The evolution of Rapid Burster outbursts     

R. Guerriero    D. W. Fox    J. Kommers    W. H. G. Lewin    R. Rutledge    C. B. Moore    E. Morgan    J. Van Paradijs    M. Van der Klis    L. Bildsten  T. Dotani 《Monthly notices of the Royal Astronomical Society》1999,307(1):179-189

We describe the evolutionary progression of an outburst of the Rapid Burster. Four outbursts have been observed with the Rossi X-Ray Timing Explorer between 1996 February and 1998 May, and our observations are consistent with a standard evolution over the course of each. An outburst can be divided into two distinct phases. Phase I is dominated by type I bursts, with a strong persistent emission component; it lasts for 15–20 d. Phase II is characterized by type II bursts, which occur in a variety of patterns. The light curves of time-averaged luminosity for the outbursts show some evidence for reflares, similar to those seen in soft X-ray transients. The average recurrence time for Rapid Burster outbursts during this period was 218 d, in contrast to an average ∼180‐d recurrence period observed during 1976–1983.  相似文献   

Geographic signatures of North American West Coast estuaries     

Robert Emmett  Roberto Llansó  Jan Newton  Ron Thom  Michelle Hornberger  Cheryl Morgan  Colin Levings  Andrea Copping  Paul Fishman 《Estuaries and Coasts》2000,23(6):765-792

West Coast estuaries are geologically young and composed of a variety of geomorphological types. These estuaries range from large fjords to shallow lagoons; from large to low freshwater flows. Natural hazards include E1 Niños, strong Pacific storms, and active tectonic activity. West Coast estuaries support a wide range of living resources: five salmon species, harvestable shellfish, waterfowl and marine birds, marine mammals, and a variety of algae and plants. Although populations of many of these living resources have declined (salmonids), others have increased (marine mammals). West Coast estuaries are also centers of commerce and increasingly large shipping traffic. The West Coast human population is rising faster than most other areas of the U.S. and Canada, and is distributed heavily in southern California, the San Francisco Bay area, around Puget Sound, and the Fraser River estuary. While water pollution is a problem in many of the urbanized estuaries, most estuaries do not suffer from poor water quality. Primary estuarine problems include habitat alterations, degradation, and loss; diverted freshwater flows; marine sediment contamination; and exotic species introductions. The growing West Coast economy and population are in part related to the quality of life, which is dependent on the use and enjoyment of abundant coastal natural resources.  相似文献   

The seismic anomaly beneath Iceland extends down to the mantle transition zone and no deeper   总被引：8，自引：0，他引：8  

G. R. Foulger  M. J. Pritchard  B. R. Julian  J. R. Evans  R. M. Allen  G. Nolet  W. J. Morgan  B. H. Bergsson  P. Erlendsson  S. Jakobsdóttir  S. Ragnarsson  R. Stefansson  K. Vogfjörd 《Geophysical Journal International》2000,142(3):F1-F5

A 3-D teleseismic tomography image of the upper mantle beneath Iceland of unprecedented resolution reveals a subvertical low wave speed anomaly that is cylindrical in the upper 250 km but tabular below this. Such a morphological transition is expected towards the bottom of a buoyant upwelling. Our observations thus suggest that magmatism at the Iceland hotspot is fed by flow rising from the mantle transition zone. This result contributes to the ongoing debate about whether the upper and lower mantles convect separately or as one. The image also suggests that material flows outwards from Iceland along the Reykjanes Ridge in the upper 200 km, but is blocked in the upper 150 km beneath the Tjornes Fracture Zone. This provides direct observational support for the theory that fracture zones dam lateral flow along ridges.  相似文献   

中英海滨污染美学的比较研究     

罗伯特·摩根  刘洪 《海岸工程》1999,18(4):35-45

在西方从美学的角度研究海滨环境质量是近３０年的事,近些年来发展更快,发表了一系列文章,对滨海环境的评价主要脸地形地貌、生态植被和水体的环境质量,其中尤其是对水体的评价显得更为重要,将水质分为优质水和劣质水。优质水体表现为洁净、生物丰富、有鱼、水鸟、没有垃圾等;而劣质水则呈深色、漂浮着垃圾、泡沫、死鱼、水底为油污,缺少生物,发出令人难以接受的恶臭等气味。传统环境质量监测可用物理、化学、生物方法进行,并有一系列量化指标,而美学对地貌、环境、垃圾、泡沫、噪声的评价就比较困难,没有统一的标准,只能凭感官、视觉和心理反应做出评价。例如,对海滨垃圾的测量方法也处在初级阶段,不同的机构,不同的研究人员采用不同的方法,在时空上进行对比也非常困难。通常我们采用了采访和问卷的两种调查方式,同时配合出示彩色图片或录相来进行,由于人们的文化背景、经不同,对环境质量从美学上认识有一定差距,故对环境、海岸评价带来一定困难,但原始的、未被破坏的自然景观,被西方人认为是最美,近就是油画、水彩画半自然风光做成永恒主题,经久不衰的原因。中国海滨污染从美学上进行研究还没有起步,,但客观上,我们已经感受到了污染的存在,例如,沿海滨的垃圾,瓢浮在海面上的塑料袋、油污、泡沫、海草、死鱼等;许多污水不经处理直接入海,散发着恶臭……这一切都对生态环境,特别是滨海旅游业发展造成了严重影响,随着中国工业的快速发展,在海岸带局地区造成的污染还将加重。法律保护广大群众的健康,面对法规和广大公众关心海滨环境的压力,英国政府和企业投巨资以便改进滨海环境质量,威尔士最大的自来水公司威尔士水公司于１９９５－２０００年将花费６亿英镑（１０亿美元）用于改进威尔士的污水处理系统（只有２万km^2面积３００万人口）。中国可以从英国经济发展过程中吸取教训,控制好烟、尘、油和固体废弃物向海洋的倾倒。否则,要付出沉痛的代价的。  相似文献