Observations of the response time of high-latitude ionospheric convection to variations in the interplanetary magnetic field using EISCAT and IMP-8 data   总被引：1，自引：0，他引：1  

H. Khan  S. W. H. Cowley 《Annales Geophysicae》1999,17(10):1306-1335

We have combined ∼300 h of tristatic measurements of the field-perpendicular F region ionospheric flow measured overhead at Tromsø by the EISCAT UHF radar, with simultaneous IMP-8 measurements of the solar wind and interplanetary magnetic field (IMF) upstream of the Earth’s magnetosphere, in order to examine the response time of the ionospheric flow to changes in the north-south component of the IMF (B_z). In calculating the flow response delay, the time taken by field changes observed by the spacecraft to first effect the ionosphere has been carefully estimated and subtracted from the response time. Two analysis methods have been employed. In the first, the flow data were divided into 2 h-intervals of magnetic local time (MLT) and cross-correlated with the “half-wave rectifier” function V²B_s, where V is the solar wind speed, and B_s is equal to IMF B_z if the latter is negative, and is zero otherwise. Response delays, determined from the time lag of the peak value of the cross-correlation coefficient, were computed versus MLT for both the east-west and north-south components of flow. The combined data set suggests minimum delays at ∼1400 MLT, with increased response times on the nightside. For the 12-h sector centred on 1400 MLT, the weighted average response delay was found to be 1.3 ± 0.8 min, while for the 12-h sector centred on 0200 MLT the weighted average delay was found to increase to 8.8 ± 1.7 min. In the second method we first inspected the IMF data for sharp and enduring (at least ∼5 min) changes in polarity of the north-south component, and then examined concurrent EISCAT flow data to determine the onset time of the corresponding enhancement or decay of the flow. For the case in which the flow response was timed from whichever of the flow components responded first, minimum response delays were again found at ∼1400 MLT, with average delays of 4.8 ± 0.5 min for the 12-h sector centred on 1400 MLT, increasing to 9.2 ± 0.8 min on the nightside. The response delay is thus found to be reasonably small at all local times, but typically ∼6 min longer on the nightside compared with the dayside. In order to make an estimate of the ionospheric information propagation speed implied by these results, we have fitted a simple theoretical curve to the delay data which assumes that information concerning the excitation and decay of flow propagates with constant speed away from some point on the equatorward edge of the dayside open-closed field line boundary, taken to lie at 77° magnetic latitude. For the combined cross-correlation results the best-fit epicentre of information propagation was found to be at 1400 MLT, with an information propagation phase speed of 9.0 km s⁻¹. For the combined event analysis, the best-fit epicentre was also found to be located at 1400 MLT, with a phase speed of 6.8 km s⁻¹.  相似文献   

Large-Eddy Simulation of air Pollution Dispersion in the Nocturnal Cloud-Topped Atmospheric Boundary Layer     

Zbigniew Sorbjan  Marek Uliasz 《Boundary-Layer Meteorology》1999,91(1):145-157

Effects of stratocumulus clouds on the dispersion of contaminants are studied in the nocturnal atmospheric boundary layer. The study is based on a large-eddy simulation (LES) model with a bulk parametrization of clouds. Computations include Lagrangian calculations of atmospheric dispersion of a passive tracer released from point sources at various heights above the ground. The results obtained show that the vertical diffusion is non-Gaussian and depends on the location of a source in the boundary layer.  相似文献   

用近垂直地震反射方法研究莫霍面的特征与成因   总被引：11，自引：3，他引：8       下载免费PDF全文

杨宝俊  刘财  杨平华  侯广兵  冯晅 《地球物理学报》1999,42(5):617-628

利用近垂直地震反射方法研究了在中国满洲里－绥芬河地学断面域内泰康－哈尔滨约１３０ｋｍ范围内地壳结构和莫霍面特征．通过数据采集,定量处理和综合分析,得到以下认识：在野外观测中激发井深和震源能量是主要参数;在资料处理中速度分析与共反射点分布的处理是主要技术．分析莫霍震相可以得到它的层状与似层状内部结构以及起伏、断裂等外形特征．利用傅立叶谱、时频分析与瞬时信息可更细致地研究莫霍特征．影响莫霍特征与成因的因素主要为分异作用、地幔对流、相变、地球转动和板块运动;其中地幔对流和相变是形成莫霍的主要因素．  相似文献   

Visualization and Analysis of Mixing Dynamical Properties in Convecting Systems with Different Rheologies     

A. A. Ten  D. A. Yuen  Yu. Yu. Podladchikov 《Visual Geosciences》1999,4(1):1-15

We analyzed and compared the mixing properties of 2-D mantle convection models. Two rheologically different models, Newtonian and non-Newtonian (power-law), were considered with both the line and field methods. The line method is based on monitoring of passive particles joined into lines, while the field method relies on the advection of a passive scalar field. Both visual and quantitative estimates revealed that the efficiency of the Newtonian mixing is greater than the non-Newtonian. A heterogeneity placed in the non-Newtonian convection forms horizontal structures, which may persist for at least 1 Ga on the upper-mantle scale. In addition, the non-Newtonian medium reveals a lesser amount of stretching of the lines than the Newtonian material. The rate of the Newtonian stretching fits well with an exponential dependence with time, while the non-Newtonian rheology shows the stretching rate close to a power-law dependence with time. In the Newtonian medium the heterogeneity is reorganized into two unstable vertical columns, while the non-Newtonian mixing favors horizontal structures. In the latter case, these structures are sufficiently stable in both the temporal and spatial planes to explain the mantle geochemical and geophysical heterogeneities. Due to the non-linear character of power-law rheology, the non-Newtonian medium offers a “natural” scale-dependent resistance to deformation, which prevents efficient mixing at the intermediate length scales.  相似文献   

1999年5月5日兰州强对流天气分析   总被引：1，自引：0，他引：1  

杨民  杨建才  伏晓红  陈洁 《干旱气象》1999,(3)

自１９９８ 年１０ 月１２ 日至１９９９ 年３ 月２５ 日,兰州市经历了１６２ｄ 无明显降雨的干旱时段,旱情十分严重。１９９９ 年５ 月５ 日晚,兰州附近发生的雷阵雨是今年第一场降水量超过１０ｍ ｍ 的降水,也是今年的第一场强对流天气,具有雷暴过程长,降水时段及区域集中的特点。从天气动力学成因方面,对此次过程进行了分析,提出兰州附近发生强对流天气的预报着眼点。  相似文献   

Hawaiian volcanic propagation and Hawaiian swell asymmetry: evidence of northwestward flow of the deep upper mantle     

R. T. Cox 《Tectonophysics》1999,310(1-4):69-79

Bathymetry and the geoid anomaly of the northern flank of the Hawaiian swell is broader and higher than the southern flank, and it is characterized by higher heat flow than the axis or southern flank. It is here proposed that the northern flank of the Hawaiian swell has been augmented by heat conducted from the hotspot conduit into the upper mantle then transported northward of the volcanic axis by flow in the upper mantle (325°) that is more northerly than Pacific plate motion (292°). By assuming that the deep upper mantle is decoupled from the Pacific plate and is flowing at 325° to the northwest, changes in direction and rate of volcanic propagation and in geochemistry along individual volcanic segments of the Hawaiian volcanic chain can be interpreted in terms of tank experiment results showing that a volcanic hotspot conduit breaks into diapirs when tilted by mantle flow. Hawaiian volcanoes are aligned in en-echelon segments, and the Hawaiian Islands are the two most recent segments. For an individual segment, older northwestern volcanoes are aligned nearly parallel to the 292° plate motion direction, and they propagated to the southeast at approximately the same rate as the 92 km/m.y. speed of northwestward plate motion. In contrast, the alignment of the younger southeastern volcanoes is close to 325°, and they show a conspicuous acceleration in propagation of volcanism marked by out-of-sequence eruptions. Within the model proposed here, diapirs rise from instability nodes that develop along the tilted conduit of a mantle hotspot plume as it is sheared in the direction of deep upper-mantle flow and each diapir gives rise to a single volcanic center. As tilting progresses, diapirs form at lower levels along the conduit in more upstream positions of the mantle flow zone, rise sequentially into the decoupled lithosphere, erupt sequentially, and are translated in the direction of plate motion (older, northwestern Hawaiian Islands). Eventually, flow in the highly tilted conduit is impeded to the degree that the remaining upstream conduit breaks into a number of diapirs that rise together into the lithosphere. These late diapirs, translated as a group aligned in the direction of horizontal mantle flow, erupt over a relatively short time span and show out-of-sequence volcanism (younger, southeastern Hawaiian Islands). At this stage, a new cycle of rising and tilting will initiate the next en-echelon segment.  相似文献   

Probing solar convection     

M. Brüggen 《Monthly notices of the Royal Astronomical Society》2000,312(4):887-896

In the solar convection zone, acoustic waves are scattered by turbulent sound speed fluctuations. In this paper the scattering of waves by convective cells is treated using Rytov's technique. Particular care is taken to include diffraction effects, which are important, especially for high-degree modes that are confined to the surface layers of the Sun. The scattering leads to damping of the waves and causes a phase shift. Damping manifests itself in the width of the spectral peak of p-mode eigenfrequencies. The contribution of scattering to the linewidths is estimated and the sensitivity of the results to the assumed spectrum of the turbulence is studied. Finally, the theoretical predictions are compared with recently measured linewidths of high-degree modes.  相似文献   

Flattening of the oceanic topography and geoid: thermal versus dynamic origin     

Marie&#;Pierre Doin  Luce Fleitout 《Geophysical Journal International》2000,143(3):582-594

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三峡坝区非对称共轭低缓角度剪破裂系   总被引：1，自引：0，他引：1       下载免费PDF全文

索书田  石林等长 《地球科学》2000,25(3):283-289

在岩石摩擦流变学理论指导下, 运用构造解析方法, 在三峡坝区岩体内多期的、复杂的破裂系统中, 识别出一个不对称的共轭低缓角度剪破裂系统.几何学和运动学标志证明, 这个影响大坝深层和浅层抗滑稳定性的共轭剪切破裂系统形成较早, 受岩体非均一性制约, 被后期破裂系统切错和改造, 是高流体压力控制下非对称共轴变形的结果.   相似文献   

热带对流活动与长江中下游和华北地区夏季旱涝的关系   总被引：14，自引：0，他引：14  

葛旭阳  李砚华  蒋尚城 《南京气象学院学报》2000,23(3):391-398

利用ＯＬＰ资料,结合ＮＣＥＰ／ＮＣＡＲ再分析资料,研究了长江中下游、华北地区旱、涝年热带对流活动特征,着重讨论了ＩＴＣＺ位置、季节内低频振荡与两地区旱、涝气候异常的联系,结果发现：１月赤道太平洋上对流活动的异常与夏季两地区旱涝有密切的关系,冬季赤道太平洋对流活动东强西弱（东弱西强）,则对应夏季长江中下游地区易偏涝（旱）,华北地区锡偏旱（涝）;ＩＴＣＺ的位置异常偏南（北）时,夏季长江中下游地区易偏涝  相似文献