Variability Of Ionospheric Total Electron Content Research Articles

Performance evaluation of ionospheric models over equatorial Indian region

An extensive year-long evaluation of Total Electron Content (TEC) obtained from NCAR Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM) simulations along with TEC obtained from International Reference Ionosphere (IRI-2020) is carried out with respect to observations made by a GPS TEC receiver over the dip equatorial location of Thiruvananthapuram (8.5° N, 77° E, 0.5° magnetic), in the Indian region during solar minimum period of 2005–2006. The TIE-GCM model and IRI model were run for the period of 2005 (November December) and till October 2006 for each day, and the hourly values of TEC were compared to GPS TEC observations. Overall the temporal and seasonal characteristics of TEC over Thiruvananthapuram were simulated reasonably well by the TIE-GCM model and IRI model. The skill scores of normalized Root Mean Square Error (nRMSE), Prediction Efficiency (P.E), and Ratio between maximum and minimum values were estimated for both models to evaluate their performance. The mean percentage difference between the measured and the simulated TEC was found to be around −25% for IRI, indicating underestimation of the observations, and between −40% and −50% for TIE-GCM again indicating underestimation. A comprehensive analysis revealed that there is good conformity between the modeled and measured values for the whole observation period, with a correlation coefficient R-value of 0.8 and 0.9 for the TIE-GCM and IRI models respectively. This study brings out the competence of the two models in simulating the temporal and seasonal pattern of variability of ionospheric TEC over the equatorial Indian region.

Ionospheric Total Electron Content Variability at High Northern and Southern Latitudes

Ionospheric Total Electron Content Variability at High Northern and Southern Latitudes

Variations of ionospheric TEC due to coronal mass ejections and geomagnetic storm over New Zealand

The variations of ionospheric Total electron content (TEC) due to coronal mass ejections (CMEs) and geomagnetic storms over New Zealand were studied. TEC data were collected from dual-frequency GPS sites operated by the University NAVSTAR Consortium in New Zealand, at Ous2 (−45.86°N, 170.51°E), Dund (−45.89°N, 170.59°E), Mtjo (−43.99°N, 170.46°E), and Quar (−43.53°N, 169.81°E) stations. The findings of this study show that the variability of ionospheric TEC was more noticeable during the main phase of the storm on September 08, 2017 than the initial phase on September 07, 2017, recovery phase on September 09, 2017, and during the CME day on September 06, 2017. Also, the enhancements of change in TEC (ΔTEC) were pronounced more over the Mtjo station during the CME day and over the Quar station during the geomagnetic storm day than over other stations. On September 06, 2017, two X-class solar flares, specifically X2.2 and X9.3, were observed with quiet geomagnetic storm conditions throughout the CME day, resulting in an increase in TEC. On September 07, 2017, the M7.3, X1.3, and on September 08, 2017, the X8.1 solar flare classes with disturbed geomagnetic conditions led to a significant enhancement of TEC over the stations at 02:00 UT (13:00 LT). Finally, during the main phase of the storm, the solar wind electric field (IEF Ey) and prompt penetration electric field (PPEF) showed good correlations in which the fluctuation of PPEF, an increase in Ey, Dst minimum, and horizontal Earth’s magnetic field are well associated with strong changes in TEC.

GPS observations of ionospheric TEC variations over Nepal during 22 July 2009 solar eclipse

As the study of ionospheric behavior during various solar activities is an important task, various studies of ionospheric changes during eclipse events have been widely performed in the different regions of the globe. This paper investigates the ionospheric responses to the solar eclipse on 22 July 2009 over Nepal using the total electron content (TEC) measured by dual-frequency Global Positioning System (GPS) receivers. The time-averaged Vertical TEC (vTEC) of ten GPS stations from Nepal is analyzed and it is found that the value of ionospheric TEC decreases due to the reduction of ionizing radiation. In addition, the deviation in the TEC value on eclipse day from the mean vTEC value of the top five quietest days is found to lie in the range ~1–5 TECu at those regions which were associated with the partial eclipse shadow. On the other hand, the region with the total eclipse (BRN2 and RMTE) faced ~6–7 TECu on average reduction in the TEC value. Considering that the eclipse of 22 July 2009 occurred just at sunrise in the Nepalese zone, a maximum reduction of about 5 TECu is very significant. Higher deviation in TEC is therefore linked with the path of totality and the obscuration rate. This study reveals that the ionospheric TEC over Nepal was altered by wave-like energy and momentum transport, as well as obscuration of the solar disc due to the partial and total solar eclipse. Furthermore, the cross-correlation results presented similar type signatures of the eclipse-induced ionospheric modification over Nepal. This research work serves a crucial future reference for the comparative study of change of ionospheric TEC variability over the Nepal region during Eclipse events

Development of a monitoring system for ionospheric TEC variability before the earthquakes

Total Electron Content (TEC) in the ionosphere changes before an earthquake and is one of the important parameters in the study of earthquake precursors. Monitoring of TEC in real-time may prove an excellent input for the effective precursory study of earthquakes. In the present study, a Monitoring system was developed to integrate TEC, geomagnetic storm, and solar flare data and to carry out a TEC anomaly study before earthquakes occurred. The system integrates data from publicly available sources, carries out statistical analysis in the background to detect an anomaly and sends an email notification for the anomaly detected. The system auto-updates at 9.00 a.m. IST daily to check for any missing data in the past 30 days. The system enables observations of the ionosphere conditions before an Earthquake and may help in understanding earthquake precursors and space weather conditions. The system has been demonstrated studying the recent cluster of earthquakes that occurred during July–August 2020 in Tibet, where a series of anomalies were observed during 15, 20, 23, 25 & 28 2020; 02 & July 05, 2020; 06, 08, 10, 15, 25 & July 27, 2020, and August 16, 2020, before the earthquakes. Therefore, the study presents TEC monitoring on a daily basis and its integration with dependent variables in a single platform for effective research on earthquake precursor detection.

Modeling and analysis of ionospheric TEC variability from GPS–TEC measurements using SSA model during 24th solar cycle

Ionospheric modeling across the Indian low latitudes continues to be a challenging task because of the day-to-day variability pertaining to the underlying complex equatorial and low latitude electrodynamics. In this paper, we aimed at developing an ionospheric Total Electron Content (TEC) model based on Singular Spectrum Analysis (SSA) using the Global Positioning System (GPS)-TEC observations (from 2009 to 2019) at Bengaluru (Geographic: 13.02° N, 77.57° E; Dip Angle: 13.77° N), a low latitude station in India. SSA has the most significant advantage of extracting information from the noisy time-series data with the first-four SSA modes in the decomposition, almost converging to total variance of about 99%. The proposed SSA-TEC model has been assessed by comparing it with the GPS-TEC observations as well as the outcomes from the latest version of the International Reference Ionosphere model (IRI-2016) and International GNSS Services Global Ionosphere Maps (IGS-GIM). The results demonstrate excellent outcomes with the Root Mean Square Error (RMSE) values between observed and model estimations laying at 1.58 TECU (SSA), 7.04 TECU (IRI-2016), and 8.89 TECU (GIM) with the respective correlation coefficients 0.99, 0.91, and 0.94. The model outcomes are in good agreement with the GPS-TEC observations, presenting relatively better estimates of TEC than the IRI-2016 and GIM during the geomagnetically quiet and disturbed daytime and nighttime conditions under different seasons and solar activity. Moreover, the proposed SSA-TEC model replicates the significant changes and time distribution characteristics of the ionosphere over the low latitude location, foreseeing the scopes for its regional implementation with a wide range of operations.

GPS derived ionospheric TEC variability with different solar indices over Saudi Arab region

In this paper, we investigate the variation of ionospheric total electron content (TEC) derived from global positioning system (GPS) observations at a low latitude station in Magna (Station code: RASH, Geographic coordinate: 28°17′N, 34°47′E), Saudi Arab. The study includes a linear regression method for validating the relationship between regional TEC and solar indices during the period 2015–2017 representing the descending phase of 24th solar cycle. The results show a relatively better agreement of diurnal variation of TEC with solar extreme ultraviolet (EUV) flux (0.1–50 nm wavelength) as compared to the Sunspot number (SSN) and 10.7 cm radio flux (F10.7). Interestingly, the seasonal variation shows a higher magnitude of TEC during the winter season as compared to the summer and fall equinox seasons, which is reported as the winter anomaly over the low latitude region. The correlation coefficient between VTEC and solar indices is also higher during the winter as compared to summer season. Concerning diurnal variability, the results show a higher correlation coefficient during the local noon time (1200SAST) that gradually descends towards evening time (1800SAST) and after that decays during the night time (2100SAST) to attend the diurnal minima. This manifests the diurnal solar irradiance dependence over the low latitude region. Moreover, the lesser agreement in the linear relationship between the TEC and solar indices during the year 2015 than the year 2017 substantiates the dependence of TEC magnitude and vitiations on solar activity. The outcomes from this strengthens the regional understanding and modeling of ionospheric parameters and would complement towards the global efforts for modeling and mitigating the ionospheric effects on transionospheric communication, timing and navigation services. Above all, the intents of this study are in line with the augmentation of aviation services performances in Saudi Arab that plays a vital role in the international civil aviation and logistic trade due to its central geographic location.

Evaluation of long term solar activity effects on GPS derived TEC

The solar activity hence the solar radiance follows a long term periodic variability with eleven years periodicity, known as solar cycle. This drives the long term variability of the ionosphere. In the present problem we investigate the long term behaviour of the ionosphere with the eleven year cyclic solar activity. Under the present study we characterize the ionospheric variability by Total Electron Content (TEC) using measurements made by Global Positioning System (GPS) and solar cycle variability by various solar activity indices. We make use of five solar activity indices viz. sunspot number (Rz), solar radio Flux (F10.7 cm), EUV Flux (26-34 nm), flare index and CME occurrences. The long term variability of these solar activity indices were then compared and correlated with the variability of ionospheric TEC, at a mid latitude station, Usuda (36.13N, 138.36E), of Japan, during the solar cycle 23 and ascending phase of cycle 24. From our study, we found that long term changes in the ionospheric TEC vary synchronously with corresponding changes in the solar activity indices. The correlation analysis shows that all the solar activity indices exhibit a very strong correlation with TEC (R =0.76 -0.99). Moreover the correlation between the two is stronger in the descending phase of the solar cycle. The correlation is found to be remarkably strongest during the deep minimum of the solar cycle 24 i.e. between 2007- 2009. Also we noticed a hysteresis effect exists with solar radio flux (F10.7 cm) and solar EUV flux (26-34 nm). This effect is absent with other parameters.

Analysis of ionosphere variability over low-latitude GNSS stations during 24th solar maximum period

Global Positioning System (GPS) is a remote sensing tool of space weather and ionospheric variations. However, the interplanetary space-dependent drifts in the ionospheric irregularities cause predominant ranging errors in the GPS signals. The dynamic variability of the low-latitude ionosphere is an imperative threat to the satellite-based radio communication and navigation ranging systems. The study of temporal and spatial variations in the ionosphere has triggered new investigations in modelling, nowcasting and forecasting the ionospheric variations. Hence, in this paper, the dynamism in the day-to-day, month-to-month and seasonal variability of the ionospheric Total Electron Content (TEC) has been explored during the solar maximum period, January-December 2013, of the 24th solar cycle. The spatial and temporal variations of the ionosphere are analysed using the TEC values derived from three Indian low-latitude GPS stations, namely, Bengaluru, Guntur and Hyderabad, separated by 13–18° in latitude and 77–81° in longitude. The observed regional GPS-TEC variations are compared with the predicted TEC values of the International Reference Ionosphere (IRI-2012 and 2007) models. Ionospheric parameters such as Vertical TEC (VTEC), relative TEC deviation index and monthly variations in the grand-mean of ionosphere TEC and TEC intensity, along with the upper and lower quartiles, are adopted to investigate the ionosphere TEC variability during quiet and disturbed days. The maximum ionospheric TEC variability is found during March and September equinoxes, followed by December solstice while the minimum variability is observed during June solstice. IRI models are in reasonable agreement with GPS TEC but are overestimating during dawn hours (01:00–06:00 LT) as compared to the dusk hours. Higher percentage deviations are observed during equinoctial months than summer over EIA stations, Guntur and Hyderabad. GPS TEC variations are overestimated during dawn hours for all the seasons over Bengaluru. It has also been observed that positive storm effect (enhancement of TEC) is observed during the main phase of the March storm, 2013 (March 16–18, 2013) while both positive and negative storm effects (depletion of TEC) are registered during the main phase of the June storm, 2013 (June 28–30, 2013) at Bengaluru and Guntur, respectively. IRI-2012 model has slightly large discrepancies with the GPS-VTEC compared with the IRI-2007 model during the June storm, 2013 over Guntur station. This analysis highlights the importance of upgrading the IRI models due to their discrepancies during quiet and disturbed states of the ionosphere and developing an early warning forecast system to alert about ionosphere variability.

Is there a one-to-one correspondence between ionospheric anomalies and large earthquakes along Longmenshan faults?

Abstract. On 12 May 2008, a destructive M8.0 earthquake struck Wenchuan County (31.0° N, 103.4° E) in the Longmenshan fault zone of southwestern China. Five years later, on 20 April 2013, another terrible M7.0 earthquake struck Lushan County (30.3° N, 103.0° E) in the same fault area, only 87 km away from the epicenter of the Wenchuan earthquake. In this paper, an integrated wavelet analysis methodology is proposed to detect and diagnose ionospheric total electron content (TEC) anomalies related to seismic activities. Analytic wavelet transform is used to detect ionospheric perturbations, and then cross-wavelet analysis is used to diagnose ionospheric anomalies by gaining further insights into the dynamic relationship between the anomaly variability of ionospheric TEC and geomagnetic indices for the same set of observations. The results show that a significant ionospheric disturbance occurred on 9 May 2008 above the forthcoming epicenter, 3 days prior to the Wenchuan earthquake. However, we did not observe an ionospheric anomaly over the epicenter of the Ya'an earthquake during the 1 month period before the shock. Finally, we discuss the possible interpretations of the different seismo-ionospheric effects for the two similar earthquakes.

High‐resolution station‐based diurnal ionospheric total electron content (TEC) from dual‐frequency GPS observations

AbstractTotal electron content (TEC) estimates derived from Global Navigation Satellite System (GNSS) signal delays provide a rich source of information about the Earth's ionosphere. Networks of Global Positioning System (GPS) receivers data can be used to represent the ionosphere by a Global Ionospheric Map (GIM). Data input for GIMs is dual‐frequency GNSS‐only or a mixture of GNSS and altimetry observations. Parameterization of GNSS‐only GIMs approaches the ionosphere as a single‐layer model (SLM) to determine GPS TEC models over a region. Limitations in GNSS‐only GIM TEC are due largely to the nonhomogenous global distribution of GPS tracking stations with large data gaps over the oceans. The utility of slant GPS ionospheric‐induced path delays for high temporal resolution from a single‐station data rate offers better representation of TEC over a small region. A station‐based vertical TEC (TECV) approach modifies the traditional single‐layer model (SLM) GPS TEC method by introducing a zenith angle weighting (ZAW) filter to capture signal delays from mostly near‐zenith satellite passes. Comparison with GIMs shows the station‐dependent TEC (SD‐TEC) model exhibits robust performance under variable space weather conditions. The SD‐TEC model was applied to investigate ionospheric TEC variability during the geomagnetic storm event of 9 March 2012 at midlatitude station NJJJ located in New Jersey, USA. The high temporal resolution TEC results suggest TEC production and loss rate differences before, during, and after the storm.

Variability of ionospheric TEC during solar and geomagnetic minima (2008 and 2009): external high speed stream drivers

Abstract. We study solar wind–ionosphere coupling through the late declining phase/solar minimum and geomagnetic minimum phases during the last solar cycle (SC23) – 2008 and 2009. This interval was characterized by sequences of high-speed solar wind streams (HSSs). The concomitant geomagnetic response was moderate geomagnetic storms and high-intensity, long-duration continuous auroral activity (HILDCAA) events. The JPL Global Ionospheric Map (GIM) software and the GPS total electron content (TEC) database were used to calculate the vertical TEC (VTEC) and estimate daily averaged values in separate latitude and local time ranges. Our results show distinct low- and mid-latitude VTEC responses to HSSs during this interval, with the low-latitude daytime daily averaged values increasing by up to 33 TECU (annual average of ~20 TECU) near local noon (12:00 to 14:00 LT) in 2008. In 2009 during the minimum geomagnetic activity (MGA) interval, the response to HSSs was a maximum of ~30 TECU increases with a slightly lower average value than in 2008. There was a weak nighttime ionospheric response to the HSSs. A well-studied solar cycle declining phase interval, 10–22 October 2003, was analyzed for comparative purposes, with daytime low-latitude VTEC peak values of up to ~58 TECU (event average of ~55 TECU). The ionospheric VTEC changes during 2008–2009 were similar but ~60% less intense on average. There is an evidence of correlations of filtered daily averaged VTEC data with Ap index and solar wind speed. We use the infrared NO and CO2 emission data obtained with SABER on TIMED as a proxy for the radiation balance of the thermosphere. It is shown that infrared emissions increase during HSS events possibly due to increased energy input into the auroral region associated with HILDCAAs. The 2008–2009 HSS intervals were ~85% less intense than the 2003 early declining phase event, with annual averages of daily infrared NO emission power of ~ 3.3 × 1010 W and 2.7 × 1010 W in 2008 and 2009, respectively. The roles of disturbance dynamos caused by high-latitude winds (due to particle precipitation and Joule heating in the auroral zones) and of prompt penetrating electric fields (PPEFs) in the solar wind–ionosphere coupling during these intervals are discussed. A correlation between geoeffective interplanetary electric field components and HSS intervals is shown. Both PPEF and disturbance dynamo mechanisms could play important roles in solar wind–ionosphere coupling during prolonged (up to days) external driving within HILDCAA intervals.

Spatial correlations of day‐to‐day ionospheric total electron content variability obtained from ground‐based GPS

The spatial correlations of day‐to‐day ionospheric TEC variations for four 30‐day long periods in 2004 (January, March/April, June/July, September/October) were determined using more than 1000 ground‐based GPS receivers. The spatial correlations were obtained in a two‐step process. Initially, the day‐to‐day variability was calculated by first mapping the observed slant TEC values for each 5‐min GPS ground receiver‐satellite pair to the vertical using a simple geometrical factor and then differencing it with its corresponding value from the previous day. This resulted in more than 150 million values of day‐to‐day change in TEC (ΔTEC). Next, statistics were performed on the ΔTEC values to obtain their spatial correlations. Our study indicates strong correlations between geomagnetic conjugate points, and these correlations are larger at low latitudes (r = 0.63–0.73) than at mid‐latitudes (r = 0.32–0.43). Typical correlation lengths, defined as the angular separation at which the correlation coefficient drops to 0.7, were found to be larger at mid‐latitudes than at low latitudes. The meridional correlation lengths are about 7 degrees and 4 degrees at middle and low latitudes, respectively. The zonal correlation lengths are approximately 20 degrees at mid‐latitudes and 11 degrees at low latitudes. The correlation lengths are larger during daytime (1100–1300 MLT) than during nighttime (2300–0100 MLT). The results indicate that the spatial correlation is largely independent of season. These spatial correlations are important for understanding the physical mechanisms that cause ionospheric weather variability and are also relevant to data assimilation.

DISCUSSION

Global Positioning System (GPS) is a remote sensing tool of space weather and ionospheric variations. However, the interplanetary space-dependent drifts in the ionospheric irregularities cause predominant ranging errors in the GPS signals. The dynamic variability of the low-latitude ionosphere is an imperative threat to the satellite-based radio communication and navigation ranging systems. The study of temporal and spatial variations in the ionosphere has triggered new investigations in modelling, nowcasting and forecasting the ionospheric variations. Hence, in this paper, the dynamism in the day-to-day, month-to-month and seasonal variability of the ionospheric Total Electron Content (TEC) has been explored during the solar maximum period, January-December 2013, of the 24th solar cycle. The spatial and temporal variations of the ionosphere are analysed using the TEC values derived from three Indian low-latitude GPS stations, namely, Bengaluru, Guntur and Hyderabad, separated by 13–18° in latitude and 77–81° in longitude. The observed regional GPS-TEC variations are compared with the predicted TEC values of the International Reference Ionosphere (IRI-2012 and 2007) models. Ionospheric parameters such as Vertical TEC (VTEC), relative TEC deviation index and monthly variations in the grand-mean of ionosphere TEC and TEC intensity, along with the upper and lower quartiles, are adopted to investigate the ionosphere TEC variability during quiet and disturbed days. The maximum ionospheric TEC variability is found during March and September equinoxes, followed by December solstice while the minimum variability is observed during June solstice. IRI models are in reasonable agreement with GPS TEC but are overestimating during dawn hours (01:00–06:00 LT) as compared to the dusk hours. Higher percentage deviations are observed during equinoctial months than summer over EIA stations, Guntur and Hyderabad. GPS TEC variations are overestimated during dawn hours for all the seasons over Bengaluru. It has also been observed that positive storm effect (enhancement of TEC) is observed during the main phase of the March storm, 2013 (March 16–18, 2013) while both positive and negative storm effects (depletion of TEC) are registered during the main phase of the June storm, 2013 (June 28–30, 2013) at Bengaluru and Guntur, respectively. IRI-2012 model has slightly large discrepancies with the GPS-VTEC compared with the IRI-2007 model during the June storm, 2013 over Guntur station. This analysis highlights the importance of upgrading the IRI models due to their discrepancies during quiet and disturbed states of the ionosphere and developing an early warning forecast system to alert about ionosphere variability.