TEC Changes Research Articles

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.

Mid‐Latitude Ionospheric Response to a Weak Geomagnetic Activity Event During Solar Minimum

AbstractSalient depletions (30%–40% of the reference values) were observed in the vertical total electron content (VTEC) from the Global Navigation Satellite System (GNSS) network and the topside ionosphere electron density from Swarm satellites measurements over the Western Atlantic and Northeast American regions during a weak geomagnetic activity event (maximum Kp = 2.3) between Day‐of‐Year (DOY) 155 and 156 in 2019. These depletions coincided with thermospheric Oxygen (O) to Nitrogen (N2) column density ratio (∑O/N2) depletions observed by the Global‐scale Observations of the Limb and Disk (GOLD) mission. The National Center for Atmospheric Research (NCAR) Thermosphere–Ionosphere–Electrodynamics General Circulation Model (TIEGCM) reproduces VTEC depletions qualitatively, which is used to investigate the underlying mechanisms. Term analysis of the TIEGCM oxygen ion (O+) continuity equation showed that the VTEC depletion was dominated by chemical processes and neutral wind transport effects, which were partially offset by the transport due to E × B drifts and ambipolar diffusion. On the dayside, the contribution from the neutral wind transport was induced by a weak poleward meridional wind during the geomagnetic activity. With the decrease of electron density, the loss rate in the chemical term was reduced, which led to a large reduction of the contribution from chemical processes to electron density and TEC changes during the event, and the contribution of the neutral wind to VTEC depletion even exceeded that of chemical processes after 14:10 UT.

Ionospheric Behavior during the 10 June 2021 Annular Solar Eclipse and Its Impact on GNSS Precise Point Positioning

The main effects of the 10 June 2021 annular solar eclipse on GNSS position estimation accuracy are presented. The analysis is based on TEC measurements made by 2337 GNSS stations around the world. TEC perturbations were obtained by comparing results 2 days prior to and after the day of the event. For the analysis, global TEC maps were created using ordinary Kriging interpolation. From TEC changes, the apparent position variation was obtained using the post-processing kinematic precise point positioning with ambiguity resolution (PPP-AR) mode. We validated the TEC measurements by contrasting them with data from the Swarm-A satellite and four digiosondes in Central/South America. The TEC maps show a noticeable TEC depletion (<−60%) under the moon’s shadow. Important variations of TEC were also observed in both crests of the Equatorial Ionization Anomaly (EIA) region over the Caribbean and South America. The effects on GNSS precision were perceived not only close to the area of the eclipse but also as far as the west coast of South America (Chile) and North America (California). The number of stations with positioning errors of over 10 cm almost doubled during the event in these regions. The effects were sustained longer (∼10 h) than usually assumed.

Design of Ionospheric TEC Gradient Anomaly Detection Platform Based on GNSS

Abstract Ionospheric anomalies can seriously threaten important flight phases such as approach and landing, so accurate real-time monitoring is necessary. In this paper, an ionospheric anomaly gradient detection platform is designed for satellite navigation receiver. Based on GNSS observation data, the spatial gradient of ionospheric vertical total electron content is calculated, and anomaly ionospheric gradient is detected using generalized likelihood ratio test method. To verify the correctness of detection results, rate of TEC change index is also calculated for comparison. Simulation results show that the platform can detect ionospheric anomaly gradient state fast and accurately, and the proposed approach can used by GNSS receivers to detect ionospheric anomaly.

Co-seismic and Post-seismic Changes in ZTD and TEC of the 2015 Nepal Earthquake

Co-seismic and Post-seismic Changes in ZTD and TEC of the 2015 Nepal Earthquake

Influence of Zonal Wind Velocity Variation on Equatorial Plasma Bubble Occurrences Over Southeast Asia

AbstractThe present study aims to investigate the influence of the zonal wind velocity on equatorial plasma bubble (EPB) occurrences over Southeast Asia. The observation of the EPB occurrence is obtained from the GPS Rate of TEC change index. Meanwhile, the zonal winds were measured using a Fabry‐Perot interferometer located at Kototabang and Chiang Mai stations, and the height of F layer was acquired using an ionosonde at Chumphon station near the magnetic equator. This is the first study to report the influence of zonal wind velocity variation on EPB occurrences with the presence and absence of EPB using GPS data in the Southeast Asian sector. The results illustrated that the average magnitude of zonal wind velocity during the presence of EPB (78 ± 23 m/s) was higher than that of its absence (68 ± 21 m/s). It was observed using long‐term data analyses which led to in‐depth analyses. The analysis of temporal variation of zonal wind variation demonstrated that the zonal winds during EPB were higher in the evening compared to midnight and postmidnight periods from medium to high solar activities. The dependence of zonal wind velocity on EPB over local time was obtained based on the analysis which utilized the data collected during equinox in high solar activity. Besides that, a positive correlation was obtained between the zonal wind velocity and EPB occurrences during pre‐reversal enhancement (PRE) corroborated the effects of zonal wind influence on PRE, and thus EPB occurrences.

Agricultural Productivity Growth and Its Determinants in South and Southeast Asian Countries

Improving agricultural productivity is a priority concern in promoting the sustainable development of agriculture in developing countries. In this study, we first apply stochastic frontier analysis (SFA) to analyze the growth of agricultural total factor productivity (TFP) and its three components (technical change—TC, technical efficiency change—TEC and scale change—SC) in 15 south and southeast Asian countries covering the period 2002 to 2016. Then, the determinants of agricultural TFP growth are identified using dynamic panel data models. The results reveal that the south and southeast Asian countries witnessed an overall decline in agricultural productivity during the sample period, thereby creating concerns over sustaining future agricultural growth. Technical progress was the major source of TFP growth, but its contribution has slowed in recent years. On the other hand, declining scale change and technical efficiency change resulted in the deterioration of productivity over time. Variable levels of productivity performances were observed for individual countries, mainly driven by technological progress. Overall, southeast Asia achieved a more stable and sustained agricultural growth as compared to south Asia. Among the determinants, human capital, level of urbanization, and development flow to agriculture positively influenced agricultural TFP growth, while the level of economic development and agricultural import were negatively associated with TFP growth. Policy recommendations include the suggestions that south and southeast Asian countries should increase investment in human capital, focus on technological innovation and make use of financial assistance and development flow to agriculture to increase and sustain agricultural productivity. In addition, frontier countries of the two regions (e.g., India and Indonesia) should take the lead on regional agricultural development ventures by enhancing cooperation with neighboring countries on technological innovations, and countries facing diseconomies of scale (i.e., Afghanistan and Iran) should consider the rational reallocation of agricultural inputs.

Spatial and Temporal Evolution of Different‐Scale Ionospheric Irregularities in Central and East Siberia During the 27–28 May 2017 Geomagnetic Storm

AbstractWe present a multi‐instrumental study of ionospheric irregularities of different scales (from tens of centimeters to few kilometers) observed over the Central and East Siberia, Russia, during a moderate‐to‐strong geomagnetic storm on 27–28 May 2017. From high‐frequency (HF) and ultrahigh‐frequency (UHF) radar data, we observed an intense auroral backscatter developed right after the initial phase of the geomagnetic storm. Additionally, we examined variations of Global Positioning System (GPS)‐based ROT (rate of TEC changes, where TEC is total electron content) for available GPS receivers in the region. Ionosondes, HF, and UHF radar data exhibited a presence of intense multi‐scale ionospheric irregularities. We revealed a correlation between different‐scale Auroral/Farley‐Buneman ionospheric irregularities of the E layer during the geomagnetic storm. The combined analysis showed that an area of intense irregularities is well connected and located slightly equatorward to field‐aligned currents (FACs) and auroral oval at different stages of the geomagnetic storm. An increase and equatorward displacement of Region 1 (R1)/Region 2 (R2) FACs leads to appearance and equatorward expansion of ionospheric irregularities. During downward (upward) R1 FAC and upward (downward) R2 FAC, the eastward and upward (westward and downward) E × B drift of ionospheric irregularities occurred. Simultaneous disappearance of UHF/HF auroral backscatter and GPS ROT decrease occurred during a prolonged near noon reversal of R1 and R2 FAC directions that accompanied by R1/R2 FAC degradation and disappearance of high‐energy auroral precipitation.

Analysis of ionospheric TEC anomalies for global earthquakes during 2000-2019 with respect to earthquake magnitude (Mw≥6.0)

In this work, a relationship between ionospheric TEC anomalies and different earthquake magnitude groups before the main shocks was investigated. For this purpose, 2942 global earthquakes with Mw≥6 from 2000 to 2019 and possible ionospheric TEC anomalies that occurred before earthquakes were examined by considering 13 different index values of space weather conditions (geomagnetic storm indices and solar activity indices). Anomalies of ionospheric TEC changes were defined for 15-days before and 4-days after the earthquakes by using 15-days moving median method with the length of 15 days. Earthquakes were first grouped according to their magnitudes, and then negative and positive TEC anomalies in quiet days before the earthquakes were detected. These anomalies were observed as ∼5–13, ∼5-10, ∼5-15, ∼3-13, ∼7-15, ∼1 and ∼5-8 days ago for the earthquakes of 6.0 ≤ Mw<6.5, 6.5 ≤ Mw<7.0, 7.0 ≤ Mw<7.5, 7.5 ≤ Mw<8.0, 8.0 ≤ Mw<8.5, 8.5 ≤ Mw<9.0, and 9.0 ≤ Mw<9.5, respectively. Mean of changes in TEC anomalies of these groups is 44.2 % TECU and we detected that the number of positive anomalies in each group is larger than the number of negative anomalies. Consequently, these analyses clearly show that the day-to-day changes in TEC anomalies may supply significant precursors prior to the global earthquakes (M ≥ 6) in the short-term earthquake prediction for main shocks.

A GREY MODEL FOR SHORT-TERM PREDICTION OF THE IONOSPHERIC TEC

Abstract. Accurate prediction of TEC can significantly improve the accuracy of navigation and positioning, therefore TEC observation and prediction has become a hot spot in ionospheric research. TEC has the characteristics of nonlinearity and non-stationarity, that cannot accurately describe this change by analytic expressions. Through the analysis of TEC content changes at the same time for several consecutive days in different seasons, it can be concluded that the TEC change at the same time in a short period is relatively stable, the overall monotonous change trend has a certain correlation. Since the grey model performs better in the prediction of a small amount of data and has a high accuracy in the prediction of time series with monotonous changes, it is used in the prediction of the same time and point-to-point short-term prediction of TEC. The accuracy of the grey model is verified by the Posterior variance ratio, the Little error probability test and the relation grade. The residual correction is made for the prediction results with low prediction accuracy, by further establishing the GM (1,1) model of residual values, and the original prediction results being compensated and refined by the residual GM (1,1) model. The experimental results show that the improved model is more accurate than the grey prediction model and can reflect the changing characteristics of ionospheric TEC.

Effect of moderate geomagnetic storms on equatorial plasma bubbles over eastern Africa in the year 2012: Evolution and electrodynamics

Equatorial plasma bubbles (EPBs) are common features of the equatorial and low-latitude ionosphere and are known to cause radio wave scintillation which leads to the degradation of communication and navigation systems. Although these structures have been studied for decades, a full understanding of their evolution and dynamics remains important for space weather mitigation purposes. In this study, we present cases of EPBs occurrences around April and July 2012 geomagnetic storm periods over the African equatorial sector. The EPBs were observed from the Communications/Navigation Outage Forecasting System (C/NOFS) and generally correlated well to the ionospheric irregularities observed from the Global Positioning System total electron content (GPS-TEC) measurements (rate of TEC change, ROT). This study revealed that the evolution of the EPBs during moderate storms is controlled by the strength of the daytime equatorial electrojet (EEJ) currents regardless of the strength of the equatorial ionization anomaly (EIA), the latter is observed during the July storm case in particular. These effects were more evident during the main and part of the early recovery phases of the geomagnetic storm days considered. However, the evening hours TEC gradients between regions of the magnetic equator and ionization crests also played roles in the existence of ionospheric irregularities.

Combined GPS/GLONASS Relative Receiver DCB Estimation Using the LSQ Method and Ionospheric TEC Changes over South Korea

Combined GPS/GLONASS Relative Receiver DCB Estimation Using the LSQ Method and Ionospheric TEC Changes over South Korea

Multi-scale ionospheric irregularities occurrence over South America during the St. Patrick's day storm on March 17, 2015

During the solar cycle 24 there was a very intense geomagnetic storm due to the St. Patrick's day storm and the effects of this storm on ionosphere has become a topic of extensive space weather investigation. Up to now several aspects of the St. Patrick's ionospheric storm have been study such as the prompt penetration electric fields (PPEFs), TEC changes, electron density disturbances, plasma drift, O+ concentration modification, hemispherical asymmetry developments, equatorial ionization anomaly (EIA) modification, and ionospheric irregularities. Besides all these important studies, there are some essential aspects, which have not been addressed yet, related to the occurrence of multi-scale ionospheric irregularities. In this paper, we present and discuss the generation and suppression of multi-scale ionospheric irregularities, using the observations conducted in the Latin American Sector from 4 ionosondes, 20 dual frequency GPS stations, and JULIA radar observations during the month of March 2015, which includes the St. Patrick's day geomagnetic storm period. Suppression of large-small scales ionospheric irregularities has occurred during the main and second night of the recovery phases. However, during the first night of recovery phase there was post-midnight ionospheric irregularities.

Influence of Solar Activity on Ionospheric TEC Change

为研究太阳活动对电离层TEC变化的影响，从整体到局部分析了2000—2016年的太阳黑子数、太阳射电流量<i>F</i>_10.7指数日均值与电离层TEC的关系，并重点分析了2017年9月6日太阳爆发X9.3级特大耀斑前后15天太阳活动与电离层TEC变化的相关性.结果表明：由2000—2016年的数据整体看来，太阳黑子数、太阳<i>F</i>_10.7指数、TEC两两之间具有很强的整体相关性，但局部相关性强弱不均；此次耀斑爆发前后太阳黑子数、太阳<i>F</i>_10.7指数和TEC具有很强的正相关特性，太阳活动对TEC的影响时延约为2天；太阳活动对全球电离层TEC的影响不同步，从高纬至低纬约有1天的延迟，且对低纬度的影响远大于中高纬度.太阳活动是影响电离层TEC变化的主要原因，但局部也可能存在其他重要影响因素.

Attribution of interminimum changes in global and hemispheric total electron content

AbstractWe use a new data product of electron column density (total electron content, or TEC) maps to estimate and attribute the interminimum changes (the differences between annual averages centered on the 2008 and 1996 solar minima) in TEC global and hemispheric averages (dividing the globe in three different ways). We attribute the observed changes to corresponding changes in solar and geomagnetic activity. The new TEC map product was constructed with temporally consistent processing, and it resolves some of the apparent inconsistencies of earlier studies. The estimated global average TEC interminimum change is −19.3% ± 1.0% (2σ uncertainty), of which −9.1% is attributable to the interminimum change in solar extreme ultraviolet (EUV) irradiance (as represented by the F10.7 index), −2.2% is attributable to the change in geomagnetic activity (Kp index), and −9.3% remains unattributed. The hemispheric results are very similar to the global results, but the values tend to be slightly larger in the Southern Hemisphere, at low latitudes, and at night, compared to the opposing respective hemispheres. The interminimum changes and temporal variations of thermospheric mass density anomalies (i.e., the difference between the data and the empirical model) are very similar to those of the global and hemispheric TEC residuals, suggesting that they are driven by a common, globally distributed mechanism. Thermospheric composition changes and additional (unobserved) decreases in solar EUV irradiance are possible mechanisms behind the TEC and mass density unattributed changes; we estimate a plausible range of −6% to −13% for the solar EUV irradiance interminimum change.

Anomalous Variation in GPS TEC, Land and Ocean Parameters Prior to 3 Earthquakes

The present study reports the analysis of GPS TEC prior to 3 earthquakes (M > 6.0). The earthquakes are: (1) Loyalty Island (22°36′S, 170°54′E) on 19 January 2009 (M = 6.6), (2) Samoa Island (15°29′S, 172°5′W) on 30 August 2009 (M = 6.6), and (3) Tohoku (38°19′N, 142°22′E) on 11 March 2011 (M = 9.0). In an effort to search for a precursory signature we analysed the land and ocean parameters prior to the earthquakes, namely SLHF (Land) and SST (Ocean). The GPS TEC data indicate an anomalous behaviour from 1–13 days prior to earthquakes. The main purpose of this study was to explore and demonstrate the possibility of any changes in TEC, SST, and SLHF before, during and after the earthquakes which occurred near or beneath an ocean. This study may lead to better understanding of response of land, ocean, and ionosphere parameters prior to seismic activities.

MONITORING IONOSPHERIC VARIATION FOR A DEFINITE PERIOD TIME IN TURKEY

Abstract. Ionosphere has been studied by a number of scientists in recent years. Since GPS observations cannot provide TEC value directly, it can be estimated from combination of observations. In this study TEC values derived from GPS observations were produced variations with two hours increments from the eighth day to fifteenth day in months just after beginning of each season, namely January, April, July and October in 2014 for ZONG TUSAGA-Aktif station and 41 other stations (TUSAGA-Aktif, EUREF, IGS). TEC values computed by the Bernese 5.0 software were compared with the Global Ionosphere Model (GIM) TEC values regularly published by Center for Orbit Determination in Europe (CODE) and International Reference Ionosphere (IRI-2012) TEC values. As a result of this study, with the comparison of GPS TEC values to GIM TEC values, it has become obvious that GPS TEC values are quite similar to the GIM TEC values. The differences of TEC values derived from GPS TEC and GIM TEC change from 0.91 TECU (January) to 1.88 TECU (October). On the other hand, GPS TEC values are also compared with IRI TEC values, and found out that there is a considerable difference between the two TEC values ranging from 6.30 TECU (January) to 15.15 TECU (April). Moreover, TEC values derived from GPS measurements are attained similar to GIM TEC, but found to stray from IRI-2012 TEC values remarkably.

Latitudinal variation in the occurrence of GPS L-band scintillations associated with the day-to-day changes in TEC, h′F and the E×B drift velocity and their impact on GPS satellite signals

The present study describes the day-to-day variations in the occurrence of GPS L-band scintillations from equator to the anomaly crest location associated with the changes in TEC, h\(^{\prime }\)F and E ×B drift velocities. The GPS–TEC and S4 index data from an equatorial station, Trivandrum (8.47∘N, 76.91∘E), a low latitude station, Waltair (17.7∘N, 83.3∘E) and an anomaly crest location Kolkata (22.6∘N, 88.4∘E) during the low solar activity years of 2004 and 2005 are used. It is observed that the day-time ambient TEC is higher during scintillation days compared to that during the days on which there are no scintillations at the three different locations mentioned above. Further, the diurnal variation of TEC shows a rapid decay during 1700–2000 hr LT over the three different locations during scintillation days which is observed to be comparatively much less during no scintillation days. The average height of the F-layer in the post-sunset hours over Trivandrum is found to be higher, around 350 km during scintillation days while it is around 260 km during the days on which there is no scintillation activity. The average pre-reversal E ×B drift velocity observed around 19:00 hr LT is higher (20 m/s) during scintillation days, whereas during no scintillation days, it is found to be much less (7 m/s). Further, it is observed that the GPS receivers lose their locks whenever the S4 index exceeds 0.5 (>10 dB power level) and these loss of lock events are observed to be more around the anomaly crest location (Kolkata). It may be inferred from the present observations that the level of ambient ionization around noon-time, and a fast decay (collapse) of the ionization during afternoon hours followed by rapid increase in the height of the F-layer contributes significantly to the occurrence of scintillations. The present study further indicates that the S4 index at L-band frequencies increases with an increase in latitude maximizing around the crest of the equatorial ionization anomaly during the post-sunset hours resulting in more loss of lock events in the GPS receiver signals around the EIA crest region.

Preseismic TEC Changes for Tohoku-Oki Earthquake: Comparisons Between Simulations and Observations

Preseismic TEC Changes for Tohoku-Oki Earthquake: Comparisons Between Simulations and Observations

Obtaining Ionospheric Conditions according to Data of Navigation Satellites

Defining ionospheric conditions, the deviation of the observational value of the total electron content TEC(obs), measured by means of navigation satellites, from a median is a bench mark. According to more than 40 ionospheric stations during April 2014 it is shown that synchronism of change of deviations of TEC and critical frequency foF2 of the ionosphere is kept under quiet and moderate disturbed conditions. This fact allows to use a median of the equivalent slab thicknessτ(med) as a reliable calibration factor to calculate foF2 from TEC(obs). The efficiency coefficient of joint use ofτ(med) and TEC(obs) changes from 1.5 to 4 with average value 2.2 across the globe. The highest coefficient corresponds to middle latitudes, however the estimations obtained for high- and low-latitude areas indicate possibility to useτ(med) and TEC(obs) in these areas.