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排序方式: 共有194条查询结果,搜索用时 515 毫秒
111.
Joe Giacalone 《Planetary and Space Science》2003,51(11):659-664
Using analytic theory, test-particle simulations, and self-consistent hybrid simulations, we show that quasi-perpendicular shocks—those which propagate nearly perpendicular to the upstream magnetic field—accelerate particles directly out of the incident thermal population to energies much higher than the upstream ram energy of the plasma. It has already been established that quasi-parallel shocks—those which propagate nearly in the same direction as the upstream magnetic field—efficiently accelerate particles directly out of the incident thermal population; however, this has not yet been established for quasi-perpendicular shocks. Our results can be understood within the framework of the diffusive shock acceleration theory. We find that the accelerated-particle spectrum obtained from a more-general self-consistent hybrid plasma simulation are quantitatively consistent with a less-sophisticated test-particle simulation. The implications of this are discussed. 相似文献
112.
Jeremie Vaubaillon Pavel Koten Anastasios Margonis Juraj Toth Regina Rudawska Maria Gritsevich Joe Zender Jonathan McAuliffe Pierre-Dominique Pautet Peter Jenniskens Detlef Koschny Francois Colas Sylvain Bouley Lucie Maquet Arnaud Leroy Jean Lecacheux Jiri Borovicka Junichi Watanabe Jürgen Oberst 《Earth, Moon, and Planets》2015,114(3-4):137-157
113.
Detlef Koschny Jorge Diaz del Rio Rodrigue Piberne Marek Szumlas Joe Zender André Knöfel 《Earth, Moon, and Planets》2004,95(1-4):255-263
Both amateur and professional meteor groups are more frequently using Low-Light level TV (LLTV) systems to record meteors.
Double-station observations can yield orbit data. However, data analysis normally is still done by hand and thus time consuming.
This paper addresses the question of whether available automated tools can be used to determine reasonably accurate orbits
with minimum human intervention. The European Space Agency performed several observing campaigns to observe the Leonid meteor
stream. In November 1999, the ESA meteor group was stationed at two locations in Southern Spain, in November 2001 at two stations
close to Broome in North-Western Australia. Double-station observations with LLTV systems were conducted. The data was recorded
on S-VHS video tapes. The tapes were processed using automatic detection software from which meteor heights, velocities and
radiants were computed. This paper shows the results for the two maximum nights. The radiants determined in 1999 show a very
large scatter due to unfortunate observing geometry and inaccurate position determination since one of the cameras was moving
because of the wind. The 2001 data is excellent and the radiant was determined to be at RA = 153.96°±0.3° and Dec = 21.09°±0.2°.
The error bars for individual meteor radiants are about 0.2° to 0.4°. This demonstrates that is indeed possible to determine
good radiant positions using totally automated tools. Orbits, on the other hand, are not well defined due to the fact that
the velocity of individual meteors shows large errors. Reasons for this are described. 相似文献
114.
Joe M. Straus 《Dynamics of Atmospheres and Oceans》1976,1(1):71-76
The transition to “multi-α” solutions of the upper bounding problem for thermal convection is discussed. For convection in a fluid contained between parallel stress-free perfectly conducting boundaries, the “single-α” solutions of Straus (1973) are used to determine the Rayleigh number R at which the first transition occurs. Two upper bounding problems are treated: one valid for all values of the Prandtl number and one valid only for large Prandtl numbers. A significant difference between the two problems is noted. The former has a transition at R ~ 28200; the latter has no transition within the range of Rayleigh numbers treated here: R ? 2.3 · 105. 相似文献
115.
A numerical study of the summertime flow around the Luzon Strait 总被引:3,自引:0,他引:3
Luzon Strait, a wide channel between Taiwan and Luzon islands, connects the northern South China Sea and the Philippine Sea.
The Kuroshio, South China Sea gyre, monsoon and local topography influence circulation in the Luzon Strait area. In addition,
the fact that the South China Sea is a fairly isolated basin accounts for why its water property differs markedly from the
Kuroshio water east of Luzon. This work applies a numerical model to examine the influence of the difference in the vertical
stratification between the South China Sea and Kuroshio waters on the loop current of Kuroshio in the Luzon Strait during
summer. According to model results, the loop current’s strength in the strait reduces as the strongly stratified South China
Sea water is driven northward by the southwest winds. Numerical results also indicate that Kuroshio is separated by a nearly
meridional ridge east of Luzon Strait. The two velocity core structures of Kuroshio can also be observed in eastern Taiwan.
Moreover, the water flowing from the South China Sea contributes primarily to the near shore core of Kuroshio. 相似文献
116.
A polygon-based modeling approach to assess exposure of resources and assets to wildfire 总被引:1,自引:0,他引:1
Matthew P. Thompson Joe Scott Jeffrey D. Kaiden Julie W. Gilbertson-Day 《Natural Hazards》2013,67(2):627-644
Spatially explicit burn probability modeling is increasingly applied to assess wildfire risk and inform mitigation strategy development. Burn probabilities are typically expressed on a per-pixel basis, calculated as the number of times a pixel burns divided by the number of simulation iterations. Spatial intersection of highly valued resources and assets (HVRAs) with pixel-based burn probability estimates enables quantification of HVRA exposure to wildfire in terms of expected area burned. However, statistical expectations can mask variability in HVRA area burned across all simulated fires. We present an alternative, polygon-based formulation for deriving estimates of HVRA area burned. This effort enhances investigations into spatial patterns of fire occurrence and behavior by overlaying simulated fire perimeters with mapped HVRA polygons to estimate conditional distributions of HVRA area burned. This information can be especially useful for assessing risks where cumulative effects and the spatial pattern and extent of area burned influence HVRA response to fire. We illustrate our modeling approach and demonstrate application across real-world landscapes for two case studies: first, a comparative analysis of exposure and area burned across ten municipal watersheds on the Beaverhead-Deerlodge National Forest in Montana, USA, and second, fireshed delineation and exposure analysis of a geographically isolated and limited area of critical wildlife habitat on the Pike and San Isabel National Forests in Colorado, USA. We highlight how this information can be used to inform prioritization and mitigation decisions and can be used complementarily with more traditional pixel-based burn probability and fire intensity metrics in an expanded exposure analysis framework. 相似文献
117.
Remnant riparian vegetation,sediment and nutrient loads,and river rehabilitation in subtropical Australia
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Jon Olley Joanne Burton Virgilio Hermoso Kate Smolders Joe McMahon Belinda Thomson Andrew Watkinson 《水文研究》2015,29(10):2290-2300
A decline in the ecosystem health of Australia's Moreton Bay, a Ramsar wetland of international significance, has been attributed to sediments and nutrients derived from catchment sources. To address this decline the regional management plan has set the target of reducing the loads by 50%. Reforestation of the channel network has been proposed as the means to achieve this reduction, but the extent of revegetation required is uncertain. Here we test the hypothesis that sediment and nutrient loads from catchments decrease proportionally with the increasing proportion of the stream length draining remnant vegetation. As part of a routine regional water quality monitoring program sediment and nutrient loads were measured in 186 flow events across 22 sub‐catchments with different proportions of remnant woodland. Using multiple linear regression analysis we develop a predictive model for pollutant loads. Of the attributes examined a combination of runoff and the proportion of the stream length draining remnant vegetation was the best predictor. The sediment yield per unit area from a catchment containing no remnant vegetation is predicted to be between 50 and 200 times that of a fully vegetated channel network; total phosphorus between 25 and 60 times; total nitrogen between 1.6 and 4.1 times. There are ~48 000 km of streams in the region of which 32% drain areas of remnant vegetation. Of these 17 095 km are above the region's water storage dams. We estimate that decreasing the sediment and phosphorus loads to Moreton Bay by 50% would involve rehabilitating ~6350 km of the channel network below the dams; halving the total nitrogen load would require almost complete restoration of the channel network. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
118.
Joe M. Straus 《地球物理与天体物理流体动力学》2013,107(1):261-281
AbstractTwo upper bounding problems for thermal convection in a layer of fluid contained between perfectly conducting stress-free boundaries are treated numerically. Since the Euler equations resulting from this variational approach are simpler than the Navier-Stokes equations, they allow numerical calculations to be carried out economically to fairly large values of the Rayleigh number. The upper bounding problem formulated by Howard (1963), which yields a Nusselt number independent of Prandtl number, diverges from the correct behavior as the Rayleigh number increases. In hopes of coming closer to results of previous investigations of the Boussinesq equations of motion, a more restrictive upper bounding problem is formulated. For large Prandtl numbers the momentum equation is linearized and is used as an explicit side constraint on the variational problem, thereby forcing the solutions to more closely resemble the solutions of the Boussinesq equations. Numerical calculations at values of the Rayleigh number up to 1.5 × 105 indicate that the additional constraint decreases the upper bound on the Nusselt number; it appears that this upper bound differs by only a multiplicative factor from that calculated from solutions of the full equations of motion and may be a reasonable approximation for large Rayleigh numbers. 相似文献
119.
Sarah H. Wallace Samuel Shaw Katherine Morris Joe S. Small Adam J. Fuller Ian T. Burke 《Applied Geochemistry》2012
Strontium-90 is a beta emitting radionuclide produced during nuclear fission, and is a problem contaminant at many nuclear facilities. Transport of 90Sr in groundwaters is primarily controlled by sorption reactions with aquifer sediments. The extent of sorption is controlled by the geochemistry of the groundwater and sediment mineralogy. Here, batch sorption experiments were used to examine the sorption behaviour of 90Sr in sediment–water systems representative of the UK Sellafield nuclear site based on groundwater and contaminant fluid compositions. In experiments with low ionic strength groundwaters (<0.01 mol L−1), pH variation is the main control on sorption. The sorption edge for 90Sr was observed between pH 4 and 6 with maximum sorption occurring (Kd ∼ 103 L kg−1) at pH 6–8. At ionic strengths above 10 mmol L−1, and at pH values between 6 and 8, cation exchange processes reduced 90Sr uptake to the sediment. This exchange process explains the lower 90Sr sorption (Kd ∼ 40 L kg−1) in the presence of artificial Magnox tank liquor (IS = 29 mmol L−1). Strontium K-edge EXAFS spectra collected from sediments incubated with Sr2+ in either HCO3-buffered groundwater or artificial Magnox tank liquor, revealed a coordination environment of ∼9 O atoms at 2.58–2.61 Å after 10 days. This is equivalent to the Sr2+ hydration sphere for the aqueous ion and indicates that Sr occurs primarily in outer sphere sorption complexes. No change was observed in the Sr sorption environment with EXAFS analysis after 365 days incubation. Sequential extractions performed on sediments after 365 days also found that ∼80% of solid associated 90Sr was exchangeable with 1 M MgCl2 in all experiments. These results suggest that over long periods, 90Sr in contaminated sediments will remain primarily in weakly bound surface complexes. Therefore, if groundwater ionic strength increases (e.g. by saline intrusion related to sea level rise or by design during site remediation) then substantial remobilisation of 90Sr is to be expected. 相似文献
120.
Joydeep Bagchi Joe Jacob Gopal-Krishna Norbert Werner Nitin Wadnerkar Jaydeep Belapure A. C. Kumbharkhane 《Monthly notices of the Royal Astronomical Society》2009,399(2):601-614
We present detailed observations of MRC 0116+111, revealing a luminous, miniradio halo of ∼240-kpc diameter located at the centre of a cluster of galaxies at redshift z = 0.131 . Our optical and multiwavelength Giant Metrewave Radio Telescope and Very Large Array radio observations reveal a highly unusual radio source: showing a pair of giant (∼100-kpc diameter) bubble-like diffuse structures, that are about three times larger than the analogous extended radio emission observed in M87 – the dominant central radio galaxy in the Virgo cluster. However, in MRC 0116+111 we do not detect any ongoing active galactic nucleus (AGN) activity, such as a compact core or active radio jets feeding the plasma bubbles. The radio emitting relativistic particles and magnetic fields were probably seeded in the past by a pair of radio jets originating in the AGN of the central cD galaxy. The extremely steep high-frequency radio spectrum of the north-western bubble, located ∼100 kpc from cluster centre, indicates radiation losses, possibly because having detached, it is rising buoyantly and moving away into the putative hot intracluster medium. The other bubble, closer to the cluster centre, shows signs of ongoing particle re-acceleration. We estimate that the radio jets which inflated these two bubbles might have also fed enough energy into the intracluster medium to create an enormous system of cavities and shock fronts, and to drive a massive outflow from the AGN, which could counter-balance and even quench a cooling flow. Therefore, this source presents an excellent opportunity to understand the energetics and the dynamical evolution of radio jet inflated plasma bubbles in the hot cluster atmosphere. 相似文献