TEC Variations Research Articles

Intensity of 27-day variations in solar emission and ionospheric electron content

The rotation of the Sun modulates solar emission with 27-day variations. Large-scale structures in the solar atmosphere, like sunspots and plages, rotate with the Sun and emit at extreme ultraviolet (EUV) and microwaves that produce the varying part of the emission. As EUV emission is the main driver of ionization in the Earth ionosphere, its variations are reflected in the electron content. In this paper, we analyze wide range of data from radio emission in 245-17000 MHz range and the entire EUV spectrum to total and global electron contents (TEC and GEC, respectively) in the ionosphere to find out where the 27-day variations are most prominent. We show that the relative contribution of 27-day variations to radio emission is maximal at 1000 MHz and decreases at 245 and 17000 MHz. The variations are pronounced throughout the EUV spectrum, their intensity vary at different spectral lines and, on average, is higher than in radio emission. The distribution of the variations in global ionospheric maps (GIM) shows an intensification of 27-day variations in TEC in the North America region, which can be related to the winter anomaly as it is also more significant in this region. Temporal variations of the 27-day component of emission are similar in shape for radio and EUV emission, as well as for GEC, and have a high correlation, up to 0.8. Cycle-averaged temporal variations imply that the 27-day component weakens significantly during solar minimum but can intensify during the remaining phases of the solar cycle. We also find that the flux at 1000 MHz, which originate from free-free emission in contrast to F10.7, can be used as a proxy index of solar activity because it has the highest intensity of 27-day variations and has been observed since 1956.

Variability of Ionospheric Total Electron Content Over Morocco During the Godzilla Sand and Dust Storm of June 2020

During sand and dust storm (SDS) events, atmospheric suspension and transport of sand and dust brings a reasonable amount of electrification in the atmosphere which plays a very important role in the atmosphere-ionosphere coupling. The Godzilla SDS began on 5<sup>th</sup> June 2020 in Algeria following a decrease in pressure and spread to other areas across the Sahara between 6<sup>th</sup> and 28<sup>th</sup> June 2020. Using SDS data from Copernicus Sentinel-5P satellite mission and Vertical Total Electron Content (VTEC) data from four GNSS receiver stations: IFR1 (Ifrane Seismic), MELI (Melilla), TETN (Tetouan) and OUCA (Ouca) over Morocco, we investigate the possible ionospheric TEC variability over the four GNSS receiver stations during the Godzilla SDS event which was tracked using the Sentinel-5P Satellite mission. Solar wind parameters: Horizontal component of Interplanetary Magnetic Field (IMF-Bz), interplanetary Electric Field (IEF-Ey) and solar wind speed (V) and geomagnetic indices: Disturbance Storm Time (Dst) and Planetary K (Kp) indices were examined and showed very minimal geomagnetic influence during the period. We observed major ionospheric disturbances over the four Global Navigation Satellite System (GNSS) receiver stations on 16<sup>th</sup>, 17<sup>th</sup>, 18<sup>th</sup>, 21<sup>st</sup>, 22<sup>nd</sup>, 23<sup>rd</sup> 25<sup>th</sup> and 26<sup>th</sup> June 2020: the period with the Sentinel-5P Aerosol Index (SAI) of more than 4 as recorded by the Sentinel-5P Satellite engine. The daily VTEC values over the four GNSS receiver stations recorded continuous electron density perturbations during these days. Apart from the ionospheric TEC perturbations, significant enhancements and decreases in daily maximum VTEC values over the four GNSS receiver stations were also noted. These were attributed to the changes in the atmospheric electric fields generated by the SDS event. The VTEC plots for each day exhibited similar trends, hence exhibited the same ionospheric dynamics. VTEC depletions of depths 3 to 6 TECU over all the four GNSS receiver stations were noted on 12<sup>th</sup>, 14<sup>th</sup>, 17<sup>th</sup>, 20<sup>th</sup> and 25<sup>th</sup> June 2020. Nighttime VTEC enhancements were also noted and majorly occurred between 20:00 and 21:00 UT on 9<sup>th</sup>, 13<sup>th</sup>, 15<sup>th</sup>, 17<sup>th</sup>, 19<sup>th</sup>, 20<sup>th</sup> and 21<sup>st</sup> June 2020. This was attributed to the development of the electron avalanche processes including dust and electron absorption or losses and the active conversion to electron dissociative attachment leading to electron excitation. In conclusion, the Godzilla SDS of June 2020 led to the electron density perturbations over Morocco.

On the variations in equatorial and low-latitude GPS-TEC and assessment of NeQuick-2, IRI-2016 and IRI-2020 models in the African longitude during solar cycle 24–25

Ionospheric models play a crucial role in understanding, prediction, and mitigation of the effects of ionospheric variability on a wide range of technological and scientific applications relying on space-based services. Conversely, the models need to be routinely updated with newer datasets and specifications to account for the regional discrepancies in the changing ionospheric conditions due to various dominant localized physical and chemical processes. Although there have been ongoing improvements to the extensively utilized empirical model known as the International Reference Ionosphere (IRI), the newly emerged version (IRI-2020) needs to undergo global testing. In this research, we carried out diurnal and seasonal variations in GPS-TEC and the assessment of some ionospheric models such as IRI-2016 and its recently updated version (IRI-2020), alongside the NeQuick-2 model at 2 stations each in the East, West and South in the equatorial and low-latitude African longitudes during different phases of solar cycles 24–25 (2016 – 2021). Also, we carried out statistical analysis between GPS-TEC and the ionospheric models using Root Mean Square Error (RMSE) and Mean Absolute Error (MAE), in order to show the model with the best forecasting capability in the African region. Diurnal, seasonal and solar cycle variations in GPS-TEC, NeQuick-2, IRI-2016 and IRI-2020 were observed, showing higher magnitudes in the West, followed by the East in close range and least in the Southern sector of the African longitudes. TEC Variations in some sectors in the African longitudes show a consistent trend in this research. More importantly, there are observed regional differences within the African longitudes owing to the wider coverage of landmass in the equatorial and low latitudes. However, TEC variations in the Northern sector of Africa are not included in the present research. From our observation, NeQuick-2 and IRI-2016 models either underestimate or overestimate GPS-TEC during different phases of the solar cycles at the three sectors in the African longitudes, whereas IRI-2020 shows mostly underestimating characteristics at the three sectors irrespective of solar activity conditions during the study period. Nevertheless, the underestimation or overestimation of NeQuick-2 and IRI-2016, and the underestimation of IRI-2020 are reflected in the RMSE and MAE values. Regrettably, the predictions from IRI-2020 model are not satisfactory at any of the three sectors in the African longitudes and prompt attention of the modeling community for further investigations towards possible refinements in the model specifications.

Response to geomagnetic storm on 23–24 March 2023 long-lasting longitudinal variations of the global ionospheric TEC

This study examines the global behavior of the ionospheric TEC anomalies, i.e. their structures and temporal variability, during the initial, main and recovery phase of the geomagnetic storm occurring on 23–24 March 2023. For this purpose the global vertical TEC maps, generated by the NASA JPL IGS Ionosphere center, have been used and the geomagnetically forced anomalies have been presented in 2D (longitude-modip latitude) and polar maps. The main peculiar features of the TEC response could be summarized as: (i) the TOI-like event observed at the SH midnight hours during the main storm phase, and (ii) a clear longitude asymmetry between the TEC response at low and middle latitudes of the western and eastern hemispheres was seen; while the long-lasting positive response was evident for a full day in the western hemisphere the negative response or almost the lack of response was a characteristic of the eastern part. The study presented possible explanations of the above mentioned basic type of TEC anomalies by using the GUVY [O/N2] ratio maps and geomagnetic data from eight magnetometric stations situated at low latitudes for the separation of the effect generated by the DDEF. An attempt was made the observed TEC response during different storm phases to be reproduced. The presented reconstructions described in almost perfect way the real TEC responses and the RMSE assessments provided clear evidence for this evaluation.

TEC variations and IRI-2016, IRI-2020 and IRI-Plas performance in Mexico

The present work discusses TEC variations over Mexico and their representation by the advanced ionospheric models IRI-2016, IRI-2020 and IRI-Plas during the years of low and increasing solar activity, 2018–2022. The statistical estimates Mean Absolute Error (MAE), Root Mean Square Error (RMSE) and Normalized Root Mean Square Error (NRSME) were used to assess model performances. The earlier revealed TEC features were confirmed and new features were revealed. The hour of the diurnal TEC maximum varies in a wide range within 10–20 LT. In summer, it occurs at earlier hours than in other seasons. During the solar minimum, the annual variation (two minima and two maxima during a year) was found absent and a seasonal dependence of the lowest night TEC value is revealed. The accuracy of TEC representation depended on the model, hour of the day, season and solar activity level and suffered a notable change in 2022. In the considered period, IRI-2016 overestimated the daytime and underestimated the nighttime TEC in most cases. Essentially, IRI-2016 and IRI-2020 gave similar results. IRI-Plas overestimated the daytime TEC during the low solar activity and mostly gave adequate estimates at night. Its accuracy at day hours increased with the growth of solar activity. Quantitative estimates of the accuracy of the models are as follows. The yearly average of RMSE for IRI-2016 and IRI-2020 was minimal in 2018 and maximal in 2022. In contrast, for IRI-Plas it was minimal in 2022 and maximal in 2021. At nighttime, RMSE typically varied as follows. For IRI-2016, within (1–5) TECU during low and within (2–12) TECU during increasing solar activity; for IRI-2020 within (1–3) TECU and (1–9) TECU; and for IRI-Plas within (0.5–4) TECU and (2–9) TECU, respectively. At daytime, RSME were higher: for IRI-2016 within (4–11) TECU under low and (6–19) TECU under increasing solar activity; for IRI-2020 (4–9) TECU and (4–17) TECU; and for IRI-Plas (10–19) TECU and (6–15) TECU, respectively. The discrepancies between the modeling results and observations at low latitudes are due to the poor representation of EIA structures and topside and plasmaspheric parts in TEC. TEC annual variation is always represented by the models, but, it was practically absent during the solar minimum according to the experimental results.

A novel ionospheric TEC mapping function with azimuth parameters and its application to the Chinese region

The ionospheric mapping function (MF) for Global Navigation Satellite System (GNSS), a mutual projection method for the slant total electron content (STEC) and vertical total electron content, is one of the significant factors affecting the performance of ionospheric models. The commonly used MF assumes isotropic TEC variations and takes into account only the satellite elevation angle, which may result in significant ionospheric projection errors, especially at low elevation angles. Based on the single-layer model, we propose an additional azimuth parameter mapping function (APMF). The APMF was estimated and evaluated by the NeQuick model during the periods of January 2014 and January 2022 from the aspect of simulation and measured STEC during the periods of 2014 and 2022 from the aspect of actual measurements over China, respectively. Compared to the modified single-layer model mapping function (MSLM-MF), the experimental results indicate that (1) The APMF can significantly reduce the ionospheric projection error, and the fluctuation in errors with different azimuth angles is small. (2) According to the evaluation based on the NeQuick simulation during the TEC peak time, when the ionosphere is quite active, the upper and lower quartiles of the absolute projection error boxplot of the APMF relative to the MSLM-MF in January 2014 are reduced by 56.1% and 60.0%, respectively, and in January 2022, they are reduced by 67.7% and 65.2%, respectively. Similarly, the upper whiskers in the boxplot are reduced by 54.7% and 67.5% in January 2014 and January 2022, respectively; the APMF performance in terms of the root mean square error (RMSE) is improved by 47.0% in January 2014 and 58.3% in January 2022. (3) According to the evaluation based on the measured STEC from GNSS raw data during the TEC peak time, the upper and lower quartiles of the absolute mapping error boxplot of the APMF relative to the MSLM-MF in 2014 are reduced by 48.9% and 46.9%, respectively, while in 2022, they are reduced by 48.3% and 41.2%, respectively. The upper whiskers in the boxplot are reduced by 41.8% and 35.2% in 2014 and 2022, respectively; the APMF performance in terms of RMSE is improved by 44.6% in 2014 and 39.2% in 2022.

Investigating the effect of TEC variability on a dual-band GNSS receiver's geographic location using carrier phase measurement

Investigating the effect of TEC variability on a dual-band GNSS receiver's geographic location using carrier phase measurement

Lower Ionospheric variations during the intense tectonic activity in the broader area of Greece on October of 2020

In this paper, we investigate the Lower ionospheric variations from TEC observations during the intense seismic activity of October 2020 in the area of Greece. The data were analysed using both, statistical analysis of TEC variations in order to detect uneven gross variations and Discrete Fourier analysis in order to investigate the TEC turbulence. The results of this investigation indicate that the High-Frequency limit fo of the ionospheric turbulence content, increases as aproaching the occurrence time of the earthquake, pointing to the earthquake epicenter, in accordance with our previous investigations. We conclude that the Lithosphere Atmosphere Ionosphere Coupling, LAIC, mechanism through acoustic or gravity waves could explain this phenomenology. In addition, the statistical analysis shows an excess greater than 3𝜎 from the mean TEC values, one and seven days before the earthquake. Since no major disturbance of the geomagnetic field occured during these days, we conclude that we probably observed precursory ionospheric variations in accordance to analogous findings from the variation of VH/VHF electromagnetic wave propagrations over strong earthquake areas.

Modeling Total Electron Content and Critical Frequency at F1 and E Layer Boundary

This work investigates the variation of the total electron content TEC and the critical frequency fo in the boundary zone of the F1 and E layers at the low-latitude in the ionosphere. This study takes place at the Ouagadougou station (12.4°N and 358.5°E), in West Africa during the quiet geomagnetic activity of solar cycle 23. Ionosphere is the upper layer of the Earth's atmosphere ionized mainly by solar X- and UV-rays, extending from around 80km altitude up to 1000km [1] [2]. Ultraviolet light from the sun ionizes the atoms and molecules in the Earth's upper atmosphere [3]. For this study we use the 2016 version of International Reference Ionosphere (IRI) model. The quiet periods of maximum and minimum phase of solar cycle 23 are considered [4] [5]. From this study, it emerges that at the E and F1 layer boundary zone, TEC and fo increase during the day as solar irradiance increases and decrease as solar irradiance decreases.

Analysis of Ionospheric TEC Variations and Prediction of TEC during Earthquakes Using Ordinary Kriging Based Surrogate Model

Analysis of Ionospheric TEC Variations and Prediction of TEC during Earthquakes Using Ordinary Kriging Based Surrogate Model

Signature of thunderstorm induced acoustic and gravity waves at low latitude Indian sector

This study reports thunderstorm-generated acoustic and gravity waves from a tropical location using newly launched Indian navigation system NavIC for the first time. The detection of acoustic wave components in travelling ionospheric disturbance (TID) measurement is very significant. In compared to acoustic wave gravity wave signature is more ubiquitous because gravity wave propagates in thermospheric altitude by dissipative filtering process. The turbulent nature in plasma characteristics due to strong vertical drift and plasma motion in lower latitude region generate strong scintillation effect and plasma bubble. Large temperature gradient at thermospheric altitude also provides a strong attenuation effect in this region for comparatively high frequency acoustic wave. We have found the TID signatures on TEC variation for acoustic wave up to 0.4 TECU for both the days whereas gravity wave induced signatures went up to 0.8 TECU. Measurements using GPS signal validate signatures obtained from the NavIC signal. The study shows the potential of application of NavIC system for middle atmosphere as well as thermospheric study and understanding of coupling process between various atmospheric layers in tropical region which is not well known so far.

Ionospheric Response to the 6 February 2023 Turkey–Syria Earthquake

Two strong earthquakes occurred in Turkey on 6 February 2023, at 01:17:34 (nighttime, Mw = 7.8) and at 10:24:50 UT (daytime, Mw = 7.5). The seismo-ionospheric impact is an important part of the near-Earth environment state. This paper provides the first results on the ionospheric effects associated with the aforementioned earthquakes. We used data from global navigation satellite system (GNSS) receivers and ionosondes. We found that both earthquakes generated circle disturbance in the ionosphere, detected by GNSS data. The amplitude of the ionospheric response caused by daytime M7.5 earthquake exceeded by five times that caused by nighttime M7.8 earthquake: 0.5 TECU/min and 0.1 TECU/min, respectively, according to the ROTI data. The velocities of the earthquake-related ionospheric waves were ~2000 m/s, as measured by ROTI, for the M7.5 earthquake. TEC variations with 2–10 min periods showed velocities from 1500 to 900 m/s as disturbances evolved. Ionospheric disturbances occurred around epicenters and propagated to the south by means of 2–10 min TEC variations. ROTI data showed a more symmetric distribution with irregularities observed both to the South and to the North from 10:24:50 UT epicenter. The ionospheric effects were recorded over 750 km from the epicenters. Ionosonde located 420/490 km from the epicenters did not catch ionospheric effects. The results show significant asymmetry in the propagation of coseismic ionospheric disturbances. We observed coseismic ionospheric disturbances associated with Rayleigh mode and acoustic modes, but we did not observe disturbances associated with acoustic gravity mode.

Anomalous Variations in Ionosphere TEC Before the Earthquakes of 2021 in the Different Parts of the Globe

The present article shows the satellite-based TEC data analysis for 6 earthquakes (M ≥ 6.0) from specific parts of the earth that occurred in 2021 during the low solar radiation period. In search of pre-earthquake signatures in the ionosphere, the study of the data of GPS-based TEC before the earthquake is needed. L1 and L2 frequency delays have shown STEC and VTEC formulations for Ionosphere electron density measurements. The TEC data shows unusual behavior from 1 to 30 days before an earthquake. The primary goal of this article is to investigate and establish the correlation of lithosphere atmosphere coupling before earthquakes. Dst and F10.7 solar flux parameters have shown solar radiation. Additionally, TEC data also represented as heat maps to confirm their location at the epicenter. This research might help us understand how ionosphere properties respond to seismic activity in the earth’s crust. HIGHLIGHTS There is a growing interest in studying the seismo-ionospheric relationship today An analysis of ionospheric density changes prior to earthquake is presented in this paper We have observed anomaly in TEC [riot o seismic condition, while There were no changes in solar indices. The results of TEC enhancements led us to believe they were responsible for these high TECs GRAPHICAL ABSTRACT

Koudougou (Burkina Faso, Africa), GPS-TEC Response to Recurrent Geomagnetic Storms during Solar Cycle 24 Declining Phase

In this paper, we presented the effect of moderate geomagnetic storms on the TEC variation at the Koudougou station (Geo Lat 12° 15′N; Geo Long: -2° 20′E) in Burkina Faso (Africa) during the descending phase of solar cycle 24. For this purpose, four moderate geomagnetic storms without storm sudden commencement (SSC) or sudden impulse (SI) that occurred on May 13, 2015 (Dst: -76 nT), June 08, 2015 (Dst: -73 nT), September 11, 2015 (Dst: -80 nT), and May 08-09, 2016 (Dst: -88nT), were considered. These moderate storms were found to be associated with transients induced by fast solar winds. At the Koudougou station, TEC variation shows a positive response to the different moderate geomagnetic storms studied, with increases of order of 2-21 TECU around 1300-1500 UT except for September 11, 2015, TEC variation which shows sometimes negative responses at a few hours (mainly at night). TEC increases observed are a function of geomagnetic parameter (magnitude and polarity) variation. Storm-induced electric field and neutral winds are the main drivers of TEC changes observed during the selected geomagnetic storms. In addition, it was found that the TEC peak on storm day behaves differently compared to the days before and after the storm depending on whether Dst is positive or negative before southward inversion. Indeed, a TEC small peak relative to the days before and after the storm is observed when Dst is negative before southward inversion, and a larger peak occurs in the opposite case. The reasons for these differences are not investigated in this paper.

Statistical and Wavelet Transform-Based Study of the Latitudinal Ionospheric Response to an Annular Solar Eclipse on June 21, 2020

The ionosphere is a very complex and variable part of the atmosphere and it is controlled by solar activity. A solar eclipse is one of the phenomena which depicts a major impact on the ionosphere. In this study, we have analyzed the TEC data of 11 IGS-TEC stations (including one GPS station namely Agra) corresponding to a solar eclipse of June 21, 2020 for the duration of June 07–21, 2020. The TEC variations show lower values on the eclipse’s day in comparison to the other days from the mean of each station except some of the stations like Agra (≈2 TECU), BHR4 (≈1TECU), IISC (≈0.5TECU) have shown the enhanced TEC variations. These results are examined by applying wavelet transform techniques such as continuous wavelet transforms (CWTs), and wavelet decomposition over the average, addition, and multiplication of TEC data of 11 stations for the duration of 9:30 AM to 3:30 PM on the eclipse’s day. These results match very well with our statistical results and depict a better representation of the TEC variations during the solar eclipse. The wavelet decomposition of TEC variation has provided that TEC is affected by solar eclipse globally. These TEC variations are interpreted in terms of the mechanisms available in the literature.

Investigating the effect of TEC variability on a dual-band GNSS receiver’s geographic location using carrier phase measurement

Investigating the effect of TEC variability on a dual-band GNSS receiver’s geographic location using carrier phase measurement

The Variation of Ionospheric TEC During Solar Eclipse on March 20, 2015

Bu çalışma, 20 Mart 2015 tarihindeki güneş tutulması sırasında toplam elektron içeriğinin (TEİ) değişimini araştırmayı amaçlamaktadır. Bu amaçla, beş farklı GPS alıcısından elde edilen TEİ değerleri [(MORP (55.21 N, 1.68 W), ONSA (57.39 N) , 11.92 D), MAR6 (60.59 K, 17.26 D), METS(60.22 K, 24.40 D), TRO1(69.66 K, 18.94 D)] yüzde sapma hesabı kullanılarak güneş tutulması sırasında iyonosferik değişimi belirlemek için incelendi. Güneş tutulmasının etkisini net bir şekilde görebilmek için, referans gün olarak jeomanyetik olarak sakin (30 Mart) ve tedirgin (19 Mart) günlerdeki TEİ değişiklikleri de incelenmiştir. Yüzde sapma hesaplarına bakıldığında tüm istasyonlarda TEC değerlerinde gün batımına benzer bir davranış gözlemlenmekte olup MORP, METS, MAR6, ONSA ve TRO1 istasyonlarında güneş tutulmasına bağlı olarak sırasıyla %16,45 %6,53, %7,99, %8,12 ve %7,22oranında azalma olduğu sonucuna varılmıştır. Her iki referans gününde de TEC değerlerinde bir artış vardır. Ancak tedirgin gündeki artış yüzdesi sakin güne göre daha fazladır. Tutulma büyüklüğü %80'den büyük olan bu beş istasyonda iyonosferik TEİ'deki azalmaların tutulmadan kaynaklandığı ifade edilebilir.

A method to determine exact wave parameters of TID

Total electron content measurements by using dual-frequency signals of global navigation satellite systems (GNSS) makes it possible to obtain the global distribution of electron density of ionosphere with high spatial and temporal resolution. Such high spatial and temporal resolution allows to explore of small-scale traveling ionospheric disturbances generated by terrestrial geophysical events, including seismic activity, solar terminator passage, and atmospheric cyclones. One of the features of measuring the total electron content of the ionosphere with GNSS is that the measurements are made at the line of intersection of the satellite-receiver beam with the layer of maximum ionization of the ionosphere at height of ≈ 300 km. At the same time, due to the orbital motion of the satellites and the Earth rotation, the ionospheric points at which the measurements are providing carrying out a movement relative to each other, relative to the Earth and relative to the traveling ionospheric disturbances. Such a relative motion of the measurement points causes the occurrence of the Doppler effect and leads to a distortion of the wave parameters of the total electron content variations. In particular, the determination of the period of traveling ionospheric wave disturbances on the basis of time series leads to large distortions depending on the used satellite, the time and coordinates of the receiver. This paper describes a method for determining the exact wave parameters –frequency, wavelength, and propagation velocity of traveling ionospheric wave disturbances, based on the use of godochrones to analyze TEC variations. The difference between the wave parameters measured by the proposed method and from the time series of TEC data is shown on an example of wave disturbances generated by the passage of solar terminator.

Analysis of Ionospheric TEC Variations Due to X, M & C Class Solar Flares during the Years 2003 to 2018 and Comparison with IRI Models

Analysis of Ionospheric TEC Variations Due to X, M & C Class Solar Flares during the Years 2003 to 2018 and Comparison with IRI Models

On the GPS TEC variability for full solar cycle and its comparison with IRI-2016 model

On the GPS TEC variability for full solar cycle and its comparison with IRI-2016 model