Equatorial Ionization Anomaly Region Research Articles

Bi-LSTM based vertical total electron content prediction at low-latitude equatorial ionization anomaly region of South India

In this study, we consider Bi-directional Long Short Term Memory (Bi-LSTM) model based Vertical Total Electron Content (VTEC) prediction over Thanjavur (Geographic 10.72˚ N, 79.01˚ E, Geomagnetic 2.34˚ N, 152.19˚ E) Global Positioning System (GPS) station. This station is located at low latitude Equatorial Ionization Anomaly (EIA) region of 2˚ geomagnetic dip latitude and has unique ionospheric dynamics. In this region, the VTEC prediction is crucial and challenging for space weather and the Sixth Generation (6G) Internet of Space (IoS) application to support early warning systems and future spatial data transmissions. A Deep Learning (DL) model based on Bi-LSTM was developed and trained for F10.7 and Dst index for predicting the VTEC. This study highlights the prediction of VTEC for any day that includes solstice and equinox time frames. The Bi-LSTM has an improvement of 28 % in mean absolute error (MAE), 48% in mean square error (MSE) and 24% in root mean square error (RMSE) as compared to the conventional Long Short Term Memory (LSTM) network. Hence, this Bi-LSTM model can be helpful to predict the VTEC in the EIA region and may be helpful to extrapolate over the unmeasured grid region of ocean and land.

A Novel Technique for High-Precision Ionospheric VTEC Estimation and Prediction at the Equatorial Ionization Anomaly Region: A Case Study over Haikou Station

This paper introduces a novel technique that uses observation data from GNSS to estimate the ionospheric vertical total electron content (VTEC) using the Kriging–Kalman method. The technique provides a method to validate the accuracy of the Ionospheric VTEC analysis within the Equatorial Ionization anomaly region. The technique developed uses GNSS VTEC alongside solar parameters, such as solar radio flux (F10.7 cm), Disturbance Storm Time (Dst) and other data, and Long Short Term Memory (LSTM) Networks to predict the occurrence time of the ionospheric equatorial anomaly and ionospheric VTEC changes. The LSTM method was applied to GNSS data from Haikou Station. A comparison of this technique with the neural network (NN) model and International Reference Ionosphere model shows that the LSTM outperforms all of them at VTEC estimation and prediction. The results, which are based on the root mean square error (RMSE) between GNSS VTEC and GIM VTEC outside the equatorial anomaly region, was 1.42 TECU, and the results of GNSS VTEC and VTEC from Beidou geostationary orbit satellite, which lies inside the equatorial ionization anomaly region, was 1.92 TECU. The method developed can be used in VTEC prediction and estimation in real time space operations.

Ionospheric storm effects in the EIA region in the American and Asian-Australian sectors during geomagnetic storms of October 2016 and September 2017

The interrelationships between a geomagnetic storm and Earth’s ionosphere are highly variable and complex. This paper studies the ionospheric storm effects (ISEs) and plasma redistributions in the American and Asian-Australian sectors during the geomagnetic storms of 13 October 2016 and 8 September 2017. The hourly NmF2 variations in the equatorial ionization anomaly (EIA) region and horizontal magnetic field (H) data in each sector were employed to unveil the plasma redistributions during the storms. The hourly analysis presents more revealing ISEs on EIA structures, showing evidence of sectorial difference during the two storms. During the SSC period, negative storm, NS, dominated as ISE around the magnetic equator, negative storm effects were less significant beyond the equator, and positive storm, PS, was significantly dominant at higher latitudes. During the main phase, the ionization is enhanced/depleted with time in the American/Asian-Australian sector for the 13 Oct. 2016 storm, and a reversed ISE is recorded for the 8 September 2017 storm. The ISE during the October 2016 storm, was negative at the equator and positive at the crest regions. However, these effects were reversed during the September 2017 storm in the American sector. The PS effect was more pronounced in the Asian-Australian sector than in the American sector in the northern hemisphere and was weaker near the equator. During the recovery phase, PS was dominant during the Oct 2016 storm in the American sector. NS was extensively observed during Sept. 2017 storm except for around 1200 – 2300 UT when PS was registered around the dip equator. The EIA structure is stronger with a noticeable PS impact in the American sector than it appears in the Asian-Australian sector during the storm of Oct. 2016. The differences in ISEs observed during the two storms could be attributed to differences in the properties of solar wind driver gases and associated ring current mechanisms. PPEF and DDEF are the primary causes of ISEs, but other storm-time forces were also discussed.

Linkage of Equatorial Ionization Anomaly With the Day‐To‐Day Occurrence of Postsunset Irregularities and Scintillation in Low‐Latitude Region Around 110°E

The zonal electric field and meridional wind are two major factors to affect the formation of both equatorial ionization anomaly (EIA) and plasma irregularities in equatorial and low-latitude regions, the correlation between the EIA characteristics and the occurrence of both irregularities and the sequent scintillation is always concerned. In this study, we investigated the relationship between the day-to-day occurrence of scintillation based on ground-based GPS observations at Sanya (18.3°N, 109.6°E; dip lat.: 12.8°N) and Hongkong (22.4°N, 114.2°E; dip lat.: 16.1°N), China, and day-to-day variations of EIA inferred from the Gravity Recovery and Climate Experiment observations in equinoctial months (March-April and September-October) during high solar activities (2012–2014). The results indicated that EIA strength can be linked with the day-to-day occurrence of post-sunset irregularities and scintillation, and post-sunset scintillation would be likely to occur as electron density above the EIA trough is low and EIA is strong. There are other key factors to affect the initiation and evolution of irregularities besides zonal electric field and meridional wind, such as seed perturbations or wave-like structures. The day-to-day variations of electron density in EIA regions, EIA strength and EIA asymmetry were not well correlated quantitatively with the scintillation intensity representing by daily S4 maximum in low-latitude region. These results can help us understand and predict the day-to-day variability of irregularities and scintillation in equatorial and low-latitude regions.

Modelling of [formula omitted] using artificial neural network over Equatorial Ionization Anomaly (EIA) region stations

The critical frequency of F2-layer in the Equatorial Ionization Anomaly (EIA) region is one of the most challenging ionospheric characteristics to model. We illustrate the efforts to model monthly hourly medians of foF2 using Artificial Neural Network (ANN) over five EIA crest stations between 23.1° N – 30.5° N of Eastern longitudes during 1958–2008 (as per data availability per station). The ANN based model was designed to obtain foF2 values as output by inputting seven parameters defining diurnal (hour), seasonal (month), geographical (latitude & longitude) and solar cycle (year, F10.7 & F10.7P indices) dependency of foF2. With this input space, the model uses a hidden layer of 96 neurons to generate output foF2, producing a mean squared error of less than 1.0MHz2. Additionally, the convergence of the model improved on introducing derived solar radio flux (F10.7P) by 6.0%. After training and validation, the model predicts foF2 values over the locations under study for unseen testing data of 1989, 1998 & 2008 as a representation of high, moderate and low solar activity, respectively. Our ANN based climatological model gave a Root Mean Square Error (RMSE) of 0.956 MHz and goodness of fit, r2=0.929 for the testing data. Data-model comparison with ANN model is better than the International Reference Ionosphere (IRI) estimations of foF2 with CCIR (IRIC) and URSI (IRIU) coefficients. IRIC and IRIU produced RMSE of 1.901 MHz (r2=0.732) and 1.926 MHz (r2=0.716) for the same testing data set, respectively. The designed ANN based model successfully captures the climatological features of foF2 in the EIA region. After necessary modifications, the model may be applied for predicting daily hourly values, particularly in the Asian-longitudinal range over Pakistan, in future.

Solar and interplanetary events that drove two CIR-related geomagnetic storms of 1 June 2013 and 7 October 2015, and their ionospheric responses at the American and African equatorial ionization Anomaly regions

This study investigates the sequence of solar and interplanetary events that drove the 1 June 2013 and October 2015 geomagnetic storms and how the American (68°–78oE) and African (32°–42oE) Equatorial Ionization Anomaly (EIA) regions responded to them. We constructed the EIA structures by using Total Electron Content (TEC) and ionospheric irregularities derived from Global Navigation Satellite System (GNSS) receivers along with the study locations. We also analyzed disturbed time ionospheric electric field and model data alongside the GNSS data. The 1 June 2013 geomagnetic storm was driven by a combination of a weak CME and HSSs from solar coronal holes, while the 7 October 2015 storm was solely driven by HSSs. Storm-time hemispherical asymmetry in ionospheric TEC and irregularities distributions was consistently observed. Storm with minimum SYM-H value at day-side locations caused enhancement in plasma ionization and pole-ward movement of EIA crests, while storm with minimum SYM-H value at night-side locations caused reduction in plasma ionization and equator-ward movement of EIA crests. The phase of responses of the ionosphere to geomagnetic storms depends on the local time of storm’s onset and local time of the storm’s main phase minimum which also determine the orientation of Prompt Penetration Electric Field (PPEF). At storm’s onset time in the low latitude regions, the main storm-induced electric field is PPEF. Daytime eastward PPEF intensified plasma fountain to increase the EIA crests locations, while nighttime westward PPEF reversed plasma fountain to cause equator-ward collapse of the EIA crests. However, around the storm’s recovery phase, under southward turning of IMF Bz, depending on their orientations, PPEF and Disturbed Dynamo Electric Field (DDEF) collectively influenced low latitude ionosphere. Eastward PPEF at the Pre-Reversal Enhancement (PRE) time enhanced irregularities generation, while westward DDEF at PRE time inhibited irregularities generation. The season of storm’s occurrence is also a factor that dictates ionospheric response to a storm, for instance, the 7 October storm (SYM-H −124 nT) influenced the ionosphere more than the 1 June storm (SYM-H −137 nT). Both storms had long recovery phase. On pre-storm days, we observed stronger and well-developed EIA crests over the American sector than over the African sector.

Ionospheric irregularities during disturbed geomagnetic conditions over Argentinian EIA region

Ionospheric irregularities can severely degrade radio communication and navigation systems. Geomagnetic storms may affect the generation of these irregularities in a way that is not yet fully understood. To improve the forecasting of this phenomenon, we need to study the ionosphere in different regions of the world, and in particular in the equatorial ionization anomaly (EIA) where irregularities are usually more intense. This study analyses the effect of geomagnetic storms on ionospheric irregularities. We examined the occurrence of irregularities at the southern crest of the EIA in Argentina (Tucumán, 26.9°S, 294.6°E, dip latitude 15.5°S) during three intense and one moderate geomagnetic storm of different solar sources, between 2015 and 2018. We used data from an ionosonde, a Global Positioning System receiver and magnetometers. Ionogram spread-F, the F-layer bottom side (h'F), the critical frequency of the F2-layer (foF2), the rate of TEC index and the S4 scintillation index were analysed. The data show irregularities were present as range spread-F and moderate TEC fluctuations in one storm: 27 May 2017 (a coronal mass ejection CME-driven storm occurred on local winter), and were absent in the other events. We suggest that eastward disturbance dynamo electric field and over-shielding prompt penetration electric fields may create favourable conditions for developing these irregularities, whereas westward storm time electric fields might inhibit the growth of irregularities during the other storms considered. During co-rotating interaction region CIR-driven storms, the westward disturbance dynamo electric field may be associated with the non-occurrence of irregularities.

Lunar Tidal Effect on Equatorial Ionization Anomaly Region in China Low Latitude

AbstractThe equatorial ionization anomaly (EIA) crest derived from GPS observations, F2‐layer peak height (hmF2), critical frequency (foF2), and equatorial electrojet (EEJ) in China low latitude are used to study the lunar tidal effect on EIA region for the years from 2006 to 2015. A predominant 14.76‐day periodic component in EIA crest, hmF2, and EEJ concurrently appears in some seasonal intervals, which coincides with the half of the lunar revolution period (29.53 days) and the lunar phase, indicating that the 14.76‐day periodic oscillation in EIA region is modulated by the lunar tide. This 14.76‐day periodic oscillation occurs in all Northern Hemisphere (NH) winter and corresponds to their respective sudden stratospheric warming (SSW) period from 2006 to 2015. In addition, this 14.76‐day periodic oscillation also occurs in other seasons for some years, for example, May and August, in which wave power is sometimes comparable to that in SSW period. The occurrence of the 14.76‐day periodic oscillation and its wave power during non‐SSW seasons in the solar maximum years (2012–2015) is more frequent and higher than that in the solar minimum years (2008–2010). Our results suggest that, besides the NH SSW condition, the solar activity and some other unclear factors are also responsible for the lunar tide enhancement in EIA region.

Conjugate Ionospheric Perturbation During the 2017 Solar Eclipse

AbstractWe report new findings of total electron content (TEC) perturbations in the southern hemisphere at conjugate locations to the northern eclipse on August 21, 2017. We identified a persistent conjugate TEC depletion by 10%–15% during the eclipse time, elongating along magnetic latitudes with at least ∼5° latitudinal width. As the Moon's shadow swept southward, this conjugate depletion moved northward and became most pronounced at lower magnetic latitudes (>−20°N). This depletion was coincident with a weakening of the southern crest of the equatorial ionization anomaly (EIA), while the northern EIA crest stayed almost undisturbed or was slightly enhanced. We suggest these conjugate perturbations were associated with dramatic eclipse initiated plasma pressure reductions in the flux tubes, with a large portion of shorter tubes located at low latitudes underneath the Moon's shadow. These short L‐shell tubes intersect with the F region ionosphere at low and equatorial latitudes. The plasma pressure gradient was markedly skewed northward in the flux tubes at low and equatorial latitudes, as was the neutral pressure. These effects caused a general northward motion tendency for plasma within the flux tubes, and inhibited normal southward diffusion of equatorial fountain plasma into the southern EIA region. We also identified posteclipse ionospheric disturbances likely associated with the global propagation of eclipse‐induced traveling atmospheric disturbances in alignment with the Moon's shadow moving direction.

Six‐Day Periodic Variation in Equatorial Ionization Anomaly Region

AbstractUsing the location of equatorial ionization anomaly (EIA) crest derived from GPS observations and CHAllenging Minisatellite Payload (CHAMP) satellite measurement, along with the F2 layer peak height (hmF2), critical frequency (foF2), and the equatorial electrojet (EEJ) in India and China longitudinal sectors, we found the obvious 6‐day oscillation in these ionospheric parameters during 16 April to 31 May 2003 and 21 September to 26 October 2004. The tidal amplitudes of EEJ are enhanced during the positive phase of the 6‐day waves and are weakened during its negative phase. Since the strong 6‐day planetary wave activities in mesosphere and lower thermosphere (MLT) region during these two events have been reported by earlier studies, this 6‐day oscillation in EIA region can be due to the interaction of enhanced planetary waves with the tidal wind. Moreover, the temporal variations of this 6‐day wave are consistent and in phase in these ionospheric parameters, indicating that the 6‐day planetary wave modulates electric fields via E region wind dynamo, thereby altering the vertical height and horizontal distribution of electron densities through upward E × B drift, which results in the periodical variations in these ionospheric parameters in F region. These observation results provide insight into how the 6‐day planetary waves modulate the ionospheric morphological structure.

Ionospheric Responses at Low Latitudes to the Annular Solar Eclipse on 21 June 2020

AbstractIn this study, we utilized both ground‐based and space‐borne observations including total electron content (TEC) from Beidou geostationary satellites, two‐dimensional TEC maps from the worldwide dense Global Navigation Satellite System receivers, ionosondes, and in situ electron density (Ne) and electron temperature (Te) from both Swarm and China Seismo‐Electromagnetic Satellite satellites, to investigate the low‐latitude ionospheric responses to the annular solar eclipse on 21 June 2020. The decrease in TEC during the eclipse at low latitudes showed a local time dependence with the largest depletions in the noon and afternoon sectors. It was also found that the TEC depletions at different latitudes in the equatorial ionization anomaly (EIA) region over the East Asian sector cannot solely be explained by the solar flux changes associated with the obscuration rate. The differences in TEC reduction between stations can be more than a factor of 2 at latitudes with the same obscuration rate of over 90%. Compared with TEC variations in the Northern Hemisphere, the TEC also underwent a considerable decrease in the EIA region in the conjugate hemisphere without eclipse shadow. Meanwhile, the hmF2 near the magnetic equator increased around the onset of the eclipse, indicating an enhancement of the eastward equatorial electric field. Furthermore, the TEC decrease during the eclipse in the EIA region in both hemispheres lasted for a long period of more than 7 hr after the eclipse, with a TEC depletion of 2–6 TEC units. The Ne from Swarm and China Seismo‐Electromagnetic Satellite satellites showed a complicated variation after the eclipse, whereas no visible change was observed in Te. The enhanced equatorial electric field, neutral wind changes, and the associated plasma transport act together to generate the observed ionospheric effects at low latitudes during the eclipse. Our results also suggest that the eclipse‐induced perturbations of dynamic processes can continue to impact the ionosphere after the eclipse.

Validation of CSES RO measurements using ionosonde and ISR observations

The radio occultation (RO) measurements obtained by GOR payload on board the CSES (the first China Seismo-Electromagnetic Satellite) satellite are validated using the observations from five ionosonde stations located at different latitudes and one incoherent scatter radar (ISR) observatory located at mid-latitude. CSES RO peak values (NmF2) and peak heights (hmF2) are compared with the corresponding measurements from the five ionosonde stations, and RO electron density profiles (EDP) are compared with the corresponding ISR measurements. The results show that: (1) CSES RO NmF2 measurements are generally very consistent with that from ionosondes with a correlation coefficient of 0.9448. Obvious underestimation and overestimation are shown for the daytime NmF2 measurements obtained at and near EIA (Equatorial Ionization Anomaly) crest regions, and different error variation patterns are shown: the error at the EIA peak region increases with increase of the observations, while the error at the region near the EIA crest is almost a constant. Attention should be paid to these daytime measurements obtained at and near EIA region in applications. (2) CSES RO hmF2 measurements are in good agreement with that from ionosondes with a correlation coefficient of 0.8357. There is a tendency the daytime hmF2s are overestimated and the nighttime hmF2s are underestimated. (3) CSES RO EDP measurements generally show good consistency with that from ISR measurements. The RO measurements obtained within the peak height range (200–400 km) have the best performance, but errors increase out of this altitudes range. The error for the altitude of the E layer is relatively large, therefore RO measurements are less reliable for this altitude.

The characteristics of terdiurnal tides in the ionosphere

Studying the characteristics of terdiurnal tides in the ionosphere provides an important pathway to understand the dynamic coupling between the low atmosphere and the ionosphere. Based on the TEC data from the Madrigal database at the Massachusetts Institute of Technology Haystack Observatory, a decomposition and nonlinear fitting method is used to derive ionospheric terdiurnal tides. Statistical analysis is then carried out to study the variability of ionospheric terdiurnal tides. Both the absolute and relative amplitudes of terdiurnal tides are large near the equator and in the equatorial ionization anomaly (EIA) region. The relative amplitude of terdiurnal tides in the Northern Hemisphere is larger than that in the Southern Hemisphere at low latitude, exhibiting hemispheric asymmetry. The amplitudes of terdiurnal tides at magnetic middle latitudes (∼35 °N MLat) at the east hemisphere (142 °E), west hemisphere (90 °W) and zero degree longitude sites all have annual, semiannual and 4-month cycles. Furthermore, our analysis shows that there are longitudinal variations of terdiurnal tides and these variations appear to be dependent on solar activity. Terdiurnal tides show two peaks near the equinoxes, one in March and April, and the other between October and November. The terdiurnal tides with characteristics of large amplitudes in the equator and EIA regions, and their correlation with the diurnal and semidiurnal components suggest that terdiurnal tides and their intra-annual variations are likely related to the nonlinear interaction between diurnal and semidiurnal tides, which also show strong intra-annual variations.

Quiet Time Ionopheric Irregularities Over the African Equatorial Ionization Anomaly Region

AbstractThis paper investigated variations of quiet time ionospheric irregularities over the African equatorial ionization anomaly using the rate of change of total electron content index. Irregularities were quantified in terms of percentage occurrence and examined along with parameter of the anomaly, mainly its strength and the asymmetry of the crests as well as equatorial electric field derived from the real‐time equatorial electric field model and meridional wind obtained from the horizontal wind model. Irregularities occurred from 19:30 to 03:00 LT with a time difference of 1 hr between both crests. The highest occurrences were registered in April: 91.67%, 75.00%, and 96.43% for the northern crest, trough, and southern crest, respectively. Seasonally, stronger anomaly (>20 total electron content unit) in addition to the highest equatorial electric field value in the dusk sector corresponded with the equinoctial higher occurrence rate in both hemispheres, while stronger meridional wind and farthest crests location accounted for the least occurrence in winter. The summer occurrence rate was favored by reduced meridional wind, smaller crests location, and late time of prereversal enhancement at the magnetic equator. There was a significant asymmetry in irregularities over the crests in both hemispheres with stronger and greater occurrence rate over the southern crest. Also, irregularities strength and occurrence rate were similar over the northern crest and trough in summer. In addition to dusk‐sector activity, irregularities occurred during postmidnight in summer. Simultaneous variations of irregularities at the crests and trough also highlighted the contribution of nonequatorial processes to their formation at the crests.

Performance analysis of Neural Networks with IRI-2016 and IRI-2012 models over Indian low-latitude GPS stations

The Global Positioning System (GPS) applications are highly vulnerable to the ionospheric space weather effects. Modelling and forecasting the ionospheric effects such as time delays for GPS signals are important for real time alerts of space weather effects on GPS services. In the present work, the performance of Neural Networks (NN) model is compared with International Reference Ionosphere (IRI) models. The ionospheric Total Electron Content (TEC) observations have been collected during 2015 year, descending phase of 24th solar cycle, over three Indian low latitude GPS stations namely, Bengaluru (Geographic coordinates: 13.02°N and 77.57°E), near to geomagnetic equator, Guntur (Geographic coordinates: 16.37°N and 80.37°E), which is at Equatorial Ionization Anomaly (EIA) and Lucknow (Geographic coordinates: 26.83°N and 80.92°E), which is beyond EIA region. The performance of NN model in predicting the ionospheric TEC values is compared with IRI (IRI-2012 and IRI-2016) models during test period, October–December 2015 over three Indian low latitude regions. It is observed that IRI models (IRI-2012 and IRI-2016) have shown more the temporal differences with GPS-VTEC during sunrise hours compared to sunset hours over three low latitude regions. The performance of IRI-2016 model has apparently better than IRI-2012 model. However, it is observed that IRI-2016 model has large discrepancies over Bengaluru and Guntur station due to high VTEC fluctuations at equatorial and low latitudes. The NN models are well predicted the measured diurnal mean VTEC variations with the less errors, ±5 TECU but the differences of IRI models are ±15 TECU over all the three stations. Later, GPS-data for 10 years, 2009–2018, is collected over Bengaluru station during 24th solar cycle. The performance of NN model is validated during 2016, 2017 and 2018 years over Bengaluru GPS station. The error measurements and experimental results reported that the measured GPS-VTEC values are well predicted by NN model compared to IRI-2016 model over equatorial and low latitude GPS stations.

Quasi-10 d wave modulation of an equatorial ionization anomaly during the Southern Hemisphere stratospheric warming of 2002

Abstract. The present paper studies the perturbations in an equatorial ionization anomaly (EIA) region during the Southern Hemisphere (SH) sudden stratospheric warming (SSW) of 2002, using the location of EIA crests derived from global positioning system (GPS) station observations, the total electron content (TEC) obtained by the International GNSS (global navigation satellite system) Service (IGS) global ionospheric TEC map (GIMs) and the equatorial electrojet (EEJ) estimated by the geomagnetic field in the Asian sector. The results indicate the existence of an obvious quasi-10 d periodic oscillation in the location and TEC of the northern and southern EIA crest. An eastward phase progression of the quasi-10 d wave producing the SH SSW of 2002 is also identified in polar stratospheric temperature. Previous studies have shown that a strong quasi-10 d planetary wave with zonal wave numbers s=1 extended from the lower stratosphere to the mesosphere and lower thermosphere during the SH SSW of 2002 (Palo et al., 2005). Moreover, the EEJ driven by the equatorial zonal electric field exhibits quasi-10 d oscillation, suggesting the enhanced quasi-10 d planetary wave associated with SSW penetrates into the ionosphere E region and produces oscillation in the EIA region through modulating the E-region electric fields. Our results reveal some newer features of ionospheric variation that have not been reported during Northern Hemisphere (NH) SSWs.

Performance of NavIC for studying the ionosphere at an EIA region in India

This paper emphasizes on NavIC’s performance in ionospheric studies over the Indian subcontinent region. The study is performed using data of one year (2017–18) at IIT Indore, a location near the northern crest of Equatorial Ionization Anomaly (EIA). It has been observed that even without the individual error corrections, the results are within ±20% of NavIC VTEC estimates observed over the 1° × 1° grid of IPP surrounding the GPS VTEC estimates for most of the time. Additionally, ionospheric response during two distinct geomagnetic storms (September 08 and 28, 2017) at the same location and other IGS stations covering the Indian subcontinent using both GPS and NavIC has also been presented. This analysis revealed similar variations in TEC during the geomagnetic storms of September 2017, indicating the suitability of NavIC to study space weather events along with the ionospheric studies over the Indian subcontinent.

Multi-Instrument Observations of the Atmospheric and Ionospheric Response to the 2013 Sudden Stratospheric Warming Over Eastern Asia Region

We investigate the atmospheric and ionospheric response to the 2013 sudden stratospheric warming (SSW) by using multiple instruments located in Eastern Asia. Three meteor radars and five ionosondes are used to investigate the mesospheric zonal wind fields and ionospheric parameters of F-layer virtual height (h'F), F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -layer peak height (h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), and critical frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) at midand low-latitudes (10.7°N to 40.3°N). The vertical total electron content (TEC) data derived from the groundbased global positioning system receiver network are analyzed to study the ionospheric perturbations in the equatorial ionization anomaly (EIA) region. The changes in equatorial electrojet (EEJ) are observed by using the magnetometer data from stations on and off the magnetic equator. The variations of the h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at Sanya and EEJ strength presented the semidiurnal pattern with increase/decrease and eastward/westward currents in the morning/afternoon hours. In addition, the EIA crest moved poleward/equatorward in the morning/afternoon. The f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> showed the most significant enhancements during daytime at Wuhan and Shaoyang but the f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at Sanya and Chumphon reduced mildly. Most importantly, based on the time-period wavelet analysis, the diurnal tidal components in the f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> over Beijing, Wuhan, and Sanya seemed similar those in zonal winds and the semidiurnal tides in the low-latitude h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> showed the similar temporal variations as those in EEJ strength during the later phase of SSW. Therefore, apart from the local tides propagating from lower atmosphere having influence on the mid- and low-latitude ionosphere directly during the early phase, the equatorial fountain effect modulated by the enhanced tides also disturbed the EIA region.

Signal Frequency Dependence of Ionospheric Scintillations: An Indicator of Irregularity Spectrum Characteristics

AbstractMultifrequency scintillation observations show that strong amplitude scintillations on very high frequency (VHF) signals are accompanied by weak L‐band scintillations near the dip equator and strong L‐band scintillations in equatorial ionization anomaly (EIA) region. For several decades this has been attributed to higher ambient plasma density in the EIA region. Recent work suggests that occurrence of stronger L‐band scintillations in the EIA region requires that the intermediate‐scale (~100 m to few km) ionospheric irregularity spectrum in this region be significantly shallower than that in the equatorial region. However, this has not been established so far. Signal frequency dependence of amplitude scintillations is characterized by the frequency exponent that determines the dependence on signal frequency of the S4 index: the standard deviation of normalized intensity fluctuations. In this paper, theoretical calculations of the frequency exponent for VHF and L‐band signals are carried out using different irregularity spectra and compared with observations in order to characterize power‐law irregularity spectra in different latitude zones. Intermediate‐scale irregularities in the equatorial and low‐latitude region, which are aligned with the geomagnetic field, are described by a two‐dimensional model. Model simulations show that the frequency exponent derived from S4 indices for VHF and L‐band signals is determined only by the characteristics of the irregularity spectrum and hence can be utilized to identify the nature of the power‐law irregularity spectrum. Frequency exponents computed using VHF and L‐band observations near the dip equator and EIA regions show distinct patterns, which clearly indicate steep and shallow irregularity spectra, respectively, in these regions.

An Observational Review on influence of Intense Geomagnetic Storm on Positional Accuracy of NavIC/IRNSS System

ABSTRACT The positional accuracy of Navigation with the Indian Constellation (NavIC)/Indian Regional Navigation Satellite System (IRNSS) in the low latitudes of the Indian region during the intense geomagnetic storm of 8 September 2017 being reported first time. The existence of an intense geomagnetic storm is verified with several geomagnetic indices and interplanetary field parameters. The Total Electron Content (TEC) maps from the International GNSS Service (IGS) data and the thermosphere O/N2 ratio maps from the Global Ultra Violet Imager (GUVI) are also used in the study. The one week of data (3–9 September 2017) are collected based on equatorial region (Trivandrum) and Equatorial Ionization Anomaly (EIA) region (e.g. Surat, Bombay, Hyderabad and Gandhinagar), from India, using the Accord NavIC/IRNSS dual-frequency (L5 and S-band) receivers. For the intense storm day, a comparative study is being carried out for TEC, ionospheric delay and positional accuracy for L5 band. The observations perceive the positive impact in the equatorial region while the negative impact for EIA region, with reference to quiet days of the observed week. The average ∼70% and further ∼25% NavIC/IRNSS system positional accuracy is improved by applying ionospheric correction and augmenting NavIC/IRNSS with GPS system respectively. This study is very much important in satellite-based navigation application for Precise Point Positioning (PPP).