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This work presents the application of Neural Networks (NNs) techniques to develop an empirical model for foF2, the critical frequency of the F2 ionospheric layer. Prediction of foF2 parameter is important because of the paucity of data in some areas of the globe (due to uneven distributions of ground- based measuring equipment) and, also, because of the roles it plays in high frequency communications and navigation purposes. Hourly values of foF2 corresponding to the period 1985–2007 and obtained from 105 World-wide distributed ionospheric stations, have been considered in this work. A statistical analysis of the difference between NN model-derived and the corresponding experimental foF2 values was carried out for eleven (11) selected ionospheric stations during periods of very high (year 2000), relatively low (year 2006) and very low (year 2008) solar activity. In particular, the ionospheric behavior during the years 2000 and 2008 appeared to be very challenging to be captured for any empirical model. For comparison purposes, the same kind of statistical analysis was performed with foF2 values as obtained from the International Reference Ionosphere (IRI) model and from NeQuick 2 model. Results of the statistical comparison confirmed that the neural network modelling approach is suitable to describe in a climatological way the main characteristics of the ionosphere F2 layer peak frequency, also over the extended geographic region where the additional sensor stations were located. The results obtained also indicate that the NN predictions and NeQuick 2 algorithm driven by solar flux (F10.7) index have similar performance. [ABSTRACT FROM AUTHOR]

• This work presents of a new approach for using altimeter TEC data to study the ionospheric WA. • The outcomes agree with historical results on the locations of WA active regions. • A phenomenon leading to WA events during high solar activity was possibly identified. The ionospheric Winter Anomaly (WA) is a phenomenon discovered almost 100 years ago. It can be defined as the situations in which over a given location the winter ionization level is larger than the summer ionization level, for approximately the same solar activity conditions. Along the decades, a lot of studies have been carried out and relevant theories have been formulated to explain this phenomenon, but some aspects of the nature and physical principles of the WA are still a matter of discussion. The first objective of this work was to introduce a new technique for studying the WA, at any given location over the Oceans, based on the 27-year series of altimeter (TOPEX, Jason1, Jason 2 and Jason3 satellites)-derived Total Electron Content (TEC) data. The second objective of this work was to validate the new technique by presenting a comparative analysis between the results obtained with the new technique and those already published. This goal has been accomplished by studying the WA events over the 50°N and 50°S geographic parallels using the 27-year long altimeter data series. The results have indicated clear and systematic WA occurrences over the Northern Hemisphere and a more complex situation over the Southern Hemisphere, with two regions having different behaviors. These results are in full agreement with the findings published in relevant previous works. The last objective of this work was to present some original findings related to WA events. Taking advantage of the new technique, it has been shown that the WA might be interpreted as a high solar activity period consequence of a yearly periodic phenomenon. This phenomenon is always present, also during low solar activity years, even when WA effects are not visible. During winters over the Southern Hemisphere, it systematically produces (at any solar activity level) low maximum TEC values in the (180°E–360°E) longitudinal region and high maximum TEC values in the (0°E–180°E) longitudinal sector. Precisely, this last sector matches with the well-known region over which WA events are observed for high solar activity. [ABSTRACT FROM AUTHOR]

TOPEX/Poseidon, Jason-1, Jason-2 and Jason-3 altimeter missions have provided 27 + years of uninterrupted Total Electron Content (TEC) measurements since 1992, with unprecedented precision. Nevertheless, the issue of a possible systematic bias in the data was identified immediately after first TOPEX measurements were compared with measurements from other sources. The bias issue has remained open for decades, and it has increased in complexity because each new mission had its different bias. The purpose of this paper is to assess the problem of TEC bias of altimeters. Two approaches have been followed. The first one relied on the TEC data series of the four altimeters to determine inter-mission systematic biases using the last available data versions for each mission. The second approach consisted of inspecting the missions’ official reports to trace changes of the inter-mission and inter-version biases, including biases relative to DORIS ionospheric measurements. Both approaches have converged and resulted in the determination of a reference frame where missions, instruments and ionospheric reference levels could be compared. This reference frame was also used to analyze results published in representative papers during the last decades, including ionospheric data from the ENVISAT mission. This reference frame could help to assess TEC levels of the announced new data version of Jason-2, Jason-3 and the imminent Jason-CS/Sentinel missions. The main conclusion of this work is that Jason-1, ‘E’ data version, defines a TEC reference level which is compatible with most of the results found in the literature. Highlights This contribution provides evidence in favor of TEC data series from Jason-1, ‘E’ version, as the most accurate reference level among altimeter missions and versions. This work tries to assess a long-lasting issue about the existence of systematic biases among ionospheric data series obtained from different satellite altimeters. This work establishes a TEC reference frame where it is possible to ‘map’ already published results and interpret their meanings. This work provides a frame where any possible bias in the upcoming ‘F’ data version series of Jason-2 and Jason-3, and of the imminent Jason-CS/Sentinel-6A mission could be detected. [ABSTRACT FROM AUTHOR]

• This work proposes a different way to study the Winter Anomaly. • This work introduces the concept of equivalent ionization coefficients. • Thecoefficients are site and month dependent. The Winter Anomaly (WA) is an ionospheric phenomenon, particularly related to the F2-layer, that is classically defined as the situation in which the ionization level during winter is higher than during summer for a certain location. This situation is anomalous because it contradicts what would be naturally expected with summer's ionization levels higher than winter ones. This phenomenon has been a matter of study since the early decades of the XX century. This contribution tackles the study of the WA based on long time series (up to two solar cycles) of Total Electron Content (TEC) measurements over a globally distributed network of stations. The work done relies on a threefold strategy including: a classical approach based on the comparison of the winter and summer maximum TEC values, that confirmed the results previously documented; an intermediate approach with maximum TEC values modelled as linear functions of the solar radiation level, that allowed to identify stations where the WA is likely to be observed (and at what solar radiation level) and where is not; a final original approach where the maximum TEC values are modelled using Chapman's function for the seasonal variations, linear regressions for the solar activity dependence and the introduction of site and month dependent equivalent ionization coefficients for the remaining effects. The main conclusions are that the coefficients for winter months seem to depend mainly on geomagnetic latitude, to increase towards high latitudes and to be asymmetric between hemisphere. In accordance to these findings, the occurrences of WA effects would be regulated by proper combinations of these three effects. [ABSTRACT FROM AUTHOR]

Ionospheric estimation is becoming more and more important for the new multifrequency positioning algorithms, since they can help to improve greatly the convergence time for acquiring a good positioning error. In this paper, an open source tool to estimate precise ionospheric estimates is presented, namely ESA UGI (Unified-GNSS-Ionosphere). The presentation is done jointly with a methodology to test ionospheric model using a modified NeQuick to generate synthetic data. The results with different option of the ESA UGI shows that it has a good performance below 1 TECU (Total Electron Content Units) in vTEC (vertical Total Electron Content) RMS (Root Mean Squared) for European networks, around 2 TECU in a well-covered African region and between 1 and 6 TECU globally with this synthetic data. It shows as well the capability of changing between different ionosphere models (voxel, multilayer and spherical harmonics) and configuration options. Finally, a test with uncombined PPP actual data is presented showing that instantaneous convergence below 30 cm in 3D RMS position error are achievable in a well sounded area in Europe. [ABSTRACT FROM AUTHOR]

Abstract The NeQuick 2 has been implemented as a climatological model with the goal to represent the typical median conditions of the ionosphere. Several investigations have been made to test its performances at global and regional scale. In order to provide a 3D specification of the electron density of the ionosphere at a given geographic location and time, specific data ingestion techniques, based on NeQuick 2 adaptation to Total Electron Content (TEC) data, have been introduced. In the present work, the performance of the NeQuick 2 model for climate prediction of TEC has been evaluated in the west African equatorial region. Several types of solar indexes such as the smoothed monthly mean sunspot number (R12), the daily solar radio flux at 10.7 cm wavelengths (F10.7) and the daily sunspot number (Rz) have been used as input parameters for the model. The monthly medians of the modeled vertical TEC (vTEC) have been compared to the corresponding monthly medians of the vTEC obtained from two Global Navigation Satellite System (GNSS) stations located in Côte d'Ivoire. Furthermore, to assess the "weather-like" capabilities of the model, a data ingestion technique has been implemented through the computation of an effective parameter Az at a single GNSS station in Côte d'Ivoire (Yamoussoukro). For geomagnetically disturbed and quiet days, this Az parameter has been used as input for NeQuick to estimate the vertical TEC values in another station (Abidjan) located at about 200 km from the previous one. Additional tests based on the evaluation of Az at the station of Cotonou in Benin allowed analysing the model capability of reconstructing TEC at different distances (up to 850 km from the reference station) according to the north-south and east-west directions of displacement. The statistical analysis has been based on relative frequencies distribution of the difference between reconstructed and the corresponding observed TEC. Highlights • There isn't any substantial degradation in the NeQuick 2 performance with data ingestion technique during disturbed days. • TEC mismodelings are less than 5 TECu within 800 km from the reference station on daytime but up to 20 TECu after sunset. • The worsening of NeQuick 2 performance with data ingestion is more remarkable along north-south than east-west directions. [ABSTRACT FROM AUTHOR]

• Low-latitude topside ionosphere mismodelling made by NeQuick. • Improving of the NeQuick topside representation through COSMIC-1 data. • Different topside tests with different values of the NeQuick topside parameters. This paper deals with the well-known mismodelling characterizing the NeQuick topside ionosphere at low latitudes, i.e., the fact that the model keeps the two electron density humps typical of the equatorial ionization anomaly as the altitude increases from the height of the F2-layer electron density peak, without merging them in a single peak above the geomagnetic equator as expected. This is because the NeQuick topside ionosphere modelling strongly depends on several bottomside ionosphere parameters, which causes an essential coupling between the topside and the bottomside that in many cases behave differently. This means that this kind of topside ionosphere modelling may lead to inaccurate results, as it is the maintenance at low latitudes of the electron density double hump structure in the topside as the altitude increases. On the base of some recently published results, this paper analyzes the role played by the three NeQuick scale height parameters H 0 , g and r in the description of the electron density above the F2 layer peak height. The results of this work pave the way for a possible solution of this low-latitude NeQuick topside ionosphere mismodelling. [ABSTRACT FROM AUTHOR]

Inverse Abel transform is the most common method to invert radio occultation (RO) data in the ionosphere and it is based on the assumption of the spherical symmetry for the electron density distribution in the vicinity of an occultation event. It is understood that this ‘spherical symmetry hypothesis’ could fail, above all, in the presence of strong horizontal electron density gradients. As a consequence, in some cases wrong electron density profiles could be obtained. In this work, in order to incorporate the knowledge of horizontal gradients, we have suggested an inversion technique based on the adaption of the empirical ionospheric model, NeQuick2, to RO-derived TEC. The method relies on the minimization of a cost function involving experimental and model-derived TEC data to determine NeQuick2 input parameters (effective local ionization parameters) at specific locations and times. These parameters are then used to obtain the electron density profile along the tangent point (TP) positions associated with the relevant RO event using NeQuick2. The main focus of our research has been laid on the mitigation of spherical symmetry effects from RO data inversion without using external data such as data from global ionospheric maps (GIM). By using RO data from Constellation Observing System for Meteorology Ionosphere and Climate (FORMOSAT-3/COSMIC) mission and manually scaled peak density data from a network of ionosondes along Asian and American longitudinal sectors, we have obtained a global improvement of 5% with 7% in Asian longitudinal sector (considering the data used in this work), in the retrieval of peak electron density (NmF2) with model-assisted inversion as compared to the Abel inversion. Mean errors of NmF2 in Asian longitudinal sector are calculated to be much higher compared to American sector. [ABSTRACT FROM AUTHOR]

Abstract: The latest version of IRI includes various options for the computation of the topside electron density profile. One of the possible choices is based on NeQuick model. Its inclusion in IRI has been made transferring all the formulations used in NeQuick model. In details, an Epstein layer function is used to describe the electron density profile and the topside shape is controlled by an empirical parameter, connected to the NeQuick F2 bottomside thickness parameter, B2bot. It is computed also in this IRI topside option in order to maintain self-consistency with its original formulation. This paper analyses the possibility of using the IRI bottomside parameters for this option and its impact on the profile and TEC. The case of experimental peak values given as input is also analysed. [Copyright &y& Elsevier]

Abstract: NeQuick ionospheric electron density model produces the full electron density profile in the ionosphere using the F2 layer peak values (foF2 and hmF2) as anchor points. Each part of the profile is modeled using Epstein layer formalism. Simple empirical relations are used to compute the thicknesses of each semi-Epstein layer. It has been observed that when NeQuick model is used to estimate total electron content at low latitudes the modeled values tend to underestimate the observed ones. Beside the F2 peak values, the most important profile parameter is the thickness of the F2 layer bottomside. The present study focuses on NeQuick model behavior at low latitudes comparing modeled profiles parameters with the ones extracted from experimental data mostly from African and Indian sector at different levels of solar activity and different time of the day. Possible model improvements are discussed. [Copyright &y& Elsevier]

Abstract: NeQuick is a three-dimensional and time dependent ionospheric electron density model developed at the Aeronomy and Radiopropagation Laboratory of the Abdus Salam International Centre for Theoretical Physics (ICTP), Trieste, Italy and at the Institute for Geophysics, Astrophysics and Meteorology of the University of Graz, Austria. It is a quick-run model particularly tailored for trans-ionospheric applications that allows one to calculate the electron concentration at any given location in the ionosphere and thus the total electron content (TEC) along any ground-to-satellite ray-path by means of numerical integration. Taking advantage of the increasing amount of available data, the model formulation is continuously updated to improve NeQuick capabilities to provide representations of the ionosphere at global scales. Recently, major changes have been introduced in the model topside formulation and important modifications have also been introduced in the bottomside description. In addition, specific revisions have been applied to the computer package associated to NeQuick in order to improve its computational efficiency. It has therefore been considered appropriate to finalize all the model developments in a new version of the NeQuick. In the present work the main features of NeQuick 2 are illustrated and some results related to validation tests are reported. [Copyright &y& Elsevier]

Abstract: In the presence of electron density gradients the thin shell approximation for the ionosphere, used together with a simple mapping function to convert slant total electron content (TEC) to vertical TEC, could lead to TEC conversion errors. These “mapping function errors” can therefore be used to detect the electron density gradients in the ionosphere. In the present work GPS derived slant TEC data have been used to investigate the effects of the electron density gradients in the middle and low latitude ionosphere under geomagnetic quiet and disturbed conditions. In particular the data corresponding to the geographic area of the American Sector for the days 5–7 April 2000 have been used to perform a complete analysis of mapping function errors based on the “coinciding pierce point technique”. The results clearly illustrate the electron density gradient effects according to the locations considered and to the actual levels of disturbance of the ionosphere. In addition, the possibility to assess an ionospheric shell height able to minimize the mapping function errors has been verified. [Copyright &y& Elsevier]

Highlights: [•] Spherical symmetry hypothesis of Onion-peeling inversion algorithm produces erroneous results. [•] Proposed asymmetry level index provides good indication of global ionospheric asymmetry. [•] A good correlation has been found between asymmetry level index and Onion-peeling related errors (in vertical electron density profiles). [Copyright &y& Elsevier]

Abstract: Electron density obtained by IRI (topside options NeQuick and IRI-Corr) and NeQuick models in their standard versions have been compared with plasma density values measured by F13 and F15 DMSP satellites for years of different solar activities. A statistical study of the differences between modeled and experimental data has been carried out to investigate each model performance. In general, the models present a similar behavior. Nevertheless it has been found that the IRI-Corr topside option of IRI overestimates electron density at high latitudes and this trend increases with the solar activity. A few examples of representative half-orbit passes are given to illustrate the differences found. [Copyright &y& Elsevier]

Abstract: The NeQuick electron density model of the ionosphere is designed for trans-ionospheric propagation applications. The model topside has been revised on the basis of ISIS 2 topside sounder profiles, producing a new formulation of its empirical shape parameter. Comparisons between experimental slant TEC data and values modelled using both versions of NeQuick topside showed that in general we have obtained a distinct improvement. However, during some months of the year and at low latitudes, the new topside formulation does not produce improvements on the slant TEC estimates. We discuss the likely reasons for this behaviour including assessment of merits and shortcomings of the ISIS 2 data in low latitudes. The topside sounder on Intercosmos 19 satellite extensively sounded the equatorial region during a period of high solar activity, which was less covered by ISIS 2. This paper presents comparisons of NeQuick and topside sounders profiles at low latitudes using Intercosmos 19 satellite data. [Copyright &y& Elsevier]

Highlights • Assessment of ionospheric correction models for SBAS applications over African. • The residual error along the IGPs over the region were evaluated models used. • The Bounding errors were as well estimated with the same algorithms. • The models assessed employed NeQuick2 output for case test. Abstract It is a known fact that ionosphere is the largest and the least predictable among the sources of error limiting the reliability and accuracy of Global Navigation Satellite Systems (GNSS) and its regional augmentation systems like Satellite Based Augmentation System (SBAS) in a safety-of-life application. The situation becomes worse in the Equatorial Ionization Anomaly (EIA) region, where the daytime ionization distribution is modified by the fountain effect that develops a crest of electron density at around ±15° to ±20° of the magnetic equator and a trough at the magnetic equator during the local noon hours. Related to this phenomenon is the appearance of ionosphere irregularities and plasma bubbles after local sunset. These may degrade further the quality of service obtained from the GNSS/SBAS system of the said periods. Considering the present operational augmentation systems, the accuracy and integrity of the ionosphere corrections estimate decreases as the level of disturbances increases. In order to provide a correct ionosphere correction to the user of GNSS operating in African EIA region and meet the integrity requirements, a certified ionosphere correction model that accurately characterizes EIA gradient with the full capacity to over-bound the residual error will be needed. An irregularities detector and a decorrelation adaptor are essential in an algorithm usable for African sub-Saharan SBAS operation. The algorithm should be able to cater to the equatorial plasma vertical drifts, diurnal and seasonal variability of the ionosphere electron density and also should take into account the large spatial and temporal gradients in the region. This study presents the assessment of the ionosphere threat model with single and multi-layer algorithm, using modified planar fit and Kriging approaches. [ABSTRACT FROM AUTHOR]

Modelling the Earth's ionospheric characteristics is the focal task for the ionospheric community to mitigate its effect on the radio communication, and satellite navigation. However, several aspects of modelling are still challenging, for example, the storm time characteristics. This paper presents modelling efforts of TEC taking into account solar and geomagnetic activity, time of the day and day of the year using neural networks (NNs) modelling technique. The NNs have been designed with GPS-TEC measured data from low and mid-latitude GPS stations. The training was conducted using the data obtained for the period from 2011 to 2014. The model prediction accuracy was evaluated using data of year 2015. The model results show that diurnal and seasonal trend of the GPS-TEC is well reproduced by the model for the two stations. The seasonal characteristics of GPS-TEC is compared with NN and NeQuick 2 models prediction when the latter one is driven by the monthly average value of solar flux. It is found that NN model performs better than the corresponding NeQuick 2 model for low latitude region. For the mid-latitude both NN and NeQuick 2 models reproduce the average characteristics of TEC variability quite successfully. An attempt of one day ahead forecast of TEC at the two locations has been made by introducing as drivers previous day solar flux and geomagnetic index values. The results show that a reasonable day ahead forecast of local TEC can be achieved. [ABSTRACT FROM AUTHOR]

Using vertical total electron content (VTEC) measurements obtained from GPS satellite signals the capability of the NeQuick 2 and IRI Plas models to predict VTEC over the low latitude and South American sector is analyzed. In the present work both models were used to calculate VTEC up to the height of GPS satellites. Also, comparisons between the performance of IRI Plas and IRI 2007 have been done. The data correspond to June solstice and September equinox 1999 (high solar activity) and they were obtained at nine stations. The considered latitude range extends from 18.4°N to −64.7°N and the longitude ranges from 281.3°E to 295.9°E in the South American sector. The greatest discrepancies among model predictions and the measured VTEC are obtained at low latitudes stations placed in the equatorial anomaly region. Underestimations as strong as 40 TECU [1 TECU = 10 16  m −2 ] can be observed at BOGT station for September equinox, when NeQuick2 model is used. The obtained results also show that: (a) for June solstice, in general the performance of IRI Plas for low latitude stations is better than that of NeQuick2 and, vice versa, for highest latitudes the performance of NeQuick2 is better than that of IRI Plas. For the stations TUCU and SANT both models have good performance; (b) for September equinox the performances of the models do not follow a clearly defined pattern as in the other season. However, it can be seen that for the region placed between the Northern peak and the valley of the equatorial anomaly, in general, the performance of IRI Plas is better than that of NeQuick2 for hours of maximum ionization. From TUCU to the South, the best TEC predictions are given by NeQuick2. The source of the observed deviations of the models has been explored in terms of CCIR foF2 determination in the available ionosonde stations in the region. Discrepancies can be also related to an unrealistic shape of the vertical electron density profile and or an erroneous prediction of the plasmaspheric contribution to the vertical total electron content. Moreover, the results of this study could be suggesting that in the case of NeQuick, the underestimation trend could be due to the lack of a proper plasmaspheric model in its topside representation. In contrast, the plasmaspheric model included in IRI, leads to clear overestimations of GPS derived TEC. [ABSTRACT FROM AUTHOR]

A Satellite Based Augmentation System (SBAS) is designed to improve Global Navigation Satellite Systems (GNSS) in terms of integrity, accuracy, availability and continuity. The main limitation to SBAS performance optimization is the ionosphere, and this is more critical in low latitude. During geomagnetically disturbed periods the role of storm-time winds is important because they modify the atmospheric composition toward low latitudes. An index of ionospheric disturbance, the relative percentage of deviation of the vertical Total Electron Content (TEC) from the quiet level (DvTEC) at each station was evaluated to study positive and negative phases of the geomagnetic storms. The rate of change of TEC index (ROTI) over all the GNSS stations was estimated to evaluate equatorial ionospheric gradients and irregularities. From the study it is observed that the positive deviations are more frequent than negative ones. The availability map, which is the mean of the combine Vertical Protection Level (VPL) and Horizontal Protection Level (HPL) are used for the SBAS performance. The cases of moderate and minor storms studied during the months of July and October 2013 showed that the SBAS system performance during the disturbed periods depends on the local time in which the storm occurs, geographic longitude and other phenomena that need further study. During the storm-time conditions considered, three out of seven geomagnetic storms indicated good SBAS performance and exceed monthly average of the availability map, three geomagnetic storms reduced the system performance below monthly average while one does not have effect on SBAS system performance in respect to monthly average. The present study indicates ROTI as a better proxy than geomagnetic indices for the assessment of storm-time effects on GNSS-SBAS performance. [ABSTRACT FROM AUTHOR]

Global Navigation Satellite Systems (GNSS) have become a powerful tool use in surveying and mapping, air and maritime navigation, ionospheric/space weather research and other applications. However, in some cases, its maximum efficiency could not be attained due to some uncorrelated errors associated with the system measurements, which is caused mainly by the dispersive nature of the ionosphere. Ionosphere has been represented using the total number of electrons along the signal path at a particular height known as Total Electron Content (TEC). However, there are many methods to estimate TEC but the outputs are not uniform, which could be due to the peculiarity in characterizing the biases inside the observables (measurements), and sometimes could be associated to the influence of mapping function. The errors in TEC estimation could lead to wrong conclusion and this could be more critical in case of safety-of-life application. This work investigated the performance of Ciraolo's and Gopi's GNSS-TEC calibration techniques, during 5 geomagnetic quiet and disturbed conditions in the month of October 2013, at the grid points located in low and middle latitudes. The data used are obtained from the GNSS ground-based receivers located at Borriana in Spain (40 $$^{\circ }$$ N, 0 $$^{\circ }$$ E; mid latitude) and Accra in Ghana (5.50 $$^{\circ }$$ N, −0.20 $$^{\circ }$$ E; low latitude). The results of the calibrated TEC are compared with the TEC obtained from European Geostationary Navigation Overlay System Processing Set (EGNOS PS) TEC algorithm, which is considered as a reference data. The TEC derived from Global Ionospheric Maps (GIM) through International GNSS service (IGS) was also examined at the same grid points. The results obtained in this work showed that Ciraolo's calibration technique (a calibration technique based on carrier-phase measurements only) estimates TEC better at middle latitude in comparison to Gopi's technique (a calibration technique based on code and carrier-phase measurements). At the same time, Gopi's calibration was also found more reliable in low latitude than Ciraolo's technique. In addition, the TEC derived from IGS GIM seems to be much reliable in middle-latitude than in low-latitude region. [ABSTRACT FROM AUTHOR]

Investigating the effects of the Equatorial Ionization Anomaly (EIA) ionosphere and space weather on Global Navigation Satellite Systems (GNSS) is very crucial, and a key to successful implementation of a GNSS augmentation system (SBAS) over the equatorial and low-latitude regions. A possible ionospheric vertical delay (GIVD, Grid Ionospheric Vertical Delay) broadcast at a Ionospheric Grid Point (IGP) and its confidence bounds errors (GIVE, Grid Ionospheric Vertical Error) are analyzed and compared with the ionospheric vertical delay estimated at a nearby user location over the West African Sub-Saharan region. Since African sub-Saharan ionosphere falls within the EIA region, which is always characterized by a disturbance in form of irregularities after sunset, and the disturbance is even more during the geomagnetically quiet conditions unlike middle latitudes, the need to have a reliable ionospheric threat model to cater for the nighttime ionospheric plasma irregularities for the future SBAS user is essential. The study was done during the most quiet and disturbed geomagnetic conditions on October 2013. A specific low latitude EGNOS-like algorithm, based on single thin layer model, was engaged to simulate SBAS message in the study. Our preliminary results indicate that, the estimated GIVE detects and protects a potential SBAS user against sampled ionospheric plasma irregularities over the region with a steep increment in GIVE to non-monitored after local sunset to post midnight. This corresponds to the onset of the usual ionospheric plasma irregularities in the region. The results further confirm that the effects of the geomagnetic storms on the ionosphere are not consistent in affecting GNSS applications over the region. Finally, this paper suggests further work to be investigated in order to improve the threat integrity model activity, and thereby enhance the availability of the future SBAS over African sub-Saharan region. [ABSTRACT FROM AUTHOR]

Abstract: This paper examines the performances of NeQuick 2 and IRI-2007 ionospheric empirical models in describing the monthly median characteristics of the equatorial region ionosphere. This is carried out by comparing the vertical total electron content (vTEC) obtained from ground Global Positioning System (GPS) receivers and NmF2 obtained from digisonde with the corresponding values computed using these models. Four years worth of data have been considered in this study. We have shown that the performances of both models are better during the medium solar activity period than low solar activity period. The present investigation has depicted that both models overestimate the observed vTEC during low solar activity period. The modeled and experimental NmF2 has shown good agreement; but the modeled and experimental vTEC has shown significant discrepancy. This discrepancy is shown mainly due to inadequately computed ionospheric slab thicknesses using IRI-2007 and NeQuick 2. However, the chi-square test showed that both models are describing the observed vTEC with 99.9% of confidence level except in the time intervals when the double peaks of modeled vTEC are obtained. In addition, we have shown that both models have a relatively minimal mismodeling during the December solstices. [Copyright &y& Elsevier]

Abstract: It is well known that ionospheric perturbations are characterised by strong horizontal gradients and rapid changes of the ionisation. Thus, space weather induced severe ionosphere perturbations can cause serious technological problems in Global Navigation Satellite Systems (GNSS) such as GPS. During the severe ionosphere storm period of 29–31 October 2003, reported were several significant malfunctions due to the adverse effects of the ionosphere perturbations such as interruption of the WAAS service and degradation of mid-latitudes GPS reference services. To properly warn service users of such effects, a quick evaluation of the current signal propagation conditions expressed in a suitable ionospheric perturbation index would be of great benefit. Preliminary results of a comparative study of ionospheric gradients including vertical sounding and Total Electron Content (TEC) data are presented. Strong enhancements of latitudinal gradients and temporal changes of the ionisation are observed over Europe during the 29–30 October storm period. The potential use of spatial gradients and rate of change of foF2 and TEC characterising the actual perturbation degree of the ionosphere is discussed. It has been found that perturbation induced spatial gradients of TEC and foF2 strongly enhance during the ionospheric storm on 29 October over the Central European region in particular in North–South direction exceeding the gradients in East–West direction by a factor of 2. [Copyright &y& Elsevier]

Abstract: The IRI model offers a choice of options for the computation of the electron density profile and electron content (TEC). Recently new options for the topside electron density profile have been developed, which have a strong impact on TEC. Therefore it is important to test massively the IRI and the new options with experimental data. A large number of permanent stations record dual frequency GPS data from which it is possible to obtain TEC values. Thirty-one worldwide distributed stations have been selected to investigate the capabilities of the IRI to reproduce experimental TEC. Data for years 2000 (high solar activity) and 2004 (medium solar activity) have been analyzed computing modeled values with the IRI-2001 and the IRI-2007-NeQuick topside options. It is found that IRI-2007-NeQuick option generally improves the estimate of the slant TEC, especially in the case of high latitudes stations during high solar activity. [Copyright &y& Elsevier]

Abstract: TEC values obtained from TOPEX satellite were compared with the International Reference Ionosphere (IRI) 2001 model estimates. The present work also shows results of the IRI model with the option of a new topside electron density distribution (NeQuick model). TOPEX TEC measurements, which include years of high and middle to low solar activity (2000 and 2004), were analyzed by binning the region covered by the satellite (±66°) every five degrees of modip. In general, there is good agreement between IRI predictions and Topex measurements. Cases with large disagreements are observed at low and high latitudes during high solar activity. Comparing the model predictions using the default IRI2001 model and the NeQuick topside option show that the default IRI 2001 version represents the observed data in a more realistic way, but appears to be less reliable at high and low latitudes in some cases. [Copyright &y& Elsevier]

Abstract: The topside ionosphere modeling suffers from the relative scarcity of experimental electron density data. Different modeling approaches have been applied in empirical models, like IRI, and profilers, like NeQuick. Comparing ISIS-2 topside electron density profiles with IRI and NeQuick models weak points in their topside formulation are identified and analysed. The IRI topside consists in two constant gradient sections, with a transition height kept at a fixed height in its domain. Tests of transition height and gradient modifications are presented, based on experimental evidences and model efforts. NeQuick model has a topside formulation based on a semi-Epstein layer, governed by an empirical shape parameter k. An alternative formulation is presented based on k values derived from experimental profiles. [Copyright &y& Elsevier]

Dual frequency Global Positioning System (GPS) receivers provide integrated total electron content (TEC) along the ray path (slant TEC, affected by a bias). By inverting this observable, it is possible to obtain the vertical total electron content with some assumptions about the horizontal structure of the ionosphere.The large number of permanent receivers distributed around the world provide enough information to obtain such TEC observables with high spatial and temporal resolutions. Nevertheless, the geometry (mainly vertical) of the ground GPS observations does not allow to solve the vertical structure of electron density of the ionosphere.Mixing different kinds of complementary data in a tomographic context helps to overcome this problem. Several works have obtained successful results achieved by combining occultation and ground GPS data to estimate the local three-dimensional structure of ionospheric electron density. This paper proposes the use of just ground data to obtain similar or better results. To do this, the ground GPS data are mixed with vertical profiles of electron density derived from ionosonde data instead of GPS occultation observations.In this paper, the complementarity between vertical profiles of electron density (estimated using the NeQuick model) and ground GPS data (from GPS IGS permanent network) are shown as well as the performance of the resulting combination. [Copyright &y& Elsevier]

Prolonged exposure of 3T3-L1 adipocytes to micromolar concentrations of H2O2 results in an impaired response to the acute metabolic effects of insulin. In this study, we further characterized the mechanisms by which oxidative stress impairs insulin stimulation of glucose transport activity. Although insulin induced a 2.5-fold increase in plasma membrane GLUT4 content and a 50% reduction in its abundance in the low-density microsomal (LDM) fraction in control cells, oxidation completely prevented these responses. The net effect of insulin on 2-deoxyglucose uptake activity was reduced in oxidized cells and could be attributed to GLUT1 translocation. Insulin stimulation of insulin receptor substrate (IRS) 1 tyrosine phosphorylation and the association of IRS-1 with phosphatidylinositol (PI) 3-kinase were not impaired by oxidative stress. However, a 1.9-fold increase in the LDM content of the p85 subunit of PI 3-kinase after insulin stimulation was observed in control, but not in oxidized, cells. Moreover, although insulin induced an increase in IRS-1-associated PI 3-kinase activity in the LDM in control cells, this effect was prevented by oxidation. These findings suggest that prolonged low-grade oxidative stress impairs insulin-stimulated GLUT4 translocation, potentially by interfering with compartment-specific activation of PI 3-kinase. [ABSTRACT FROM AUTHOR]

In this paper, a neural network based regional ionospheric model is developed using GPS-TEC data from 01 January 2012 to 31 December 2015. For this purpose, nine GPS station TEC data in the time intervals 2012 to 2014 were used to determine model parameters. TEC data obtained in various years and geographical locations which are excluded in the training time are used to validate the performance of the model. For the first case, TEC data from each station in the year 2015 is used to validate the performance of the model. In the second case, GPS observations at Metu, Robe, and Serb stations are used to investigate the model's performance in the year 2012, 2014 and 2013–2014, respectively. In both cases, to validate the accuracy and quality of the model, GPS-TEC values were compared with the predicted TEC. The results indicate that the proposed model can capture most of the spatio-temporal variations of the regional TEC. The present model reproduces the observed hourly TEC with RMS values that lie around 3 to 6.05 TECU at different geographical locations for both one hour and one day ahead prediction. For one day ahead prediction, a comparison of the NN method using NeQuick 2 model outputs with GPS derived measurements have also been conducted. The results indicate that the NN TEC model proposed has a good performance in representing TEC variations compared to climate NeQuick 2 model. • A new regional TEC model based on NN has been developed for the East African region. • The model is able to capture the features of temporal–spatial distribution of the regional TEC. • The regional NN model has demonstrated a good performance compared with global empirical model NeQuick 2 under quiet conditions. [ABSTRACT FROM AUTHOR]