Tomographic reconstruction of the ionospheric electron density as a function of space and time

Electron density distribution is the major determining parameter of the ionosphere. Computerized Ionospheric Tomography (CIT) is a method to reconstruct ionospheric electron density image by computing Total Electron Content (TEC) values from the recorded Global Positioning Satellite System (GPS) signals. Due to the multi-scale variability of the ionosphere and inherent biases and errors in the computation of TEC, CIT constitutes an underdetermined ill-posed inverse problem. In this study, a novel Singular Value Decomposition (SVD) based CIT reconstruction technique is proposed for the imaging of electron density in both space (latitude, longitude, altitude) and time. The underlying model is obtained from International Reference Ionosphere (IRI) and the necessary measurements are obtained from earth based and satellite based GPS recordings. Based on the IRI-2007 model, a basis is formed by SVD for the required location and the time of interest. Selecting the first few basis vectors corresponding to the most significant singular values, the 3-D CIT is formulated as a weighted least squares estimation problem of the basis coefficients. By providing significant regularization to the tomographic inversion problem with limited projections, the proposed technique provides robust and reliable 3-D reconstructions of ionospheric electron density.     

Testing the improvement of performance of the IRI model in the estimation of TEC over the mid-latitude American regions

This paper mainly discusses the improvement of performance of the International Reference Ionosphere (IRI) model in estimating the variation of the Vertical Total Electron Content (VTEC) over the mid latitude American regions during the relatively low (2008–2010) and relatively high (2012) solar activity years. This has been conducted employing the VTEC values obtained from the dual frequency ground based Global Positioning System (GPS) receivers located at Mineral Area Community College, MACC (37.85°N, 269.52°W) and Mississippi County Airport, MAIR (36.85°N, 270.64°W), and the latest versions of the IRI online model (IRI 2007, IRI 2012 and IRI 2016). The study mainly focuses to compare the trend of variability of the monthly and seasonal modeled VTEC values (IRI 2007 VTEC, IRI 2012 VTEC and IRI 2016 VTEC) with the corresponding measured VTEC values (GPS VTEC). The overall results show that the IRI VTEC values (almost in all versions of the model) are generally smaller than the GPS VTEC except after about 15:00 UT (09:00 LT) in the December solstice when the Sun shifts to the high solar activity. On the contrary, overestimations of the VTEC values by the model are observed in traversing from the low solar activity (2008) to high solar activity (2012) phase, especially after about 15:00 UT (09::00 LT) with the IRI 2016 version showing the highest. In general, the IRI 2007 and IRI 2012 versions show similar monthly and seasonal underestimations or overestimations showing that the two versions have almost similar performance. The IRI 2016 version is generally better in capturing both the diurnal and arithmetic mean GPS VTEC values with some exceptional months and seasons as compared to those of the IRI 2007 and IRI 2012 versions.     

Using GPS-TEC data to calibrate VTEC computed with the IRI model over Nigeria

This paper presents the development of a Total Electron Content (TEC) map for the Nigerian ionosphere. In this work, TEC measurements obtained from the AFRL-SCINDA GPS (Air Force Research Laboratory-Scintillation Network Decision Aid, Global Positioning System) equipment installed at Nsukka (6.87°N, 7.38°E) are used to adapt the International Reference Ionosphere (IRI) model for the Nigerian Ionosphere. The map is being developed as a computer program (implemented in the MATLAB programming language) that shows spatial and temporal representations of TEC for the Nigerian ionosphere. The method is aimed at showing how the IRI model can be used to estimate VTEC over wide areas by incorporating GPS measurements. This method is validated by using GPS VTEC data collected from a station in Ilorin (8.50°N, 4.55°E).     

Quasi-biennial oscillation in GPS VTEC measurements

The quasi-biennial oscillation, QBO, a well known periodicity in the equatorial stratospheric zonal winds, is also found in ionospheric parameters and in solar and geomagnetic activity indices. Many authors speculated about the link between the QBO in solar and geomagnetic activity and the QBO in atmospheric parameters. In this work we analyze the presence of the QBO in the ionosphere using the Vertical Total Electron Content (VTEC) values obtained from Global Navigation Satellite System (GNSS) measurements during the period 1999–2012. In particular, we used IONEX files, i.e. the International GNSS Service (IGS) ionospheric products. IONEX provide VTEC values around the world at 2-h intervals. From these data we compute global and zonal averages of VTEC at different local times at mid and equatorial geomagnetic latitudes. VTEC and Extreme Ultra Violet (EUV) solar flux time series are analyzed using a wavelet multi resolution analysis. In all cases the QBO is detected among other expected periodicities.     

Comparison of standard TEC models with a Neural Network based TEC model using multistation GPS TEC around the northern crest of Equatorial Ionization Anomaly in the Indian longitude sector during the low and moderate solar activity levels of the 24th solar cycle

The highest Total Electron Content (TEC) values in the world normally occur at Equatorial Ionization Anomaly (EIA) region resulting in largest ionospheric range delay values observed for any potential Space Based Augmentation System (SBAS). Reliable forecasting of TEC is crucial for satellite based navigation systems. The day to day variability of the location of the anomaly peak and its intensity is very large. This imposes severe limitations on the applicability of commonly used ionospheric models to the low latitude regions. It is necessary to generate a mathematical ionospheric forecasting and mapping model for TEC based on physical ionospheric influencing parameters. A model, IRPE-TEC, has been developed based on real time low latitude total electron content data using GPS measurements from a number of stations situated around the northern crest of the EIA during 2007 through 2011 to predict the vertical TEC values during the low and moderate solar activity levels of the 24th solar cycle. This model is compared with standard ionospheric models like International Reference Ionosphere (IRI) and Parameterized Ionospheric Model (PIM) to establish its applicability in the equatorial region for accurate predictions.     

基于BDS/GPS和经验模型的区域电离层建模

基于北斗卫星导航系统（BDS）和全球定位系统（GPS）实测电离层穿刺点（IPP）数据，结合国际参考电离层（IRI）经验模型历史数据，提出一种对区域二维电离层总电子含量（TEC）进行高精度建模的方法.针对缺乏穿刺点的区域内短时间电离层建模时精度较低且各时段穿刺点空间分布不同的问题，该方法使用IRI模型在建模区域内均匀添加虚拟穿刺点数据，并根据与实测穿刺点的距离，使用构造的权重计算公式赋予其动态权重值，通过加权最小二乘法进行球谐模型参数解算.与欧洲定轨中心（CODE）发布的全球电离层图（GIM）进行数据比对发现，相对于只使用BDS/GPS实测穿刺点数据的建模方法，利用本文建模方法计算获得的垂直总电子含量（VTEC）值对缺乏实测穿刺点的区域精度有明显的提升.      

Solar and magnetic control on night-time enhancement in TEC near the crest of the Equatorial Ionization Anomaly

The ionospheric Total Electron Content (TECs), derived by dual frequency signals from the Global Positioning System (GPS) recorded near the Indian equatorial anomaly region, Bhopal (23.2°N, 77.4°E, Geomagnetic 14.2°N) were analyzed for the period of January, 2005 to February, 2008. The work deals with monthly, diurnal, solar and magnetic activity variations on night-time enhancement in TEC. From a total of 157 night-time enhancements, 75 occur during pre-midnight and 82 post-midnight hours. The occurrence of night-time enhancement in TEC is utmost during summer months, followed by equinox and winter months. The occurrence of night-time enhancement in TEC decreases with increase in solar and magnetic activities. We observed that peak size and half amplitude duration are positively correlated, while time of occurrence of night-time enhancement in TEC and time of peak enhancement are negatively correlated with solar activity. The peak size, half amplitude duration, time of peak enhancement and time of occurrence of night-time enhancement in TEC shows negative correlation with magnetic activity. The results have been compared with the earlier ones and discussed in terms of possible source mechanism responsible for the enhancement at anomaly crest region.     

Low solar activity variability and IRI 2007 predictability of equatorial Africa GPS TEC

Diurnal, seasonal and latitudinal variations of Vertical Total Electron Content (VTEC) over the equatorial region of the African continent and a comparison with IRI-2007 derived TEC (IRI-TEC), using all three options (namely; NeQuick, IRI01-corr and IRI-2001), are presented in this paper. The variability and comparison are presented for 2009, a year of low solar activity, using data from thirteen Global Positioning System (GPS) receivers. VTEC values were grouped into four seasons namely March Equinox (February, March, April), June Solstice (May, June, July), September Equinox (August, September, October), and December Solstice (November, December, January). VTEC generally increases from 06h00 LT and reaches its maximum value at approximately 15h00–17h00 LT during all seasons and at all locations. The NeQuick and IRI01-corr options of the IRI model predict reasonably well the observed diurnal and seasonal variation patterns of VTEC values. However, the IRI-2001 option gave a relatively poor prediction when compared with the other options. The post-midnight and post-sunset deviations between modeled and observed VTEC could arise because NmF2 or the shape of the electron density profile, or both, are not well predicted by the model; hence some improvements are still required in order to obtain improved predictions of TEC over the equatorial region of the Africa sector.     

TEC derived from some GPS stations in Nigeria and comparison with the IRI and NeQuick models

Total electron content (TEC) measured simultaneously using Global Positioning System (GPS) ionospheric monitors installed at some locations in Nigeria during the year 2011 (Rz = 55.7) was used to study the diurnal, seasonal, and annual TEC variations. The TEC exhibits daytime maximum, seasonal variation and semiannual variations. Measured TEC were compared with those predicted by the improved versions of the International Reference Ionosphere (IRI) and NeQuick models. The models followed the diurnal and seasonal variation patterns of the observed values of TEC. However, IRI model produced better estimates of TEC than NeQuick at all locations.     

GPS derived TEC and foF2 variability at an equatorial station and the performance of IRI-model

The ionosphere induces a time delay in transionospheric radio signals such as the Global Positioning System (GPS) signal. The Total Electron Content (TEC) is a key parameter in the mitigation of ionospheric effects on transionospheric signals. The delay in GPS signal induced by the ionosphere is proportional to TEC along the path from the GPS satellite to a receiver. The diurnal monthly and seasonal variations of ionospheric electron content were studied during the year 2010, a year of extreme solar minimum (F10.7 = 81 solar flux unit), with data from the GPS receiver and the Digisonde Portable Sounder (DPS) collocated at Ilorin (Geog. Lat. 8.50°N, Long. 4.50°E, dip −7.9°). The diurnal monthly variation shows steady increases in TEC and F2-layer critical frequency (foF2) from pre-dawn minimum to afternoon maximum and then decreases after sunset. TEC show significant seasonal variation during the daytime between 0900 and 1900 UT (LT = UT + 1 h) with a maximum during the March equinox (about 35 TECU) and minimum during the June solstice (about 24 TECU). The GPS-TEC and foF2 values reveal a weak seasonal anomaly and equinoctial asymmetry during the daytime. The variations observed find their explanations in the amount of solar radiation and neutral gas composition. The measured TEC and foF2 values were compared with last two versions of the International Reference Ionosphere (IRI-2007 and IRI-2012) model predictions using the NeQuick and CCIR (International Radio Consultative Committee) options respectively in the model. In general, the two models give foF2 close to the experimental values, whereas significant discrepancies are found in the predictions of TEC from the models especially during the daytime. The error in height dependent thickness parameter, daytime underestimation of equatorial drift and contributions of electrons from altitudes above 2000 km have been suggested as the possible causes.     

Validation of IRI-2012 TEC model over Ethiopia during solar minimum (2009) and solar maximum (2013) phases

This paper investigates the capacity of the latest version of the International Reference Ionosphere (IRI-2012) model in predicting the vertical Total Electron Content (vTEC) over Ethiopian regions during solar minimum (2009) and solar maximum (2013) phases. This has been carried out by comparing the IRI-2012 modeled and experimental vTEC inferred from eight ground based dual frequency GPS (Global Positioning System) receivers installed recently at different regions of the country. In this work, the diurnal, monthly and seasonal variation in the measured vTEC have been analyzed and compared with the IRI-2012 modeled vTEC. During the solar minimum phase, the lowest and highest diurnal peak of the experimental vTEC are observed in July and October, respectively. In general, the diurnal variability of vTEC has shown minimum values around 0300 UT (0600 LT) and maximum values between around 1000 and 1300 UT (1300 and 1600 LT) during both solar activity phases. Moreover, the maximum and minimum monthly and seasonal mean hourly vTEC values are observed in October and July and in the March equinox and June solstice, respectively. It is also shown that the IRI-2012-model better predicts the diurnal vTEC in the time interval of about 0000–0300 UT (0300–0600 LT) during the solar minimum phase. However, the model generally overestimates the diurnal vTEC except in the time interval of about 0900–1500 UT (1200–1800 LT) during the solar maximum phase. The overall result of this work shows that the diurnal vTEC prediction performance of the model is generally better during the solar minimum phase than during solar maximum phase. Regarding the monthly and seasonal prediction capacity of the model, there is a good agreement between the modeled and measured monthly and seasonal mean hourly vTEC values in January and December solstice, respectively. Another result of the work depicts that unlike the GPS–TEC the IRI-2012 TEC does not respond to the effect resulted from geomagnetic storms.     

Analysis of global TEC prediction performance of IRI-PLAS model

The International Reference Ionosphere (IRI) empirical model provides valuable data for many fields including space and navigation applications. Since the IRI model gives the ionospheric parameters in the altitude range from 50?km to 2000?km, researchers focused on the IRI-PLAS model which is the plasmasphere extension of the IRI model. In this study, Total Electron Content (TEC) prediction performance of the IRI-PLAS model was examined at a global scale using the location of globally distributed 9 IGS stations. Besides the long term (01.01.2015–31.12.2015) behavior of the model, TEC predictions during the equinox and solstice days of 2014–2017 were also tested. IRI-PLAS-TEC values were examined in comparison with GPS-TEC data. Hourly interval of yearly profile exhibits that when the geomagnetic and solar active days are ignored, differences between IRI-PLAS-TEC and GPS-TEC are rather small (～2–3 TECU) at stations in the northern hemisphere, generally ～4–5 TECU level at the southern hemisphere stations and reaching above 10 TECU for few hours. While the IRI-PLAS-TEC generally overestimates the GPS-TEC at southern hemisphere stations during quiet days, the model-derived TEC underestimates GPS-TEC during solar active days. IRI-PLAS-TEC and GPS-TEC values exhibit similar trend for the equinoxes 21 March and 23 September which refer equivalent conditions.     

Regional modeling of the ionosphere using adaptive neuro-fuzzy inference system in Iran

In recent years, new techniques and algorithms such as Artificial Neural Networks (ANNs), Fuzzy Inference Systems (FIS) and Genetic Algorithm (GA) have been used as alternative statistical tools in modeling and forecasting issues. These methods have been extensively used in the field of geosciences and atmospheric physics. The main purpose of this paper is to combine FIS and ANNs for local modeling of the ionosphere Total Electron Content (TEC) in Iran. An Adaptive Neuro-Fuzzy Inference System (ANFIS) is developed for TEC modeling. Also, Multi-Layer Perceptron ANN (MLP-ANN) and ANN based on Radial Base Functions (RBF) have been designed for analyzing ANFIS results. Observations of 29 Global Positioning System (GPS) stations from the Iranian Permanent GPS Network (IPGN) have been used in 3 different seasons in 2015 and 2016. These stations are located at geomagnetic low latitudes region. Out of these 29 stations, 24 stations for training and 5 stations for testing and validating were selected. The relative and absolute errors have been used to evaluate the accuracy of the proposed model. Also, the results of this paper are compared with the International Reference Ionosphere model (IRI2016). The maximum values of the average relative error for RBF, MLP-ANN, ANFIS and IRI2016 methods are 13.88%, 11.79%, 10.06%, and 18.34%, respectively. Also, the maximum values of the average absolute error for these methods are 2.38, 2.21, 1.5 and 3.36 TECU, respectively. Comparison of diurnal predicted TEC from the ANFIS, RBF, MLP-ANN and IRI2016 models with GPS-TEC revealed that the ANFIS provides more accurate predictions than the other methods in the test area.     

A morphological study of GPS-TEC data at Agra and their comparison with the IRI model

The temporal and seasonal variations of Total Electron Content (TEC) are studied at Agra (Geographic Lat. 27.17°N, Long. 78.89°E, Dip: 41.4°), India, which is in the equatorial anomaly region, for a period of 12 months from 01 January to 31 December, 2007 using a Global Positioning System (GPS) receiver. The mean TEC values show a minimum at 0500 h LT (LT = UT + 5.5 h) and a peak value at about 1400 h LT. The lowest TEC values are observed in winter whereas largest values are observed in equinox and summer. Anomalous variations are found during the period of magnetic disturbances. These results are compared with the TEC derived from IRI-2007 using three different options of topside electron density, NeQuick, IRI01-corr, and IRI-2001. A good agreement is found between the TEC obtained at Agra and those derived from IRI models.     

一种基于GNSS数据的全球电离层不规则结构活动表征方法

针对如何利用GNSS(Global Navigation Satellite System)数据进行电离层扰动监测的问题,提出了一种基于GNSS数据表征全球电离层扰动的方法.利用大约400个GNSS地面站点的观测数据,计算总电子含量(Total Electron Content,TEC)变化率的标准差——ROTI(Ra...     

A study on TEC reduction during the tail phase of the 21st June 2020 annular solar eclipse

Ionospheric response during the annular solar eclipse of June 21, 2020, has been examined in terms of the Total Electron Content (TEC) obtained from six Global Positioning System (GPS) receivers positioned in the Chinese-Taiwanese region. We have shown TEC variation from satellites designated by PRNs (Pseudo-Random Noise code) 2, 6, and 19. PRN wise TEC trend was observed to depend upon satellite-pass trajectory to the receiver's location during the eclipse period. A time lag of ~15–30 min is also observed in maximum TEC decrement after the phase of maximum eclipse. Instead of the percentage of eclipse magnitude, a reduction in TEC is seen more for the station for which the orbital track of respective satellites was in closer view relative to receivers for more hours of eclipse window. Additionally, the eclipse day diurnal variations are compared with the pre-eclipse day TEC trend, and observed results show a clear decrease in TEC values at all chosen stations after the eclipse onset then reached the lowest value a few minutes afterward the maximum eclipse phase.     

Characteristics of GOCE orbits based on Satellite Laser Ranging

The Gravity field and steady-state Ocean Circulation Explorer (GOCE) was the first European Space Agency’s (ESA) Earth Explorer core mission. Through its extremely low, about 260?km above the Earth, circular, sun-synchronous orbit, the satellite gained high spatial resolution and accuracy gravity gradient, and ocean circulation data. Global Positioning System (GPS) receivers, mounted on the spacecraft, allowed the determination of reduced-dynamic and kinematic GOCE orbits, whereas Laser Retroreflector Array (LRA) dedicated to Satellite Laser Ranging (SLR) allowed an independent validation of GPS-derived orbits. In this paper, residuals between different GPS-based orbit types and SLR observations are used to investigate the sensitivity and the influence of solar, geomagnetic, and ionospheric activities on the quality of kinematic and reduced-dynamic GOCE orbits. We also analyze the quality of data provided by individual SLR sites, by detecting time biases using ascending and descending sun-synchronous GOCE orbit passes, and the residual analysis of the measurement characteristics, i.e., the dependency of SLR residuals as a function of nadir and horizontal angles. Results show a substantial vulnerability of kinematic orbit solutions to the solar F10.7 index and the ionospheric activity measured by the variations of the Total Electron Content (TEC) values. The sensitivity of kinematic orbits to the three-hour-range KP index is rather minor. The reduced-dynamic orbits are almost insensitive to indices describing ionospheric, solar, and geomagnetic activities. The investigation of individual SLR sites shows that some of them are affected by time bias errors, whereas other demonstrate systematics, such as a dependency between observation residuals and the satellite nadir angle or the horizontal azimuth angle from the SLR station to the direction of the satellite.     

不同太阳活动条件下电离层形态对估算GPS系统硬件延迟的影响

利用两个中纬度台站GPS观测数据提取的GPS卫星硬件延迟,分析了不同太阳活动情况下估算的硬件延迟稳定性和统计特征,结合同期电离层观测数据,研究了电离层状态对硬件延迟估算结果的影响.研究结果表明,基于太阳活动高年(2001年)GPS观测数据估算的硬件延迟稳定性要低于太阳活动低年GPS观测数据的估算结果,利用2001年GPS数据估算的卫星硬件延迟年标准偏差(RMS)平均值约为1TECU,而2009年GPS数据估算的卫星硬件延迟年标准偏差平均值约为0.8TECU.通过对2001年和2009年北京地区电离层F₂层最大电子密度(N_mF₂)变化性分析,结合GPS硬件延迟估算方法对电离层时空变化条件的要求,认为硬件延迟稳定性与太阳活动强度的联系是由不同太阳活动条件下电离层变化的强度差异引起的.      

45°(N)纬度带的Klobuchar like电离层延迟季节修正模型与评估

电离层延迟是全球卫星导航系统（GNSS）的主要误差源之一。对于装配GNSS单频接收机的航空器,选择简单有效的Klobuchar广播电离层模型来改正电离层延迟误差,其修正率为50%～60%。针对45°（N）纬度带,提出了更高电离层修正需求。考虑到季节因素对中高纬度地区电离层的显著影响,利用GIMs（Global Ionospheric Maps）分析了昼夜中TEC（Total Electron Content）的峰值和谷值随季节（年积日）的变化,建立了一种适用于45°（N）纬度带的Klobuchar like电离层模型。该模型不增加广播模型系数,新模型的夜间和VTEC高峰时电离层修正率分别达到了82%和80%,表明在穿刺点集中的45°(N)纬度地区使用该模型可以更精确地描述该地区的电离层,帮助航空器实现更高精度的定位。