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It has been a concern of space scientists and engineers for many decades to have a well-defined proxy to characterize the fluctuations experienced by radio-based signals propagating through the ionosphere. Scintillation indices were deduced using the signal intensity to show the level of fluctuations in phase and amplitude of radio signal passing through the ionosphere. Subsequently, more than two decades now, the rate of change of Total Electron Content (TEC) index (ROTI) was as well used to characterize the level of fluctuations associated to ionospheric irregularities on radio signals. This paper examines the level of relationship between the rate of change of TEC index (ROTI) and the ionospheric amplitude scintillation (S4) within the belt of equatorial ionization anomaly of West African sector. The study was carried out using the Scintillation Network Decision Aid (SCINDA) Global Positioning System (GPS) data obtained from ground-based stations located at Dakar (Senegal) and Cape Verde. The comparative analysis of ROTI and S4 was done with normalized ROTI. The results obtained have shown that the magnitude of normalized ROTI and S4 corresponds very well, higher in Cape Verde compared to Dakar. It was observed that the correlation coefficient could be ∼0.6 when all the satellites are used. However, the relationship increases with individual satellite in which the coefficient of correlation could be up to ∼0.97 and R2 ∼ 0.94. It can be concluded that the relationship between the S4 and ROTI depends on the location and the level of ionospheric disturbance condition as observed in this work. [ABSTRACT FROM AUTHOR]

The response of the ionosphere in the Indian region to the extreme space weather events of 17–19 March 2015 is studied using Global Positioning System (GPS) Total electron Content (TEC) measurements, Thermosphere Ionosphere Electrodynamics-General Circulation Model (TIE-GCM) simulations and High Frequency (HF) Radar observations. The data from Thiruvananthapuram (8.5° N, 77° E geographic, 0.3° N magnetic latitude) and northern hemispheric midlatitude station Hanle (32.80° N, 79° E geographic, 23.9° N magnetic latitude) have been analyzed for the above event. The TEC over Thiruvananthapuram, a dip equatorial location exhibits a decrease during the post sunset hours on 17 March 2015 and is followed by the generation of F region irregularities at an altitude of 600 km, as observed by HF radar as well as TEC scintillations. The TEC over the mid latitude station of Hanle decreased during the storm recovery phase on 18March 2015 in response to negative storm effects. The extent of decrease in TEC as observed during the recovery phase could not be simulated by TIE-GCM over Hanle. This difference is attributed to the enhanced Joule heating taking place in the polar region during St. Patrick's Day storm and the associated composition changes. An anomalous heating scenario was introduced and the simulated TIE-GCM results were found to match the observations. The present study clearly brings out the impact of 17 March 2015, St. Patrick's Day geomagnetic storm on the ionization distribution over equatorial as well as mid latitude stations over the Indian region using observations as well as TIE-GCM simulations and the evolutionary features of ionospheric irregularities over equatorial location using TEC and HF radar observations. The role played by thermosphere ionosphere coupling in modulating the ionization variability over equatorial as well as mid latitude region during periods of extreme geomagnetic forcing is highlighted. [ABSTRACT FROM AUTHOR]

Coherent backscatter radar observations made at the Jicamarca Radio Observatory (JRO) have contributed significantly to our understanding of equatorial F-region irregularities. Radar observations, however, have been made predominantly at the Very-High Frequency (VHF) band (50 MHz), which corresponds to measurements of 3-m field-aligned irregularities. The deployment of the 14-panel version of the Advanced Modular Incoherent Scatter Radar (AMISR-14) at Jicamarca provided an opportunity for observations of Ultra-High Frequency (UHF - 445 MHz) echoes which correspond to measurements of irregularities with 0.34 m scale sizes. Here, we present what we believe to be the first report describing the quiet-time climatology of sub-meter equatorial F-region irregularities derived from UHF radar measurements. The measurements were made between August 2021 and February 2023 using a 10-beam AMISR-14 mode that scanned the F-region in the magnetic equatorial plane. The results show how F-region sub-meter irregularities respond to variations in season and solar flux conditions. The results also confirm, experimentally, that the occurrence of UHF F-region echoes is controlled by the occurrence of equatorial spread F (ESF). Higher occurrence rates were observed during pre-midnight hours and during Equinox and December solstice. Reduced occurrence rates were observed during June solstice. The results show that an increase in solar flux was followed by an increase in the altitude where noticeable occurrence rates (≳ 10%) start and in the maximum altitude of these occurrence rates. The observations also show that occurrence rates lasted longer (in local time) during low solar flux conditions. Comparisons with collocated VHF radar observations showed that, despite differences in radar parameters, observation days, and the scale size (one order of magnitude) of the scattering irregularities, the two systems show similar climatological variations with only minor differences in the absolute occurrence rates. Finally, the analysis of the occurrence rates for different beams did not show substantial climatological variations over local (within a few 100s of km) zonal distances around JRO. We point out, however, that observations on a single day can show strong local variations in echo detection and intensity within the AMISR-14 field of view due to the intrinsic development and decay of ESF structures. • We report for the first time the climatology of nighttime F-region echoes observed by a UHF radar at the magnetic equator. • We present the response of the occurrence rate of UHF echoes to variations in season and solar flux that are related to ESF. • We show that the climatology of irregularities from AMISR-14 is similar to that observed by the collocated VHF JULIA radar. • Analysis of the 10-beam observations confirms that the ESF echo climatology does not change within local zonal distances. [ABSTRACT FROM AUTHOR]

The presence of large and small-scale irregularities TID of the ionosphere in the path of radio wave propagation from the navigation satellite to the GPS receiver is the main challenge for the application of ionospheric research to improve positioning accuracy using single-frequency receivers. In this paper, in contrast to the traditional methods of compensating for the error associated with the ionospheric delay (the Klobuchar method and the use of TEC values), we propose the calculation of the ionospheric delay by the ray tracing method in a model that is as close as possible to the real conditions. To achieve this proximity, a model is proposed that includes various types of TID irregularities. The corresponding algorithms are presented and the numerical results are given. It is shown that single large-scale TIDs can lead to errors of 10–30 m, errors due to small-scale TIDs do not exceed 2 m. © 2001 Elsevier Science. All rights reserved. • Positioning systems are highly dependent on the state of the ionosphere. • Ionospheric delay is a major source of positioning errors. • The greatest influence is exerted by irregularities. • A method is proposed for direct calculation of delays in a model with irregularities. • The core effect is influence of single large-scale irregularities. [ABSTRACT FROM AUTHOR]

• Equatorial F-region irregularities (EFI) occurred within ± 22° magnetic latitudes. • Severe EFI are most probable in equinoxes and around the crest region. • Moderate EFI often occur in solstices, around the equator/poleward edge of crest. Trans-ionospheric signals such as the Global Positioning System (GPS) signals propagating through irregular ionospheric plasma structures may experience phase variation and amplitude fades causing degradation in system performance. This paper investigates the latitudinal structure of equatorial F-region irregularities (EFI) in the African region at different seasons using a GPS-based proxy index (rate of change of TEC index (ROTI)). The results obtained indicate the dependence of EFI on local time, season and latitude. Its occurrence time ranges between 19:00 and 0:00LT depending on the season. Its appearance is earlier in equinoxes, particularly in March equinox, and late in June solstice. Further, its occurrence lies generally within ± 22° magnetic latitudes for all seasons. The probability of occurrence, which is observed to be generally higher in equinoxes than solstices, is almost comparable across the entire latitudes where it was observed in equinoxes while the observations in solstices indicate a dip around the magnetic equator. Furthermore, the average seasonal value of ROTI indicates a clear latitudinal dependence; with appearance within ± (0°-12°) magnetic latitude in solstices while it extends up to 18° in equinoxes, particularly in the south. While the hemispheric asymmetry in the average seasonal values is favored by the hemispheric asymmetry of latitudinal TEC profile, the TEC gradient could help delineate the latitude of its peak occurrence. In addition, severe irregularities are mostly observed in equinoxes than in solstices across all latitudes of occurrence and are most pronounced around the crest region mainly in the south. Conversely, moderate irregularities are typically observed in solstices and are most frequent around the magnetic equator and the poleward edge of equatorial ionization anomaly (EIA) region in the northern hemisphere. [ABSTRACT FROM AUTHOR]

This study investigates the quiet-time and storm-time variations of the African equatorial and low latitude ionosphere during 2009–2015. We obtained TEC observation data from four Global Navigation Satellite System (GNSS) stations over the African equatorial and low latitude, namely: Asab (ASAB, geographic coordinates 13.0°N, 42.65°E; geomagnetic coordinates 4.85°N, 114.34°E), Arba Minch University, (ARMI, 6.03°N, 37.56°E; 3.06°S, 109.29°E), Yamoussoukro (YKRO, 6.87°N, 5.24oW; 2.84°S, 67.41°E) and Mbarara (MBAR, 0.60°S, 30.74°E; 10.27°S, 102.36°E). Seven selected major Geomagnetic Storms (GSs) over solar cycle 24 were investigated and the responses of TEC and Rate of change of TEC Index (ROTI) as proxy for ionospheric irregularities to the storms were also studied. We recorded weaker ionospheric plasma density distributions around the magnetic equator (trough) than at the inner flank of the Equatorial Ionization Anomaly (EIA) crests: A clear evidence of EIA formation. Furthermore, hemispherical asymmetry was observed in the latitudinal distribution of TEC, with consistently higher distribution in the Northern Hemisphere (NH) than Southern Hemisphere (SH). TEC and ionospheric irregularities showed clear dependence on solar activity. Negative responses dominated GSs that occurred at night-time and positive effects for those that occurred during the daytime. The enhancement or inhibition of irregularities during GS is strongly dependent on the time of GS unfolding. As previously established, the daily maximum distribution of TEC at 1400–1600 LT and minimum at 0400–0600 LT were validated by this study, using long time (7 years) data. Seasonally, TEC showed semi-annual pattern of variations, with highest values in equinoxes, and lowest in solstices. [ABSTRACT FROM AUTHOR]

Abstract This paper investigates the plasma irregularities at different longitudes in the month of March 2015; a period that consists of both quiet and disturbed geomagnetic conditions. The average rate of change of TEC index (ROTI ave), derived from Global Positioning System (GPS) measurements obtained at South America, Africa, Asia and Oceania equatorial regions, was used as indicator. The observations revealed significant longitudinal differences for both quiet and disturbed conditions. The quiet-time observations indicate that irregularities were most frequent in the American and African sectors, it is rarely observed in the Asian sector and mostly absent in the Oceania longitudes. The strength is however observed to decrease eastward i.e. it is most prominent in the American sector (up to ∼1.6 TECU/min.) and absent in the Oceania longitudes. The results of the investigation of the 17 March, 2015 storm event revealed that the storm appeared not to hinder the development of irregularities in all the stations in the America sector during the night following the main phase. However, significant longitudinal variation is observed within the sector on the first night following the storm's recovery. In the African sector, the storm inhibits the development of irregularities in all the stations during the storm days considered: a development that is fundamentally different from the America sector. Generally, no significant storm effect is observed in the Asian and Oceania stations considered. The storm-time longitudinal variations of irregularities have been partly attributed to the storm timing and significant longitudinal difference in the action of storm-induced related drivers. Highlights • The magnitude of irregularities decreases from eastward for quiet-time condition. • Notable longitudinal variations of irregularities during the 17 March 2015 storm. • The storm-induced drivers and storm timing play major roles during the storm event. [ABSTRACT FROM AUTHOR]

The increased analytical sensitivity of capillary electrophoresis detects additional irregularities that are suspicious for a monoclonal component. This is most noticeable in the beta-1-, beta-2- and gamma-globulin fractions. The causes of non-monoclonal irregularities are manifold, but are rarely reported back to the ordering physician. This article reviews the basic concepts to correctly identify irregularities, monoclonal and oligoclonal peaks by capillary electrophoresis. It then focuses on detecting and reporting typical non-monoclonal irregularities according to their electrophoresis fractions as well as their possible clinical implications. [ABSTRACT FROM AUTHOR]

The quality of climate data is of extreme relevance, since these data are used in many different contexts. However, few climate time series are free from non-natural irregularities. These inhomogeneities are related to the process of collecting, digitising, processing, transferring, storing and transmitting climate data series. For instance, they can be caused by changes of measuring instrumentation, observing practices or relocation of weather stations. In order to avoid errors and bias in the results of analysis that use those data, it is particularly important to detect and remove those non-natural irregularities prior to their use. Moreover, due to the increase of storage capacity, the recent gathering of massive amounts of weather data implies also a toilsome effort to guarantee its quality. The process of detection and correction of irregularities is named homogenisation. A comprehensive summary and description of the available homogenisation methods is critical to climatologists and other experts, who are looking for a homogenisation method wholly considered as the best. The effectiveness of homogenisation methods depends on the type, temporal resolution and spatial variability of the climatic variable. Several comparison studies have been published so far. However, due to the absence of time series where irregularities are known, only a few of those comparisons indicate the level of success of the homogenisation methods. This article reviews the characteristics of the most important procedures used in the homogenisation of climatic variables based on a thorough literature research. It also summarises many methods applications in order to illustrate their applicability, which may help climatologists and other experts to identify adequate method(s) for their particular needs. This review study also describes comparison studies, which evaluated the efficiency of homogenisation methods, and provides a summary of conclusions and lessons learned regarding good practices for the use of homogenisation methods. [ABSTRACT FROM AUTHOR]

The strength and dilation of rock joints in the field cannot be evaluated solely on the basis of parameters scaled from laboratory data, but also requires assessment of large-scale irregularities not present in the laboratory sample. A constitutive model for rock joints has been developed that considers the dilation and strength along both small-scale joint roughness scaled from laboratory data, and large-scale waviness determined from geologic observations. The model’s performance is illustrated by providing its correlation with experimental results taken from literature. The degradation in dilation and post-peak strength along small-scale irregularities is modeled using the plastic work done in shear, and the degradation along large-scale irregularities is modeled using a sinusoidal function. A dimensionless product of plastic work, rock strength, and wavelength of irregularities has been developed which fits the direct shear test results. An approach to scaling shear strength and shear displacement from laboratory to field-scale is also suggested. [ABSTRACT FROM AUTHOR]

The use of the X-ray technique and the X-ray densitometry to determine pellet particle distribution and to understand the biomass compaction and its effects in pellet properties has been limited. The present work evaluates the quality of pellets manufactured with several lignocellulosic materials by using X-ray photography for studying surface cracks and irregularities, and by using X-ray densitometry to evaluate density and its variation in longitudinal and transversal directions. Density values and their variation were correlated to the pellets' mechanical properties (mechanical durability and compression resistance). It was found that X-ray photography may be applied to evaluate the presence of cracks and irregularities in the pellets' surface; however, these are not indicators of pellet durability or compression resistance. Moreover, density evaluation by the X-ray densitometry technique allowed the determination of the pellets' mechanical resistance and durability. A negative correlation was observed between the force at break and the coefficient of variation of density. No correlation was found between the mechanical durability and the average density or its variation. According to the above results, X-ray technique can be utilized to study the pellet quality. [ABSTRACT FROM AUTHOR]

This paper presents a new technique based on a collocation method using cubic splines for second order elliptic equation with irregularities in one dimension and two dimensions. The differential equation is first collocated at the two smooth sub domains divided by the interface. We extend the sub domains from the interior of the domain and then the scheme at the interface is developed by patching them up. The scheme obtained gives the second order accurate solution at the interface as well as at the regular points. Second order accuracy for the approximations of the first order and the second order derivative of the solution can also be seen from the experiments performed. Numerical experiments for 2D problems also demonstrate the second order accuracy of the present scheme for the solution u and the derivatives u x , u x x and the mixed derivative u x y. The approach to derive the interface relations, established in this paper for elliptic interface problems, can be helpful to derive high order accurate numerical methods. Numerical tests exhibit the super convergent properties of the scheme. [ABSTRACT FROM AUTHOR]

Abstract: The plasma densities detected by the Planar Langmuir Probe (PLP) on board the Communication/Navigation Outage Forecasting System (C/NOFS) satellite have been analyzed in detail from launch in April 2008 through December 2009 period. The initial period just after launch which corresponds to one of the quietest periods in the space era, exhibited broad plasma decreases (BPDs) which we have reported previously. In order to treat the data quantitatively, we have detrended the PLP data by using the International Reference Ionosphere (IRI) model to remove variations in density due to changes in spacecraft altitude and latitude along the orbit. In this paper we present results of a statistical analysis of C/NOFS detrended plasma densities as well as neutral densities from the Gravity Recovery and Climate Experiment (GRACE) measured during the same period. The results show periodic structures in both charged and neutral species, most evident in limited local times. These structures resemble wave-4 nonmigrating tides. BPDs persist in the detrended data and appear as one of the minima in the longitudinal signatures, most strikingly on the nightside. Statistical averages of meridional velocities from the Ion Velocity Meter (IVM), also on C/NOFS, during the same period shows that the locations of BPDs coincide with measurements of large downward-directed velocities. [Copyright &y& Elsevier]

Abstract: The objective of this work was to investigate the fatigue performance of a 95 mm 2 copper conductor. The compacting procedure applied during the manufacturing process causes geometrical irregularities in the wires. Specimens from different layers have been tested in tension-tension mode with a stress-ratio, R = 0.1. The irregularities have been measured and through numerical studies applied to assess the resulting bending stresses in tension loading. When this is accounted for, the fatigue behavior of the core and the outer layer seems to be similar. This indicates that the difference in fatigue data observed between different layers of wires can be explained by surface irregularities. However, this needs to be supported by more data. [Copyright &y& Elsevier]

Abstract: Due to resource constraints, securities regulators cannot find or punish all firms that have conducted irregular or even illegal activities (hereafter referred to as fraud). Those who study securities regulations can only find the instances of fraud that have been punished, not those that have not been punished, and it is these unknown cases that would make the best control sample for studies of enforcement action criteria. China’s mandatory management earnings forecasts solve this sampling problem. In the A-share market, firms that have not forecasted as mandated are likely in a position to be punished by securities regulators or are attempting to escape punishment, and their identification allows researchers to build suitable study and control samples when examining securities regulations. Our results indicate that enforcement actions taken by securities regulators are selective. The probability that a firm will be punished for irregular management forecasting is significantly related to proxies for survival rates. Specifically, fraudulent firms with lower return on assets (ROAs) or higher cash flow risk are more likely to be punished. Further analysis shows that selective enforcement of regulations has had little positive effect on the quality of listed firms’ management forecasts. [Copyright &y& Elsevier]

Abstract: This study employs a digisonde and a GPS receiver at Wuhan (30.5°N, 114.4°E), China to observe spread F, GPS phase fluctuations, and medium-scale traveling ionospheric disturbances (MSTIDs) at nighttime in 2000. The MSTIDs are derived from the perturbations of total electron content (TEC). In this study, we explored the seasonal and nighttime occurrence rates as well as the one-to-one correspondences for these phenomena. The results show that, for the seasonal variations, three phenomena all are highly active in summer, which confirms that irregularities over Wuhan mainly relate to MSTIDs. Moreover, all spread F types have a minor occurrence peak in winter but none for the MSTIDs and the GPS phase fluctuations. Besides, none of large GPS phase fluctuations event had occurred during observation periods, which indicates that the strength of irregularities related to MSTIDs are weaker than that of equatorial plasma bubbles. For the nighttime variations, the frequency spread F occurs earlier than other spread F types which implies that disturbances start at the altitude near the F-region peak. For the one-to-one correspondences, the occurrence rates of each spread F type and GPS phase fluctuations are low during MSTIDs occurred. This implies that smaller scale irregularities which are observed as spread F or GPS phase fluctuations may only appear in some phase of MSTIDs. [Copyright &y& Elsevier]

Abstract: The probability of occurrence of spread-F can be modeled and predicted using neural networks (NNs). This paper presents a feasibility study into the development of a NN based model for the prediction of the probability of occurrence of spread-F over selected equatorial stations within the Brazilian sector. The input space included the day number (seasonal variation), hour (diurnal variation), sunspot number (measure of the solar activity), magnetic index (measure of the magnetic activity) and magnetic position. Twelve years of spread-F data from Brazil (covering the period 1978–1989) measured at the equatorial site Fortaleza (3.9°S, 38.45°W) and low latitude site Cachoeira Paulista (22.6°S, 45.0°W) are used in the development of an input space and NN architecture for the model. Spread-F data that is believed to be related to plasma bubble developments (range spread-F) was used in the development of the model. The model results show the probability of spread-F occurrence as a function of local time, season and latitude. Results from the Brazilian Sector NN (BSNN) based model are presented in this paper, as well as a comparative analysis with a Brazilian model developed for the same purpose. [Copyright &y& Elsevier]

Abstract: Discussions continue on uplift/subsidence in Denmark/southern Scandinavia. The patterns in time scales of 10s, 100s and 1000s of years show some similarities and some differences. Geodetic observations exist for all of Scandinavia. Those describe the phenomenon in 10–100s of years scale. Earthquake observations in seismology are of relevance to the same time scales. Geological studies of dated shore lines describe the postglacial earth-surface motion in a quite different time scale of 100–1000s of years. There is a need for integration of these observations geographically. Present integration of the data of various time scales is that the overall uplift pattern of all time scales 10s, 100s, 1000s and 10,000s years are coincident. But a few geodynamics irregularities are found. Further observations in the various time scales will be collected together with process descriptions of the observed similarities and differences in the DynaQlim project. [Copyright &y& Elsevier]

Abstract: We present a statistical study of the diurnal variations of the E-region backscatter occurrence observed by 15 SuperDARN radars over 3 years. The diurnal variation of echo occurrence observed by each radar is examined for three levels of geomagnetic activity. It is found that the E-region echo occurrence for individual radars exhibits very similar diurnal variations for low geomagnetic activity, with the differences between radars increasing with activity. This indicates that local processes become more important in the generation of the E-region echoes during disturbed periods. It is also shown that the majority of E-region backscatter structures take the form of diffuse echo bands rather than discrete echo patches, with the dominance of diffuse echoes increasing with activity. Finally, a hypothesis that the echo occurrence is controlled by the radar''s coverage of the auroral oval is tested using the electron and ion oval models derived from the DMSP satellite observations. [Copyright &y& Elsevier]

Scintillations are caused by ionospheric irregularities and can affect the propagation of trans-ionospheric radio signals. One way to understand and predict the impact of such irregularities on Global Navigation Satellite System (GNSS) signals is through the spatial/temporal characterization of the scintillation's climatology during different phases of a solar cycle covering different latitudes and longitudes. This characterization is performed using amplitude scintillation index S 4 , during the full solar cycle 24, in the South American (SA) sector. The investigation considers the diurnal, daily, and seasonal variation of S 4 index for climatological purpose, and the goal of this study is to investigate the scintillations covering a large spatial scale during the full solar cycle 24. The characterization shows a latitudinal asymmetry, whereas at the south, the scintillations were more frequent and their peak was more distant from the magnetic equator, which can be attributed by the South Atlantic Magnetic Anomaly (SAMA), and/or by the transequatorial meridional neutral winds. It also shows a longitudinal asymmetry, where the scintillations at the eastern sector occurred between November and February, while at the western sector, they occurred during the months of October, November, February and March, which can be attributed to the difference between the magnetic and geographic equators. The occurrence of scintillations during two distinct geomagnetic storms with similar storm time in the SA sector is also presented. • During solar minimum, weak scintillations occur in a region of ∼10°, centered at 15°-20° N/S, mainly during the equinoxes. • During solar maximum, intense scintillations have a coverage of ∼30° centered at 20°-30° N/S, during equinoxes and summer. • Ionospheric scintillations are more frequent at the southern crest of Equatorial Ionization Anomaly. • Ionospheric scintillations at the easter sector of South America (SA) occurred between November and February. • Ionospheric scintillations at the western sector of SA occurred during the months of October, November, February and March. [ABSTRACT FROM AUTHOR]

Abstract: The effects of geomagnetic storm on GPS ionospheric scintillations are studied here using GPS scintillation data recorded at Sanya (18.3°N, 109.5°E; geomagnetic: 7.6°N, 180.8°E), the southmost station in the Chinese longitude region. GPS scintillation/TEC and DMSP data are utilized to show the development of irregularities during the period year 2005 (solar minimum). Statistical analysis of K planetary index (Kp) and amplitude scintillation index (S4) indicates that most storms of the year did not trigger the scintillation occurrence at Sanya. However, cases of scintillation occurring during moderate and strong storm (Dst<−100) periods show clearly that the development of irregularities producing scintillations can be triggered by geomagnetic storms during the low scintillation occurrence season. The effects (trigger or not trigger/inhibit) depend on the maximum dDst/dt determined local time sector, and can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance electric fields. For station Sanya, the maximum dDst/dt determined local time is near the noon (or post-midnight) sector for most storms of the year 2005, which inhibited (or did not trigger) the post-sunset (or post-midnight) scintillation occurrence and then led to the phenomena that the statistical results presented. [Copyright &y& Elsevier]

Abstract: Ionospheric scintillation results from a single frequency global positioning system (GPS) receiver have been presented in this paper. Ionospheric scintillation is rapid variation in the amplitude and phase of radio signals caused by irregularities in the ionosphere. Ionosphere contains large amplitude variations over spatial scales from few cm to 100s of km. It is observed that VHF–UHF communications as well as automated navigation and precision positioning via GPS are affected by scintillations. Scintillations do not have major effects on mid latitude regions, but low latitude scintillations are the greatest cause of GPS position errors. In the present work, we study the effect of ionospheric scintillations on GPS signal at low latitude station, Chiang Rai, Thailand. The data were analyzed from January 2001 to December 2001. Results show that scintillation has a significant problem at this latitude and position error increases during active scintillation condition, which causes loss of lock on several satellites. [Copyright &y& Elsevier]

Abstract: The aim of this study is to simulate the dynamic vertical response of a vehicle traversing rigid rails and a railway bridge. This is achieved by using the authors''wheel-rail contact element (WRC) to model the dynamic interaction that exists between a sprung wheel, using a Hertzian spring, and the rail. The objective in creating these elements was to model the rail and wheel irregularities, which was not a feature of the contact elements within the ANSYS finite element program. In this paper the numerical results generated using the authors'' WRC element are identical to the results generated using the commercial contact element of ANSYS for a smooth rail condition. In the case of irregular rails, the numerical results generated using the authors'' WRC elements compare very favourably with the results from the literature. [Copyright &y& Elsevier]

Abstract: Scintillation measurements of the 150MHz NNSS beacon satellites at two midlatitude locations spaced approximately 400km east–west revealed that as the east–west zenith angle increased the scintillation index decreased. The explanation of this observation is that the F region irregularities are in the form of (approximately) north–south magnetically field aligned sheets. The likely formation mechanism of the irregularities probably involves the Perkins instability. [Copyright &y& Elsevier]

A network of global positioning system based on single (L1) frequency receivers has been installed in Brazil for the purpose of monitoring ionospheric L-band scintillation. In this paper, we study the characteristics of L-band amplitude scintillations at two different location in Brazil, namely Sao Luis, located at magnetic equator, and Sao Jose dos Campos located under the equatorial anomaly peak. The detailed results are presented for data collected between October 2001 and January 2002. We generally found that the percentage occurrence of the scintillation activity increases from the magnetic equator to the equatorial anomaly peaks. [Copyright &y& Elsevier]

It is common knowledge that the night-time ionosphere at the middle and moderately high latitudes is sustained by the charged particle flux arriving from plasmasphere along magnetic field lines. The downward directed particle velocity in this flux decreases with the decreasing height. It provides a potential source for plasma instability. This should be interpreted in such a way that the initial density perturbations increase with the time, and are enhanced as they propagate in space. In this paper we have carried out investigations of topside ionosphere plasma stability on the basis of solving the dispersion equation for low-frequency waves in a weakly inhomogeneous medium. Also analysis have been made of propagation of the waves within ray approximation. Is shown, that of irregularities, extending down accrue on intensity up to heights of a maximum of layer F2, and are loss in low layers of ionosphere. [Copyright &y& Elsevier]

The observations of meter-scale, field-aligned irregularities made at the Chung-Li VHF radar are presented. Applying radar interferometry, the temporal and spatial variations of backscatter positions show the appearance of quasi-periodic (QP) drift motions, with wavelengths of about 10–15 km, and wave frequencies close to the Brunt–Vaisala frequency. This periodicity also reflects the so-called QP echoes shown in the range-time-intensity (RTI) plots. In-beam images of scatters are also analyzed in this paper, results show that the scattering structures are mainly organized along wave fronts. But, in some cases, the quasi-point target scatters are responsible for the striations in the RTI plots. [Copyright &y& Elsevier]

The ionosphere often becomes turbulent and develops electron density irregularities. These irregularities scatter radio waves to cause amplitude and phase scintillation and affect satellite communication and GPS navigation systems. The effects are most intense in the equatorial region, moderate at high latitudes and minimum at middle latitudes. The thermosphere and the ionosphere seem to internally control the generation of irregularities in the equatorial region and its forcing by solar transients is an additional modulating factor. On the other hand, the irregularity generation mechanisms in the high-latitude ionosphere seem to be driven by magnetospheric processes and, therefore, high-latitude scintillations can be tracked by following the trail of energy from the sun in the form of solar flares and coronal mass ejections. The development of a global specification and forecast system for scintillation is needed in view of our increased reliance on space-based communication and navigation systems, which are vulnerable to ionospheric scintillation. Such scintillation specification systems are being developed for the equatorial region. An equatorial satellite equipped with an appropriate suite of sensors, capable of detecting ionospheric irregularities and tracking the drivers that control the formation of ionospheric irregularities, has also been planned for the purpose of specifying and forecasting equatorial scintillations. In the polar region, scintillation specification and forecast systems are yet to emerge although modeling and observations of polar cap plasma structures, their convection and associated irregularities have advanced greatly in recent years. Global scintillation observations made during the S-RAMP Space Weather Month in September 1999 are currently being analyzed to study the effects of magnetic storms on communication and navigation systems. [Copyright &y& Elsevier]

During the International Equatorial Electrojet Year (1993–1994), irregularities have been observed with a multi-frequency zenithal HF radar in the whole F region from 130 up to 300 km of the daytime equatorial ionosphere. They appear as amplitude fluctuations with time scales increasing from few seconds in the lower F1 region up to tens of seconds in the upper F2 region. Fluctuations are specially strong in the F1 region, their amplitude decreases with increasing altitudes. F region irregularities are kilometric, they are observed simultaneously with E region type II irregularities, associated with gradient drift processes, but there is no coherency between irregularities in the E and F regions. Doppler spectra and echolocation indicate oblique scattering from east–west direction. Measurements show the existence of an altitude gradient in the horizontal velocities, varying from 150 m/s at 142 km to 190 m/s at 236 km in the F region. Some mechanisms, already proposed to explain equatorial irregularities are discussed as possible mechanisms at the origin of the observed irregularities, they are however concluded unsuitable. The irregularity origin remains then not understood, suggesting that the electrojet structure could be more complex than expected. [Copyright &y& Elsevier]

This paper discusses the physical nature of neutral wind-induced processes. A simple “pictorial” explanation of the neutral-wind-driven gradient drift and thermal instabilities, originally suggested by Kagan and Kelley (Geophys. Res. Lett. 25 (1998) 4141; J. Geophys. Res. 105 (2000) 5291), is developed for the first time. A formula for the growth rate of a neutral-wind-driven gradient drift instability applicable for an arbitrary latitude is presented. The effectiveness of the neutral wind approach to the problem of midlatitude backscatter in the ionospheric E region is discussed. The paper concludes that there are cases when one may experimentally distinguish between electric field- and neutral wind-driven processes and shows that the backscatter character is defined by either the neutral behavior or the structure of ionization clouds but not by the gradient drift process itself. [Copyright &y& Elsevier]

This paper is concerned with short lived (<2 s) meteor trails which produced VHF radar echoes, spread in range >10 km. The Doppler spectrum of these echoes in each range bin is very broad and asymmetric with many discrete peaks. Spatial domain Interferometer observations of Spread Meteor Echoes were made at the Indian National MST Radar Facility to locate the echoing regions on the trail producing the discrete peaks in the broad Doppler spectra of each range bin. We report that the mean direction of arrival was found to be similar for all Doppler frequencies in each range bin and varies for successive bins by much less than one degree. The echoing regions on the trail are located nearly parallel to the radar beam direction. Further, the directions of arrival were found to be not necessarily normal to the geomagnetic field. The observations are discussed in terms of ionization structures developed within the meteor trail. [Copyright &y& Elsevier]

This paper investigates the seismic performance of steel moment resisting frames, exploring the potential for energy dissipation not only in the beams but in the beam-to-column joints too. Such an approach practically doubles the possible locations for dissipative zones in the structure but on the other hand places requirements for ductile behaviour on the connections. A design methodology that facilitates the formation of plastic hinges in the beam-to-column joints and the beams at the same time, is presented and examined in comparison to typically designed frames with rigid joints. A number of 24, 2D regular and irregular frame configurations are studied numerically, employing both nonlinear static and dynamic analyses. The examined configurations involve either irregular mass distribution across the floors or stiffness discontinuities of the framing across the floors and the bays. The results indicate a great reduction in ductility demands for the frames with energy dissipation in the beams and the joints concurrently, while other metrics of their seismic response, such as interstorey drifts and lateral capacity remain practically unaffected. • Frames with joint energy dissipation drastically drop their ductility demands. • Dissipation zones in beams and joints together is an effective seismic design. • Total hysteresis is reduced when joint energy dissipation occurs. • Irregularities did not affect negatively the benefits. [ABSTRACT FROM AUTHOR]