrew C. Becker Lynne Jones Andrew A. West Steve Kent Robert H. Lupton Mark Claire Gillian R. Knapp Tom Quinn James E. Gunn Donald P. Schneider 《Icarus》2012,218(1):571-584
We present the ensemble properties of 31 comets (27 resolved and 4 unresolved) observed by the Sloan Digital Sky Survey (SDSS). This sample of comets represents about 1 comet per 10 million SDSS photometric objects. Five-band (u, g, r, i, z) photometry is used to determine the comets’ colors, sizes, surface brightness profiles, and rates of dust production in terms of the Afρ formalism. We find that the cumulative luminosity function for the Jupiter Family Comets in our sample is well fit by a power law of the form N(<H) ∝ 10(0.49±0.05)H for H < 18, with evidence of a much shallower fit N(<H) ∝ 10(0.19±0.03)H for the faint (14.5 < H < 18) comets. The resolved comets show an extremely narrow distribution of colors (0.57 ± 0.05 in g ? r for example), which are statistically indistinguishable from that of the Jupiter Trojans. Further, there is no evidence of correlation between color and physical, dynamical, or observational parameters for the observed comets. 相似文献
83.
Jennifer A. Jay Matthew E. Pritchard Michael E. West Douglas Christensen Matthew Haney Estela Minaya Mayel Sunagua Stephen R. McNutt Mario Zabala 《Bulletin of Volcanology》2012,74(4):817-837
Using a network of 15 seismometers around the inflating Uturuncu Volcano from April 2009 to 2010, we find an average rate
of about three local volcano-tectonic earthquakes per day, and swarms of 5–60 events a few times per month with local magnitudes
ranging from −1.2 to 3.7. The earthquake depths are near sea level, more than 10 km above the geodetically inferred inflation
source and the Altiplano Puna Magma Body. The Mw 8.8 Maule earthquake on 27 February 2010 triggered hundreds of earthquakes
at Uturuncu with the onset of the Love and Rayleigh waves and again with the passage of the X2/X3 overtone phases of Rayleigh
waves. This is one of the first incidences in which triggering has been observed from multiple surface wave trains. The earthquakes
are oriented NW–SE similar to the regional faults and lineaments. The b value of the catalog is 0.49, consistent with a tectonic origin of the earthquakes. We perform ambient noise tomography using
Love wave cross-correlations to image a low-velocity zone at 1.9 to 3.9 km depth below the surface centered slightly north
of the summit. The low velocities are perhaps related to the hydrothermal system and the low-velocity zone is spatially correlated
with earthquake locations. The earthquake rate appears to vary with time—a seismic deployment from 1996 to 1997 reveals 1–5
earthquakes per day, whereas 60 events/day were seen during 5 days using one seismometer in 2003. However, differences in
analysis methods and magnitudes of completeness do not allow direct comparison of these seismicity rates. The rate of seismic
activity at Uturuncu is higher than at other well-monitored inflating volcanoes during periods of repose. The frequent swarms
and triggered earthquakes suggest the hydrothermal system is metastable. 相似文献
84.
A. C. Becker E. Agol N. M. Silvestri J. J. Bochanski C. Laws A. A. West G. Basri V. Belokurov D. M. Bramich J. M. Carpenter P. Challis K. R. Covey R. M. Cutri N. W. Evans M. Fellhauer A. Garg G. Gilmore P. Hewett P. Plavchan D. P. Schneider C. L. Slesnick S. Vidrih L. M. Walkowicz D. B. Zucker 《Monthly notices of the Royal Astronomical Society》2008,386(1):416-424
We report on Two-Micron All-Sky Survey (2MASS) J01542930+0053266, a faint eclipsing system composed of two M dwarfs. The variability of this system was originally discovered during a pilot study of the 2MASS Calibration Point Source Working Data base. Additional photometry from the Sloan Digital Sky Survey yields an eight-passband light curve from which we derive an orbital period of 2.639 0157 ± 0.000 0016 d. Spectroscopic followup confirms our photometric classification of the system, which is likely composed of M0 and M1 dwarfs. Radial velocity measurements allow us to derive the masses (M1 = 0.66 ± 0.03 M⊙ ; M2 = 0.62 ± 0.03 M⊙ ) and radii (R1 = 0.64 ± 0.08 R⊙ ; R2 = 0.61 ± 0.09 R⊙ ) of the components, which are consistent with empirical mass–radius relationships for low-mass stars in binary systems. We perform Monte Carlo simulations of the light curves which allow us to uncover complicated degeneracies between the system parameters. Both stars show evidence of Hα emission, something not common in early-type M dwarfs. This suggests that binarity may influence the magnetic activity properties of low-mass stars; activity in the binary may persist long after the dynamos in their isolated counterparts have decayed, yielding a new potential foreground of flaring activity for next generation variability surveys. 相似文献
85.
The cooling of plasmas in closed coronal loops by thermal conduction is important when considering their detectability at
X-ray and EUV wavelengths. A non-local formalism of thermal conduction originating in laboratory plasmas is used and it is
shown that while the effect is unlikely to be important for loops that are in a steady state, it does play a significant role
in loops that are impulsively heated (e.g., by nanoflares). Such loops are “under-dense”, and so hot electrons have a relatively long mean free path. Analytic and numerical
models are presented, and it is shown that conduction cooling times are lengthened quite considerably. A comparison of various
cooling times with ionisation times is also presented, and it is noted that this conductive physics may enhance the chances
of observing hot nanoflare-heated plasma. 相似文献
86.
Banded kokopu populations were surveyed seasonally over 1 year in five Coromandel streams to determine the influence of food supply on diet, growth, abundance, and movement. Streams with high benthic invertebrate abundance and drift of aquatic origin had high fish abundance. However, terrestrial drift was the dominant food for adult banded kokopu over all seasons and streams, on average comprising 75% of their diet by number and 89% by weight. Annual growth rates of tagged individuals ranged from 3 mm to 16.5 mm, with smaller fish growing faster than large fish. Low growth rates of large fish were consistent with the recapture of a fish that had only grown from 195 mm to 215 mm in 7 years. Our observations confirmed the strong association between banded kokopu, pool habitat, and cover, and also indicated that high fish abundance restricted growth rates and movement and increased interaction. 相似文献
87.
ZHOU Xinyuan WANG Zhaoming YANG Haijun ZHANG Lijuan HAN Jianf WANG Zhenyu 《《地质学报》英文版》2009,83(1):179-188
The Tazhong reef-flat oil-gas field is the first large-scale Ordovician organic reef type oil-gas field found in China. Its organic reefs were developed in the early Late Ordovician Lianglitag Formation, and are the first large reefs of the coral-stromatoporoid hermatypic community found in China. The organic reefs and platform-margin grain banks constitute a reef-flat complex, mainly consisting of biolithites and grainstones. The biolithites can be classified into the framestone, bafflestone, bindstone etc. The main body of the complex lies around the wells from Tazhong-24 to Tazhong-82, trending northwest, with the thickness from 100 to 300 m, length about 220 km and width 5-10 km. It is a reef-flat lithologic hydrocarbon reservoir, with a very complex hydrocarbon distribution: being a gas condensate reservoir as a whole with local oil reservoirs. The hydrocarbon distribution is controlled by the reef complex, generally located in the upper 100-200 m part of the complex, and largely in a banded shape along the complex. On the profile, the reservoir shows a stratified feature, with an altitude difference of almost 2200 m from southeast to northwest. The petroleum accumulation is controlled by karst reservoir beds and the northeast strike-slip fault belt. The total geologic reserves had reached 297.667 Mt by 2007. 相似文献
88.
A species-selective trawl for demersal gadoid fisheries 总被引:3,自引:0,他引:3
89.
CHEN Bailin LIU Jianmin ZHANG DaInstitute of Geomechanics CAGS Beijing and WU Jianshe West Fujian Geological Party of FBEDGMR Sanming 《《地质学报》英文版》1999,73(4):438-446
Low-angle faults include those occurring in thrust-nappe structures in a compressive setting and the detachment of metamorphic core complexes in an extensional setting. All low-angle faults have their own particularities. The low-angle fault plays an important role in controlling over some endogenetic metallic ore deposits. Based on studies of the Xiaoban gold deposit, Xinzhou gold deposit, and Longfengchang polymetallic ore deposit, and comparisons with other mines, the authors conclude the ore-controlling implications of low-angle faults as follows. (1) Because of high temperature and high pressure, as well as strong ductile deformation, the internal energy of the elements rises in the large-scale deep ductile low-angle faults, which causes the elements to activate and differentiate from the source rocks, forming ore-bearing hydrothermal solution, and bring mineralization to happen. (2) When rising from depths and flowing along the low-angle faults, the ore-bearing hydrothermal solution will alter and 相似文献
90.