Post-noon two-minute period pulsating aurora and their relationship to the dayside convection pattern     

S. E. Milan  T. K. Yeoman  M. Lester  J. Moen  P. E. Sandholt 《Annales Geophysicae》1999,17(7):877-891

Poleward-moving auroral forms, as observed by meridian-scanning photometers, in the vicinity of the cusp region are generally assumed to be the optical signature of flux transfer events. Another class of quasi-continuous, short period (1–2 min) wave-like auroral emission has been identified, closely co-located with the convection reversal boundary in the post-noon sector, which is similar in appearance to such cusp aurora. It is suggested that these short period wave-like auroral emissions, the optical signature of boundary plasma sheet precipitation in the region 1 field-aligned current system, are associated with ULF magnetohydrodynamic wave activity, which is observed simultaneously by ground magnetometer stations. This association with ULF wave activity is strengthened by the observation of several harmonic frequencies in the pulsation spectrum, each an overtone of the fundamental standing wave resonance frequency.  相似文献   

Waveform and packet structure of lion roars     

W. Baumjohann  R. A. Treumann  E. Georgescu  G. Haerendel  K.-H. Fornacon  U. Auster 《Annales Geophysicae》1999,17(12):1528-1534

The Equator-S magnetometer is very sensitive and has a sampling rate of normally 128 Hz. The high sampling rate allows for the first time fluxgate magnetometer measurements of ELF waves between the ion cyclotron and the lower hybrid frequencies in the equatorial dayside magnetosheath. The so-called lion roars, typically seen by the Equator-S magnetometer at the bottom of the magnetic troughs of magnetosheath mirror waves, are near-monochromatic packets of electron whistler waves lasting for a few wave cycles only, typically 0.25 s. They are right-hand circularly polarized waves with typical amplitudes of 0.5–1 nT at around one tenth of the electron gyrofrequency. The cone angle between wave vector and ambient field is usually smaller than 1.5°.  相似文献   

利用地震面波和重力资料联合反演地壳—上地幔三维密度结构的方法探讨   总被引：2，自引：0，他引：2  

刘崇兵  张禹慎 《测绘学报》1999,28(2):103-109

本文探讨了利用地震面波和重力资料联合反演地壳－地幔三维密度结构的反演问题。首先建立了地震面波和重力资料的观测方程,然后应用广义线性反线理论给出了反演问题的解。模型试验结果表明,与单纯面波反演的结果相比,联合反演的解在分析率和方差两个方面都有改善,当加入扰动重力数据时,在０－３００ｋｍ深度范围内联合反演的结果明显好于单纯面波反演的结果。  相似文献   

正压模式中的非绝热波动     

黄大川  刘式适 《热带气象学报》1999,15(1):26-37

应用含有非绝热因子的正压模式方程组,着重分析了对流凝结加热、蒸发－风反馈和ＣＩＳＫ机制等非绝热因子对低纬Ｒｏｓｓｂｙ波和Ｋｅｌｖｉｎ波的影响。既获得了圆频率的表达式,又分析了波的周期与稳定性。结果表明：非绝热因子的存在不但扩大了绝热波的周期,而且改变了波的稳定性。因此,可以认为：所谓季节内振荡（３０—６０天的周期振荡）等低频振荡实际上是一类非绝热波,这类非绝热波是引起中长期天气变化及短期气候演变的重要因素。  相似文献   

Changes in the internal spatial structure of post-communist Prague*     

Luděk Sýkora 《GeoJournal》1999,49(1):79-89

The article overviews the most important changes in the internal urban structure of Prague since 1989. Post-communist urban development has been influenced by government-directed reforms of political and economic system, internationalisation and globalisation, public policies favouring unregulated market development, economic restructuring in terms of deindustrialisation and growth of producer services, and increasing social differentiation. The three most transparent processes of urban change in Prague have been (1) commercialisation of the historical core; (2) revitalisation of some inner city neighbourhoods; and (3) residential and commercial suburbanisation in the outer city. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

The application of a shift theorem analysis technique to multipoint measurements     

M. E. Dieckmann  S. C. Chapman 《Annales Geophysicae》1999,17(3):321-327

A Fourier domain technique has been proposed previously which, in principle, quantifies the extent to which multipoint in-situ measurements can identify whether or not an observed structure is time stationary in its rest frame. Once a structure, sampled for example by four spacecraft, is shown to be quasi-stationary in its rest frame, the structure’s velocity vector can be determined with respect to the sampling spacecraft. We investigate the properties of this technique, which we will refer to as a stationarity test, by applying it to two point measurements of a simulated boundary layer. The boundary layer was evolved using a PIC (particle in cell) electromagnetic code. Initial and boundary conditions were chosen such, that two cases could be considered, i.e. a spacecraft pair moving through (1) a time stationary boundary structure and (2) a boundary structure which is evolving (expanding) in time. The code also introduces noise in the simulated data time series which is uncorrelated between the two spacecraft. We demonstrate that, provided that the time series is Hanning windowed, the test is effective in determining the relative velocity between the boundary layer and spacecraft and in determining the range of frequencies over which the data can be treated as time stationary or time evolving. This work presents a first step towards understanding the effectiveness of this technique, as required in order for it to be applied to multispacecraft data.  相似文献   

Pc3 pulsations during variable IMF conditions     

U. Villante  S. Lepidi  P. Francia  M. Vellante  A. Meloni  R. P. Lepping  F. Mariani 《Annales Geophysicae》1999,17(4):490-496

Pc3 geomagnetic field fluctuations detected at low latitude (L’Aquila, Italy) during the passage of a high velocity solar wind stream, characterized by variable interplanetary magnetic field conditions, are analyzed. Higher frequency resonant fluctuations and lower frequency phenomena are simultaneously observed; the intermittent appearance and the variable frequency of the longer period modes can be well interpreted in terms of the variable IMF elements; moreover their polarization characteristics are consistent with an origin related to external waves propagating in antisunward direction. A comparison with simultaneous observations performed at Terra Nova Bay (Antarctica) provides additional evidence for a clear relationship between the IMF and Pc3 pulsations also at very high latitudes.  相似文献   

Mudbank regime off the Kerala coast during monsoon and non-monsoon seasons     

R. Tatavarti  A. C. Narayana  P. Manoj Kumar  Shyam Chand 《Journal of Earth System Science》1999,108(1):57-68

Field experiments conducted in the nearshore ocean to understand the dynamics of mudbank off Kerala, south-west coast of India, are highlighted. Real time monitoring of the nearshore ocean off Purakkad, Kerala was accomplished using pressure transducers for nearshore surface wave measurements, and current sensors for nearshore velocity measurements. Comprehensive information on the spatial structure of mudbank was obtained from aerial surveys. Extensive data collected on surface waves and currents in the nearshore ocean, indicate that the infra-gravity (IG) waves (leaky modes and trapped edge wave modes), and far infra-gravity (FIG) waves coupled with strong shoreline reflections and undertow play an important role in the dynamics associated with the mudbanks off Kerala during the monsoon season. During the non-monsoon season evidence for progressive edge waves in the infragravity frequency band, an energetic gravity wave band and a strong undertow with weak reflections was observed.  相似文献   

不规则波远破波对直墙的作用   总被引：2，自引：0，他引：2  

李玉成  刘大中  齐桂萍  苏小军 《海洋学报》1999,21(2):99-107

基于物模试验、理论推理、因次分析和工程实例验证等综合分析方法,就不规则波远破波对直墙作用力的计算方法进行了研究,用推荐的方法得到的结果与工程实例符合较好.  相似文献   

A Possible Mechanism of Ocean Finestructure Part Ⅰ: Energy and Coherence     

范植松  方欣华 《中国海洋大学学报(自然科学版)》1999,(2)

依据在作者的前文（１９９８）中对于考虑旋转向量水平分量（ｆ～）的大洋内波方程求出的渐近解,文中对Ｓ场（考虑ｆ～条件下的总的内波场）,ＳＩ场（不计ｆ～影响的内波场）和ＳＦ场（Ｓ场中与ｆ～相关联的部分）导出了能量谱和相干谱。此理论谱与Ｍｕｌｅｒ等（１９７８）的ＩＷＥＸ实验的观测结果相比较表明,ＳＦ场的水平动能谱和铅直位移谱基本上与水平流细结构和铅直位移细结构的观测谱一致。作者推测ＳＦ场对应于中纬度大洋可逆细结构。但是对于相干谱的观测结果,在不考虑内波非线性相互作用和破碎过程的条件下,仅考虑ＳＦ场的影响还不能给予满意的解释。  相似文献