Ion composition measurements and modelling at altitudes from 140 to 350 km using EISCAT measurements

This work aims at processing the data of CP1 and CP2 programs of EISCAT ionospheric radar from 1987 to 1994 using the –full profile method which allows to solve the –temperature-composition ambiguity problem in the lower F region. The program of data analysis was developed in the CEPHAG in 1995–1996. To improve this program, we implemented another analytical function to model the ion composition profile. This new function better reflects the real profile of the composition. Secondly, we chose the best method to select the initial conditions for the –full profile procedure. A statistical analysis of the results was made to obtain the averages of various parameters: electron concentration and temperature, ion temperature, composition and bulk velocity. The aim is to obtain models of the parameter behaviour defining the ion composition profiles: z₅₀ (transition altitude between atomic and molecular ions) and dz (width of the profile), for various seasons and for high and low solar activities. These models are then compared to other models. To explain the principal features of parameters z₅₀ and dz, we made an analysis of the processes leading to composition changes and related them to production and electron density profile. A new experimental model of ion composition is now available.     

On the measurement of global radiation using black and white atmometers

Summary The difference in the evaporation from black and white Livingston atmometers was compared with the global radiation as measured by a Bellani spherical pyranometer. Results show that one unit (cm³) of difference in the evaporation corresponds to 20±3 cal/cm² of global radiation. The total of 95 daily values of radiation measured by pairs of atmometers differed from the corresponding Bellani figure by 2.3%. The operation of the atmometers and the accuracy of the results are discussed. The method is considered sufficiently accurate for general climatological purposes.Zusammenfassung Die Differenz in der Verdunstung, welche von schwarzen und weissen Livingston Atmometern angezeigt wird, wurde mit der Globalstrahlung, gemessen mit einem Bellani Kugelpyranometer, verglichen. Die Ergebnisse lehren, dass die Einheit (cm³) der Differenz in der Verdunstung 20±3 cal/cm² der Globalstrahlung entspricht. Der Unterschied der Summe von 95 Tageswerten der Strahlung, ermittelt auf der Verdunstung von weissen und schwarzen Atmometern, und jener, gemessen mit dem Bellani Kugelpyranometer, betrug 2.3%. Die Handhabung der Atmometer und die Genauigkeit der Resultate wird diskutiert. Die Methode scheint für klimatologische Zwecke hinreichend genau.     

A revised procedure for discharge measurement by means of the salt dilution method

Discharge measurement by means of injection of a NaCl-solution and integration of the electrical conductivity as a function of time is a traditional and well-documented method for use in turbulent streams. The ease of the chemical measurements permits results to be calculated in the field. This work has shown that dry fine-grained salt may substitute the salt solution. Correction factors for water temperature and background conductivity are derived from experiments. A simple procedure for calculation of the discharge is described.     

The relationship between electric fields, conductances and currents in the high-latitude ionosphere: a statistical study using EISCAT data

The relationship between electric fields, height-integrated conductivities and electric currents in the high-latitude nightside electrojet region is known to be complex. The tristatic nature of the EISCAT UHF radar facility provides an excellent means of exploring this interrelationship as it enables simultaneous estimates to be made of the full electric field vector and the ionospheric Hall and Pedersen conductances, further allowing the determination of both field-perpendicular electric current components. Over 1300 h of common programme observations by the UHF radar system provide the basis of a statistical study of electric fields, conductances and currents in the high-latitude ionosphere, from which preliminary results are presented. Times at which there is significant solar contribution to the ionospheric conductances have been excluded by limiting the observations according to solar zenith angle. Initial results indicate that, in general, the times of peak conductance, identified from the entire set of EISCAT observations, do not correspond to the times of the largest electric field values; the relative contribution of ionospheric conductance and electric field to the electrojet currents therefore depends critically on local time, a conclusion which corroborates work by previous authors. Simultaneous measurements confirm a tendency for a decrease in both Hall and Pedersen conductances to be accompanied by an increase in the electric field, at least for moderate and large electric field value, a tendency which is also identified to some extent in the ratio of the conductances, which acts as an indicator of the energy of precipitating particles.     

A New Method for Research on Magnetically Oriented Ionospheric Inhomogeneities using a Program of Radio-System Determination

Geomagnetism and Aeronomy - The work proposes methods for the study of artificial, magnetically oriented, ionosphere inhomogeneities and for its practical implementation. The developed approach...     

Selection of the best fit flood frequency distribution and parameter estimation procedure: a case study for Tasmania in Australia

Selection of a flood frequency distribution and associated parameter estimation procedure is an important step in flood frequency analysis. This is however a difficult task due to problems in selecting the best fit distribution from a large number of candidate distributions and parameter estimation procedures available in the literature. This paper presents a case study with flood data from Tasmania in Australia, which examines four model selection criteria: Akaike Information Criterion (AIC), Akaike Information Criterion—second order variant (AIC_c), Bayesian Information Criterion (BIC) and a modified Anderson–Darling Criterion (ADC). It has been found from the Monte Carlo simulation that ADC is more successful in recognizing the parent distribution correctly than the AIC and BIC when the parent is a three-parameter distribution. On the other hand, AIC and BIC are better in recognizing the parent distribution correctly when the parent is a two-parameter distribution. From the seven different probability distributions examined for Tasmania, it has been found that two-parameter distributions are preferable to three-parameter ones for Tasmania, with Log Normal appears to be the best selection. The paper also evaluates three most widely used parameter estimation procedures for the Log Normal distribution: method of moments (MOM), method of maximum likelihood (MLE) and Bayesian Markov Chain Monte Carlo method (BAY). It has been found that the BAY procedure provides better parameter estimates for the Log Normal distribution, which results in flood quantile estimates with smaller bias and standard error as compared to the MOM and MLE. The findings from this study would be useful in flood frequency analyses in other Australian states and other countries in particular, when selecting an appropriate probability distribution from a number of alternatives.     

Revisiting reservoir storage–yield relationships using a global streamflow database

Annual and monthly streamflows for 729 rivers from a global data set are used to assess the adequacy of five techniques to estimate the relationship between reservoir capacity, target draft (or yield) and reliability of supply. The techniques examined are extended deficit analysis (EDA), behaviour analysis, sequent peak algorithm (SPA), Vogel and Stedinger empirical (lognormal) method and Phien empirical (Gamma) method. In addition, a technique to adjust SPA using annual flows to account for within-year variations is assessed. Of our nine conclusions the key ones are, firstly, EDA is a useful procedure to estimate streamflow deficits and, hence, reservoir capacity for a given reliability of supply. Secondly, the behaviour method is suitable to estimate storage but has limitations if an annual time step is adopted. Thirdly, in contrast to EDA and behaviour which are based on time series of flows, if only annual statistics are available, the Vogel and Stedinger empirical method compares favorably with more detailed simulation approaches.     

Assessment of global TEC mapping using a three-dimensional electron density model

Dual-frequency transmissions from the Global Positioning System satellites can be used to measure and map ionospheric total electron content (TEC) on global scales. Using data exclusively from ground-based GPS networks, global ionosphere mapping has been successfully applied using either two- or three-dimensional techniques. Two-dimensional TEC maps retrieve a horizontally-varying distribution of total electron content, assuming a fixed vertical electron density profile. In three-dimensional mapping, both the horizontal and vertical distribution density are adjusted to fit the data. We describe a three-dimensional TEC mapping algorithm that uses three independent constant-density slabs stacked vertically to model the electron density, and compare with a more conventional two-dimensional approach using a single slab. One apparent benefit of the new method is reduction in a level error of the TEC maps, which decreased by 1.7 TECU using the three-dimensional retrieval on simulated data (1 TEC Unit corresponds to 10¹⁶ electrons/m²). Another benefit of the multilayer approach is improved slant TEC modeling. Using actual data from an equatorial site at Cocos Islands (96.8E, 12.2 S), three slab modeling improved estimates of slant TEC by a factor of 2 for elevation angles between 10 and 20° (9 versus 4.4 TECU, root-mean-square). However, the global structure of the vertical TEC retrievals we analyzed did not improve using three-dimensional modeling. This may be due to a critical approximation shared by both techniques that TEC persists unchanged at a given local time. This assumption is required to produce global maps from observations acquired from widely scattered ground receivers. Further improving the retrieval of global TEC structure with ground-based data probably requires improved dynamical models of TEC behavior. New data available from GPS receivers in low Earth orbit is also promising.     

Assessing uncertainty in refraction seismic traveltime inversion using a global inversion strategy

To analyse and invert refraction seismic travel time data, different approaches and techniques have been proposed. One common approach is to invert first‐break travel times employing local optimization approaches. However, these approaches result in a single velocity model, and it is difficult to assess the quality and to quantify uncertainties and non‐uniqueness of the found solution. To address these problems, we propose an inversion strategy relying on a global optimization approach known as particle swarm optimization. With this approach we generate an ensemble of acceptable velocity models, i.e., models explaining our data equally well. We test and evaluate our approach using synthetic seismic travel times and field data collected across a creeping hillslope in the Austrian Alps. Our synthetic study mimics a layered near‐surface environment, including a sharp velocity increase with depth and complex refractor topography. Analysing the generated ensemble of acceptable solutions using different statistical measures demonstrates that our inversion strategy is able to reconstruct the input velocity model, including reasonable, quantitative estimates of uncertainty. Our field data set is inverted, employing the same strategy, and we further compare our results with the velocity model obtained by a standard local optimization approach and the information from a nearby borehole. This comparison shows that both inversion strategies result in geologically reasonable models (in agreement with the borehole information). However, analysing the model variability of the ensemble generated using our global approach indicates that the result of the local optimization approach is part of this model ensemble. Our results show the benefit of employing a global inversion strategy to generate near‐surface velocity models from refraction seismic data sets, especially in cases where no detailed a priori information regarding subsurface structures and velocity variations is available.     

基于单台加速度记录的混合全局优化HVSR反演场地浅层速度结构

利用水平与竖向谱比（HVSR）方法反演场地速度结构是国际上迅速发展的研究领域.HVSR反演计算实质是一个土层场地模型空间搜索的全局优化问题,当模型搜索空间的复杂程度增大时,目前常用的搜索算法收敛速度慢,计算效率较低.本文实现了一种结合遗传和模拟退火方法优点的混合全局优化HVSR反演算法,通过理论模型和竖向台阵实测数据的检验,表明该算法能获得很好的反演效果,较好地解决了蒙特卡罗方法收敛速度慢,遗传算法收敛早熟和模拟退火算法搜索效率低的问题.本文在此基础上讨论了单台加速度S波记录用于场地速度结构HVSR反演的适用性,为基于单个地震台的地震观测记录反演浅层速度结构提供了一种高效且较为准确的反演方法.

     

Analysis of the positive ionospheric response to a moderate geomagnetic storm using a global numerical model

Current theories of F-layer storms are discussed using numerical simulations with the Upper Atmosphere Model, a global self-consistent, time dependent numerical model of the thermosphere-ionosphere-plasmasphere-magnetosphere system including electrodynamical coupling effects. A case study of a moderate geomagnetic storm at low solar activity during the northern winter solstice exemplifies the complex storm phenomena. The study focuses on positive ionospheric storm effects in relation to thermospheric disturbances in general and thermospheric composition changes in particular. It investigates the dynamical effects of both neutral meridional winds and electric fields caused by the disturbance dynamo effect. The penetration of short-time electric fields of magnetospheric origin during storm intensification phases is shown for the first time in this model study. Comparisons of the calculated thermospheric composition changes with satellite observations of AE-C and ESRO-4 during storm time show a good agreement. The empirical MSISE90 model, however, is less consistent with the simulations. It does not show the equatorward propagation of the disturbances and predicts that they have a gentler latitudinal gradient. Both theoretical and experimental data reveal that although the ratio of [O]/[N₂] at high latitudes decreases significantly during the magnetic storm compared with the quiet time level, at mid to low latitudes it does not increase (at fixed altitudes) above the quiet reference level. Meanwhile, the ionospheric storm is positive there. We conclude that the positive phase of the ionospheric storm is mainly due to uplifting of ionospheric F2-region plasma at mid latitudes and its equatorward movement at low latitudes along geomagnetic field lines caused by large-scale neutral wind circulation and the passage of travelling atmospheric disturbances (TADs). The calculated zonal electric field disturbances also help to create the positive ionospheric disturbances both at middle and low latitudes. Minor contributions arise from the general density enhancement of all constituents during geomagnetic storms, which favours ion production processes above ion losses at fixed height under day-light conditions.     

Damage detection of a thin plate using modal curvature via macrostrain measurement

Plate structures are employed as important structural components in many engineering applications. Hence, assessing the structural conditions of in-service plate structures is critical to monitoring global structural health. Modal curvature-based damage detection techniques have recently garnered considerable attention from the research community, and have become a promising vibration-based structural health monitoring solution. However, computing errors arise when calculating modal curvatures from lateral mode shapes, which result from unavoidable measurement errors in the mode shapes as identified from lateral vibration signals; this makes curvature-based algorithms that use a lateral measurement only theoretically feasible, but practically infeasible. Therefore, in this study, long-gauge fiber Bragg grating strain sensors are employed to obtain a modal curvature without a numerical differentiation procedure in order to circumvent the computing errors. Several damage indices based on modal curvatures that were developed to locate beam damage are employed. Both numerical and experimental studies are performed to validate the proposed approach. However, although previous studies have reported relative success with the application of these damage indices on a simple beam, only one damage index demonstrated the capability to locate damage when the stiffness of the local region changed near the sensor.     

利用掩星数据研究全球对流层顶断裂带变化

应用掩星数据研究全球对流层顶断裂带的变化.介绍了温度递减率定义下的对流层顶的判别原理和算法,并且把由掩星数据判别的对流层顶与探空仪数据获得的对流层顶作比对,验证掩星对流层顶数据的可靠性.进一步给出了通过生成的对流层顶数据获得对流层顶断裂带的原理和算法.选用2006年7月到2008年12月COSMIC掩星数据,分析了南北...     

Absolute measurement ofD by means of a proton magnetometer

In a large coil with vertical axis the current is adjusted so thatZ=0. A smaller coil of a new type with four sets of cylindrical turns is placed with its axis horizontal. ThisD-coil is provided with a telescope pointing at a mark nearly in the magnetic meridian. A proton magnetometer sensor is placed in the common centre of the coils. Two series of readings are taken with theD-coil in the erect and inverted position respectively. Variations ofD, H, F, andD-coil current are recorded. A simple formula gives the mean value ofD.     

Throat aurora observed by DMSP/SSUSI in a global view

Throat aurora was defined based on the ground observations near local noon and has been suggested to be the ground signature of an indentation on the subsolar magnetopause. A global view of the auroral oval with throat aurora will be critical for inferring global processes at the magnetopause, but it has never been achieved. Using imaging spectrograph observations from Defense Meteorological Satellite Program satellites, for the first time, here we show typical throat auroras in a global view and reveal some important observational facts as follows.(1) The throat auroras can be as long as ～8 degrees in latitudinal direction,which is hardly to be fully seen in the ground-based camera.(2) The plasma flows and field aligned currents associated with throat aurora show consistences with previous radar observations, which have been suggested to be the observational evidence of magnetopause reconnection.(3) Most importantly, we confirmed that the electron and ion precipitations associated with throat aurora are always spatially separated, i.e., electrons in the east and ions in the west. The observational results not only establish a new picture of the aurora oval near local noon, but also provide important support to a conceptual model of throat aurora.     

逐步逼近曲化平方法研究

位场资料的常规处理解释方法是建立在平面数据理论之上的,然而实际的位场数据大多为曲面数据,若把曲面数据当成平面数据进行处理,必然导致很大的误差,因此有必要进行“曲化平”处理. 曲化平目前存在的主要问题是计算精度低、计算量大,因此,研究快速、精度高且适合大数据量处理的曲化平方法具有重要的价值. 本文在已有的泰勒级数曲化平方法基础上提出了逐步逼近技术和平均平面技术,使得曲面位场资料处理的精度得到了很大提高. 最后通过理论模型和实际资料的处理验证了该方法的应用效果.     

A regional gravimetric Moho recovery under Tibet using gravitational potential data from a satellite global model

The Moho information under Tibet Plateau is important for a better understanding of the geodynamic processes associated with the continental collision of the Indian and Eurasian tectonic plates and subsequent formation of Himalayan and Tibetan orogens. However, under the central and western parts of Tibet, the existing Moho models are still relatively inaccurate due to a sparse and irregular distribution of seismic surveys. To overcome this problem, the gravimetric data could be used to interpolate the Moho information, where seismic data are missing. In this study, we apply the gravimetric method for a regional Moho recovery under Tibet. Compared to existing methods that use either the gravity or gravity-gradient data, the method presented here utilizes a more generic definition based on a functional relation between the Moho depth and the gravitational potential. Since the gravity and gravity-gradient data have more regional support than the potential field, a numerical test is conducted to find an optimal data area extension that is needed to solve a regional inversion problem in order to reduce errors caused by disregarding the far-zone contribution. Our analysis shows that for the potential field such extension should be at least 25°, while 5° for the gravity and only about 1° for the gravity gradient. The comparison of our gravimetric result with the CRUST1.0 seismic model shows differences at the level of expected accuracy of the gravimetric method of about 5 km and without the presence of significant bias.