Comparison of observed TEC values with IRI-2007 TEC and IRI-2007 TEC with optional foF2 measurements predictions at an equatorial region,Chumphon, Thailand

In this research, as part of working towards improving the IRI over equatorial region, the total electron content (TEC) derived from GPS measurements and IRI-2007 TEC predictions at Chumphon station (10.72°N, 99.37°E), Thailand, during 2004–2006 is analyzed. The seasonal variation of the IRI-2007 TEC predictions is compared with the TEC from the IRI-2007 TEC model with the option of the actual F2 plasma frequency (foF2) measurements as well as the TEC from the GPS and International GNSS service (IGS). The Chumphon station is located at the equatorial region and the low latitude of 3.22°N. For a declining phase of the solar cycle (2004–2006), the study shows that the IRI-2007 TEC underestimates the IRI-2007 TEC with the foF2 observation at the nighttime by about 5 TECU. The maximum differences are about 15 TECU during daytime and 5 TECU during nighttime. The overestimation is more evident at daytime than at nighttime. When compared in terms of the root-mean square error (RMSE), we find that the highest RMSE between GPS TEC and IRI 2007 TEC is 14.840 TECU at 1230 LT in 2004 and the lowest average between them is 1.318 TECU at 0630 LT in 2006. The noon bite-out phenomena are clearly seen in the IRI-2007 TEC with and without optional foF2 measurements, but not on the GPS TEC and IGS TEC. The IRI TEC with optional foF2 measurements gives the lowest RMSE values between IRI TEC predicted and TEC measurement. However, the TEC measurements (GPS TEC and IGS TEC) are more correct to use at Chumphon station.     

Variation of ionospheric total electron content in Indian low latitude region of the equatorial anomaly during May 2007–April 2008

The ionospheric total electron content (TEC), derived by analyzing dual frequency signals from the Global Positioning System (GPS) recorded near the Indian equatorial anomaly region, Varanasi (geomagnetic latitude 14°, 55′N, geomagnetic longitude 154°E) is studied. Specifically, we studied monthly, seasonal and annual variations as well as solar and geomagnetic effects on the equatorial ionospheric anomaly (EIA) during the solar minimum period from May 2007 to April 2008. It is found that the daily maximum TEC near equatorial anomaly crest yield their maximum values during the equinox months and their minimum values during the summer. Using monthly averaged peak magnitude of TEC, a clear semiannual variation is seen with two maxima occurring in both spring and autumn. Statistical studies indicate that the variation of EIA crest in TEC is poorly correlated with Dst-index (r = −0.03) but correlated well with Kp-index (r = 0.82). The EIA crest in TEC is found to be more developed around 12:30 LT.     

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.     

Equatorial Ionospheric Anomaly (EIA) and comparison with IRI model during descending phase of solar activity (2005–2009)

The ionospheric variability at equatorial and low latitude region is known to be extreme as compared to mid latitude region. In this study the ionospheric total electron content (TEC), is derived by analyzing dual frequency Global Positioning System (GPS) data recorded at two stations separated by 325 km near the Indian equatorial anomaly region, Varanasi (Geog latitude 25°, 16^/ N, longitude 82°, 59^/ E, Geomagnetic latitude 16°, 08^/ N) and Kanpur (Geog latitude 26°, 18^/ N, longitude 80°, 12^/ E, Geomagnetic latitude 17°, 18^/ N). Specifically, we studied monthly, seasonal and annual variations as well as solar and geomagnetic effects on the equatorial ionospheric anomaly (EIA) during the descending phase of solar activity from 2005 to 2009. It is found that the maximum TEC (EIA) near equatorial anomaly crest yield their maximum values during the equinox months and their minimum values during the summer. Using monthly averaged peak magnitude of TEC, a clear semi-annual variation is seen with two maxima occurring in both spring and autumn. Results also showed the presence of winter anomaly or seasonal anomaly in the EIA crest throughout the period 2005–2009 only except during the deep solar minimum year 2007–2008. The correlation analysis indicate that the variation of EIA crest is more affected by solar activity compared to geomagnetic activity with maximum dependence on the solar EUV flux, which is attributed to direct link of EUV flux on the formation of ionosphere and main agent of the ionization. The statistical mean occurrence of EIA crest in TEC during the year from 2005 to 2009 is found to around 12:54 LT hour and at 21.12° N geographic latitude. The crest of EIA shifts towards lower latitudes and the rate of shift of the crest latitude during this period is found to be 0.87° N/per year. The comparison between IRI models with observation during this period has been made and comparison is poor with increasing solar activity with maximum difference during the year 2005.     

Application of BDS-GEO for studying TEC variability in equatorial ionosphere on different time scales

The Earth's ionosphere and especially its equatorial part is a highly dynamical medium. Geostationary satellites are known to be a powerful tool for ionospheric studies. Recent developments in BDS-GEO satellites allow such studies on the new level due to the best noise pattern in TEC estimations, which corresponds to those of GPS/GLONASS systems. Here we used BDS-GEO satellites to demonstrate their capability for studying equatorial ionosphere variability on different time scales. Analyzing data from the equatorial SIN1 IGS station we present seasonal variations in geostationary slant TEC for the periods of high (October 2013 - October 2014) and low (January 2017 - January 2018) solar activity, which show semi-annual periodicity with amplitudes about 10 TECU during solar maximum and about 5 TECU during the solar minimum. The 27-day variations are also prominent in geostationary slant TEC variations, which correlates quite well with the variations in solar extreme UV radiation. We found semi-annual pattern in small scale ionospheric disturbances evaluated based on geostationary ROTI index: maximal values correspond to spring and fall equinoxes and minimum values correspond to summer and winter solstices. The seasonal asymmetry in ROTI values was observed: spring equinox values were almost twice as higher than fall equinox ones. We also present results on the 2017 May 28–29 G3 geomagnetic storm, when ～30 TECU positive anomaly was recorded, minor and final major sudden stratospheric warmings in February and March 2016, with positive daytime TEC anomalies up to 15–20 TECU, as well as the 2017 September 6 X9.3 solar flare with 2 TECU/min TEC rate. Our results show the large potential of geostationary TEC estimations with BDS-GEO signals for continuous monitoring of space weather effects in low-latitude and equatorial ionosphere.     

Traveling ionospheric disturbances observed at South African midlatitudes during the 29–31 October 2003 geomagnetically disturbed period

This paper presents traveling ionospheric disturbances (TIDs) observations from GPS measurements over the South African region during the geomagnetically disturbed period of 29–31 October 2003. Two receiver arrays, which were along two distinct longitudinal sectors of about 18°-20° and 27°-28° were used in order to investigate the amplitude, periods and virtual propagation characteristics of the storm induced ionospheric disturbances. The study revealed a large sudden TEC increase on 28 October 2003, the day before the first of the two major storms studied here, that was recorded simultaneously by all the receivers used. This pre-storm enhancement was linked to an X-class solar flare, auroral/magnetospheric activities and vertical plasma drift, based on the behaviour of the geomagnetic storm and auroral indices as well as strong equatorial electrojet. Diurnal trends of the TEC and foF2 measurements revealed that the geomagnetic storm caused a negative ionospheric storm; these parameters were depleted between 29 and 31 October 2003. Large scale traveling ionospheric disturbances were observed on the days of the geomagnetic storms (29 and 31 October 2003), using line-of-sight vertical TEC (vTEC) measurements from individual satellites. Amplitude and dominant periods of these structures varied between 0.08–2.16 TECU, and 1.07–2.13 h respectively. The wave structures were observed to propagate towards the equator with velocities between 587.04 and 1635.09 m/s.     

Ionospheric total electron content response to annular solar eclipse on June 21, 2020

The effects of physical events on the ionosphere structure is an important field of study, especially for navigation and radio communication. The paper presents the spatio-temporal ionospheric TEC response to the recent annular solar eclipse on June 21, 2020, which spans across two continents, Africa and Asia, and 14 countries. This eclipse took place on the same day as the June Solstice. The Global Navigation Satellite System (GNSS) based TEC data of the Global Ionosphere Maps (GIMs), 9 International GNSS Service (IGS) stations and FORMOSAT-7/COSMIC-2 (F7/C2) were utilized to analyze TEC response during the eclipse. The phases of the TEC time series were determined by taking the difference of the observed TEC values on eclipse day from the previous 5-day median TEC values. The results showed clear depletions in the TEC time series on June 21. These decreases were between 1 and 9 TECU (15–60%) depending on the location of IGS stations. The depletions are relatively higher at the stations close to the path of annular eclipse than those farther away. Furthermore, a reduction of about ?10 TECU in the form of an equatorial plasma bubble (EPB) was observed in GIMs at ~20° away from the equator towards northpole, between 08:00–11:00 UT where its maximum phase is located in southeast Japan. Additionally, an overall depletion of ~10% was observed in F7/C2 derived TEC at an altitude of 240 km (hmF2) in all regions affected by the solar eclipse, whereas, significant TEC fluctuations between the altitudes of 100 km ? 140 km were analyzed using the Savitzky-Golay smoothing filter. To prove TEC depletions are not caused by space weather, the variation of the sunspot number (SSN), solar wind (V_SW), disturbance storm-time (Dst), and Kp indices were investigated from 16th to 22nd June. The quiet space weather before and during the solar eclipse proved that the observed depletions in the TEC time series and profiles were caused by the annular solar eclipse.     

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.      

Cross testing of ionosphere models IRI-2001 and IRI-2007, data from satellite altimeters (Topex/Poseidon and Jason-1) and global ionosphere maps

In this study, we use a great body of statistical data covering the entire 23rd solar cycle to cross test data of satellite altimeters, Global Ionosphere Maps and the International Reference Ionosphere models, IRI-2001 and IRI-2007. It is revealed that experimental TEC values of the satellite altimeters regularly exceed the model ones by ∼3 TECU (1 TECU = 10¹⁶ m⁻²). The best possible value of difference between TECs obtained from altimeter and GIM-map data significantly differs for different laboratories: the maximum for CODG data falls on 2.5 TECU, ESAG – 3 TECU, JPLG – 0 TECU, UPCG – 2 TECU. The dependence of experimental and model data root-mean-square deviation on the F10.7 index is shown to be nearly linear. IRI-2001 and IRI-2007 relative errors are characterized by considerable 11-year and annual variations. Given the geomagnetic planetary index K_p under 7, IRI-2001 and IRI-2007 reproduce TEC in the ionosphere with an accuracy of ∼30% relative to measurement data from satellite altimeters. The amplitude of absolute error variations resulting from the difference in ionization enhancement between the model and the real ionosphere during the morning solar terminator transit is ∼5 TECU.     

Response of the EIA ionosphere to the 7-8 May 2005 geomagnetic storm

In this paper, response of low latitude ionosphere to a moderate geomagnetic storm of 7–8 May 2005 (SSC: 1920 UT on 7 May with Sym-H minimum, ∼−112 nT around 1600 UT on 8 May) has been investigated using the GPS measurements from a near EIA crest region, Rajkot (Geog. 22.29°N, 70.74°E, Geomag.14°), India. We found a decrease in total electron content (TEC) in 12 h after the onset of the storm, an increase during and after 6 h of Sym-H deep minimum with a decrease below its usual-day level on the second day during the recovery phase of the storm. On 8 May, an increase of TEC is observed after sunset and during post-midnight hours (maximum up to 170%) with the formation of ionospheric plasma bubbles followed by a nearly simultaneous onset of scintillations at L-band frequencies following the time of rapid decrease in Sym-H index (−30 nT/h around 1300 UT).     

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.     

Total electron content monitoring using triple frequency GNSS: Results with Giove-A/-B data

Triple frequency GNSS will be fully operational within the next decade, opening opportunities for new applications. Dual frequency GNSS already allow to study the ionosphere through the estimation of Total Electron Content (TEC). However, the precision is limited by the ambiguity resolution process. This paper studies a triple frequency TEC monitoring technique in which the use of Geometry-Free and Iono-Free linear combinations improves the ambiguity resolution process and therefore the precision of TEC. We have tested it on a set of triple frequency Giove-A/-B data from January and December 2008. The conclusions achieved are (1) TEC values are affected by an error of about 2–2.5 TECU produced through the ambiguity resolution process; (2) the error caused by the Geometric Free phase combination delays (hardware, multipath, noise, antenna phase center) on TEC is about 0.2 TECU; (3) the total error on TEC approximately reach 2–3 TECU.     

A real-time ionospheric model based on GNSS Precise Point Positioning

This paper proposes a method of real-time monitoring and modeling the ionospheric Total Electron Content (TEC) by Precise Point Positioning (PPP). Firstly, the ionospheric TEC and receiver’s Differential Code Biases (DCB) are estimated with the undifferenced raw observation in real-time, then the ionospheric TEC model is established based on the Single Layer Model (SLM) assumption and the recovered ionospheric TEC. In this study, phase observations with high precision are directly used instead of phase smoothed code observations. In addition, the DCB estimation is separated from the establishment of the ionospheric model which will limit the impacts of the SLM assumption impacts. The ionospheric model is established at every epoch for real time application. The method is validated with three different GNSS networks on a local, regional, and global basis. The results show that the method is feasible and effective, the real-time ionosphere and DCB results are very consistent with the IGS final products, with a bias of 1–2 TECU and 0.4 ns respectively.     

Plasmaspheric electron content derived from GPS TEC and FORMOSAT-3/COSMIC measurements: Solar minimum condition

The plasmaspheric electron content (PEC) was estimated by comparison of GPS TEC observations and FORMOSAT-3/COSMIC radio occultation measurements at the extended solar minimum of cycle 23/24. Results are retrieved for different seasons (equinoxes and solstices) of the year 2009. COSMIC-derived electron density profiles were integrated up to the height of 700 km in order to retrieve estimates of ionospheric electron content (IEC). Global maps of monthly median values of COSMIC IEC were constructed by use of spherical harmonics expansion. The comparison between two independent measurements was performed by analysis of the global distribution of electron content estimates, as well as by selection specific points corresponded to mid-latitudes of Northern America, Europe, Asia and the Southern Hemisphere. The analysis found that both kinds of observations show rather similar diurnal behavior during all seasons, certainly with GPS TEC estimates larger than corresponded COSMIC IEC values. It was shown that during daytime both GPS TEC and COSMIC IEC values were generally lower at winter than in summer solstice practically over all specific points. The estimates of PEC (h > 700 km) were obtained as a difference between GPS TEC and COSMIC IEC values. Results of comparative study revealed that for mid-latitudinal points PEC estimates varied weakly with the time of a day and reached the value of several TECU for the condition of solar minimum. Percentage contribution of PEC to GPS TEC indicated the clear dependence from the time with maximal values (more than 50–60%) during night-time and lesser values (25–45%) during day-time.     

Towards a GPS-based TEC prediction model for Southern Africa with feed forward networks

In this paper, first results from a national Global Positioning System (GPS) based total electron content (TEC) prediction model over South Africa are presented. Data for 10 GPS receiver stations distributed through out the country were used to train a feed forward neural network (NN) over an interval of at most five years. In the NN training, validating and testing processes, five factors which are well known to influence TEC variability namely diurnal variation, seasonal variation, magnetic activity, solar activity and the geographic position of the GPS receivers were included in the NN model. The database consisted of 1-min data and therefore the NN model developed can be used to forecast TEC values 1 min in advance. Results from the NN national model (NM) were compared with hourly TEC values generated by the earlier developed NN single station models (SSMs) at Sutherland (32.38°S, 20.81°E) and Springbok (29.67°S, 17.88°E), to predict TEC variations over the Cape Town (33.95°S, 18.47°E) and Upington (28.41°S, 21.26°E) stations, respectively, during equinoxes and solstices. This revealed that, on average, the NM led to an improvement in TEC prediction accuracy compared to the SSMs for the considered testing periods.     

Empirical model of the TEC response to the geomagnetic activity over the North American region

The paper presents an empirical model of the total electron content (TEC) response to the geomagnetic activity described by the K_p-index. The model is built on the basis of TEC measurements covering the region of North America (50°W–150°W, 10°N–60°N) for the period of time between October 2004 and December 2009. By using a 2D (latitude-time) cross-correlation analysis it is found that the ionospheric response to the geomagnetic activity over the considered geographic region and at low solar activity revealed both positive and negative phases of response. The both phases of the ionospheric response have different duration and time delay with respect to the geomagnetic storm. It was found that these two parameters of the ionospheric response depend on the season and geographical latitude. The presence of two phases, positive and negative, of the ionospheric response imposed the implementation of two different time delay constants in order to properly describe the two different delayed reactions. The seasonal dependence of the TEC response to geomagnetic storms is characterized by predominantly positive response in winter with a short (usually ∼5–6 h) time delay as well as mainly negative response in summer with a long (larger than 15 h) time delay. While the TEC response in March and October is more close to the winter one the response in April and September is similar to the summer one.     

Improving regional ionospheric TEC mapping based on RBF interpolation

Radial basis function (RBF) interpolation with multi-quadric is developed to perform ionospheric total electron content (TEC) mapping for the Chinese region between 15°N ~ 40°N and 100°E ~ 125°E. TEC measurements from the Centre for Orbit Determination in Europe (CODE) covering the solar maximum year 2011 are used to investigate the performance of the proposed RBF interpolation method. The differences between the RBF interpolated TEC and the CODE TEC are within 0.5 TECU and the root mean square error (RMSE) is very small when 49 data points are used. The maximum difference is ~5 TECU and the error is less than 1 TECU with 25 samples. Our study suggests that a random distribution of measurement points gives smaller RMSEs than a homogenous distribution when the number of sample points is low. The study indicates that RBF interpolation offers a powerful and reliable tool for ionospheric TEC mapping.     

Day-to-day variability of equatorial anomaly in GPS-TEC during low solar activity period

The ionospheric total electron content (TEC) in the northern hemispheric equatorial ionization anomaly (EIA) crest region is investigated by using dual-frequency signals of the Global Positioning System (GPS) acquired from Rajkot (Geog. Lat. 22.29°N, Geog. Long. 70.74°E; Geom. Lat. 14.21°N, Geom. Long. 144.90°E), India. The day-to-day variability of EIA characteristics is examined during low solar activity period (F10.7∼83 sfu). It is found that the daily maximum TEC at EIA crest exhibits a day-to-day and strong semi-annual variability. The seasonal anomaly and equinoctial asymmetry in TEC at EIA is found non-existent and weaker, respectively. We found a moderate and positive correlation of daily magnitude of crest, Ic with daily F10.7 and EUV fluxes with a correlation coefficient of 0.43 and 0.33, respectively indicating an existence of a short-term relation between TEC at EIA and the solar radiation even during low solar activity period. The correlation of daily Ic with Dst index is also moderate (r = −0.35), whereas no correlation is found with the daily Kp index (r = 0.14) respectively. We found that the magnitude of EIA crest is moderately correlated with solar flux in all seasons except winter where it is weakly related (0.27). The magnitude of EIA crest is also found highly related with EEJ strength in spring (r = 0.69) and summer (r = 0.65) than autumn (0.5) and winter (r = 0.47), though EEJ is stronger in autumn than spring.     

The variability and IRI2007-predictability of hmF2 over South Africa

This paper presents an investigation into the variability and predictability of the maximum height of the ionospheric F2 layer, hmF2 over the South African region. Data from three South African stations, namely Madimbo (22.4°S, 26.5°E, dip angle: −61.47°), Grahamstown (33.3°S, 26.5°E, dip angle: −64.08°) and Louisvale (28.5°S, 21.2°E, dip angle: −65.44°) were used in this study. The results indicate that hmF2 shows a larger variability around midnight than during the daytime for all seasons. Monthly median hmF2 values were used in all cases and were compared with predictions from the IRI-2007 model, using the URSI (Union Radio-Scientifique Internationale) coefficient option. The analysis covers the diurnal and seasonal hourly hmF2 values for the selected months and time sectors e.g. January, July, April and October for 2003 and 2005. The time ranges between (03h00–23h00 UT; LT = UT + 2h) representing the local sunrise, midday, sunset and midnight hours. The time covers sunrise, midday, sunrise, and midnight hours (03–06h00 UT, 07–11h00 UT, sunrise 16–18h00 UT and 22–23h00 UT; LT = UT + 2h). The dependence of the results on solar activity levels was also investigated. The IRI-2007 predictions follow fairly well the diurnal and seasonal variation patterns of the observed hmF2 values at all the stations. However, the IRI-2007 model overestimates and underestimates the hmF2 value during different months for all the solar activity periods.