Magnetic cycles of the planet-hosting star τ Bootis – II. A second magnetic polarity reversal     

R. Fares  J.-F. Donati  C. Moutou  D. Bohlender  C. Catala  M. Deleuil  E. Shkolnik  A. C. Cameron  M. M. Jardine  G. A. H. Walker 《Monthly notices of the Royal Astronomical Society》2009,398(3):1383-1391

In this paper, we present new spectropolarimetric observations of the planet-hosting star τ Bootis, using ESPaDOnS and Narval spectropolarimeters at Canada–France–Hawaii Telescope and Telescope Bernard Lyot, respectively.
We detected the magnetic field of the star at three epochs in 2008. It has a weak magnetic field of only a few gauss, oscillating between a predominant toroidal component in January and a dominant poloidal component in June and July. A magnetic polarity reversal was observed relative to the magnetic topology in 2007 June. This is the second such reversal observed in 2 years on this star, suggesting that τ Boo has a magnetic cycle of about 2 years. This is the first detection of a magnetic cycle for a star other than the Sun. The role of the close-in massive planet in the short activity cycle of the star is questioned.
τ Boo has a strong differential rotation, a common trend for stars with shallow convective envelope. At latitude 40°, the surface layer of the star rotates in 3.31 d, equal to the orbital period. Synchronization suggests that the tidal effects induced by the planet may be strong enough to force at least the thin convective envelope into corotation.
τ Boo shows variability in the Ca  ii H & K and Hα throughout the night and on a night-to-night time-scale. We do not detect enhancement in the activity of the star that may be related to the conjunction of the planet. Further data are needed to conclude about the activity enhancement due to the planet.  相似文献   

A comprehensive program for countermeasures against potentially hazardous objects (PHOs)     

W. F. Huebner  L. N. Johnson  D. C. Boice  P. Bradley  S. Chocron  A. Ghosh  P. T. Giguere  R. Goldstein  J. A. Guzik  J. J. Keady  J. Mukherjee  W. Patrick  C. Plesko  J. D. Walker  K. Wohletz 《Solar System Research》2009,43(4):334-342

At the hundredth anniversary of the Tunguska event in Siberia it is appropriate to discuss measures to avoid such occurrences in the future. Recent discussions about detecting, tracking, cataloguing, and characterizing near-Earth objects (NEOs) center on objects larger than about 140 m in size. However, objects smaller than 100 m are more frequent and can cause significant regional destruction of civil infrastructures and population centers. The cosmic object responsible for the Tunguska event provides a graphic example: although it is thought to have been only about 50 to 60 m in size, it devastated an area of about 2000 km². Ongoing surveys aimed at early detection of a potentially hazardous object (PHO: asteroid or comet nucleus that approaches the Earth’s orbit within 0.05 AU) are only a first step toward applying countermeasures to prevent an impact on Earth. Because “early” may mean only a few weeks or days in the case of a Tunguska-sized object or a longperiod comet, deflecting the object by changing its orbit is beyond the means of current technology, and destruction and dispersal of its fragments may be the only reasonable solution. Highly capable countermeasures- always at the ready—are essential to defending against an object with such short warning time, and therefore short reaction time between discovery and impending impact. We present an outline for a comprehensive plan for countermeasures that includes smaller (Tunguska-sized) objects and long-period comets, focuses on short warning times, uses non-nuclear methods (e.g., hyper-velocity impactor devices and conventional explosives) whenever possible, uses nuclear munitions only when needed, and launches from the ground. The plan calls for international collaboration for action against a truly global threat.  相似文献   

On the Heat Islands of Washington,DC, and New York City,NY     

Bruce B. Hicks  William J. Callahan  Mark A. Hoekzema 《Boundary-Layer Meteorology》2010,135(2):291-300

Data collected in 2007 from a dense commercial network (operated by AWS Convergence Technologies, Inc.) of roof-mounted temperature sensors are used to explore the heat island characteristics of Washington, DC, and New York City, NY. Considerable spatial detail is revealed, but aggregating data in annuli centered on assumed central locations in the business districts of the two cities reveals that the heat islands extend out to more than 30 km, with the New York City island being somewhat larger. The results from both arrays reveal the influence of the surroundings, with large scatter of daytime results being characteristic of sites with the greatest local surface inhomogeneity. Nighttime data are more ordered, and suggest that surface air temperatures decrease by about 0.02°C km⁻¹ for the Washington case, and 0.04°C km⁻¹ for New York, with the winter behaviour being more pronounced than for other seasons. Scatter of the data in the daytime is a common feature for all seasons, but mainly for those with the strongest insolation. Comparison between working day and weekend temperatures provides convincing verification that the air responds quite slowly to changes in surface (radiometric) temperatures, with distance constants of the order of many tens of km. There appears to be a small wind speed effect, which is evident in the nighttime data but is largely obscured by scatter for the daytime.  相似文献   

东北太平洋Explorer Ridge热液羽状流位温浊度异常和物质能量通量估算     

郭双喜  鲁远征  岑显荣  屈玲  Sharon L. Walker  周生启 《海洋学报》2017,39(12):1-11

深海热液流体与周围海水之间存在明显的物理和化学差异,通过检测海水的位温浊度异常是探测深海热液活动的重要手段之一。本文采用"海底火山带项目（Submarine Ring of Fire 2002）"拖曳式温盐深测量仪数据资料,研究了东北太平洋Explorer Ridge热液场的水文特征及物质能量通量的释放。结果表明Explorer Ridge热液场热液羽状流中性浮力层所在深度范围约为1 600~1 900 m,距离海底的高度约为200 m,最大位温、盐度和浊度异常分别为0.04℃、0.004和0.18 NTU;中性浮力层热液羽状流帽呈椭圆结构,其长轴与洋中脊线重合,羽状流帽总面积约为27 km2;热液羽状流在中性层范围内存在明显的分层现象,通过经验公式计算得到Explorer Ridge热液场观测范围内热液喷口的总的浮力通量为6.19×10-2 m4/s3,平均值为2.063×10-2 m4/s3;总的体积通量为9.884×10-2 m3/s,平均值为3.295×10-2 m3/s;总的热通量为194.9 MW,平均值为64.967 MW。  相似文献   

Global contraction of planetary bodies due to despinning: Application to Mercury and Iapetus     

Isamu Matsuyama  Bruce G. Bills 《Icarus》2010,209(2):271-279

We extend previous work on the global tectonic patterns generated by despinning with a self-consistent treatment of the isotropic despinning contraction that has been ignored. We provide simple analytic approximations that quantify the effect of the isotropic despinning contraction on the global shape and tectonic pattern. The isotropic despinning contraction of Mercury is ∼93 m (T/1 day)⁻², where T is the initial rotation period. If we take into account both the isotropic contraction and the degree-2 deformations associated with despinning, the preponderance of compressional tectonic features on Mercury’s surface requires an additional isotropic contraction ?1 km (T/1 day)⁻², presumably due to cooling of the interior and growth of the solid inner core. The isotropic despinning contraction of Iapetus is ∼9 m (T/16 h)⁻², and it is not sensitive to the presence of a core or the thickness of the elastic lithosphere. The tectonic pattern expected for despinning, including the isotropic contraction, does not explain Iapetus’ ridge. Furthermore, the ridge remains unexplained with the addition of any isotropic compressional stresses, including those generating by cooling.  相似文献   

Sediment variability in offshore sampling areas     

Bruce L. Godfriaux 《新西兰海洋与淡水研究杂志》2013,47(4):323-329

From two to four different sediment types were found in each of six offshore sampling areas in western Bay of Plenty, New Zealand. The size of the areas ranged from 7.7 km² to 64.6 km², and the average depths ranged from 36 m to 168 m. Sediments may thus vary widely within areas plotted as homogeneous on large‐scale (1:200 000) sediment charts.  相似文献   

Remote Acoustic Habitat Assessment Techniques Used to Characterize the Quality and Extent of Oyster Bottom in the Chesapeake Bay     

G. F. Smith  D. G. Bruce  E. B. Roach 《Marine Geodesy》2013,36(3):171-189

Underwater acoustic technologies may be used for remote sensing of estuarine and coastal benthic habitat in addition to more traditional methods of assessing the seabed. These systems use reflected sound energy to identify surface objects, texture, and subbottom density discontinuities, and to classify benthic habitat. We evaluated the ability of subbottom profiling systems, side scan sonar, and acoustic seabed classification systems (ASCS) to assess oyster habitat in the Chesapeake Bay. Our criteria were the systems' abilities to assess the quality and quantity of oyster shell resources and to integrate with Geographical Information Systems (GIS) technology for the charting of the seabed. Although all systems examined have previously been used for benthic habitat assessment, we found that ASCS offered the most promise as a stand alone system for mapping complex and heterogeneous habitat typical of oyster bottoms. The results from this study suggest that ASCS technology is highly suited for the identification and charting of oyster shell as well as distinguishing among different combinations of shell and fine sediments. Such systems also offer excellent linkage with GIS display and analysis capability.  相似文献   

The scale of forest transition: Amazonia and the Atlantic forests of Brazil     

Robert Walker 《Applied geography (Sevenoaks, England)》2012,32(1):12-20

This article adapts a general equilibrium model that provides a spatial solution for land use, labor allocation, and product markets in a two good economy. The adaptation, based on von Thünen, considers the multi-regional case, and solves for two regions, one industrial and the other, a newly opened agricultural frontier. The conceptual framework is considered with reference to Brazil, where forest recovery in the Atlantic Rainforest occurs simultaneously with forest losses in Amazonia. Simulation results of the theoretical model are given, demonstrating the impacts of comparative advantage in regional agriculture on the spatial system. The main theoretical interest of the article, aside from providing a formal spatial statement, is to define a distinction between aggregate forest transition (A-FT), when the area of all forests in a multi-regional system increases with the advent of trade relations, and regional forest transition (R-FT), when forest recovery is spatially constrained, and depends on forest losses elsewhere. Thus, the article addresses the role of scale in defining forest transition, and does so by representing spatial dynamics with a formal model. It also suggests that forest transition privileges one biome at the expense of others, and that a concept of landscape turnaround is more germane from a wildlands conservation perspective. The article closes with a discussion of Brazil, and how its forests in Amazonia and along the Atlantic will fare in the coming years.  相似文献   

Assessment of pingo distribution and morphometry using an IfSAR derived digital surface model, western Arctic Coastal Plain, Northern Alaska     

Benjamin M. Jones  Guido GrosseKenneth M. Hinkel  Christopher D. ArpShane Walker  Richard A. BeckJohn P. Galloway 《Geomorphology》2012,138(1):1-14

Pingos are circular to elongate ice-cored mounds that form by injection and freezing of pressurized water in near-surface permafrost. Here we use a digital surface model (DSM) derived from an airborne Interferometric Synthetic Aperture Radar (IfSAR) system to assess the distribution and morphometry of pingos within a 40,000 km² area on the western Arctic Coastal Plain of northern Alaska. We have identified 1247 pingo forms in the study region, ranging in height from 2 to 21 m, with a mean height of 4.6 m. Pingos in this region are of hydrostatic origin, with 98% located within 995 drained lake basins, most of which are underlain by thick eolian sand deposits. The highest pingo density (0.18 km^− 2) occurs where streams have reworked these deposits. Morphometric analyses indicate that most pingos are small to medium in size (< 200 m diameter), gently to moderately sloping (< 30°), circular to slightly elongate (mean circularity index of 0.88), and of relatively low height (2 to 5 m). However, 57 pingos stand higher than 10 m, 26 have a maximum slope greater than 30°, and 42 are larger than 200 m in diameter. Comparison with a legacy pingo dataset based on 1950s stereo-pair photography indicates that 66 may have partially or completely collapsed over the last half-century. However, we mapped over 400 pingos not identified in the legacy dataset, and identified only three higher than 2 m to have formed between ca. 1955 and ca. 2005, indicating that caution should be taken when comparing contemporary and legacy datasets derived by different techniques. This comprehensive database of pingo location and morphometry based on an IfSAR DSM may prove useful for land and resource managers as well as aid in the identification of pingo-like features on Mars.  相似文献   

Intensification of seasonal extremes given a 2°C global warming target     

Bruce T. Anderson 《Climatic change》2012,112(2):325-337

Current international efforts to reduce greenhouse gas emissions and limit human-induced global-mean near-surface temperature increases to 2°C, relative to the pre-industrial era, are intended to avoid possibly significant and dangerous impacts to physical, biological, and socio-economic systems. However, it is unknown how these various systems will respond to such a temperature increase because their relevant spatial scales are much different than those represented by numerical global climate models—the standard tool for climate change studies. This deficiency can be addressed by using higher-resolution regional climate models, but at great computational expense. The research presented here seeks to determine how a 2°C global-mean temperature increase might change the frequency of seasonal temperature extremes, both in the United States and around the globe, without necessarily resorting to these computationally-intensive model experiments. Results indicate that in many locations the regional temperature increases that accompany a 2°C increase in global mean temperatures are significantly larger than the interannual-to-decadal variations in seasonal-mean temperatures; in these locations a 2°C global mean temperature increase results in seasonal-mean temperatures that consistently exceed the most extreme values experienced during the second half of the 20th Century. Further, results indicate that many tropical regions, despite having relatively modest overall temperature increases, will have the most substantial increase in number of hot extremes. These results highlight that extremes very well could become the norm, even given the 2°C temperature increase target.  相似文献