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排序方式: 共有1032条查询结果,搜索用时 15 毫秒
231.
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K. I. Kellermann Y. Y. Kovalev M. L. Lister D. C. Homan M. Kadler M. H. Cohen E. Ros J. A. Zensus R. C. Vermeulen M. F. Aller H. D. Aller 《Astrophysics and Space Science》2007,311(1-3):231-239
We discuss results from a decade long program to study the fine-scale structure and the kinematics of relativistic AGN jets
with the aim of better understanding the acceleration and collimation of the relativistic plasma forming AGN jets. From the
observed distribution of brightness temperature, apparent velocity, flux density, time variability, and apparent luminosity,
the intrinsic properties of the jets including Lorentz factor, luminosity, orientation, and brightness temperature are discussed.
Special attention is given to the jet in M87, which has been studied over a wide range of wavelengths and which, due to its
proximity, is observed with excellent spatial resolution.
Most radio jets appear quite linear, but we also observe curved non-linear jets and non-radial motions. Sometimes, different
features in a given jet appear to follow the same curved path but there is evidence for ballistic trajectories as well. The
data are best fit with a distribution of Lorentz factors extending up to γ∼30 and intrinsic luminosity up to ∼1026 W Hz−1. In general, gamma-ray quasars may have somewhat larger Lorentz factors than non gamma-ray quasars. Initially the observed
brightness temperature near the base of the jet extend up to ∼5×1013 K which is well in excess of the inverse Compton limit and corresponds to a large excess of particle energy over magnetic
energy. However, more typically, the observed brightness temperatures are ∼2×1011 K, i.e., closer to equipartition. 相似文献
233.
Hemispheric-scale climate response to Northern Eurasia land surface characteristics and snow anomalies 总被引:3,自引:0,他引:3
This paper presents a synopsis of recently published studies by the co-authors, which show that several land surface characteristics unique to Northern Eurasia are responsible for facilitating a causal relationship between autumn snow anomalies in this region and subsequent hemispheric winter climate patterns. The large size and extratropical location of the contiguous Eurasian land mass results in broad, continental-scale interannual snow cover extent and depth variations throughout autumn and winter, and corresponding diabatic heating anomalies. These surface anomalies occur in the presence of a large region of stationary wave activity, produced in part by the orographic barriers that separate northern/central Eurasia from southern/eastern Eurasia. This co-location of snow-forced anomalies and ambient wave energy is unique to Northern Eurasia, and initiates a teleconnection pathway involving stationary wave–mean flow interaction throughout the troposphere and stratosphere, ultimately resulting in a modulation of the winter Arctic Oscillation (AO). Complementary new results are also presented which show that partial snow cover extent or snow depth only anomalies in Northern Eurasia are insufficient to initiate the teleconnection pathway and produce a winter AO signal. This synopsis provides a useful interpretation of the earlier studies in the specific context of Northern Eurasia regional climate and environmental change. 相似文献
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Jason M. Dortch Lewis A. Owen William C. Haneberg Marc W. Caffee Craig Dietsch Ulrich Kamp 《Quaternary Science Reviews》2009,28(11-12):1037-1054
Four large landslides, each with a debris volume >106 m3, in the Himalaya and Transhimalaya of northern India were examined, mapped, and dated using 10Be terrestrial cosmogenic radionuclide surface exposure dating. The landslides date to 7.7±1.0 ka (Darcha), 7.9±0.8 ka (Patseo), 6.6±0.4 ka (Kelang Serai), and 8.5±0.5 ka (Chilam). Comparison of slip surface dips and physically reasonable angles of internal friction suggests that the landslides may have been triggered by increased pore water pressure, seismic shaking, or a combination of these two processes. However, the steepness of discontinuities in the Darcha rock-slope, suggests that it was more likely to have started as a consequence of gravitationally-induced buckling of planar slabs. Deglaciation of the region occurred more than 2000 years before the Darcha, Patseo, and Kelang Serai landslides; it is unlikely that glacial debuttressing was responsible for triggering the landslides. The four landslides, their causes, potential triggers and mechanisms, and their ages are compared to 12 previously dated large landslides in the region. Fourteen of the 16 dated landslides occurred during periods of intensified monsoons. Seismic shaking, however, cannot be ruled out as a mechanism for landslide initiation, because the Himalaya has experienced great earthquakes on centennial to millennial timescales. The average Holocene landscape lowering due to large landslides for the Lahul region, which contains the Darcha, Patseo, and Kelang Serai landslides, is ~0.12 mm/yr. Previously published large-landslide landscape-lowering rates for the Himalaya differ significantly. Furthermore, regional glacial and fluvial denudation rates for the Himalaya are more than an order of magnitude greater. This difference highlights the lack of large-landslide data, lack of chronology, problems associated with single catchment/large landslide-based calculations, and the need for regional landscape-lowering determinations over a standardized time period. 相似文献
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Jason F. Shogren 《Climatic change》2000,45(3-4):489-491