The effect of total solar eclipse of October 3, 2005, on the total electron content over Europe

GPS observations from EUREF permanent GPS network were used to observe the response of TEC (Total Electron Content) to the total solar eclipse on October 3, 2005, under quiet geomagnetic conditions of the daytime ionosphere. The effect of the eclipse was detected in diurnal variations and more distinctly in the variations of TEC along individual satellite passes. The trough-like variations with a gradual decrease and followed by an increase of TEC at the time of the eclipse were observed over a large region. The depression of TEC amounted to 3–4 TECU. The maximum depression was observed over all stations located at the maximum path of the solar eclipse. The delay of a minimum level of TEC with respect to the maximum phase of the eclipse was about 20–30 min.     

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.     

Demonstration of the use of the Doppler Orbitography and Radio positioning Integrated by Satellite (DORIS) measurements to validate GPS ionospheric imaging

There is a lack of independent ionospheric data that can be used to validate GPS imaging results at mid latitudes over severe storm times. Doppler Orbitography and Radio positioning Integrated by Satellite (DORIS), a global network of dual-frequency ground to satellite observations, provides this missing data and here is employed as verification to show the accuracy of the ionospheric GPS images in terms of the total electron content (TEC). In this paper, the large-scale ionospheric structures that appeared during the strong geomagnetic storm of 20 November 2003 are reconstructed with a GPS tomographic algorithm, known as MIDAS, and validated with DORIS TEC measurements. The main trough shown in an extreme equatorward position in the ionospheric imaging over mainland Europe is confirmed by DORIS satellite measurements. Throughout the disturbed day, the variations of relative slant TECs between DORIS data and MIDAS results agree quite well, with the average of the mean differences about 2 TECu. We conclude that as a valuable supplement to GPS data, DORIS ionospheric measurements can be used to analyse TEC variations with a relatively high resolution, ∼10 s in time and tens of kilometres in space. This will be very helpful for identification of some highly dynamic structures in the ionosphere found at mid-latitudes, such as the main trough, TID (Travelling Ionospheric Disturbances) and SED (Storm Enhanced Density), and could be used as a valuable auxiliary data source in ionospheric imaging.     

Influence of the 27-day solar flux variations on the ionosphere parameters measured at Irkutsk in 2003–2005

We present an investigation of the influence of the 27-day solar flux variations, caused by solar rotation, on the ionosphere parameters such as the F2 layer critical frequency (foF2) and the total electron content (TEC). Our observational data were obtained with the Irkutsk Digisonde (DPS-4) located at 52.3 North and 104.3 East during the period from 2003 to 2005. In addition, we use TEC data from the Global Ionosphere Maps (GIM) based on Global Positioning System (GPS) satellites. The solar radiation flux at a wavelength of 10.7 cm (F10.7 index) is used as an index characterizing the solar activity level. A good correlation between observed ionosphere parameters and solar activity variations is found especially in autumn-to-winter season. We estimate the impact of the 27-day solar flux variations on the day-to-day variability and determine the time delay of the ionosphere response.     

Analysis of electron content variations over Japan during solar minimum: Observations and modeling

Comparative analysis of GPS TEC data and FORMOSAT-3/COSMIC radio occultation measurements was carried out for Japan region during period of the extremely prolonged solar minimum of cycle 23/24. COSMIC data for different seasons corresponded to equinox and solstices of the years 2007–2009 were analyzed. All selected electron density profiles were integrated up to the height of 700 km (altitude of COSMIC satellites), the monthly median estimates of Ionospheric Electron Content (IEC) were retrieved with use of spherical harmonics expansion. Monthly medians of TEC values were calculated from diurnal variations of GPS TEC estimates during considered month. Joint analysis of GPS TEC and COSMIC data allows us to extract and estimate electron content corresponded to the ionosphere (its bottom and topside parts) and the plasmasphere (h > 700 km) for different seasons of 2007–2009. Percentage contribution of ECpl to GPS TEC indicates the clear dependence from the time and varies from a minimum of about 25–50% during day-time to the value of 50–75% at night-time. Contribution of both bottom-side and topside IEC has minimal values during winter season in compare with summer season (for both day- and night-time). On average bottom-side IEC contributes about 5–10% of GPS TEC during night and about 20–27% during day-time. Topside IEC contributes about 15–20% of GPS TEC during night and about 35–40% during day-time. The obtained results were compared with TEC, IEC and ECpl estimates retrieved by Standard Plasmasphere–Ionosphere Model that has the plasmasphere extension up to 20,000 km (GPS orbit).     

Estimation and analysis of GPS receiver differential code biases using KGN in Korean Peninsula

The total electron content (TEC) estimation by the Global Positioning System (GPS) can be seriously affected by the differential code biases (DCB), referred to as inter-frequency biases (IFB), of the satellite and receiver so that an accuracy of GPS–TEC value is dependent on the error of DCBs estimation. In this paper, we proposed the singular value decomposition (SVD) method to estimate the DCB of GPS satellites and receivers using the Korean GPS network (KGN) in South Korea. The receiver DCBs of about 49 GPS reference stations in KGN were determined for the accurate estimation of the regional ionospheric TEC. They obtained from the daily solution have large biases ranging from +5 to +27 ns for geomagnetic quiet days. The receiver DCB of SUWN reference station was compared with the estimates of IGS and JPL global ionosphere map (GIM). The results have shown comparatively good agreement at the level within 0.2 ns. After correction of receiver DCBs and knowing the satellite DCBs, the comparison between the behavior of the estimated TEC and that of GIMs was performed for consecutive three days. We showed that there is a good agreement between KASI model and GIMs.     

GPS total electron content and surface latent heat flux variations before the 11 March 2011 M9.0 Sendai earthquake

The present paper describes the variations of the GPS total electron content (TEC) from the International GNSS service network and surface latent heat flux (SLHF) from the Scientific Computing Division of the National Center for Atmospheric Research (NCAR) before the 11 March 2011 M9.0 Sendai earthquake, respectively. The analysis shows pronounced enhancements in the GPS TEC and SLHF a few days prior to the earthquake event. The maximum increase in the GPS TEC was about 30 TECu with an extended spatial distribution on a geomagnetically quiet day (Dst ? −20 nT, between two moderate geomagnetic storms), 8 March, 3 days prior to the earthquake. This giant positive disturbance was possibly associated with the impending disastrous earthquake and contributed from the enhanced solar radiation. Moreover, there were several anomalous regions of SLHF on the global map, but an area of enhanced SLHF very close to the epicenter. The purpose of this paper is to report the existence of the changes in surface and ionosphere, and show the potential application of multi-source data to identify seismic precursors.     

Modeling medium-scale TEC structures,observed by Belgian GPS receivers network

GALOCAD project “Development of a Galileo Local Component for the nowcasting and forecasting of atmospheric disturbances affecting the integrity of high precision Galileo applications” aims to perform a detailed study on ionospheric small- and medium-scale structures and to assess the influence of these structures on the reliability of Galileo precise positioning applications. GPS-derived TEC (total electron content) is obtained from the Belgium Dense Network (BDN), consisting of 67 permanent GPS stations. An empirical 3-D model is developed for studying these ionospheric structures. The model, named LLT model, described temporal variations of TEC in latitude/longitude frame (46°, 52°)N and (−1°, 11°)E. The spatial variations of TEC are modeled by Tchebishev base functions, while the temporal variations are described by a trigonometric basis. To fit the model to the data, the observed area is divided into bins with (1° × 1°) geographic scale and 6 min on time axis. LLT model is made flexible, with varying number of coefficients along each axis. This allows different degree of smoothing, which is the key element of the present approach. Model runs with higher number of coefficients, capturing in details medium-scale TEC structures are subtracted from results obtained with smaller number of coefficients; the latter represent the background ionosphere. The residual structures are localized and followed as they travel across the observed area. In this way, the size, velocity, and direction of the irregular structures are obtained.     

Estimation of GPS instrumental biases from small scale network

With 4 GPS receivers located in the equatorial anomaly region in southeast China, this paper proposes a grid-based algorithm to determine the GPS satellites and receivers biases, and at the same time to derive the total electron content (TEC) with time resolution of 15 min and spatial resolution of 1° by 3.5° in latitude and longitude. By assuming that the TEC is identical at any point within a given grid block and the biases do not vary within a day, the algorithm arranges unknown biases and TECs with slant path TEC from the 4 receivers’ observations into a set of equations. Then the instrumental biases and the TECs are determined by using the least squares fitting technique. The performance of the method is examined by applying it to the GPS receiver chain observations selected from 16 geomagnetically quiet days in four seasons of 2006. It is found that the fitting agrees with the data very well, with goodness of fit ranging from 0.452 TECU to 1.914 TECU. Having a mean of 0.9 ns, the standard deviations for most of the GPS satellite biases are less than 1.0 ns for the 16 days. The GPS receiver biases are more stable than that of the GPS satellites. The standard deviation in the 4 receiver bias is from 0.370 ns to 0.855 ns, with a mean of 0.5 ns. Moreover, the instrumental biases are highly correlated with those derived from CODE and JPL with independent methods. The typical precision of the derived TEC is 5 TECU by a conservative estimation. These results indicate that the proposed algorithm is valid and qualified for small scale GPS network.     

An annual variation analysis of the ionospheric spatial gradient over a regional area for GNSS applications

An ionospheric spatial gradient represents the ionosphere delay difference between different locations, and its variation over a specific area is important for implementing differential GNSS systems. An estimation method for the ionospheric spatial gradient over a small regional area is proposed. A plate map model is implemented for the direct estimation of the gradients. Nine years of GPS data were processed to figure out the annual variation of the mean gradient at the mid-geomagnetic latitude of 30° N. Gradients along the north–south direction have a mean of 0.65 mm/km and follow solar-cycle variations.     

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 scintillation and TEC depletion near the northern crest of equatorial anomaly over South China

This study presents a statistical analysis of GPS L-band scintillation with data observed from July 2008 to March 2012 at the northern crest of equatorial anomaly stations in Guangzhou and Shenzhen of South China. The variations of the scintillation with local time, season, solar activity and duration of scintillation patches were investigated. The relationship between the scintillation and TEC depletion was also reported. Our results revealed that GPS scintillation occurred from 19:30 LT (pre-midnight) to 03:00 LT (post-midnight). During quiet solar activity years, the scintillation was only observed in pre-midnight hours of equinox months and patches durations were mostly less than 60 min. During high solar activity years, more scintillation occurred in the pre-midnight hours of equinox and winter months; and GPS scintillation started to occur in the post-midnight hours of summer and winter. The duration of scintillation patches extended to 180 min in high solar activity years. Solar activity had a larger effect to strong scintillations (S₄ > 0.6) than to weak scintillations (0.6 ? S₄ > 0.2). Strong scintillations were accompanied by TEC depletion especially in equinox months. We also discussed the relationship between TEC depletion and plasma bubble.     

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.     

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.     

Validation of University of New Brunswick Ionospheric Modeling Technique with ionosonde TEC estimation over South Africa

For more than a decade, ionospheric research over South Africa has been carried out using data from ionosondes geographically located at Madimbo (28.38°S, 30.88°E), Grahamstown (33.32°S, 26.50°E), and Louisvale (28.51°S, 21.24°E). The objective has been modelling the bottomside ionospheric characteristics using neural networks. The use of Global Navigation Satellite System (GNSS) data is described as a new technique to monitor the dynamics and variations of the ionosphere over South Africa, with possible future application in high frequency radio communication. For this task, the University of New Brunswick Ionospheric Modelling Technique (UNB-IMT) was applied to compute midday (10:00 UT) GNSS-derived total electron content (GTEC). GTEC values were computed using GNSS data for stations located near ionosondes for the years 2002 and 2005 near solar maximum and minimum, respectively. The GTEC was compared with the midday ionosonde-derived TEC (ITEC) measurements to validate the UNB-IMT results. It was found that the variation trends of GTEC and ITEC over all stations are in good agreement and show a pronounced seasonal variation for the period near solar maximum, with maximum values (∼80 TECU) around autumn and spring equinoxes, and minimum values (∼22 TECU) around winter and summer. Furthermore, the residual ΔTEC = GTEC − ITEC was computed. It was evident that ΔTEC, which is believed to correspond to plasmaspheric electron content, showed a pronounced seasonal variation with maximum values (∼20 TECU) around equinoxes and minimum (∼5 TECU) around winter near solar maximum. The equivalent ionospheric and total slab thicknesses were also computed and comprehensively discussed. The results verified the use of UNB-IMT as one of the tools for future ionospheric TEC research over South Africa.     

An investigation of ionospheric disturbances over South Africa during the magnetic storm on 15 May 2005

The effects of the 15 May 2005 severe geomagnetic storm on the South African ionosphere are studied using ground-based and satellite observations. Ionospheric disturbances have less frequently been investigated over mid-latitude regions. Recently, a number of studies investigated their evolution and generation over these regions. This paper reports on the first investigation of travelling ionospheric disturbances (TIDs) over mid-latitude South Africa. Using global positioning system (GPS)-derived total electron content (TEC) variations from the South African network of dual frequency GPS receivers, we were able to examine the effects of the disturbance on the TEC. During this storm, two TEC enhancements were observed at low- and mid-latitudes: the first enhancement was observed between 30–45°S geomagnetic latitudes associated with equatorward neutral winds and the passage of a TID, while the second TEC enhancement is associated with a second TID. In addition, the F-region critical frequency (foF2) values observed at two ionosonde stations show response features that differ from those of the TEC during the disturbance period. The dissimilarity between the TEC and the foF2 suggests that two competing drivers may have existed, i.e., the westward electric field and equatorward neutral wind effects.     

Earthquake induced dynamics at the ionosphere in presence of magnetic storm

The modifications induced in the dynamics of the ionosphere by the major Japan earthquake (EQ) of March 11, 2011 (epicenter at 38.322°N, 142.369°E, M = 8.9) in presence of a magnetic storm are examined by mapping latitudinal variations of F-layer ionization density (NmF2) from 22 stations covering the epicenter zone. The changes forced into the Total Electron Content (TEC) by the major EQ in the magnetic storm ambiance are also examined from the GPS data collected at Guwahati (26° 10′ N, 91° 45’ E), situated in the major fault system of East Asia. The contributions of pre-seismic electric field as well as of magnetic storm time electric field in the observed density variations are brought into the ambit of discussion. The influence of lower atmosphere in shaping TEC features during the study case is highlighted. The effects of solar activity on density variations during such complex ambiances are also addressed.     

A nonlinear background removal method for seismo-ionospheric anomaly analysis under a complex solar activity scenario: A case study of the M9.0 Tohoku earthquake

A precise determination of ionospheric total electron content (TEC) anomaly variations that are likely associated with large earthquakes as observed by global positioning system (GPS) requires the elimination of the ionospheric effect from irregular solar electromagnetic radiation. In particular, revealing the seismo-ionospheric anomalies when earthquakes occurred during periods of high solar activity is of utmost importance. To overcome this constraint, a multiresolution time series processing technique based on wavelet transform applicable to global ionosphere map (GIM) TEC data was used to remove the nonlinear effect from solar radiation for the earthquake that struck Tohoku, Japan, on 11 March, 2011. As a result, it was found that the extracted TEC have a good correlation with the measured solar extreme ultraviolet flux in 26–34 nm (EUV_26–34) and the 10.7 cm solar radio flux (F_10.7). After removing the influence of solar radiation origin in GIM TEC, the analysis results show that the TEC around the forthcoming epicenter and its conjugate were significantly enhanced in the afternoon period of 8 March 2011, 3 days before the earthquake. The spatial distributions of the TEC anomalous and extreme enhancements indicate that the earthquake preparation process had brought with a TEC anomaly area of size approximately 1650 and 5700 km in the latitudinal and longitudinal directions, respectively.     

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.     

Diagnostics of equatorial and low latitude ionosphere by TEC mapping over Brazil

The total electron content (TEC) in the equatorial and low-latitude ionosphere over Brazil was monitored in two dimensions by using 2011 data from the ground-based global navigation satellite system (GNSS) receiver network operated by the Brazilian Institute for Geography and Statistics. It was possible to monitor the spatial and temporal variations in TEC over Brazil continuously during both day and night with a temporal interval of 10 min and a spatial resolution of about 400 km. The daytime equatorial ionization anomaly (EIA) and post-sunset plasma enhancement (PS-EIA) were monitored over an area corresponding to a longitudinal extension of 4000 km in South America. Considerable day-to-day variation was observed in EIA and PS-EIA. A large latitudinal and longitudinal gradient of TEC indicated a significant ionospheric range error in application of the GNSS positioning system. Large-scale plasma bubbles after sunset were also mapped over a wide range. Depletions with longitudinally separated by more than 800 km were observed. They were extended by more than 2000 km along the magnetic field lines and drifted eastward. It is expected that 2-dimensional TEC mapping can serve as a useful tool for diagnosing ionospheric weather, such as temporal and spatial variation in the equatorial plasma trough and crest, and particularly for monitoring the dynamics of plasma bubbles.