Vertical Total Electron Content Variations Research Articles

Spatiotemporal Evolution and Drivers of the Four Ionospheric Storms over the American Sector during the August 2018 Geomagnetic Storm

The spatiotemporal variations and mechanisms of the ionospheric storms over the American sector during the August 2018 geomagnetic storm are investigated. One positive and one negative ionospheric storm occurred in North America and two positive storms were observed in South America. The ionosphere showed prominent hemispheric asymmetries during the four storms. The maximum VTEC (vertical total electron content) variation was more than 15 TECU during the positive storms and about −10 during the negative storm. The GUVI (Global Ultraviolet Imager) oxygen (O) to nitrogen (N2) column density ratio (∑O/N2) and SuperDARN (Super Dual Auroral Radar Network) polar cap potential results showed that the electric field variations played a decisive role in generating the North American negative storm while the thermspheric composition changes were responsible for the North American positive storm and the two South America positive storms.

An artificial neural network model in predicting VTEC over central Anatolia in Turkey

This research investigates the capability of artificial neural networks to predict vertical total electron content (VTEC) over central Anatolia in Turkey. The VTEC dataset was derived from the 19 permanent Global Positioning System (GPS) stations belonging to the Turkish National Permanent GPS Network-Active (TUSAGA-Aktif) and International Global Navigation Satellite System Service (IGS) networks. The study area is located at 32.6°E-37.5°E and 36.0°N-42.0°N. Considering the factors inducing VTEC variations in the ionosphere, an artificial neural network (NN) with seven input neurons in a multi-layer perceptron model is proposed. The KURU and ANMU GPS stations from the TUSAGA-Aktif network are selected to implement the proposed neural network model. Based on the root mean square error (RMSE) results from 50 simulation tests, the hidden layer in the NN model is designed with 41 neurons since the lowest RMSE is achieved in this attempt. According to the correlation coefficients, absolute and relative errors, the NN VTEC provides better predictions for hourly and quarterly GPS VTEC. In addition, this paper demonstrates that the NN VTEC model shows better performance than the global IRI2016 model. Regarding the spatial contribution of the GPS network to TEC prediction, the KURU station performs better than ANMU station in fitting with the proposed NN model in the station-based comparison.

Characteristics of Low-Latitude Ionosphere Activity and Deterioration of TEC Model during the 7–9 September 2017 Magnetic Storm

Under the influence of space weather, abnormal disturbances in the ionosphere will distort the ionosphere model seriously and affect the global navigation satellite system negatively. This study analyzes the ionospheric activity characteristics and the ionospheric model performance in low latitude during a strong geomagnetic storm from 7 to 9 September 2017. The research goals are to determine the abnormal behavior of the ionosphere during the geomagnetic storm and to refine the ionosphere model in the low latitude. In the experiment, the vertical total electron content (VTEC) peak value at low latitudes caused by this geomagnetic storm was significantly higher than that on the geomagnetic quiet day, and the VTEC peak value increased by approximately 75%. In the main phase of the geomagnetic storm, the degree of VTEC variation with longitude is significantly higher than that of the geomagnetic quiet day. The VTEC variation trend in the northern hemisphere is more severe than that in the southern hemisphere. In the region where VTEC decreases with longitude, the VTEC in the northern hemisphere is higher than that in the southern hemisphere on the same longitude at low latitudes, and this phenomenon is not significantly affected by the geomagnetic disturbance of the recovery phase. During the geomagnetic storm, the daily minimum value of VTEC at different latitudes was basically the same, approximately 5 TECU, indicating that the nighttime VTEC of the ionosphere in low latitudes was weakly affected by latitude and geomagnetic storms. Geomagnetic disturbances during geomagnetic storms will lead to anomalous features of the “Fountain effect” in the ionosphere at low latitudes. In addition, this geomagnetic storm event caused the accuracy of spherical harmonics (SH), polynomial, and ICE models to decrease by 7.12%, 27.87%, and 48.56%, respectively, and caused serious distortion, which is negative VTEC values fitted by the polynomial model.

The Geomagnetic Storm Time Response of the Mid Latitude Ionosphere During Solar Cycle 24

AbstractThis paper mainly assesses the geomagnetic storm time response of the Africa, America, Asia and Europe mid latitude ionosphere in relation to the variability of the total electron content during solar cycle 24. This is carried out by considering the vertical total electron content (VTEC) data derived from the Global Positioning System (GPS), the recent International Reference Ionosphere (IRI 2016) and IRI‐Extended to the Plasmasphere (IRI‐Plas 2017) models in different geomagnetic storm days during solar cycle 24 (1 June 2013, 17 March 2015, and 28 May 2017). The effects of the 1 June 2013 and 17 March 2015 (2015 St. Patrick's Day) geomagnetic storms which resulted in intensification and fluctuation of the GPS VTEC variations were significantly enhanced in the European sector. On the other hand, the effect of the storm that occurred on 28 May 2017 was mostly pronounced in the American and Asian sectors. Both models are not able to adequately respond to the effects resulting from the geomagnetic storm. The VTEC calculated by the IRI models also generally decrease on most of the hours when the storm on option is applied during the main and recovery phases. Nevertheless, at the Asian station, the IRI‐Plas 2017 VTEC values tend to increase on most of the hours during the 2015 St. Patrick's Day storm. Moreover, as a result of enhancement of the VTEC, the satellite signal propagation during the positive geomagnetic storms is largely affected by the VTEC fluctuation.

Ionospheric GPS-TEC responses from equatorial region to the EIA crest in the South American sector under intense space weather conditions

We present and discuss the ionospheric F-region observations from equator to the equatorial ionization anomaly (EIA) regions over the South American sector during an intense space weather event occurred between 27 and May 29, 2017. During this geomagnetic storm, the symmetric-H (SYM-H) reached a minimum of −142 nT at ∼0700 UT on May 28, 2017. For this investigation, we analyze the vertical total electron content (VTEC) observations from a chain of nearly 120 Global Positioning System (GPS) stations. Magnetometer measurements obtained at two stations in the low latitude regions are also presented. The observations do not indicate prompt penetration electric field (PPEF) effects in the VTEC variations. Magnetometer's observations over Cuiabá (CBA) and Cachoeira Paulista (CXP) in central west and south parts of Brazil, respectively, have shown a strong cross-correlation with SYM-H in the period between 3 and 48 h. The results also show positive ionospheric storm phase during the recovery phase on May 28, 2017. Positive effect during the recovery phase of the geomagnetic storm is possibly associated with effects of disturbances winds. During the recovery phase, a strong intensification of the EIA took place, possibly related to an additional ionization effect. The VTEC values show differences between the west and east sectors. This indicates that the EIA crest is stronger in the east sector than in the west sector, possibly due to the combination of disturbance wind effects and geomagnetic field geometry where in the east sector the field lines are more inclined.

Responses of the Indian Equatorial Ionization Anomaly to two CME-induced geomagnetic storms during the peak phase of solar cycle 24

This work analyzes the geo-effectiveness of Coronal Mass Ejection- (CME-) induced storms by investigating the responses of ionospheric Vertical Total Electron Content (VTEC) and the Equatorial Ionization Anomaly (EIA) over the Indian sector to two storms. One of the storms occurred on February 19, 2014 (SYM-H: −120 nT), while the other occurred on June 23, 2015 (SYM-H: −204 nT). Both storms were driven by full halo CMEs. Global TEC maps were used to characterize VTEC variations during the storms. June 23, 2015 storm was characterized with stronger solar progenitors, right from its origin, although the VTEC response to the storm was not influenced by their strong progenitors. The CMEs that caused the selected storms are large (Halo CMEs). We inferred that irrespective of the strength of solar origin of a storm, the response of ionization distribution over equatorial and low-latitude regions to it depends on the season of storm occurrence, local time of the storm onset, and PPEF orientation. From the VTEC variations for the three Indian stations namely, Trivandrum (geographic latitude: 8.52°N, geographic longitude: 76.94°E, magnetic latitude: 0.37°N), Hyderabad (17.39°N, 78.49°E, 10.15°N) and Delhi (28.70°N, 77.10°E, 22.70°N), we observed that EIA disturbances were more prominent over Hyderabad than over Delhi. The February 19, 2014 storm was characterized by a localized EIA crest at latitude a little above Hyderabad, while in June 23, 2015 storm localized EIA crest was observed directly on Hyderabad. IRI-2016 model generally underestimated VTEC at the three Indian equatorial and low-latitude locations. Solar cycle 24 was characterized with low heliospheric pressure due to its weak polar field strength. The lower pressure allowed CMEs to expand greatly as they transit through space. As they expand, the strengths of the magnetic field inside them decrease, and such lower-strength magnetic fields cause geomagnetic storms that are less geoeffective, even when their solar/interplanetary progenitors are strong and healthy. This associated weak polar field strength of solar cycle 24 caused weak fountain effect with the attendant inability to exhibit storm-time super-fountain effect in the dayside of the equatorial/low-latitude regions.

Transient variations of vertical total electron content at low latitude during the period 2013–2017

Abstract We use GPS networks to measure the vertical Total Electron Content (VTEC) variations at low latitude, in three longitude sectors: America, Europe-Africa and Asia, collected during the period 2013–2017. This period corresponds to the increasing phase of the solar cycle 24 (SC#24) observed around 2013–2014 as well as the decreasing phase around 2014–2017. Our results discussed a morphological analysis of regular variations in ionization during different phases of solar activity: daytime variations, seasonal and semi-annual variations and variations based on the solar cycle 24 in three longitude sectors. In all longitude sectors, the highest VTEC values are displayed during the two months of the spring, located after sunrise and before sunset. The lowest values are found during the summer and winter seasons. We found that the winter anomaly and the presence of equinoxial peaks are the most pronounced effects in VTECs in the increasing and decreasing phase of the SC#24. A strong asymmetry is detected between equinoxial peaks and the location of peaks occurring in March/April and October/November at maximum in the solar flux variations during the increase phase. We show that the daily VTEC maximum values were registered between 14:00 and 16:00 LT and the minimum values between 4:00 and 6:00 LT. Double ionization peak in the morning and evening is observed in VTEC annual variations, due to the proximity of the equatorial fountain stations. From the statistical analysis part, we observed practically the same distribution of the different classes of VTEC (two peaks, bell-shaped and plateau-shaped) variations in the three sectors of longitude. These observations indicate longitudinal variation in the presence of the winter anomaly in the Equatorial Ionized Anomaly (EIA) region. Additionally, we can note a longitudinal variation of the spring-autumn VTEC asymmetry in the EIA region during the five years 2013–2017. We observe also that the occurrence of nocturnal peak recorded around 19 local time (LT) shows the same characteristics as the vertical drift E × B (B: magnetic field is perpendicular to E: electric field.) with respect to solar cycle, season and longitude. Three essential characteristics we noted: 1) the occurrence of the nocturnal peak generally follows the solar cycle. 2) The occurrence of the nocturnal peak is generally stronger at the equinoxes than at the solstices. 3) The occurrence of the nocturnal peak is stronger in the Europe-Africa and America sectors than in the Asia sector. As a result, nocturnal peak occurrence is well related to the PRE at the origin of the GNSS signal scintillations.

Evidence for the Significant Differences in Response Times of Equatorial Ionization Anomaly Crest Corresponding to Plasma Fountains During Daytime and Post‐Sunset Hours

AbstractBased on 10 years' (2010–2019) of vertical total electron content (VTEC) data from Ahmedabad (23.0°N, 72.6°E, dip angle 35.2°) and campaign based OI 630.0 nm airglow intensity measurements from Mt. Abu (24.6°N, 72.7°E, dip angle 38.0°), it is shown that plasma density over the equatorial ionization anomaly (EIA) crest region increases in varying degrees during post‐sunset hours (2000–2100 LT) in magnetically quiet periods. The post‐sunset peak in VTEC precedes the corresponding peak in airglow intensity. By comparing post‐sunset VTEC enhancements with ionosonde observations from Tirunelveli (8.7°N, 77.7°E, dip angle 1.7°), it is shown that pre‐reversal enhancement (PRE) of the zonal electric field causes these enhancements over the EIA crest region. These observations are supported by TEC measurements by GAGAN (GPS Aided Geo Augmented navigation), the Indian Satellite‐based Augmentation System (SBAS). Comparison of average VTEC variations with global empirical model drifts reveals that the post‐sunset enhancements in VTEC occurs ∼1.7 h after the PRE and are significant only during December solstice and equinoctial months in high solar activity years similar to seasonal variations in PRE amplitudes. This time delay (response time of EIA crest) is almost half compared to the average response time (3–4 h) associated with the daytime fountain. Based on the latitudinal gradient in SBAS‐TEC, it is proposed that the PRE drives plasma from 5°N to 10°N magnetic latitudes to the EIA crest region leading to shorter response time. These results show the important role of the PRE in conditioning the EIA crest region.

Testing and validating IRI-2016 model over Ethiopian ionosphere

This paper focuses on the validation of the latest version of the International Reference Ionosphere (IRI-2016) model in predicting the vertical total electron content (VTEC) variation over Ethiopian ionospheric region during a high solar activity (2012-2016) phase. The diurnal, monthly and seasonal VTEC variations were analyzed from dual-frequency global positioning system (GPS) and IRI-2016 model at the Debark station. The diurnal, monthly and seasonal maximum TEC values measured between around 9:00 and 18:00 UT (12:00 and 21:00 LT) hours and the minimum TEC values measured before 9:00 (12:00 LT) and after 18:00 UT (21:00 LT). The overall results show that IRI-2016 model generally overestimates the diurnal, monthly and seasonal mean VTEC values during the high solar activity (2012-2016) phase. The model prediction generally follows that the monthly and seasonal variations of the measured VTEC values in equinoctial months are higher than solstice months. The monthly mean and seasonal mean values of IRI-TEC and GPS-TEC mostly correlates before early morning and after evening hours and also slightly IRI-TEC underestimates starting from morning hours up to evening hours. The magnetic storm effects on GPS-TEC are associated with fluctuations. However, the IRI-TEC values show smooth pattern during both in storm on and off options. Consequently, the IRI-2016 model does not correctly respond to the effects of the resulting storm disturbance.

Independent temporal and spatial variation analysis of ionospheric TEC over Asia-Pacific area based on BDS GEO satellites

GPS has been widely used to monitor ionospheric total electron content (TEC) variations. However, GPS-derived TEC contains ionospheric horizontal gradient information due to the motion of GPS satellites. As a result, the temporal and spatial variation information is mixed. With the advent of the geostationary earth orbit (GEO) of the BeiDou navigation satellite system (BDS), observations from BDS GEO satellites provide a new opportunity for independent research on the temporal and spatial variations of the ionospheric TEC. Furthermore, the GEO satellite also enables continuous and long-term TEC monitoring. Due to its higher orbit, the BDS GEO-derived TEC also contains more plasmaspheric electron content. In this study, the temporal and spatial variation characteristics of the ionospheric TEC derived from the BDS GEO satellites over the Asia-Pacific area are investigated. The achievable TEC theoretical accuracy from the BDS GEO measurements is first deduced. The TEC time series at 26 BDS stations over the Asia-Pacific area is then analyzed. Results acquired from the vertical TEC (VTEC) observations in 2017 indicate obvious periodic variations and spatial variations with a semi-annual anomaly. Through an analysis of the VTEC series over a span of five years, it is revealed that the long-term TEC variations are quite dependent on the solar activity intensity in terms of the F10.7 index. Besides, the BDS GEO-derived VTEC variation trend is consistent with those acquired from the global ionospheric map (GIM) and the NeQuick model. However, significant biases exist at some stations due to uneven accuracy of the two global ionospheric models. The maximum RMS discrepancies reach 4.3 TECU and 9.9 TECU for the GIM and NeQuick, respectively.

Jeomanyetik fırtına süresince NeQuick2 Modelinin Performansı

The aim of this study is to evaluate the performance of NeQuick 2 model during geomagnetic storm process. For this purpose, vertical total electron content (VTEC) values obtained from NeQuick 2 model and GPS (Global Positioning System) receiver were compared for five stations selected from 5 different latitude regions during 05-09 September 2017. Comparison was made by correlation, root mean square error (RMSE) and difference taking methods. The variation of VTEC values obtained by NeQuick 2 and GPS is generally similar, although there are phase shifts and amplitude differences. The maximum correlation was observed in the equator region on the quiet days before the storm (September 5-6), but the correlation coefficient decreased as it moved away from the equator. The maximum correlation coefficient was calculated at the mid-latitude of the southern hemisphere during the initial, main and return phases of the storm. In addition, the lowest correlation was determined at high latitudes of the northern hemisphere. At all stations (except NYAL station on 05, 07, 08 / 09/2017), the maximum value of the calculated difference is smaller than the minimum value. When the GPS VTEC values are higher than the NeQuick 2 model values, the difference is generally low, on the contrary when the NeQuick 2 model values are greater than the GPS VTEC values, the difference is quite large. This shows that the NeQuick 2 model generally overestimates the TEC values

Performance of NeQuick-2, IRI-Plas 2017 and GIM models over Ethiopia during varying solar activity periods

This paper investigates the performance of NeQuick-2, the latest version of International Reference Ionosphere IRI extended to the Plasmasphere (IRI-Plas 2017), and Global Ionospheric Map (GIM) models over four stations deployed in Ethiopia during medium, high, and low solar activity periods. The monthly and seasonal mean diurnal variations of Vertical Total Electron Content (VTEC) derived from NeQuick-2, IRI-Plas 2017 and GIM models were compared with the GPS-VTEC values in terms of Root-Mean-Square Errors (RMSE) and correlation coefficients (r). It was found that the monthly and seasonal mean diurnal VTEC variations agree fairly well with the GPS-VTEC at all stations although with some offsets. For the entire years of the study, the highest monthly mean VTEC variation is observed during equinox months, whereas minimum in the solstice months. The NeQuick-2 model monthly and seasonal mean VTEC output were generally better than IRI-Plas 2017 and GIM models output in most of the stations during medium solar activity year of 2012. For all models, the significant discrepancy (i.e, overestimations or underestimations) is observed during daytimes and seasons of high solar activities. In some cases, the IRI-Plas 2017 model displayed poor performance during medium and low solar activity periods, while it showed a better agreement for some stations (e.g. asos, armi, and bdmt) during high solar activity phase (especially in March equinox and December solstice). The performance of GIM model was good for most of the stations during low-solar activity year of 2016 and during the study years for adis, while it has poor performance during the periods of high solar activity year of 2014. All models correlated well, although GIM model was highly correlated with the GPS-VTEC value (r ≥0.99). The IRI-Plas 2017 model correlated better with the GPS-VTEC than NeQuick-2 model during the study years. Moreover, the performance of IRI-Plas 2017 model was better during high than low and medium periods of solar activity years. The GIM model has better performance for most stations (e.g. adis, asos, and bdmt) over the study region during low-solar activity period.

Ionospheric VTEC variations over Pakistan in the descending phase of solar activity during 2016–17

This paper presents the variations of the ionospheric Vertical Total Electron Content (VTEC) observed over Pakistan at the verge of low- to mid-latitude regions during the years 2016–17 of the descending phase of the solar cycle. The study is conducted by considering the ionospheric measurements from dual frequency Global Navigation Satellite System (GNSS) receivers permanently installed at Islamabad (geomagnetic Lat. 25.44°N, Long. 148.83°E), Multan (geomagnetic Lat. 22.13°N, Long. 146.91°E) and Quetta (geomagnetic Lat. 22.50°N, Long. 142.73°E). The diurnal, seasonal and annual variations of VTEC over Pakistan are examined in the context of geomagnetic storm during 2016. This study shows high values during the March and September equinoctial months and lower values during the summer and winter solstices from VTEC estimations. Furthermore, higher, moderate and lower VTEC variations are recorded during the seasonal analysis in the equinoxes, summer solstice and winter solstice, respectively. The maximum seasonal VTEC values are observed during the post-sunrise hours between 11:00–17:00 LT and the minimum values are recorded during the post-midnight hours between 02:00–05:00 LT during each season at all the stations. Moreover, the effect of geomagnetic storm is detected in the ionospheric VTEC of the three different stations, which occurred on 13 October 2016. The initial phase of the storm caused no prominent effect on VTEC, while an enhancement in VTEC is registered in the main and recovery phases of the storm. The recorded VTEC from three different stations is correlated with the indices of the geomagnetic storm.

Mid latitude ionospheric TEC modeling and the IRI model validation during the recent high solar activity (2013–2015)

This paper discusses the variability of the modelled Vertical Total Electron Content (VTEC) and performance of the latest versions of the International Reference Ionosphere (IRI) model in the estimation of TEC over the mid-latitude regions in the recent high solar activity (2013–2015) years. This is conducted by comparing the pattern of the variations of the VTEC obtained from five ground based Global Positioning System (GPS) receivers installed at different mid-latitude regions and the latest versions of the IRI model (IRI 2007, IRI 2012 and IRI 2016). It has been observed that the measured (GPS-derived) and modelled (IRI 2007, IRI 2012 and IRI 2016) monthly and seasonal diurnal variability of VTEC show the lowest values at around 10:00 UT (04:00 LT) and the highest values at around 20:00 UT (14:00 LT). Moreover, both the measured and modelled VTEC variations generally follow the pattern of the variation of the solar activity, showing enhancement in shifting from 2013 to 2014, and drop again in 2015, with some exceptional months. In the years 2013–2015, the highest measured and modelled seasonal arithmetic mean VTEC values are observed in the March equinox in 2014; while, the lowest measured and modelled VTEC values are observed in the December solstice and June solstice, respectively in 2015. It has also been shown that the IRI 2016 VTEC values are generally larger than those of the IRI 2007 and IRI 2012 VTEC values and tend to respond to variation of the GPS VTEC values better than others, especially in the equinoctial and June solstice months. Moreover, when compared to the IRI 2007 and IRI 2012 versions, the smallest root-mean-square deviations are observed in using the IRI 2016 version, showing that the IRI 2016 version is generally better in capturing the VTEC values with some exceptional months (especially in the December solstice months). Hence, in the December solstice months, the IRI 2007 and IRI 2012 versions are generally better in estimating the VTEC variations as compared to the IRI 2016 version. Moreover, the root-mean-square deviations obtained (either due to overestimations or underestimations) in the IRI 2007 and IRI 2012 versions are very close to each other, proving that the two versions show similar performance in TEC estimation.

Ionospheric TEC variation over Manama, Bahrain and comparison with NeQuick-2 model

This paper presents the ionospheric total electron content (TEC) variation over Manama city of Bahrain using GNSS observables from International GNSS Services (IGS) station at Manama, Bahrain during the period from January 2016 to December 2017. To understand the behavior of TEC variability, the diurnal, monthly and seasonal vertical TEC (VTEC) variations are studied and subsequently compared with NeQuick-2 model estimations. Diurnal VTEC variation reveals a diurnal peak level of about 14 TECU around 09.00 UT and declines thereafter to reach a level of about 5 TECU around 20.00 UT. However, the comparison study confirms the NeQuick-2 model overestimating the GNSS-VTEC irrespective of time of observation during the day. Monthly behavior of VTEC variation reflects that the NeQuick model is well estimated with GNSS-VTEC during November 2016 and January, February and October months in 2017. The magnitude of variations depicts about 30 to 35 TECU (the highest values) during September 2017 and three continuous months February to April in 2016, while it shows about 10 TECU (the lowest value) during January and December months of 2017, November and December months of 2016. Seasonal VTEC variation shows the underestimation of NeQuick-2 model during the June solstice for the years 2016 and 2017, about 10 TECU. It is notable that NeQuick model shows almost good agreement during the March equinox and December solstice in spite of departure in magnitude difference.

Study on the plasmaspheric Weddell Sea Anomaly based on COSMIC onboard GPS measurements

This study investigated the morphology of the Plasmaspheric Weddell Sea Anomaly (PWSA). Measurements by GPS receivers onboard COSMIC satellites were used to determine the slant total electron content (STEC) along signal propagation paths during 2007–2017, which were converted to the zenith direction using a specific plasmaspheric projection function to obtain vertical total electron content (VTEC). The characteristics of seasonal variation of the PWSA between the four seasons were examined under conditions of high and low solar activity that corresponded to values of the F10.7 index of >120 and ≤ 120, respectively. To investigate seasonal variation of plasmaspheric VTEC, maps of geographic latitude versus geographic longitude were constructed by binning the data into 5° latitudinal grids and 15° longitudinal grids. The median value of VTEC in each grid was calculated for each season under low and high solar activity conditions. The results showed that the WSA phenomenon could also be observed in the plasmasphere (altitude ≥ 800 km) as well as in the ionosphere. The anomaly is most prominent in winter under conditions of high solar activity, and it also can be found in spring and autumn, although its amplitude is relatively small. The equatorward neutral wind is the critical driver for PWSA formation. In addition, during the polar summer, high geographic latitudes are sunlit during the entire day, leading to prolonged photoionization. This is the most essential process for the existence of the nighttime maximum in the VTEC diurnal variation at the geographic latitudes of the PWSA.

Ionospheric temporal variations over the region of Turkey: a study based on long-time TEC observations

In this study, daily mean vertical total electron content (VTEC) and daily mean 2-h VTEC values were obtained from Center for Orbit Determination in Europe–Global Ionosphere Maps (CODE–GIM) data over the region of Turkey between January 1, 2003, and December 31, 2016. The time interval is sufficient to reflect temporal changes of the ionosphere. The daily mean VTEC data was used to analyze the space weather effects on the VTEC variability and daily mean 2-h VTEC data was utilized to see the pattern of the diurnal, monthly, seasonal and yearly variation of VTEC values. The highest correlation was found between VTEC and F10.7 (r = 0.83). Totally, 40 major geomagnetic storms were identified that 45% of the storms are caused a decrease and 55% of the storms are caused an increase in VTEC variation. The maximum VTEC is shown at 13:00 LT and the minimum VTEC is shown at 03:00 LT according to diurnal variation of the 14-year mean 2-h VTEC. The maximum VTEC is shown on April and the minimum VTEC is shown on July according to diurnal variation of monthly mean VTEC. Diurnal variation of seasonal mean VTEC and its standard deviations are higher in equinox than solstices. Diurnal variation of yearly mean VTEC has a significant change from low to high solar activity periods.

Response of the total electron content at Brazilian low latitudes to corotating interaction region and high-speed streams during solar minimum 2008

In this work, we investigate the Brazilian low-latitude ionospheric response to two corotating interaction regions (CIRs) and high-speed streams (HSSs) events during the solar minimum of solar cycle 23, in 2008. The studied intervals are enclosed in the whole heliospheric interval, studied by other authors, for distinct longitudinal sectors. CIRs/HSSs are structures commonly observed during the descending and low solar activity, and they are related to the occurrence of coronal holes. These events cause weak-to-moderate recurrent geomagnetic storms characterized by negative excursions of the interplanetary magnetic field, IMF_Bz, as well as long-duration auroral activity, considered as a favorable scenario for continuous prompt penetration interplanetary electric field (PPEF). In this study, we used the vertical total electron content (VTEC) calculated from GPS receivers database from the Brazilian Continuous Monitoring Network managed by the Brazilian Institute of Geography and Statistics. Moreover, we analyzed the F-layer peak height, hmF2 and the critical plasma frequency, foF2, taken from a Digisonde installed at the southern crest of the equatorial ionization anomaly, in Cachoeira Paulista, CP. It was observed that during the CIRs/HSSs-driven geomagnetic disturbances VTEC increased more than 120% over the quiet times averaged values, which is comparable to intense geomagnetic storms. On the other hand, VTEC decreases were also observed during the recovery phase of the storm. Spectral analysis using gapped wavelet technique (GWT) revealed periodicities of 7, 9, 13.5 days, which are sub-harmonics of the solar rotation period, ~ 27 days. These periods in VTEC are closely associated with those observed in solar and geomagnetic indices such as Vsw, IMF_Bz and AE during CIRs/HSSs intervals. We discuss PPEF associated to IMF_Bz reconnection processes and the auroral activity as the most probable causes for the VTEC variations. These results can be of interest for studies related to space weather monitoring, modeling and forecasting, especially during low solar activity.

Analysis of Ionospheric TEC Variation over Manama, Bahrain, and Comparison with IRI-2012 and IRI-2016 Models

In this paper, we investigate the ionospheric total electron content (TEC) variations over Manama, Bahrain, through Global Positioning System (GPS) observations from an International GPS Services (IGS) station (BHR3, 26.21 $$^{\circ }$$ N, 50.61 $$^{\circ }$$ E) during the period January 2016 to October 2017. The estimated vertical TEC (VTEC) from International Reference Ionosphere (IRI-2012 and IRI-2016) models is compared with the GPS-derived TEC to evaluate their effectiveness over the region. Diurnal variation of VTEC demonstrates highest value ( $$\sim $$ 20 TECU) around 10.00 UT, which thereafter declines to attain the lowest value after midnight. However, both the versions of IRI model overestimated GPS-VTEC during daytime and underestimated during nighttime with exception to few hours surrounding dusk and dawn period when the IRI models almost approximate the observed value. The monthly behavior of VTEC indicates the highest value ( $$\sim $$ 30 TECU) of GPS-VTEC during February 2016 to April 2016, while the lowest value ( $$\sim $$ 17 TECU) in GPS-VTEC is obtained during November 2016 to January 2017. The highest percentage deviation between the IRI models and GPS-VTEC is observed to be up to 40% during July 2016 which further increases to reach a level of even more than 60% in nighttime in majority of the months. The underestimation of IRI models during the summer season of the year 2016 is found $$\sim $$ 9 TECU, and same season for the year 2017 is found $$\sim $$ 5 TECU. The difference between IRI-2016 and IRI-2012 indicates the improvements in IRI-2016, though it needs further refinements in the present version for relatively realistic predictions irrespective of latitudes and longitudes. Our results may be useful for navigational and modeling improvements over the Bahrain territory.

Mathematical modelling of ionospheric TEC from Turkish permanent GNSS Network (TPGN) observables during 2009–2017 and predictability of NeQuick and Kriging models

The present study examines the ionospheric Total Electron Content (TEC) variations in the lower mid-latitude Turkish region from the Turkish permanent GNSS network (TPGN) and International GNSS Services (IGS) observations during the years 2009 to 2017. The corresponding vertical TEC (VTEC) predicted by Kriging and NeQuick-2 models are evaluated to realize their efficacy over the country. We studied the diurnal, seasonal and spatial pattern of VTEC variation and tried to estimate by a new mathematical model using the long term of 9 years VTEC data. The diurnal variation of VTEC demonstrates a normal trend with its gradual enhancement from dawn to attain a peak around 09:00–14.00 UT and reaching the minimum level after 22.00 UT. The seasonal behavior of VTEC indicates a strong semi-annual variation of VTEC with maxima in September equinox followed by March equinox and minima in June solstice followed by December solstice. Also, the spatial variation in VTEC depicts a meaningful longitudinal/latitudinal pattern altering with seasons. It decreases longitudinally from the west to the east during March equinox and June solstice increases with latitude. The comparative analysis among the GNSS-VTEC, Kriging, NeQuick and the proposed mathematical model are evaluated with the help one way ANOVA test. The analysis shows that the null hypothesis of the models during storm and quiet days are accepted and suggesting that all models are statistically significantly equivalent from each other. We believe the outcomes from this study would complement towards a relatively better understanding of the lower mid-latitude VTEC variation over the Turkish region and analogous latitudes over the globe.