Observations of the upper frequency cutoffs of the auroral kilometric radiation     

J. Hanasz  R. Schreiber  H. de Feraudy  M. M. Mogilevsky  T. V. Romantsova 《Annales Geophysicae》1998,16(9):1097-1104

Intense auroral kilometric radiation (AKR) is being frequently observed with POLRAD from the Auroral Probe (Interball-2). Observations of the abrupt upper frequency cutoffs (UFCs) in the spectra of AKR are reported. The UFCs can be observed at a frequency range from 300 to 700 kHz, corresponding to AKR generation altitudes from approximately 4800 to 2100 km, and are distributed in magnetic local time (MLT) hours similarly to the AKR events, with a maximum at 1 h MLT. The observed frequency extent of the UFCs is 12 kHz, and is often determined by the instrumental resolution (4 kHz). It is suggested that the UFC may be associated with an abrupt switching on of the generation mechanism, when the electron density becomes sufficiently low inside a plasma depletion at an altitude where the ratio of f_pe/f_ce crosses some threshold value. The steepness of the UFCs can imply a non-linear process of generation. The estimated distance of the e-folding field aligned wave amplification is between 3 and 8 km. The UFCs are sometimes, though very seldom (10%), accompanied by narrow band (less than 4 kHz) ridges of radiation observed at the cutoff frequency. They are smoothly drifting in frequncy for several minutes. The power density of radiation in the ridge can be up to 2 orders of magnitude stronger than in the accompanying wide band emission of AKR. The ridge at UFC can imply either energy concentration at the source bottom, or focusing, if specific conditions for the escape of the radiation are assumed.  相似文献   

Inversion, error analysis, and validation of GPS/MET occultation data     

A. K. Steiner  G. Kirchengast  H. P. Ladreiter 《Annales Geophysicae》1998,17(1):122-138

The global positioning system meteorology (GPS/MET) experiment was the first practical demonstration of global navigation satellite system (GNSS)-based active limb sounding employing the radio occultation technique. This method measures, as principal observable and with millimetric accuracy, the excess phase path (relative to propagation in vacuum) of GNSS-transmitted radio waves caused by refraction during passage through the Earth’s neutral atmosphere and ionosphere in limb geometry. It shows great potential utility for weather and climate system studies in providing an unique combination of global coverage, high vertical resolution and accuracy, long-term stability, and all-weather capability. We first describe our GPS/MET data processing scheme from excess phases via bending angles to the neutral atmospheric parameters refractivity, density, pressure and temperature. Special emphasis is given to ionospheric correction methodology and the inversion of bending angles to refractivities, where we introduce a matrix inversion technique (instead of the usual integral inversion). The matrix technique is shown to lead to identical results as integral inversion but is more directly extendable to inversion by optimal estimation. The quality of GPS/MET-derived profiles is analyzed with an error estimation analysis employing a Monte Carlo technique. We consider statistical errors together with systematic errors due to upper-boundary initialization of the retrieval by a priori bending angles. Perfect initialization and properly smoothed statistical errors allow for better than 1 K temperature retrieval accuracy up to the stratopause. No initialization and statistical errors yield better than 1 K accuracy up to 30 km but less than 3 K accuracy above 40 km. Given imperfect initialization, biases ≫ 2 K propagate down to below 30 km height in unfavorable realistic cases. Furthermore, results of a statistical validation of GPS/MET profiles through comparison with atmospheric analyses of the European Centre for Medium-range Weather Forecasts (ECMWF) are presented. The comparisons indicate the high utility of the occultation data in that very good agreement of upper troposphere/lower stratosphere temperature (better than 1.5 K rms, ≪ 0.5 K bias) is found for a region (Europe+USA) where the ECMWF analyses are known to be good, but poorer agreement for a region (Southern Pacific) where the analyses are known to be degraded.  相似文献   

The spatial structure of the dayside ionospheric trough     

S. E. Pryse  L. Kersley  M. J. Williams  I. K. Walker 《Annales Geophysicae》1998,16(10):1169-1179

Tomographic imaging provides a powerful technique for obtaining images of the spatial distribution of ionospheric electron density at polar latitudes. The method, which involves monitoring radio transmissions from the Navy Navigation Satellite System at a meridional chain of ground receivers, has particular potential for complementing temporal measurements by other observing techniques such as the EISCAT incoherent-scatter radar facility. Tomographic reconstructions are presented here from a two-week campaign in November 1995 that show large-scale structuring of the polar ionosphere. Measurements by the EISCAT radar confirm the authenticity of the technique and provide additional information of the plasma electron and ion temperatures. The dayside trough, persistently observed at high latitudes during a geomagnetically quiet period but migrating to lower latitudes with increasing activity, is discussed in relationship to the pattern of the polarcap convection.  相似文献   

Geophysical phenomena during an ionospheric modification experiment at Tromsø, Norway     

N. F. Blagoveshchenskaya  V. A. Kornienko  A. V. Petlenko  A. Brekke  M. T. Rietveld 《Annales Geophysicae》1998,16(10):1212-1225

We present an analysis of phenomena observed by HF distance-diagnostic tools located in St. Petersburg combined with multi-instrument observation at Tromsø in the HF modified ionosphere during a magnetospheric substorm. The observed phenomena that occurred during the Tromsø heating experiment in the nightside auroral E_s region of the ionosphere depend on the phase of substorm. The heating excited small-scale field-aligned irregularities in the E region responsible for field-aligned scattering of diagnostic HF waves. The equipment used in the experiment was sensitive to electron density irregularities with wavelengths 12–15 m across the geomagnetic field lines. Analysis of the Doppler measurement data shows the appearance of quasiperiodic variations with a Doppler frequency shift, f_d and periods about 100–120 s during the heating cycle coinciding in time with the first substorm activation and initiation of the upward field-aligned currents. A relationship between wave variations in f_d and magnetic pulsations in the Y-component of the geomagnetic field at Tromsø was detected. The analysis of the magnetic field variations from the IMAGE magnetometer stations shows that ULF waves occurred, not only at Tromsø, but in the adjacent area bounded by geographical latitudes from 70.5° to 68° and longitudes from 16° to 27°. It is suggested that the ULF observed can result from superposition of the natural and heater-induced ULF waves. During the substorm expansion a strong stimulated electromagnetic emission (SEE) at the third harmonic of the downshifted maximum frequency was found. It is believed that SEE is accompanied by excitation of the VLF waves penetrating into magneto-sphere and stimulating the precipitation of the energetic electrons (10–40 keV) of about 1-min duration. This is due to a cyclotron resonant interaction of natural precipitating electrons (1–10 keV) with heater-induced whistler waves in the magnetosphere. It is reasonable to suppose that a new substorm activation, exactly above Tromsø, was closely connected with the heater-induced precipitation of energetic electrons.  相似文献   

应用“八·五”短临预报方法分析福建地区地震     

林世敏 《福建地震》1998,(2)

本文利用“八·五”短临预报的地震学攻关成果来分析福建地区的几个震例,检验其在福建地区的适用情况。  相似文献   

当前欧美海洋科学研究动态评析     

王辉 《地球科学进展》1998,13(3):261-264

重点介绍了当前欧美资助海洋科学研究的基本情况、优先资助领域和重要研究计划等,对欧美当前海洋科学研究的基本特点做了简要分析;并提出了我国下世纪初的海洋科学研究优先发展领域。  相似文献   

滨海社区空间演变分形研究——以厦门市高浦社区为例          下载免费PDF全文

俞晓牮  赵志庆  李焱龙  林松 《应用海洋学学报》2019,38(4):540-548

滨海城市社区的空间形态及其海岸线表现出一定程度的复杂性,发展量化描述方法对于其可发展研究有重要的理论与应用价值。本研究基于分形理论,以滨海城市厦门的高浦社区为研究对象,利用计盒法对该滨海社区空间形态和边界进行复杂性分析和分形维计算,研究社区空间形态复杂性演变规律。结果表明:作为典型海湾渔村的高浦社区空间形态具有分数维,呈现分形结构特性。自1989年至今,该社区空间形态的分形维数呈现增长态势;社区边界的分形维数呈现下降态势。社区空间形态的分形维数与社区建筑占地面积、建筑总面积、建筑密度、容积率都表现出相关性。这表明滨海社区在快速城市化进程中,社区规模急剧扩张可能会导致空间形态和边界复杂性的显著变化。  相似文献   

现代人地关系与人地系统科学   总被引：23，自引：5，他引：23  

刘彦随 《地理科学》2020,40(8):1221-1234

人地关系地域系统理论系统提出30 a来，对促进地理学综合研究、学科建设和服务国家重大战略决策发挥了重要的科学支撑与导向作用。深入解析了人地关系地域系统理论的科学内涵及时代价值，诠释了现代人地系统的类型与环境，提出了“人地圈”与人地系统科学研究的主要内容和前沿领域。初步研究表明：① 现代人地系统具有复杂性、地域性和动态性特征，人?地交互作用过程、格局及其综合效应正在发生深刻变化，地球表层人地系统成为现代地学综合研究的核心内容和重要主题。② 科学认知和有效协调人地关系，亟需深入探究人地系统耦合格局与机理，探明人地关系地域系统类型、结构及其动力机制。依据城乡关系将人地关系地域类型划分为城市地域系统、城乡融合系统、乡村地域系统。乡村地域系统可细分为农业系统、村庄系统、乡域系统、城镇系统等子系统，分别对应于作土关系、人居关系、居业关系、产城关系。③ 现代人类活动强烈地作用于地球表层人地系统，形成了人地系统耦合与交互作用的地表圈层——“人地圈”，其实质是现代人类活动与地表环境相互联系、耦合渗透而形成的自然–经济–技术综合体或人地协同体。④ 人地系统科学或人地科学是研究人地系统耦合机理、演变过程及其复杂交互效应的新型交叉学科。它是现代地理科学与地球系统科学的深度交叉和聚焦，以现代人地圈系统为对象，致力于探究人类活动改造和影响地表环境系统的状态，以及人地系统交互作用与耦合规律、人地协同体形成机理与演化过程。人地系统耦合与可持续发展是人地系统科学的研究核心。传承创新人地关系地域系统理论和发展人地系统科学，更能凸显地球表层人类的主体性、人地协同的过程性和可持续发展的战略性，为人地系统协调与可持续发展决策提供科学指导。  相似文献   

海洋自然资本评估中生物能值转换率的研究          下载免费PDF全文

丁洁琼  魏方懿  叶观琼  曾江宁  林颖  韩宇 《应用海洋学学报》2020,39(2):253-260

自然资本评估是研究生态系统服务与社会经济发展的重要方式。由于海洋具有特殊性和复杂性、开放性、流动性和多层次耦合性等特点,海洋自然资本评估方法的研究进展缓慢。能值分析理论通过能值转换率将生态系统内流动和储存的各种不同类别的能量和物质转换为同一标准的能值,可衡量和比较不同等级的能量价值,系统完整地反映自然资本价值,这一特点是其他评估方法所缺乏的。为精确量化海洋生物资本价值,本研究引入海洋食物网信息能流图和生态系统能量传递规律,提出了生物能值转换率的经验公式,即Tn=T1·E^1-nL(T1为初级生产者的太阳能值转换率,n为营养级,EL为林德曼效率)。为验证经验公式的可行性,选用相关案例进行了研究,得到海洋食物网中不同食性生物所处营养级及其相应的能值转换率。经验公式有一定误差,具体应用时应通过调研文献和相关数据,确定关键参数的取值范围以提高准确性。本研究得出的经验公式简化了海洋生物能值转换率的计算方法,促进了能值分析理论在海洋自然资本评估中的进一步应用。  相似文献   

知识产权贸易下的全球地缘科技格局及其演化   总被引：1，自引：3，他引：1  

段德忠  杜德斌  谌颖 《地理研究》2019,38(9):2115-2128

中国崛起,关键在于科技崛起。在当前技术针对与封锁的国际环境中,探讨建构以中国为核心的全球创新网络和科学制定中国未来科技战略是当前创新地理学和地缘政治学交叉之下研究的重点课题,而明晰全球地缘科技格局及其演变过程则是基础所在。本文基于2001—2015年全球国家（地区）间的知识产权进出口数据,在研究全球知识产权贸易格局和网络结构的基础上,探讨了全球地缘科技体系的时空演化特征,得出以下结论：① 全球知识产权贸易网络两极分化显著,是一典型的小世界网络,以美国为核心的金字塔结构不断夯实;② 基于敏感性和脆弱性模型阐释的全球知识产权贸易的非对称依赖性进一步验证了美国是全球地缘科技格局的核心,且其核心地位不断巩固和加强。  相似文献