GPS Total Electron Content Research Articles

Evaluation of the improvement of IRI-2016 over IRI-2012 at the India low-latitude region during the ascending phase of cycle 24

This paper investigated the performance of the latest International Reference Ionosphere model (IRI-2016) over that of IRI-2012 in predicting total electron content (TEC) at three different stations in the Indian region. The data used were Global Positioning System (GPS) data collected during the ascending phase of solar cycle 24 over three low-latitude stations in India namely; Bangalore (13.02°N Geographic latitude, 77.57°E Geographic longitude), Hyderabad (17.25°N Geographic latitude, 78.30°E Geographic longitude) and Surat (21.16°N Geographic latitude, 72.78°E Geographic longitude). Monthly, the seasonal and annual variability of GPS-TEC have been compared with those derived from International Reference Ionosphere IRI-2016 and IRI-2012 with two different options of topside electron density: NeQuick and IRI01-corr. It is observed that both versions of IRI (i.e., IRI-2012 and IRI-2016) predict the GPS-TEC with some deviations, the latest version of IRI (IRI-2016) predicted the TEC similar to those predicted by IRI-2012 for all the seasons at all stations except for morning hours (0500 LT to 1000 LT). This shows that the effect of the updated version is seen only during morning hours and also that there is no change in TEC values by IRI-2016 from those predicted by IRI-2012 for the rest of the time of the day in the Indian low latitude region. The semiannual variations in the daytime maximum values of TEC are clearly observed from both GPS and model-derived TEC values with two peaks around March-April and September-October months of each year. Further, the Correlation of TEC derived by IRI-2016 and IRI-2012 with EUV and F10.7 shows similar results. This shows that the solar input to the IRI-2016 is similar to IRI 2012. There is no significant difference observed in TEC, bottom-side thickness (B0) and shape (B1) parameter predictions by both the versions of the IRI model. However, a clear improvement is visible in hmF2 and NmF2 predictions by IRI-2016 to that by IRI-2012. The SHU-2015 option of the IRI-2016 gives a better prediction of NmF2 for all the months at low latitude station Ahmedabad compare to AMTB 2013.

Comparison of GPS TEC with modelled values from IRI 2016 and IRI-PLAS over Istanbul, Turkey

The Total Electron Content (TEC) prediction performances of the empirical IRI model and IRI-PLAS model were investigated by comparing the GPS-based TEC values provided by the IONOLAB group. TEC values were obtained on equinox (March 21 and September 23) and solstice (June 21, and December 21) days in low (2009), medium (2012) and high (2015) solar activity periods at Istanbul, Turkey. The prediction performances of the models were statistically analyzed based on the differences between the GPS-TEC and the empirical models, considering the maximum and minimum deviations, the correlation analysis and the root mean square error (RMSE). As a result of the investigation, it is seen that the empirical models have similar predictive performances when the plasmaspheric effects are neglected, and the IRI-PLAS estimations are generally a little closer to the observed GPS-TEC values than all options of IRI-2016 model. Also, it can be said that “IRI2001”, one of the IRI-2016’s “topside” options, can make better predictions than other options and “IG” solar proxy option of IRI-PLAS model is a more appropriate option than the others in TEC calculations over Istanbul, Turkey.

Two‐Mode Ionospheric Disturbances Following the 2005 Northern California Offshore Earthquake From GPS Measurements

AbstractSeismo‐ionospheric anomalies have been reported, while its disturbance modes, patterns, and mechanism are very complex for different kinds of earthquakes. The earthquake with Mw = 7.2 occurred off the coast of Northern California with strike‐slip faulting on 15 June 2005, which may provide a new insight on ionospheric disturbance waves and patterns from dense Global Positioning System (GPS) observations. In this paper, the detailed seismic ionospheric disturbance modes and characteristics are investigated from dense GPS observations following the 2005 Northern California offshore earthquake. Around 10 min after the earthquake, significant ionospheric disturbances are observed by GPS total electron content (TEC). Furthermore, two‐mode ionospheric disturbances are clearly found with two speeds, 1.51 and 2.31 km/s. The slow‐propagating mode has an amplitude of 0.02–0.04 total electron content unit (TECU), while the fast‐propagating mode has an amplitude of 0.1–0.2 TECU. The frequency spectrogram of the slow mode is around 3.7 mHz, while typical frequency of the fast mode is about 5.3 mHz. The fast‐propagating mode is close to the speed of the seismic Rayleigh wave, which is the up propagating secondary acoustic wave triggered by the Rayleigh wave. The slow mode ionospheric disturbance is propagating horizontally as the acoustic wave at the ionospheric level, which is caused by the fault rupture in focal regions.

Variation of the TEC at a dip equatorial station, Trivandrum and a mid latitude station, Hanle during the descending phase of the solar cycle 24 (2014–2016)

Morphological variations in the Total Electron Content (TEC) of the ionosphere at a dip equatorial station, Trivandrum (8.47°N, 76.91° E) and a mid latitude station, Hanle (32.78° N, 78.95° E) during the period 2014–2016 is studied using the GPS TEC measurements from the InSWIM (Indian Network for Space Weather Impact Monitoring) network of stations. The variation of the TEC at Hanle, which is located in the same longitude sector as that of Trivandrum, is presented here for the first time. At Hanle, the TEC peaks at local noontime while its peak appears with a delay (1600 LT) at Trivandrum. The minima in the TEC at both stations appear during the early morning hours. Nighttime enhancements in the TEC are seen at Trivandrum during the equinoctial months while at Hanle they appear in the summer months. The TEC at both the stations exhibit semi-annual anomaly and equinoctial asymmetry. However, the winter anomaly is seen only at Trivandrum. The TEC at both Trivandrum and Hanle is seen to display strong solar activity dependence with the average TEC decreasing as solar activity decreases. We surmise that the observed diurnal/seasonal/annual variations in the TEC at the dip equatorial station, Trivandrum is controlled by the processes induced by electrodynamics while the TEC variations at the mid latitude station, Hanle is mostly affected by the neutral dynamical processes. The morphological comparison between the TEC at Trivandrum derived from the IRI 2012 model and GPS TEC measurements shows only a qualitative agreement. The IRI 2012 model is found to overly underestimate the daytime as well as nighttime TEC at the dip equatorial station, Trivandrum while it is well in agreement with the TEC measurements at the mid latitude station, Hanle.

Evaluation of the NeQuick model performance under different geomagnetic conditions over South Africa during the ascending phase of the solar cycle (2009–2012)

Abstract. In order to provide a scientific base to the NeQuick characterisation under disturbed conditions, the comparison of its performance for quiet and storm days is investigated in the southern mid-latitude. This investigation was realised using the two versions of the NeQuick model which were adapted to local and storm-specific response by using the critical frequency of the F2 layer (foF2) and the propagation factor (M(3000)F2) derived from three South African ionosonde measurements, Hermanus (34.40∘ S, 19.20∘ E), Grahamstown (33.30∘ S, 26.50∘ E) and Louisvale (28.50∘ S, 21.20∘ E). The number of free electrons contained within a 1 m squared column section known as total electron content (TEC) is a widely used ionospheric parameter to estimate its impact on the radio signal passing through. In this study, the TEC derived from the adapted NeQuick version is compared with observed TEC derived from Global Navigation Satellite System (GNSS) data from co-located or nearby GNSS dual-frequency receivers. The Hermanus K-index is used to select all the disturbed days (K-index ≥ 5) upon moving from low to high solar activity (from 2009 to 2012). For each disturbed day, a quiet reference day of the same month was chosen for the investigation. The study reveals that the NeQuick model shows similar reliability for both magnetic quiet and disturbed conditions, but its accuracy is affected by the solar activity. The model is much better for moderate solar activity epochs (2009 and 2010), while it exhibits a discrepancy with observations during high solar activity epochs. For instance in Hermanus, the difference between GPS TEC and NeQuick TEC (ΔTEC) is generally lower than 10 TECu in 2009, and it sometimes reaches 20 TECu in 2011 and 2012. It is also noticed that NeQuick 2 is more accurate than NeQuick 1, with an improvement in TEC estimation more significant for the high solar activity epochs. The improvement realised in the latest version of NeQuick is more than 15 % and sometimes reaches 50 %. Keywords. Ionosphere (mid-latitude ionosphere; modelling and forecasting)

Ionospheric Density Irregularities, Turbulence, and Wave Disturbances During the Total Solar Eclipse Over North America on 21 August 2017

AbstractData from ionosonde and global positioning system (GPS) total electron content (TEC) observations reveal a number of ionospheric phenomena during the total solar eclipse on 21 August 2017 over North America. The eclipse started over the West Coast at ∼16:00 UTC (∼07:53 LT) and ended over the East Coast at ∼20:00 UTC (∼14:46 LT). We identify a growth of plasma density irregularities and turbulence on the bottomside ionosphere during the eclipse totality (at ∼10:03 LT over Idaho Falls), signified by the distinct appearance of spread‐F echoes in the ionosonde data. In addition, data from the ionosonde observations also show some characteristic signatures of traveling ionospheric disturbances at ∼300‐km altitude during the eclipse. Finally, large reductions in TEC and ionospheric plasma densities (by 33–45%) due to the eclipse were observed in both the GPS TEC and ionosonde data.

Investigation on slab-thickness and B0 over an equatorial station in Africa and comparison with IRI model

The present study investigates the simultaneous morphologies of slab-thickness (τ) and thickness parameter (Bo) over Ilorin (8.500 N, 4.680 E; dip lat. 2.95), an equatorial station, during a year of low solar activity (2010). The τ is deduced from global positioning system total electron content (GPS-TEC) while the F2 peak electron density (NmF2), from digisonde portable sounder (DPS). The use of measured TEC for this type of investigation takes care of the inclusion of plasmaspheric electron content (PEC). The PEC distributions on the topside and bottomside electron density (Ne) profile add considerably to the genuine signatures of τ and Bo. Therefore, the dynamic contributions of the PEC need to be emphasized for the accurate prediction of the ionospheric models. Apart from daytime signatures of the τ and Bo which are not primarily influenced by PEC, we found that between two and 3 h of pre-sunrise and dusk periods are mainly controlled by PEC which manifests as huge peaks in the τ and Bo. Also, our investigation reveals that the Bo profile is thicker than the τ profile during the pre-sunrise periods in June indicating partial flow or halt in PEC. The result revealed approximately the same values of τ and Bo around the sunrise period suggesting the absence or negligible PEC contributions. Our investigation also shows that there are maximum and minimum of the thickness in Bo and τ during the December solstice and June solstice, respectively. We also observed a moderate sunrise enhancement in τ that is not conspicuous in the Bo signature. On the relationship between the τ and Bo, we found a significant association between τ and Bo with the highest coefficient value observed during the June seasons indicating the possibility of predicting τ in the absence of Bo and vice-versa, especially during the June season. The validation of International Reference Ionosphere (IRI) model with observed τ and Bo revealed appreciable discrepancies between the model and observed values, particularly between the IRI-τ and observed τ values. Our result revealed the underestimation of IRI-τ with respect to observed τ value in November and December.

Modeling of local ionospheric time varying characteristics based on singular value decomposition over low-latitude GPS stations

Singular Value Decomposition (SVD) model is implemented to recognize the Total Electron Content (TEC) time series of daily, temporal as well as seasonal characteristics throughout the 24th solar cycle period of the year 2015 in the study. The Vertical (vTEC) analysis has been carried out with Global Positioning System (GPS) data sets collected from five stations from India namely GNT, Guntur (16.44∘ N, 80.62∘ E), and IISC, Bangalore (12.97∘ N, 77.59∘ E), LCK2, Lucknow (26.76∘ N, 80.88∘ E), one station from Thailand namely AITB, Bangkok (14.07∘ N, 100.61∘ E), and one station from South Andaman Island namely PBR, Port Blair (11.43∘ N, 92.43∘ E), located in low latitude region. The first five singular value modes constitute about 98% of the total variance, which are linearly transformed from the observed TEC data sets. So it is viable to decrease the number of modeling parameters. The Fourier Series Analysis (FSA) is carried out to characterize the solar-cycle, annual and semi-annual dependences through modulating the first three singular values by the solar (F10.7) and geomagnetic (Ap) indices. The positive correlation coefficient (0.75) of daily averaged GPS–TEC with daily averaged F10.7 strongly supports the temporal variations of the ionospheric features depends on the solar activity. Further, the significance and reliability of the SVD model is evaluated by comparing it with GPS–TEC data and the standard global model (Standard Plasma-Spherical Ionospheric Model, SPIM and International Reference Ionosphere, IRI 2016).

Large-area tsunami signatures in ionosphere observed by GPS TEC after the 2011 Tohoku earthquake

The internal gravity waves (IGWs) produced by a tsunami can travel into the ionosphere and trigger traveling ionospheric disturbances (TIDs). In this study, we report ionospheric measurements related to the huge transpacific tsunami induced by the 2011 Tohoku earthquake. The total electron content (TEC) extracted from far-field GPS network in the US is employed to detect the TIDs induced by the tsunami. We observed the large-area tsunami-driven TIDs in TEC, which have similar propagation characteristics regarding waveform, arrival time, period, horizontal velocity and direction compared to the tsunami measurements at sea-level. Furthermore, the large-area tsunami-driven TIDs are observed in the ionosphere over not only the ocean but also land, illustrating effectively that the IGWs propagate into the ionosphere along the slant direction. In addition, the raypaths of GPS satellites along the propagation direction of IGWs can observe more significant signatures. These observed results provide a more comprehensive propagation characteristic for tsunami-driven TIDs, which can help to better know the tsunami-ionosphere coupling process.

Tsunami Wave Height Estimation from GPS‐Derived Ionospheric Data

AbstractLarge underwater earthquakes (Mw>7) can transmit part of their energy to the surrounding ocean through large seafloor motions, generating tsunamis that propagate over long distances. The forcing effect of tsunami waves on the atmosphere generates internal gravity waves that, when they reach the upper atmosphere, produce ionospheric perturbations. These perturbations are frequently observed in the total electron content (TEC) measured by multifrequency Global Navigation Satellite Systems (GNSS) such as GPS, GLONASS, and, in the future, Galileo. This paper describes the first inversion of the variation in sea level derived from GPS TEC data. We used a least squares inversion through a normal‐mode summation modeling. This technique was applied to three tsunamis in far field associated to the 2012 Haida Gwaii, 2006 Kuril Islands, and 2011 Tohoku events and for Tohoku also in close field. With the exception of the Tohoku far‐field case, for which the tsunami reconstruction by the TEC inversion is less efficient due to the ionospheric noise background associated to geomagnetic storm, which occurred on the earthquake day, we show that the peak‐to‐peak amplitude of the sea level variation inverted by this method can be compared to the tsunami wave height measured by a DART buoy with an error of less than 20%. This demonstrates that the inversion of TEC data with a tsunami normal‐mode summation approach is able to estimate quite accurately the amplitude and waveform of the first tsunami arrival.

Underutilized Spaceborne GPS Observations for Space Weather Monitoring

AbstractMany of currently operated low Earth orbit satellites, in particular Earth observing meteorological missions, are equipped with dual‐frequency Global Positioning System (GPS) receivers with a zenith‐looking antenna for tasks of precise orbit determination and timing. The already existed databases with accumulated raw GPS measurements, as a by‐product of satellite missions, can arouse specific interest for ionospheric and space weather community too. We demonstrate potential possibilities and advantages of involving underutilized spaceborne GPS measurements for Space Weather activity monitoring by specification of storm‐induced ionospheric plasma density irregularities at different altitudinal domain of the topside ionosphere. We have analyzed a dynamical response of the high‐latitude topside ionosphere to the intense geomagnetic storm of 19–21 December 2015 using up‐looking GPS measurements on board Swarm and Meteorological Operational satellite (MetOp) missions, flying at altitudes of ~500 km and ~835 km, respectively. For the first time, GPS observations on board the meteorological mission MetOp were used to reveal an occurrence of plasma irregularities at altitudes above 835 km. Our results demonstrate that during strong geomagnetic storms the intense plasma density irregularities can occur in the topside ionosphere near ~500 km altitude and can be still persisted above 835 km. Joint analysis of the Super Dual Auroral Radar Network global convection patterns, ground‐based GPS total electron content (TEC) observations, and MetOp‐derived topside TEC observations confirmed that plasma irregularities above ~835 km coincided with the plasmaspheric/magnetospheric part of the storm‐enhanced density and the polar tongue of ionization structures, which is the first direct observation of the storm‐enhanced density/tongue of ionization structure in the plasmaspheric TEC.

Performance of various predicted GNSS global ionospheric maps relative to GPS and JASON TEC data

When using predicted total electron content (TEC) products to generate preliminary real-time global ionospheric maps (GIMs), validation of these ionospheric predicted products is essential. In this study, we evaluate the accuracy of five predicted GIMs, provided by the international GNSS service (IGS), over continental and oceanic regions during the period from September 2009 to September 2015. Over continental regions, the GPS TEC data collected from 41 IGS continuous tracking stations are used as a reference data set. Over oceanic regions, the TEC data from the JASON altimeter are used for comparison. An initial performance comparison between the IGS combined final GIM product and the predicted GIMs is also included in this study. The evaluation results show that the predicted GIMs produced by CODE outperform the other predicted GIMs for all three validation results. The accuracy of the 1-day predicted GIMs, produced by the IGS associate analysis centers (IAACs), is higher than that of the 2-day predicted GIMs. Compared to the 2-day UPC predicted GIMs, the 2-day ESA predicted GIMs are observed to have slightly worse performances over ocean regions and better positioning performances over continental regions.

TEC Regional Modeling and prediction using ANN method and single frequency receiver over IRAN

In order to study in the field of the dynamics and continuous variations in the ionosphere, the ionospheric measurement tools such as ionosondes, incoherent scatter radars, satellites, and Global Positioning System (GPS) networks should be used. Total Electron Content (TEC) is a key parameter in the investigation and identification of ionosphere layer. From observations of dual frequency GPS receivers, the ionospheric TEC can be extracted. Global Ionospheric Maps (GIM) are auxiliary Maps for a study on ionosphere layer in around the world. It is necessary to produce the regional TEC map for precise studying of the ionospheric TEC. Bernese software is used to extract TEC by dual frequency GPS receivers. Regional modeling of ionospheric TEC by Artificial Neural Network (ANN) is a significant domain for prediction TEC at both single and double frequency GPS receivers. Five locations in Iran during the period of 2006-2010 were identified and used in the development of an input space and ANN architecture for the TEC modeling. The input space included the day number (seasonal variation), hour (diurnal variation), sunspot number (a measure of the solar activity) and magnetic index (a measure of magnetic activity). Based on the results, the ANN have capability and flexibility to model and to predict TEC. TEC predicted by ANN A (NN TEC) and TEC obtained from the IRI2007 version of the International Reference Ionosphere (IRI TEC) are compared during equinoxes and solstices. Results show that ANN predicts GPS TEC more accurately than the IRI over Iran. The IRI-2007 model is not a suitable method to produce TEC over IRAN.

Signature of gravity waves and ionospheric perturbations over European and North African region during the solar eclipse of 1 August 2008

Solar eclipses provide unique chances to study the ionospheric/thermospheric response to solar radiation changes. The solar eclipse of 1 August 2008 was an outstanding event since it has occurred during the deep minimum of the solar cycle 23. The ionospheric behavior during that event has been investigated using global GPS-Total Electron Content (TEC) maps, COSMIC satellite measurements and Coherent Ionospheric Doppler Radar (CIDR). We analyzed the behavior of total ionospheric ionization over European and North African region by reconstructing TEC maps and differential TEC maps. TEC estimation showed that the total ionization reduces up to 30% at high latitude region during the eclipse as a function of obscuration. COSMIC observations showed a height profile of percentage reduction in electron density with a maximum reduction at the F2 layer of about 30%. The CIDR system allowed us to study the spatial changes of the ionospheric F2 layer during the passage of the solar eclipse which revealed many interesting features of horizontally-propagating gravity waves with wavelength (λ) about 752.39 km and velocity ∼ 151.28 km/min associated with the solar eclipse. The wavelet analysis displays the existence of Wave-like Structures (WLS) of ∼5 min period at F-region altitude.

Pre-seismic ionospheric anomalies of the 2013 Mw = 7.7 Pakistan earthquake from GPS and COSMIC observations

The seismo-ionospheric anomalies may provide some insights about the earthquake. However, pre-seismic ionospheric anomalies are still challenging. In this paper, seismo-ionospheric anomalies are investigated before the September 24, 2013 (Mw = 7.7) Awaran (Pakistan) earthquake from GPS TEC (Total Electron Content) and COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) data. The TEC data are showing anomalies on September 21, 2013. The abnormality detected in the temporal data is about 10 TECU beyond the 30-day running median. The percentage deviation of the TEC anomaly on September 21, is 30% above the upper confidence interval. The anomalies prevail 5° in Latitude and 10° in Longitude over the epicenter. The spatial and temporal data of TEC showed anomalies in TEC from UT = 08 to UT = 12. In addition, the enhancement on September 21, 2013 is also very significant in COSMIC data. The results of COSMIC completely agreed with GPS TEC anomalies on September 21, 2013. The percentage deviation of the peak plasma frequency on September 21 is 5% of the normal distribution. The storm indices are quiet before and after the earthquake. The pre-sesimic ionospheric anomalies are most probably associated with the 2013 Mw = 7.7 Awaran (Pakistan) earthquake.

A Year‐Long Comparison of GPS TEC and Global Ionosphere‐Thermosphere Models

AbstractThe prevalence of GPS total electron content (TEC) observations has provided an opportunity for extensive global ionosphere‐thermosphere model validation efforts. This study presents a year‐long data‐model comparison using the Global Ionosphere‐Thermosphere Model (GITM) and the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIE‐GCM). For the entire year of 2010, each model was run and compared to GPS TEC observations. The results were binned according to season, latitude, local time, and magnetic local time. GITM was found to overestimate the TEC everywhere, except on the midlatitude nightside, due to high O/N2 ratios. TIE‐GCM produced much less TEC and had lower O/N2 ratios and neutral wind speeds. Seasonal and regional biases in the models are discussed along with ideas for model improvements and further validation efforts.

Modelling the main ionospheric trough using the Electron Density Assimilative Model (EDAM) with assimilated GPS TEC

Abstract. The main ionospheric trough is a large-scale spatial depletion in the electron density distribution at the interface between the high- and mid-latitude ionosphere. In western Europe it appears in early evening, progresses equatorward during the night, and retreats rapidly poleward at dawn. It exhibits substantial day-to-day variability and under conditions of increased geomagnetic activity it moves progressively to lower latitudes. Steep gradients on the trough-walls on either side of the trough minimum, and their variability, can cause problems for radio applications. Numerous studies have sought to characterize and quantify the trough behaviour. The Electron Density Assimilative Model (EDAM) models the ionosphere on a global scale. It assimilates observations into a background ionosphere, the International Reference Ionosphere 2007 (IRI2007), to provide a full 3-D representation of the ionospheric plasma distribution at specified times and days. This current investigation studied the capability of EDAM to model the ionosphere in the region of the main trough. Total electron content (TEC) measurements from 46 GPS stations in western Europe from September to December 2002 were assimilated into EDAM to provide a model of the ionosphere in the trough region. Vertical electron content profiles through the model revealed the trough and the detail of its structure. Statistical results are presented of the latitude of the trough minimum, TEC at the minimum and of other defined parameters that characterize the trough structure. The results are compared with previous observations made with the Navy Ionospheric Monitoring System (NIMS), and reveal the potential of EDAM to model the large-scale structure of the ionosphere. Keywords. Ionosphere (midlatitude ionosphere; modelling and forecasting) – radio science (ionospheric physics)

Nighttime Medium‐Scale Traveling Ionospheric Disturbances From Airglow Imager and Global Navigation Satellite Systems Observations

AbstractIn this study, coordinated airglow imager, GPS total electron content (TEC), and Beidou geostationary orbit (GEO) TEC observations for the first time are used to investigate the characteristics of nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) over central China. The results indicated that the features of nighttime MSTIDs from three types of observations are generally consistent, whereas the nighttime MSTID features from the Beidou GEO TEC are in better agreement with those from airglow images as compared with the GPS TEC, given that the nighttime MSTID characteristics from GPS TEC are significantly affected by Doppler effect due to satellite movement. It is also found that there are three peaks in the seasonal variations of the occurrence rate of nighttime MSTIDs in 2016. Our study revealed that the Beidou GEO satellites provided fidelity TEC observations to study the ionospheric variability.

Statistical seismo-ionospheric precursors of M7.0+ earthquakes in Circum-Pacific seismic belt by GPS TEC measurements

The Circum-Pacific seismic belt is the region heavily affected by earthquakes in the world. The relationship between earthquake (e.g., the geographic location, occurrence time, magnitude, and focal depth) and ionospheric anomalies in the belt was investigated using 100 M7.0+ earthquakes during 2006–2015. The ground-based GPS measurements and global ionosphere map (GIM) data were used for the analyses of the ionospheric variations preceding the earthquakes. The results indicated that the occurrence rate of total electron content (TEC) anomalies was proportional to the magnitude and inversely proportional to the focal depth to a certain degree, and the occurrence frequency of anomalies had a rising trend with the days getting close to the main shock. The occurrence rate of TEC anomalies in the Southern hemisphere was larger than that in the Northern hemisphere. Besides, the spatial characteristics of TEC anomalies showed that the anomalies in low-middle latitudes did not coincide with the epicenter, sometimes the anomalies were also observed in the corresponding conjugated region. However, the TEC anomalies in the high latitude usually appeared around the epicenter and within the seismogenic zone while no TEC anomalies appeared in the conjugated area. These results may have potential applications to the earthquake prediction in the Circum-Pacific seismic belt.

Linear time series modeling of GPS-derived TEC observations over the Indo-Thailand region

This paper proposes a linear time series model to represent the climatology of the ionosphere and to investigate the characteristics of hourly averaged total electron content (TEC). The GPS–TEC observation data at the Bengaluru international global navigation satellite system (GNSS) service (IGS) station (geographic $$13.02^{\circ }\hbox {N}$$ , $$77.57^{\circ }\hbox {E}$$ ; geomagnetic latitude $$4.4^{\circ }\hbox {N}$$ ) have been utilized for processing the TEC data during an extended period (2009–2016) in the $$24{\mathrm{th}}$$ solar cycle. Solar flux F10.7p index, geomagnetic Ap index, and periodic oscillation factors have been considered to construct a linear TEC model. It is evident from the results that solar activity effect on TEC is high. It reaches the maximum value ( $$\sim $$ 40 TECU) during the high solar activity (HSA) year (2014) and minimum value ( $$\sim $$ 15 TECU) during the low solar activity (LSA) year (2009). The larger magnitudes of semiannual variations are observed during the HSA periods. The geomagnetic effect on TEC is relatively low, with the highest being $$\sim $$ 4 TECU (March 2015). The magnitude of periodic variations can be seen more significantly during HSA periods (2013–2015) and less during LSA periods (2009–2011). The correlation coefficient of 0.89 between the observations and model-based estimations has been found. The RMSE between the observed TEC and model TEC values is 4.0 TECU (linear model) and 4.21 TECU (IRI2016 Model). Further, the linear TEC model has been validated at different latitudes over the northern low-latitude region. The solar component (F10.7p index) value decreases with an increase in latitude. The magnitudes of the periodic component become less significant with the increase in latitude. The influence of geomagnetic component becomes less significant at Lucknow GNSS station $$(26.76^{\circ }\hbox {N}, 80.88^{\circ }\hbox {E})$$ when compared to other GNSS stations. The hourly averaged TEC values have been considered and ionospheric features are well recovered with linear TEC model.