GPS Total Electron Content Data Research Articles

Prediction of ionospheric TEC during the occurrence of earthquakes in Indonesia using ARMA and CoK models

Predicting ionospheric Total Electron Content (TEC) variations associated with seismic activity is crucial for mitigating potential disruptions in communication networks, particularly during earthquakes. This research investigates applying two modelling techniques, Autoregressive Moving Average (ARMA) and Cokriging (CoK) based models to forecast ionospheric TEC changes linked to seismic events in Indonesia. The study focuses on two significant earthquakes: the December 2004 Sumatra earthquake and the August 2012 Sulawesi earthquake. GPS TEC data from a BAKO station near Indonesia and solar and geomagnetic data were utilized to assess the causes of TEC variations. The December 2004 Sumatra earthquake, registering a magnitude of 9.1–9.3, exhibited notable TEC variations 5 days before the event. Analysis revealed that the TEC variations were weakly linked to solar and geomagnetic activities. Both ARMA and CoK models were employed to predict TEC variations during the Earthquakes. The ARMA model demonstrated a maximum TEC prediction of 50.92 TECU and a Root Mean Square Error (RMSE) value of 6.15, while the CoK model predicted a maximum TEC of 50.68 TECU with an RMSE value of 6.14. The August 2012 Sulawesi earthquake having a magnitude of 6.6, revealed TEC anomalies 6 days before the event. For both the Sumatra and Sulawesi earthquakes, the GPS TEC variations showed weak associations with solar and geomagnetic activities but stronger correlations with the earthquake-induced electric field for the considered two stations. The ARMA model predicted a maximum TEC of 54.43 TECU with an RMSE of 3.05, while the CoK model predicted a maximum TEC of 52.90 TECU with an RMSE of 7.35. Evaluation metrics including RMSE, Mean Absolute Deviation (MAD), Relative Error, and Normalized RMSE (NRMSE) were employed to assess the accuracy and reliability of the prediction models. The results indicated that while both models captured the general trend in TEC variations, nuances emerged in their responses to seismic events. The ARMA model demonstrated heightened sensitivity to seismic disturbances, particularly evident on the day of the earthquake, whereas the CoK model exhibited more consistent performance across pre- and post-earthquake periods.

Ionosonde and GPS total electron content observations during the 26 December 2019 annular solar eclipse over Indonesia

Abstract. We report the investigation of the ionospheric response to the passage of an annular solar eclipse over Southeast Asia on 26 December 2019 using multiple sets of observations. Two ionosondes (one at Kototabang and another at Pontianak) were used to measure dynamical changes in the ionospheric layer during the event. A network of ground-based GPS receiver stations in Indonesia was used to derive the distribution of total electron content (TEC) over the region. In addition, extreme ultraviolet (EUV) images of the Sun from the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory (SDO) satellite were also analyzed to determine possible impacts of solar-active regions on the changes that occurred in the ionosphere during the eclipse. We found −1.62 and −1.90 MHz reductions (24.0 % and 27.5 % relative reduction) in foF2 during the solar eclipse over Kototabang and Pontianak, respectively. The respective TEC reductions over Kototabang and Pontianak during the eclipse were −4.34 and −5.45 TECU (24.9 % and 27.9 % relative reduction). Data from both ionosondes indicate a consistent 34–36 min delay between maximum eclipse and minimum foF2. The corresponding time delays for eclipse-related TEC reduction at these two locations were 40 and 16 min, respectively. The ionospheric F layer was found to descend with a speed of 9–19 m s−1 during the first half of the eclipse period. We also found an apparent rise in the ionospheric F-layer height near the end of the solar eclipse period, equivalent to a vertical drift velocity of 44–47 m s−1. The GPS TEC data mapping along a set of cross-sectional cuts indicates that the greatest TEC reduction actually occurred to the north of the solar-eclipse path, opposite of the direction from which the lunar shadow fell. As the central path of the solar eclipse was located just to the north of the southern equatorial ionization anomaly (EIA) crest, it is suspected that such a peculiar TEC reduction pattern was caused by plasma flow associated with the equatorial fountain effect. Net perturbations of TEC were also computed and analyzed, which revealed the presence of some wavelike fluctuations associated with the solar-eclipse event. Some of the observed TEC perturbation patterns that propagated with a velocity matching the lunar shadow may be explained in terms of nonuniform EUV illumination that arose as various active regions on the Sun went obstructed and unobstructed during the eclipse. The remaining wavelike features are likely to be traveling ionospheric disturbances (TIDs) generated by the passage of the solar eclipse on top of other diurnal factors.

Climatology of ionosphere over Nepal based on GPS total electron content data from 2008 to 2018

Abstract. In this study, we analyse the climatology of ionosphere over Nepal based on GPS-derived vertical total electron content (VTEC) observed from four stations as defined in Table 1: KKN4 (27.80∘ N, 85.27∘ E), GRHI (27.95∘ N, 82.49∘ E), JMSM (28.80∘ N, 83.74∘ E) and DLPA (28.98∘ N, 82.81∘ E) during the years 2008 to 2018. The study illustrates the diurnal, monthly, annual, seasonal and solar cycle variations in VTEC during all times of solar cycle 24. The results clearly reveal the presence of equinoctial asymmetry in TEC, which is more pronounced in maximum phases of solar cycle in the year 2014 at KKN4 station, followed by descending, ascending and minimum phases. Diurnal variations in VTEC showed the short-lived day minimum which occurs between 05:00 to 06:00 LT (local time) at all the stations considered, with diurnal peaks between 12:00 and 15:00 LT. The maximum value of TEC is observed more often during the spring equinox than the autumn equinox, with a few asymmetries. Seasonal variation in TEC is observed to be a manifestation of variations in solar flux, particularly regarding the level of solar flux in consecutive solstices.

Characterization of High‐m ULF Wave Signatures in GPS TEC Data

AbstractGPS total electron content (TEC) measurements were used to investigate high‐m ultralow frequency (ULF) waves during the recovery phase of a geomagnetic storm. ULF wave signals in TEC data show high coherence and significant common power in the wavelet coherence and cross wavelet transform analyses with magnetic field radial component data from GOES‐15. They did not cause significant ionospheric scintillation or ground magnetic signatures due to ionospheric screening effects. An automatic identification procedure is developed to identify ULF wave signature in TEC data from 10 GPS receivers on January 25, 2016. The waves were mainly distributed on the dayside and post dusk sector from ∼64° to ∼71° magnetic latitude. This is the first time that the large‐scale 2D spatial structure and temporal evolution of high‐m ULF waves are revealed, which demonstrates TEC measurements as an effective high‐m ULF wave remote sensing tool.

Ionospheric Effects During the Total Solar Eclipse Over Southeast Asia‐Pacific on 9 March 2016: Part 1. Vertical Movement of Plasma Layer and Reduction in Electron Plasma Density

AbstractWe report our investigation of ionospheric effects that occurred during a total solar eclipse over the Southeast Asia‐Pacific region on 9 March 2016. In particular, here we examine rapid uplift of the ionospheric F‐layer during the eclipse, and reductions of ionospheric plasma density in areas around the eclipse totality. This study used data from ionosondes at Biak and Guam, as well as data from a bistatic HF radio link (∼1300 km apart, cutting across the eclipse totality trajectory) between Biak and Manado. Gridded total electron content (TEC) data from the Madrigal Database were also used for a cross‐comparison. The ionosonde measurements indicate an upward vertical drift velocity in the range of 21−40 m/s in the ionospheric F‐region during the eclipse. Over Biak, foF2 decreased from 10 MHz to 6 MHz (a 40% reduction) during the eclipse. Meanwhile, foF2 over Guam during the eclipse was suppressed for a few hours; lower than the 7‐day average normal level by 3 MHz. The Madrigal GPS TEC data corroborate these ionosonde measurements. Finally, data from the Biak‐Manado HF radio link indicate that the D‐region ionosphere had diminished substantially during the eclipse.

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.

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.

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.

Comparison of IRI_PLAS and IRI_2012 Model Predictions with GPS-TEC Measurements in Different Latitude Regions

The International Reference Ionosphere (IRI) is an empirical model for providing ionospheric parameters, including Total Electron Content (TEC), electron density, electron and ion temperature etc., in the altitude range from 50 km to 2000 km. Since the IRI model is limited up to 2000 km, IRI-PLAS model, plasmasphere extension of the IRI model, was proposed by the researchers. This paper investigates the TEC prediction performance of IRI-PLAS and IRI-2012 models by comparing GPS TEC data, in different latitude regions for magnetically active and quiet days. TEC data over 9 International GNSS Service (IGS) stations, located in different latitude regions, are used for the comparison. Evaluation of the diurnal results reveals good agreement with correlation coefficient >0.9 between GPS-TEC and empirical models for the quiet day irrespectively of the latitudinal data used. However, while the differences are not relatively high in most part of the active day, they reach high level, above 30 TECu, in some part of the day.

Regional TEC model under quiet geomagnetic conditions and low-to-moderate solar activity based on CODE GIMs

Global empirical total electron content (TEC) models based on TEC maps effectively describe the average behavior of the ionosphere. However, the accuracy of these global models for a certain region may not be ideal. Due to the number and distribution of the International GNSS Service (IGS) stations, the accuracy of TEC maps is geographically different. The modeling database derived from the global TEC maps with different accuracy is likely one of the main reasons that limits the accuracy of the new models. Moreover, many anomalies in the ionosphere are geographic or geomagnetic dependent, and as such the accuracy of global models can deteriorate if these anomalies are not fully incorporated into the modeling approach. For regional models built in small areas, these influences on modeling are immensely weakened. Thus, the regional TEC models may better reflect the temporal and spatial variations of TEC. In our previous work (Feng et al., 2016), a regional TEC model TECM-NEC is proposed for northeast China. However, this model is only directed against the typical region of Mid-latitude Summer Nighttime Anomaly (MSNA) occurrence, which is meaningless in other regions without MSNA. Following the technique of TECM-NEC model, this study proposes another regional empirical TEC model for other regions in mid-latitudes. Taking a small area BeiJing–TianJin–Tangshan (JJT) region (37.5°–42.5° N, 115°–120° E) in China as an example, a regional empirical TEC model (TECM–JJT) is proposed using the TEC grid data from January 1, 1999 to June 30, 2015 provided by the Center for Orbit Determination in Europe (CODE) under quiet geomagnetic conditions. The TECM–JJT model fits the input CODE TEC data with a bias of 0.11TECU and a root mean square error of 3.26TECU. Result shows that the regional model TECM–JJT is consistent with CODE TEC data and GPS–TEC data.

A comparison of TEC predicted by IRI-2012 with that measured at three different stations in low latitude Indian region for the years (2010–2012)

The present study reports the comparison of GPS measured Total Electron Content (TEC) with that predicted by the latest IRI-2012 model at three different stations located within the Equatorial Ionization Anomaly region (EIA) in the Indian sector. The data used for the study are retrieved from three different stations, namely, Surat (geographic latitude 21.16°N, geographic longitude 72.78°E; geomagnetic latitude 12.90°N), Hyderabad (geographic latitude 17.25°N, geographic longitude 78.30°E; geomagnetic latitude 8.65°N) and Bangalore (geographic latitude 13.02°N, geographic longitude 77.57°E; geomagnetic latitude 4.58°N). The period of comparison is three years for rising solar activity from 2010 to 2012. Here it is to note that both Hyderabad and Bangalore are IGS station with the station code (HYDE and IISC respectively). The results for the comparison of seasonal variation shows a good agreement between the measured and modeled TEC for all seasons with deviation of (±15 TECU) for all three years at Surat and Bangalore and with a deviation of (±25 TECU) at Hyderabad. Both topside options NeQuick and IRI01-Corr derived nearly equal TEC at all three stations. It is observed that the GPS TEC data shows the EIA crest at (23.5°N) where as the IRI TEC predicts the EIA crest at (19.7°N) on an average for all the years 2010–2012.

Relative importance of horizontal and vertical transports to the formation of ionospheric storm‐enhanced density and polar tongue of ionization

AbstractThere are still uncertainties regarding the formation mechanisms for storm‐enhanced density (SED) in the high and subauroral latitude ionosphere. In this work, we deploy the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM) and GPS total electron content (TEC) observations to identify the principle mechanisms for SED and the tongue of ionization (TOI) through term‐by‐term analysis of the ion continuity equation and also identify the advantages and deficiencies of the TIEGCM in capturing high‐latitude and subauroral latitude ionospheric fine structures for the two geomagnetic storm events occurring on 17 March 2013 and 2015. Our results show that in the topside ionosphere, upward E × B ion drifts are most important in SED formation and are offset by antisunward neutral winds and downward ambipolar diffusion effects. In the bottomside F region ionosphere, neutral winds play a major role in generating SEDs. SED signature in TEC is mainly caused by upward E × B ion drifts that lift the ionosphere to higher altitudes where chemical recombination is slower. Horizontal E × B ion drifts play an essential role in transporting plasma from the dayside convection throat region to the polar cap to form TOIs. Inconsistencies between model results and GPS TEC data were found: (1) GPS relative TEC difference between storm time and quiet time has “holes” in the dayside ion convection entrance region, which do not appear in the model results. (2) The model tends to overestimate electron density enhancements in the polar region. Possible causes for these inconsistencies are discussed in this article.

Peculiar features of the low‐latitude and midlatitude ionospheric response to the St. Patrick's Day geomagnetic storm of 17 March 2015

AbstractThe current study aims at investigating and identifying the ionospheric effects of the geomagnetic storm that occurred during 17–19 March 2015. Incidentally, with SYM‐H hitting a minimum of −232 nT, this was the strongest storm of the current solar cycle 24. The study investigates how the storm has affected the equatorial, low‐latitude, and midlatitude ionosphere in the American and the European sectors using available ground‐based ionosonde and GPS TEC (total electron content) data. The possible effects of prompt electric field penetration is observed in both sectors during the main phase of the storm. In the American sector, the coexistence of both positive and negative ionospheric storm phases are observed at low latitudes and midlatitudes to high latitudes, respectively. The positive storm phase is mainly due to the prompt penetration electric fields. The negative storm phase in the midlatitude region is a combined effect of disturbance dynamo electric fields, the equatorward shift of the midlatitude density trough, and the equatorward compression of the plasmapause in combination with chemical compositional changes. Strong negative ionospheric storm phase is observed in both ionosonde and TEC observations during the recovery phase which also shows a strong hemispherical asymmetry. Additionally, the variation of equatorial ionization anomaly as seen through the SWARM constellation plasma measurements across different longitudes has been discussed. We, also, take a look at the performance of the IRI Real‐Time Assimilative Mapping during this storm as an ionospheric space weather tool.

The auroral ionosphere TEC response to an interplanetary shock

AbstractThis letter investigates the global total electron content (TEC) response in the auroral ionosphere to an interplanetary shock on 8 March 2012, using GPS TEC data from three pierce point chains. One is a longitudinal chain along ~65° magnetic latitude (MLAT) from ~19 magnetic local time (MLT) through dayside to 03 MLT clockwise; one meridional chain is around 14 MLT from 88° to 59° MLAT; and the third one is a chain along ~75° MLAT from ~14 to 00 MLT clockwise. The first chain clearly presents a TEC signal propagation away from ~14 MLT, indicating the shock impact location. Such a propagation is well consistent with the diffuse shock aurora propagation, and the impact location is well predicted by the shock normal direction calculated using the Geotail solar wind and interplanetary magnetic field data. The meridional chain reveals a very fast TEC signal equatorward expansion at ~45 km/s, which is the manifestation of the shock impact and further compression near the subsolar magnetopause. While TEC along the high‐latitude chain varies randomly, lacking any pattern, it is consistent with the discrete aurora dynamics along the poleward boundary of the auroral oval. These findings strongly support the shock aurora mechanisms of adiabatic compression and field‐aligned current establishment or enhancement, suggest that due to the same mechanisms a shock‐generated TEC variation is a “duplication” of the shock aurora from the global picture to the auroral forms and their dynamics, and open the door for the TEC to be an important tool to understand the solar wind and geospace coupling. These results, for the first time, reveal the prompt, intense, and global ionospheric TEC response to the interplanetary fast‐forward shock.

Assimilative model for ionospheric dynamics employing delay, Doppler, and direction of arrival measurements from multiple HF channels

AbstractWe describe the development of new HF data assimilation capabilities for our ionospheric inversion algorithm called GPSII (GPS Ionospheric Inversion). Previously existing capabilities of this algorithm included assimilation of GPS total electron content data as well as assimilation of backscatter ionograms. In the present effort we concentrated on developing assimilation tools for data related to HF propagation channels. Measurements of propagation delay, angle of arrival, and the ionosphere‐induced Doppler from any number of known propagation links can now be utilized by GPSII. The resulting ionospheric model is consistent with all assimilated measurements. This means that ray tracing simulations of the assimilated propagation links are guaranteed to be in agreement with measured data within the errors of measurement. The key theoretical element for assimilating HF data is the raypath response operator (RPRO) which describes response of raypath parameters to infinitesimal variations of electron density in the ionosphere. We construct the RPRO out of the fundamental solution of linearized ray tracing equations for a dynamic magnetoactive plasma. We demonstrate performance and internal consistency of the algorithm using propagation delay data from multiple oblique ionograms (courtesy of Defence Science and Technology Organisation, Australia) as well as with time series of near‐vertical incidence sky wave data (courtesy of the Intelligence Advanced Research Projects Activity HFGeo Program Government team). In all cases GPSII produces electron density distributions which are smooth in space and in time. We simulate the assimilated propagation links by performing ray tracing through GPSII‐produced ionosphere and observe that simulated data are indeed in agreement with assimilated measurements.

The geomagnetic storm time response of GPS total electron content in the North American sector

AbstractOver the last two decades, maps of GPS total electron content (TEC) have improved our understanding of the large perturbations in ionospheric electron density which occur during geomagnetic storms. However, previous regional and global studies of ionospheric storms have performed only a limited separation of storm time, local time, longitudinal, and seasonal effects. Using 13 years of GPS TEC data, we present a complete statistical characterization of the ionospheric response to geomagnetic storms for midlatitudes in the North American sector where dense ground receiver coverage is available. The rapid onset of a positive phase is observed across much of the dayside and evening ionosphere followed by a longer‐lasting negative phase across all latitudes and local times. Our results show clear seasonal variations in the storm time TEC, such that summer events tend to be dominated by the negative storm response while winter events exhibit a stronger initial positive phase with minimal negative storm effects. We find no discernable difference between spring and fall equinox events with both being equivalent to the average storm time response across all seasons. We also identify a prominent magnetic declination effect such that stronger dayside positive storm effects are observed in regions of negative declination (i.e., eastern North America). On the nightside, asymmetries in the TEC response are observed near the auroral oval and midlatitude trough which may be attributed to thermospheric zonal winds pushing plasma upward/downward along field lines of opposite declination.

Statistical analysis of ionospheric mid-latitude trough over the Northern Hemisphere derived from GPS total electron content data

This study statistically investigated the seasonal variation, magnetic local time (MLT) variation, geomagnetic activity dependence, and solar activity dependence of the mid-latitude trough using GPS total electron content (TEC) data from 2000 to 2014. The daily median Kp index was used to characterize the daily geomagnetic activity level. The results showed that the trough minimum position depended primarily on the geomagnetic activity, MLT, and the season. The trough depth depended primarily on the solar flux index (F107) and, to a lesser degree, on MLT. The trough depth increased as F107 increased and as the incidence angle of solar flux decreased. The trough equatorward half-width decreased as the geomagnetic activity increased. These variations in the GPS-TEC trough minimum position were compared with the variations in the TEC trough derived from the International Reference Ionosphere (IRI)-2007 model. The GPS-TEC trough minimum position changed little with respect to F107, whereas the IRI-TEC trough minimum position showed a strong F107 dependence.

Modelling total electron content during geomagnetic storm conditions using empirical orthogonal functions and neural networks

AbstractIt has been shown in ionospheric research that modelling total electron content (TEC) during storm conditions is a big challenge. In this study, TEC modelling was performed over Sutherland (32.38°S, 20.81°E, 41.09°S geomagnetic), South Africa, during storm conditions, using a combination of empirical orthogonal function (EOF) and regression analyses techniques. The neural network (NN) technique was also applied to the same TEC data set, and its output was compared with TEC modeled using the EOF model. TEC was derived from GPS observations, and a geomagnetic storm was defined for Dst≤−50 nT. The hour of the day and the day number of the year, F10.7p and A indices, were chosen as inputs for the modeling techniques to take into account diurnal and seasonal variation of TEC, solar, and geomagnetic activities, respectively. Both EOF and NN models were developed using GPS TEC data for storm days counted from 1999 to 2013 and tested on different storms. For interpolation, the EOF and NN models were validated on storms that occurred during high and low solar activity periods (storms of 2000 and 2006), while for extrapolation the validation was done for the storms of 2014 and 2015, identified based on the provisional Dst index data. A comparison of the modeled TEC with the observed TEC showed that both EOF and NN models perform well for storms with nonsignificant ionospheric TEC response and storms that occurred during period of low solar activity. For storms with significant TEC response, TEC magnitude is well captured during the nighttime and early morning, but short‐term features, TEC enhancement, and depression are not sufficiently captured by the models. Statistically, the NN model performs 12.79% better than the EOF model on average, over all storm periods considered. Furthermore, it has been shown that the EOF and NN models developed for a specific station can be used to estimate TEC over other locations within a latitudinal and longitudinal coverage of 8.7° and 10.6°, respectively. This is an important result as it reduces the data dimensionality problem for computational purposes.

A regional ionospheric TEC mapping technique over China and adjacent areas on the basis of data assimilation

AbstractIn this paper, a regional total electron content (TEC) mapping technique over China and adjacent areas (70°E–140°E and 15°N–55°N) is developed on the basis of a Kalman filter data assimilation scheme driven by Global Navigation Satellite Systems (GNSS) data from the Crustal Movement Observation Network of China and International GNSS Service. The regional TEC maps can be generated accordingly with the spatial and temporal resolution being 1°×1° and 5 min, respectively. The accuracy and quality of the TEC mapping technique have been validated through the comparison with GNSS observations, the International Reference Ionosphere model values, the global ionosphere maps from Center for Orbit Determination of Europe, and the Massachusetts Institute of Technology Automated Processing of GPS TEC data from Madrigal database. The verification results indicate that great systematic improvements can be obtained when data are assimilated into the background model, which demonstrates the effectiveness of this technique in providing accurate regional specification of the ionospheric TEC over China and adjacent areas.

EOF analysis and modeling of GPS TEC climatology over North America

AbstractAn empirical ionospheric model of the total electron content (TEC) over North America (20°–60°N, 40°–140°W) is constructed using the GPS TEC data collected by Massachusetts Institute of Technology (MIT) Haystack Observatory during the years 2001–2012. This model is based on an analysis of quiet time monthly averages using the empirical orthogonal function (EOF) decomposition technique, allowing for separation of spatial and temporal variations. The importance of different types of spatial‐temporal variations to the overall TEC variability can be well represented by the characteristics of EOF basis functions and associated principal components coefficients, with various modes. The mode one EOF decomposition constitutes 97.5% of the total variance and therefore represents the essential feature of North America spatial and diurnal variation of the TEC. The mode two EOF, as reported in an earlier study, reveals a large and significant symmetric longitudinal variation of the ionosphere, organized with respect to magnetic declination. The mode three EOF decomposition shows midlatitude latitudinal structure that varies with season in a manner very similar to the so‐called winter anomaly. Because of the quick convergence of EOF decomposition modes, the first four EOF modes are utilized for constructing a TEC empirical model. For each of the EOF modes, the temporal variations are expressed analytically in terms of local time, season, and solar activity, and the spatial variations by cubic‐spline functions. An analysis of accuracy and quality indicates that this regional empirical TEC model can reflect the majority of the quiet time monthly means, and represent characteristic temporal‐spatial variations in the North America.