Theory of convective saturation of Langmuir waves during ionospheric modification of a barium cloud     

M.V. Goldman  D.L. Newman  R.Paul Drake  Bedros B. Afeyan 《Journal of Atmospheric and Solar》1997,59(18):2335-2350

In recent experiments (Djuth, F. T., Sulzer, M. P., Elder, J. H. and Groves, K. M. (1995) Journal of Geophysical Research, 100, 17,347), a parametric decay instability was excited by an ordinarywave HF pump during an ionospheric chemical release from a rocket over Arecibo, PR, which created an artificial ‘barium ionosphere,’ with peak plasma frequency above the pump frequency, and a density gradient with a (short) 5 km scale length. Simultaneous incoherent scattering measurements revealed a strong initial asymmetry in the amplitudes of almost vertically upgoing versus downgoing measured plasma waves. We can account for this asymmetry in terms of linear convective saturation of parametrically unstable plasma waves propagating over a range of altitudes along geometric optics ray paths. Qualitative features of the frequency spectrum of the measured downgoing wave are in agreement with this model, although the theoretically predicted spectrum is narrower than observed. The observed altitude localization of the enhanced spectrum to a few range cells is consistent with the theory.  相似文献   

The role of playas in pedogenic gypsum crust formation in the Central Namib Desert: a theoretical model     

F. D. Eckardt  N. Drake  A. S. Goudie  K. White  H. Viles 《地球表面变化过程与地形》2001,26(11):1177-1193

The formation of Namibia's extensive pedogenic gypsum crusts (CaSO₄·2H₂O) is interpreted in a new light. It is suggested that gypsum primarily precipitates at isolated points of evaporitic concentration, such as inland playas, and that deflation of evaporitic‐rich gypsum dust from these playas contributes to the formation of pedogenic gypsum duricrusts on the coastal gravel plains of the Namib Desert surrounding these playas. This study establishes the nature, extent and distribution of playas in the Central Namib Desert and provides evidence for playa gypsum deflation and gravel plain deposition. Remote sensing shows the distribution of playas, captures ongoing deflation and provides evidence of gypsum deflation. It is proposed that, following primary marine aerosol deposition, both inland playas and coastal sabkhas generate gypsum which through the process of playa deflation and gravel plain redeposition contributes to the extensive pedogenic crusts found in the Namib Desert region. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

水环境汞污染及其毒理反应系统的研究进展   总被引：5，自引：0，他引：5       下载免费PDF全文

王德铭 《水科学进展》1997,8(4):359-364

论述了水环境汞污染的来源,并着重讨论了汞的甲基化及其相反过程——脱甲基化作用。研究表明,一些细菌及真菌均可在无机汞污染的环境介质,如沉积物中使甲基汞降解或使无机汞甲基化,且使其间得到某种平衡。天然水的有效水质管理需要建立受各种环境影响的定量预测模型。我国的研究表明,不但家禽和人,而且鱼的甲基汞污染也主要通过食物链富集。并对甲基汞对鱼和人的毒理效应进行了论述。  相似文献   

Glacial morphology and post-glacial contourites in northern Prince Gustav Channel (NW Weddell Sea,Antarctica)     

Camerlenghi  Angelo  Domack  Eugene  Rebesco  Michele  Gilbert  Robert  Ishman  Scott  Leventer  Amy  Brachfeld  Stefanie  Drake  Allison 《Marine Geophysical Researches》2001,22(5-6):417-443

We present the results of a marine geophysical investigation of the northern Prince Gustav Channel. By comparative analysis of multibeam bathymetric data, single channel seismic reflection profiles, underway chirp sonar data, ADCP current data and sediment coring, we define the main morphological elements of the area. In particular we define the glacial morphogenesis in relation to the excavation of inner shelf basins and troughs along structural discontinuities and lithologic boundaries. We identify streamlined surfaces that testify to the grounding of ice and past ice flow directions. These glacial forms are found only on glacial tills preserved in the deepest part of the basins, while net erosion to bedrock has occurred elsewhere. Since the Last Glacial Maximum (LGM), the relict glacial morphology has been draped by hemipelagic and diatomaceous mud, and bottom currents have played a major role in focusing sedimentation within small depocentres, that we define as contouritic drifts. Based on shallow sediment architecture and supported by direct measurements, we propose that the direction of bottom water flow is from the outer shelf into the Prince Gustav channel as a result of a combination of tidal currents and ice shelf-related thermohaline circulation.  相似文献   

环胶州湾公路(陆域)建设工程环境影响预测     

路德明  李江吉  段存贤  许龙江  林董 《中国海洋大学学报(自然科学版)》1992,(1)

提出环胶州湾公路陆域部分的环境影响评价,指出交通噪声和汽车尾气对公路沿线环境的影响,并按环保要求论证了该公路走向选线的合理性和设计的可行性。  相似文献   

Water resource conflicts in the Middle East   总被引：1，自引：0，他引：1  

Drake C 《The Journal of geography》1997,96(1):4-12

This article discusses the causes and sources of water resource conflict in the 3 major international river basins of the Middle East: the Tigris-Euphrates, the Nile, and the Jordan-Yarmuk. The physical geography of the Middle East is arid due to descending air, northeast trade winds, the southerly location, and high evaporation rates. Only Turkey, Iran, and Lebanon have adequate rainfall for population needs. Their mountainous geography and more northerly locations intercept rain and snow bearing westerly winds in winter. Parts of every other country are vulnerable to water shortages. Rainfall is irregular. Water resource conflicts are due to growing populations, economic development, rising standards of living, technological developments, political fragmentation, and poor water management. Immigration to the Jordan-Yarmuk watershed has added to population growth in this location. Over 50% of the population in the Middle East lives in urban areas where populations consume 10-12 times more water than those in rural areas. Water is wasted in irrigation schemes and huge dams with reservoirs where increased evaporation occurs. Technology results in greater water extraction of shallow groundwater and pollution of rivers and aquifers. British colonial government control led to reduced friction in most of the Nile basin. Now all ethnic groups have become more competitive and nationalistic. The Cold War restrained some of the conflict. Israel obtains 40% of its water from aquifers beneath the West Bank and Gaza. Geopolitical factors determine the mutual goodwill in managing international water. The 3 major water basins in the Middle East pose the greatest risk of water disputes. Possible solutions include conservation, better management, prioritizing uses, technological solutions, increased cooperation among co-riparians, developing better and enforceable international water laws, and reducing population growth rates.  相似文献   

Volcán Quizapu,Chilean Andes   总被引：1，自引：0，他引：1  

Wes Hildreth  Robert E Drake 《Bulletin of Volcanology》1992,54(2):93-125

Quizapu is a flank vent of the basalt-to-rhyodacite Holocene stratocone, Cerro Azul, and lies at the focus of a complex Quaternary volcanic field on the Andean volcanic front. The Quizapu vent originated in 1846 when 5 km³ of hornblende-dacite magma erupted effusively with little accompanying tephra. Between 1907 and 1932, phreatic and strombolian activity reamed out a deep crater, from which 4 km³ of dacite magma identical to that of 1846 fed the great plinian event of 10–11 April 1932. Although a total of >9 km³ of magma was thus released in 86 years, there is no discernible subsidence. As the pre-plinian crater was lined by massive lavas, 1932 enlargement was limited and the total plinian deposit contains only 0.4 wt % lithics. Areas of 5-cm and 1-cm isopachs for compacted 1932 fallout are about half of those estimated in the 1930's, yielding a revised ejecta volume of 9.5 km³. A strong inflection near the 10-cm isopach (downwind 110 km) on a plot of log Thickness vs Area^1/2 reflects slow settling of fine plinian ash — not of coignimbrite ash, as the volume of pyroclastic flows was trivial (<0.01 km³). About 17 vol.% of the fallout lies beyond the 1-cm isopach, and 82 wt% of the ejecta are finer than 1 mm. A least 18 hours of steady plinian activity produced an exceptionally uniform fall deposit. Observed column height (27–30 km) and average mass eruption rate (1.5x10⁸ kg/s) compare well with values for height and peak intensity calculated from published eruption models. The progressive aeolian fractionation of downwind ash (for which Quizapu is widely cited) is complicated by the large compositional range of 1932 juvenile pumice (52–70% SiO₂). The eruption began with andesitic scoria and ended with basaltic scoria, but >95% of the ejecta are dacitic pumice (67–68% SiO₂); minor andesitic scoria and frothier rhyodacite pumice (70% SiO₂) accompanied the dominant dacite. Phenocrysts (pl>hbopx>mt>ilmcpx) are similar in both abundance and composition in the 1846 (effusive) and 1932 (plinian) dacites. Despite the contrast in mode of eruption, bulk compositions are also indistinguishable. The only difference so far identified is a lower range of D values for 1846 hornblende, consistent with pre-eruptive degassing of the effusive batch.  相似文献   

常熟区域地震活动性规律研究     

鲁宗良  陆德明  丁建国 《地震地磁观测与研究》2003,24(5):45-52

运用常熟地方震群记录资料，讨论了常熟地方区域地震活动性规律；频次高、震级小，地震活动具有分段连续等特点，进而指出地震危险区及中强地震的先期特征。  相似文献   

Potassium-argon dating of igneous activity in the central Chilean Andes — latitude 33°S     

Robert E. Drake  Garniss Curtis  Mario Vergara   《Journal of Volcanology and Geothermal Research》1976,1(3):285-295

Potassium-argon dating of volcanic and plutonic rocks in the Andean region of central Chile has revealed previously unrecognized episodes of igneous activity during Cretaceous and Cenozoic time. These results indicate the need to re-evaluate the classic stratigraphic subdivisions that have evolved on lithologic rather than time-stratigraphic criteria.Four radiometric age groups have been identified in the coast range volcanic belt:

1. (1) Las Chilcas Formation — Early Cretaceous continental volcanic strata (120-110 m.y.).

2. (2) Lo Valle Formation — Late Cretaceous continental volcanic strata (78-65 m.y.).

3. (3) Late Oligocene extrusive volcanics (31-28 m.y.).

4. (4) Early Miocene intrusive volcanics (20.6–19.5 m.y.).

Two radiometric age groups have also been identified in the adjacent Andean Cordillera:

1. (1) Farellones Formation — continental volcanic strata (18.5–17.3 m.y.).

2. (2) Early Pliocene extrusive volcanics (5-4 m.y.).

An older group of continental volcanic strata in the Andes represented by the Abanico Formation remains undated but is intruded by plutons dated at 19.5 and 24 m.y.Available chronologic evidence indicates that volcanic activity moved eastward from the coast range volcanic belt to the Andean Cordillera between 20 and 18 m.y. ago and remained there to the present time.  相似文献   

Constraining the timing of alluvial fan response to late quaternary climatic changes, southern Tunisia     

Kevin White  Nick Drake  Andrew Millington  Stephen Stokes 《Geomorphology》1996,17(4):295-304

The evolution of the Oued es Seffaia alluvial fan during the last 50,000 years is analyzed in the light of chronometric data derived from AMS radiocarbon and optical dating techniques. These ages have enabled the temporal constraint of the distalward progression of the intersection point (the upper limit of the locus of deposition) which has resulted in the telescopic segmentation of the fan. Comparison of these data with the Late Quaternary climatic history of the Maghreb highlights several problems of interpretation; due both to our limited knowledge of regional palaeoclimates and of the response of dryland fluvial systems to climate changes. However, our data provide some evidence to suggest that fan incision and formation of telescopic segments occurs in response to changes from arid to less arid conditions, supporting the conclusions of some of the work from other areas.  相似文献