Total Electron Content Enhancement Research Articles

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.

Analysis of Ionospheric Perturbations Possibly Related to Yangbi Ms6.4 and Maduo Ms7.4 Earthquakes on 21 May 2021 in China Using GPS TEC and GIM TEC Data

On 21 May 2021 (UT), Yangbi Ms6.4 and Maduo Ms7.4 earthquakes occurred in mainland China. This paper analyzed the ionospheric perturbations possibly related to the earthquake, based on global positioning system (GPS) total electron content (TEC) and global ionosphere map (GIM) TEC data. We identified GPS TEC anomalies by the sliding quartile, based on statistical analysis. After eliminating the days with high solar activity levels and strong geomagnetic disturbances, the time series analysis of GPS TEC data showed that there were significant TEC anomalies from 5 to 10 May. TEC anomalies were mainly positive anomalies. We obtained the spatial and temporal distributions of TEC anomalies using natural neighbor interpolation (NNI). The results showed that the TEC anomalies were distributed in the seismogenic zone and surrounded the epicenters of the Maduo and Yangbi earthquakes, indicating that they may be related to the earthquakes. From the GIM TEC difference map, we found the TEC enhancement in the seismogenic zone and its magnetic conjugate area of the Maduo and Yangbi earthquakes at 10:00–12:00 (UT) on the 5 and 6 May. We discussed our results according to the lithosphere-atmosphere-ionosphere coupling mechanism. Finally, based our results, we suggested that the Yangbi and Maduo earthquakes may affect the ionosphere through seismogenic electric field and thermal anomalies generated during the process of lithosphere-atmosphere-ionosphere coupling.

Effects of solar flares on ionospheric TEC over Iceland before and during the geomagnetic storm of 8 September 2017

The effects of solar flares on ionospheric total electron content (TEC) over Iceland before and during the geomagnetic storm of September 8, 2017 were studied. Global Positioning Syste (GPS)-TEC data were obtained from dual-frequency GPS sites operated by the University NAVSTAR Consortium at Dyna (65.06°N, −16.6°E) and Kisa (64.7°N, −17.6°E) stations. The findings of this study show that the enhancement of ionospheric TEC was more noticeable over the Kisa station than over the Dyna station during the coronal mass ejection day. However, during the geomagnetic storm day, maximum enhancement of TEC was observed over the Dyna station than over the Kisa station. The values of the ionospheric TEC during the initial phase of the storm were greater than during the main and recovery phases of the storm over both stations. This might be due to the effects of X-class solar flares and shock wave preceding the arriving of interplanetary coronal mass ejections plasma. The spatial gradients of TEC between two nearby stations are similar during the main phase of the storm, but slightly different during the initial and recovery phases of the storm. Finally, solar events have a favorable influence on geomagnetic storm indices and ionospheric observables.

Spatial and Temporal Confinement of the Ionospheric Responses During the St. Patrick's Day Storm of March 2015

AbstractThe present study provides a multi‐instrument analysis of the ionospheric response to the effects of the St. Patrick's Day storm of 17–18 March 2015. Simultaneous observations from 85 global positioning system stations and multiple satellites along the past observations from the literature survey are utilized to ascertain the spatio‐temporal confinements between −20°E and 150°E longitudes. The main results include longitudinal differences in the episodic equatorward expansions of the auroral oval which is shown to relate with varying high‐latitude Joule heating and equatorward wind surges at different local times. The zonal movement of equatorial ion density depletions from initially east to west followed by a sluggish westward pattern on the night of 17 March is observed. The zonal movement is found to be better explained by the longitudinally decreasing impact of the disturbance dynamo in the nightside. The enhancements in total electron content during morning on 17 March are observed over mid‐latitudes, which are found to move equatorward in the noon time. These enhancements could be associated with the enhanced O density bulge resulting from the thermospheric expansion during the main phase. A daytime sustained and confined electron density depletion between about 100°E and 170°E is observed on 18 March which is being discussed in detail for the first time. Interestingly, large longitudinal differences are observed in the response of the disturbance dynamo, including no observable effect during morning (−20°E–20°E) but dominant afternoon and westward progression during the day of 18 March.

Convergence Effects on the Ionosphere During and After the Annular Solar Eclipse on 21 June 2020

AbstractIn this study, we analyze the observations of the ionosondes at Wuhan (30.4°N, 114.4°E, 82.4% obscuration), Xiamen (24.2°N, 118.07°E, 97.8% obscuration), and Nanning (22.7°N, 109.25°E, 81.1% obscuration), as well as the total electron content (TEC) from the global ionosphere maps (GIMs) to examine the ionospheric behaviors in the F‐region on 21 June 2020 solar eclipse day. The observations show that a TEC enhancement occurred after the major depression near the center path of the solar eclipse, and it lasts to midnight on the solar eclipse day. Notes that the occurrence of the TEC enhancement is accompanied by the prominent TEC depletion in the southern side of the center path in the nighttime. The independent in situ electron density (Ne) observation from the Swarm‐B satellite and the calibrated TEC profile from the radio occultation technique onboard the FORMOSAT‐7/Constellation Observing System for Meteorology, Ionosphere, and Climate‐2 satellites also observed the long‐lasting enhancement in the F2 region near midnight. The solar eclipse‐induced convergence effects can result in the long‐lasting ionospheric perturbations, which may further cause the spread‐F in the nighttime.

Ionospheric-Thermospheric responses to the May and September 2017 geomagnetic storms over Asian regions

This paper presents the longitudinal dependence of ionospheric responses from Global Navigation Satellite System (GNSS) derived Total Electron Content (TEC) during two intense geomagnetic storms of May and September 2017. The GNSS-TEC is retrieved from four stations installed at the verge of low to mid-latitude Asian regions of Pakistan and China. Two ionospheric enhancements were observed during the storm of May 2017. The first one at local noon–afternoon during the storm main phase on 28 May was due to the southward turning of Interplanetary Magnetic Field (IMF-Bz) and eastward Prompt Penetration Electric Field (PPEF), with the maximum TEC enhancement at Wuhan. The second one at nighttime during the recovery phase of the storm on 29 May triggered ionospheric variations, mainly due to the later southward turning of the IMF-Bz as the Asian regions, were on the nightside with the westward PPEF. Negative storm time ionospheric responses were observed on 30 May, related to the change of the thermospheric composition as O/N2 depletion. Moreover, a significant increase in TEC was recorded during the main phase of the storm on 8 September 2017. This enhancement corresponded with the eastward PPEF and an increase in the O/N2. The TEC increment was also observed during the recovery phase on 9 September in the Pakistani stations. A minor storm on 7 September also gave rise to TEC enhancements, especially in western regions. However, the negative phase was registered from 9 to 10 September at each station because of the changes in the thermospheric composition as O/N2 depletion.

Storm‐Time Observations of Traveling Ionospheric Disturbances and Ionospheric Irregularities in East Africa

AbstractIn this paper equatorward propagating large scale traveling ionospheric disturbance (LSTID) on 17 March 2015 were investigated using the Statistical Angle‐of‐Arrival and Doppler Method for GPS (SADM‐GPS) radio interferometry technique in data‐scarce East Africa. To apply the SADM‐GPS method, 5 GPS arrays each with 3 GPS receivers arranged in a triangular geometry were used. Our results show that during 15:00–18:00 UT on 17 March, TIDs with mean horizontal velocities between 161.9 and 464.4 m/s were observed. Using the wavelet analysis, the periods of TIDs in a range of 51–69 min that qualify to LSTIDs were revealed. The peak‐to‐peak phase shift of detrended total electron content (TEC) over latitudes in this study confirms the equatorward TID propagation, which was obtained by the SADM‐GPS technique. A pair of magnetometers were used to infer E × B drift and an adequate agreement was found with Swarm satellites derived plasma density that enabled us to explain the behaviour of ionospheric irregularity during the main phase of the geomagnetic storm. Moreover, significant TEC enhancement (reaching ∼20%–105%) were captured by GPS arrays during the period of TID propagation. Nevertheless, the rate of change of TEC (ROT) and ROT index (ROTI) show wavy structures that reflect TID effects over ionospheric modulations during the post noon to evening hours of 17 March 2015.

ED‐ConvLSTM: A Novel Global Ionospheric Total Electron Content Medium‐Term Forecast Model

AbstractIn this paper, we proposed an innovative encoder‐decoder structure with a convolution long short‐term memory (ED‐ConvLSTM) network to forecast global total electron content (TEC) based on the International GNSS Service (IGS) TEC maps from 2005 to 2018 with 1‐hr time cadence. The ED‐ConvLSTM model is used to forecast TEC maps 1–7 days in advance through iterations. To investigate the model's performance, we compared the model with International Reference Ionosphere (IRI2016) model in 2014 and 2018, and compared the model with 1‐day Beijing University of Aeronautics and Astronautics (BUAA) model in 2018. The results show that our 7‐day ED‐ConvLSTM model (ED‐ConvLSTM model that forecasts 7 days in advance) outperforms IRI2016 in 2014 and 2018, and our 5‐day ED‐ConvLSTM model (ED‐ConvLSTM model that forecasts 5 days in advance) outperforms 1‐day BUAA model. Furthermore, the root mean square error (RMSE) from the 1‐day ED‐ConvLSTM model with respect to the IGS TEC maps decreases by 51.5% and 43%, respectively, in 2014 and 2018 compared with that from IRI2016 model. The RMSE from the 1‐day ED‐ConvLSTM model is 20.3% lower than that from the 1‐day BUAA model in 2018. In addition, our model has the highest RMSE in the Equatorial Ionospheric Anomaly (EIA) region, but can roughly predict the features and locations of EIA. However, the model fails to forecast localized TEC enhancement and the sudden ionospheric response to the geomagnetic storms. Overall, the model shows competitive performance in medium‐term global TEC maps prediction during geomagnetic quiet periods.

Aspects related to variability of radiative cooling by NO in lower thermosphere, TEC and O/N2 correlation, and diffusion of NO into mesosphere during the Halloween storms

Nitric Oxide is a very important trace species which plays a significant role acting as a natural thermostat in Earth’s thermosphere during strong geomagnetic activity. In this paper, we present various aspects related to the variation in the NO Infrared radiative flux (IRF) exiting the thermosphere by utilizing the TIMED/SABER (Thermosphere Ionosphere Mesosphere Energetics and Dynamics/ Sounding of the Atmosphere using Broadband Emission Radiometry) observational data during the Halloween storm which occurred in late October 2003. The Halloween storm comprised of three intense-geomagnetic storms. The variability of NO infrared flux during these storm events and its connection to the strength of the geomagnetic storms were found to be different in contrast to similar super storms. The connection between the quantum of energy outflux from the upper atmosphere into space in terms of NO IRF and the duration of storms is established. The NO radiative cooling, and the closely correlated depletion in O/N2 ratio are controlled by the Joule heating intensity (proxied by AE-index). The collisional excitation rate of NO, calculated using the modelled datasets of WACCM-X (Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension), correlates well with the observed pattern of radiative emission by NO. Observational datasets from TIMED/GUVI (Global Ultra-Violet Imager) and MIT Haystack observatory madrigal GNSS (Global navigation satellite system) total electron content (TEC) database shows that the TEC and O/N2 enhancement in low-mid northern hemispheric latitudes are mainly controlled by the z-component of Interplanetary magnetic field (IMF-Bz). The penetration of eastward electric field during the storm events is found to be responsible for the overall enhancement of TEC. The contribution of enhanced day-side TEC in observed variation of O/N2 ratio by GUVI is also reported. It is also seen that during substorms related events the night-time polar region experiences more cooling due to NO than the daytime polar region. The connections between the mid- and low-latitude enhancement in NO IRF with the propagation of LSTIDs (Large-scale traveling ionospheric disturbances) in combination with the O/N2 variability, and the altitudinal variation in NO flux with the progression of the storm is also investigated. This study presents the evidence on the role of diffusion processes in the large scale enhancement of NO in the mesospheric altitudes.

On the impact of meridional wind circulation changes in the electron density distribution over the Indian equatorial and low latitude ionospheric region during a severe geomagnetic storm

Using a suite of instruments, which included a chain of ground-based dual-frequency GPS receivers, and magnetometers, we have studied the importance of thermospheric meridional wind circulation in controlling the distribution of plasma over the Indian low latitude ionospheric regions during the period of a severe geomagnetic storm. The storm on 15 May 2005, which had its onset coinciding with the local noon time sector for the Indian ionospheric zone, was a severe geomagnetic storm with symH ∼ - 305 nT. A steep increase in the Total Electron Content (TEC) of the ionosphere over the entire Indian ionospheric region was observed on May 15. The enhancement in the TEC was well correlated with the increase in ΔH at the dip-equator due to the prompt penetration of the convection electric field associated with the storm. However, contrary to the previous studies on the storm impact over low latitude regions, a clear signature of disturbance dynamo was absent on the day after the storm. Enhancements in the TEC were observed on May 16, a day after the storm, as well, though the ΔH at the dip-equator was quite below the quite-time mean. The TEC remained well above its monthly mean over the entire Indian ionospheric region during the storm recovery period. We suggest that the TEC enhancement on May 16, even though it looked like due to a prompt penetration effect, was directly related to the compositional disturbances as given by the O/N2 ratio. We conclude that the meridional wind circulation plays an important role in the distribution of electron density over the equatorial and low latitudinal region during the period of a geomagnetic storm.

A Multi-Network based Hybrid LSTM model for ionospheric anomaly detection: A case study of the Mw 7.8 Nepal earthquake

We propose a Multi-Network-based Hybrid Long Short Term Memory (N-LSTM) model for ionospheric anomaly detection to forecast highly irregular data of the ionospheric Total Electron Content (TEC). Previously purposed models were suffering from the problems of vanishing gradient, exploding gradient, uncertainty, and parameter bias. LSTM is used to overcome the vanishing and exploding gradient problems and the remaining two (uncertainty and parameter bias) are subjugated by N-LSTM. We have also discussed the implementation of the model by using the Mw 7.8 Nepal Earthquake (EQ) that occurred on April 25, 2015. The proposed model detects a significant negative anomaly about 14 days before the impending EQ. Furthermore, we find gradual increments in the TEC time-series, where the TEC values gradually increased, in terms of positive anomalies, for four continuous days (April 21–24, 2015) till the mainshock. Further analysis revealed that the nighttime TEC enhancements are more dominant than the daytimes, which persisted for 10 h (22:00–08:00 LT) in the absence of the solar flux, we named them the absolute seismic precursors. These gradual enhancements establish a perfect correlation with several atmospheric anomalies, reported in the previous studies, where the variations in different atmospheric parameters like Outgoing Longwave Radiation (OLR), air temperature, Ozone, and sub-ionospheric Very-Low Frequency (VLF), were reported for 4 consistent days till the EQ day. The N-LSTM result was compatible with a 30-day running median, which is the classical method of ionospheric anomaly detection. We also examined the planetary K-index (Kp), disturbance storm-time (Dst), solar radio flux (F10.7), and solar wind speed (VSW) indices to check the possible attribution of space weather on the ionosphere during the analysis. The results showed that the anomalies of the ionospheric TEC were more dominantly caused by the EQ phenomenon than the space weather during the nighttime hours. Also, N-LSTM provided sufficient short-term prediction performance of the ionospheric TEC and anomaly detection.

Persistent Eastward EEJ Enhancement During the Geomagnetic Storm Recovery Phases

AbstractRecent case studies disclosed that sometimes the dayside ionospheric equatorial electric field underwent persistent long‐duration (>4 hr) eastward enhancements during geomagnetic storm recovery phases, rather than anticipated decrease or reversal. However, it is still unknown the occurrence and morphology of the persistent enhancement as well as its impact on the low‐latitude ionospheric plasma distribution. For the first time, we examined the issue with superposed epoch analyses of equatorial electrojet (EEJ) and GNSS total electron content (TEC) responses in the American (49°W) and Indian (78°E) sectors. The occurrence rates of the persistent enhancement during storm recovery phases are more than 30%, with higher occurrence rates in second to fourth days of the recovery phase, equinoxes, and American sector. The corresponding averaged EEJ and TEC responses were disclosed and compared with those storm samples without the persistent enhancement. On the first day of recovery phase, the dayside equatorial ionosphere was dominated by the prevalent westward electric field and causative inhibited equatorial fountain. In contrast, after the first day, the dayside morphologies showed different patterns: (a) for the persistent cases, the large eastward EEJ and TEC enhancement indicated the enhanced eastward electric field at forenoon and causative intensified equatorial fountain. (b) For the storms without the persistent cases, the general westward EEJ and equatorial TEC enhancements were dominated. Our study indicates that dayside eastward enhancement is an important disturbed pattern of equatorial ionospheric electric field during the late recovery phases, and the enhancement generally causes the intensification of equatorial ionization anomaly.

Low-Latitude Ionospheric Responses and Coupling to the February 2014 Multiphase Geomagnetic Storm from GNSS, Magnetometers, and Space Weather Data

The ionospheric response and the associated mechanisms to geomagnetic storms are very complex, particularly during the February 2014 multiphase geomagnetic storm. In this paper, the low-latitude ionosphere responses and their coupling mechanisms, during the February 2014 multiphase geomagnetic storm, are investigated from ground-based magnetometers and global navigation satellite system (GNSS), and space weather data. The residual disturbances between the total electron content (TEC) of the International GNSS Service (IGS) global ionospheric maps (GIMs) and empirical models are used to investigate the storm-time ionospheric responses. Three clear sudden storm commencements (SSCs) on 15, 20, and 23 February are detected, and one high speed solar wind (HSSW) event on 19 February is found with the absence of classical SSC features due to a prevalent magnetospheric convection. The IRI-2012 shows insufficient performance, with no distinction between the events and overestimating approximately 20 TEC units (TECU) with respect to the actual quiet-time TEC. Furthermore, the median average of the IGS GIMs TEC during February 2014 shows enhanced values in the southern hemisphere, whereas the IRI-2012 lacks this asymmetry. Three low-latitude profiles extracted from the IGS GIM data revealed up to 20 TECU enhancements in the differential TEC. From these profiles, longer-lasting TEC enhancements are observed at the dip equator profiles than in the profiles of the equatorial ionospheric anomaly (EIA) crests. Moreover, a gradual increase in the global electron content (GEC) shows approximately 1 GEC unit of differential intensification starting from the HSSW event, while the IGS GIM profiles lack this increasing gradient, probably located at higher latitudes. The prompt penetration electric field (PPEF) and equatorial electrojet (EEJ) indices estimated from magnetometer data show strong variability after all four events, except the EEJ’s Asian sector. The low-latitude ionosphere coupling is mainly driven by the variable PPEF, DDEF (disturbance dynamo electric fields), and Joule heating. The auroral electrojet causing eastward PPEF may control the EIA expansion in the Asian sector through the dynamo mechanism, which is also reflected in the solar-quiet current intensity variability.

African and American Equatorial Ionization Anomaly (EIA) Responses to 2013 SSW Event

AbstractThis study investigates the responses of the African and American Equatorial Ionization Anomaly (EIA) regions to 2013 Sudden Stratospheric Warming (SSW) event. The Total Electron Content (TEC) data obtained from chains of Global Positioning System receivers within ±40° geomagnetic latitudes in the African and American sectors were used to construct the EIA structures for both longitudinal sectors. The responses of the EIA structures, constructed from the TEC, ΔTEC, and ionospheric irregularities data to the 2013 SSW event were investigated. During the SSW peak phase, EIA structures in both longitudinal sectors responded significantly, with the pole‐ward flow of plasma from the equator to higher crests' locations (strengthening of the EIA). Furthermore, a clear asymmetry in plasma distribution in the northern and southern crests of the EIA was observed. Generally, for the entire data span, TEC enhancements and ionospheric irregularities occurrences during SSW were more in the American sector than the African sector. The geomagnetic activity of 17 January 2013 caused negative TEC response in the African sector and positive TEC response in the American sector. Moderate storm‐induced TEC enhancements were generally lower than SSW‐induced TEC enhancements. Furthermore, solar flux‐induced TEC of 10 January 2013 was lesser than the SSW‐induced TEC of 15–16 January 2013.

Ionospheric total electron content anomaly possibly associated with the April 4, 2010 Mw7.2 Baja California earthquake

• New decomposition and nonlinear fitting method to extract TEC disturbances. • TEC increasing near epicenter on March 25, 2010, 10 days before Mw7.2 Baja California earthquake. • Both data and model show the uniqueness of the TEC enhancement on March 25. Identifying ionospheric disturbances potentially related to an earthquake is a challenging work. Based on the ionospheric total electron content (TEC) data from the madrigal database at the Haystack Observatory, Massachusetts Institute of Technology, a new decomposition and nonlinear fitting method has been developed and applied in this work to extract the TEC disturbances that are potentially related to the Mw7.2 Baja California earthquake occurred on April 4, 2010. By analyzing the TEC data for a long period of time (72 days) before and after the earthquake, we found that a unique TEC enhancement occurred in the region around the epicenter on March 25. No other significant ionospheric TEC anomalies were identified in the 72-day period around the earthquake, except some TEC disturbances that appeared to be related to the geomagnetic activity between April 1 and 6, 2010. We further analyzed the TEC data from other magnetically quiet days, and no TEC anomaly like that occurred on March 25 was detected. The TEC data calculated from a first principles model SD-WACCM-X were also analyzed using the same method as that for the observational data. No TEC anomaly was found on March 25 from the model outputs either. Thus the source of the TEC anomaly on March 25 is unlikely from the lower atmosphere waves. In this study, we show the occurrence of TEC anomaly on March 25, 10 days before the Mw7.2 Baja California earthquake and this TEC anomaly may not be explained by lower atmosphere or geomagnetic activity forcing.

Vertical characterization on global ionospheric variations during the magnetic storm in September 2017 with hierarchical subtraction method

This study investigates the global ionospheric responses at different altitudes during the geomagnetic storm in September 2017 by using the observations from IGS GIMs and GRACE, TerraSAR-X, COSMIC, Metop LEO satellite missions. According to different satellite heights, a hierarchical subtraction method is proposed to characterize the disturbance of total electron content (TEC) at the vertical regions of below 340 km , 340 ∼ 520 km , 520 ∼ 800 km and above 800 km , respectively. The integrated TEC from IRI-Plas2017 are applied to verify the feasibility and rationality of the proposed method and the electron density (Ne) profiles from the ionosonde and radio occultation (RO) measurements are used to explore the variations of Ne in different height regions. The resulting TEC disturbances ( Δ TEC) on a global scale show that: 1) There was significant increase on Ne peak in F2 region during the magnetic storm, due to the strong recombination in the lower ionosphere while more intense electron ionization in the upper ionosphere; 2) During the main phase, TEC enhancement had a tendency to drift westward, which may correlate with the twice Dst minimums. 3) The east–west and north–south hemispheric asymmetry of Δ TEC exist simultaneously in this magnetic storm. However, the former is mainly attributed to ionospheric plasma bubbles, while the latter might be the seasonal thermospheric effects or the energetic particle emissions.

Statistical Behavior of Large‐Scale Ionospheric Disturbances From High Latitudes to Mid‐Latitudes During Geomagnetic Storms Using 20‐yr GNSS‐TEC Data: Dependence on Season and Storm Intensity

AbstractTo establish the statistical behavior of ionospheric TEC variations from high latitudes to mid‐latitudes during the main and recovery phases of geomagnetic storms, we conducted a superposed epoch analysis of interplanetary magnetic field, solar wind, geomagnetic indices (AE and SYM‐H), and global navigation satellite system (GNSS)‐total electron content (TEC) data for 20 yr (2000–2019). In this study, we identify 663 geomagnetic storm events with the minimum SYM‐H value of less than −40 nT and investigate the characteristics of the TEC variations for the weak (−60 ≤ SYM‐Hmin < −40 nT), moderate (−100 ≤ SYM‐Hmin < −60 nT), and strong (−150 ≤ SYM‐Hmin < −100 nT) geomagnetic storms. The main results obtained from the present study are as follows: (a) The TEC enhancements related to the tongue of ionization (TOI), auroral oval, and storm‐enhanced density (SED) plume are more dominant in winter than in summer during the main phase of geomagnetic storms. (b) The structure of the mid‐latitude trough in the nighttime sector becomes unclear in winter. (c) The TEC depletion at auroral and mid‐latitudes (40°–70° GMLAT: geomagnetic latitude) starts to appear in the morning sector (8–10 hr GMLT: geomagnetic local time) during the main phase of geomagnetic storms and the decreased region extends in the lower latitude and GMLT directions with time. The negative storm activity tends to be enhanced significantly as the storm intensity becomes larger. The activity of the TEC depletion is dominant in summer than in winter, which is agreement with the classical ionospheric storm scenario.

Middle and low latitudes hemispheric asymmetries in ∑O/N2 and TEC during intense magnetic storms of solar cycle 24

We have investigated the global hemispheric differences in thermospheric ∑O/N 2 and its impact on the ionospheric total electron content (TEC) at mid- and low-latitudes. Four intense storms of solar cycle 24 (SC-24) have been considered, three of them occurred in Spring equinox and one in Summer solstice season. It is found that the mid-latitudes region has exhibited a large decrease in ∑O/N 2 during all the phases of the storms under consideration, which corresponds well to the observed negative storm effects. This decrease is directly related with the storm intensity. The maximum reduction in the ∑O/N 2 is observed for the St. Patrick day storm of 2015 (which was the most intense geomagnetic storm of SC-24), whereas the respective minimum decrease is found for the storm of April 2012. Strong hemispheric asymmetries, in ∑O/N 2 variation, have been observed at the mid-latitudes sector, and can be associated with the asymmetric energy input as indicated by polar cap (PC) indices. The high speed solar winds streams (HSSWs) during the recovery phases of March 2013 and 2015 storms have caused a significant reduction in ∑O/N 2 at mid-latitudes, which could not be reproduced by the coupled thermosphere-ionosphere-plasmasphere electrodynamics (CTIPe) model. On the other hand the low-latitudes region depicts an enhancement in ∑O/N 2 during all the storms except for the early recovery phases. The positive storm effect at low-latitudes agrees well with this ∑O/N 2 increase, thus indicating that the composition change is one of the major drivers of TEC enhancement at low-latitudes. The CTIPe model showed discrepancies in reproducing the satellite data for all the considered storms, especially during the recovery phases. Furthermore, the model is failed to replicate the hemispheric asymmetries at low and mid-latitudes during the main and early recovery phases.

The nighttime ionospheric response and occurrence of equatorial plasma irregularities during geomagnetic storms: a case study

Recent studies revealed that the long-lasting daytime ionospheric enhancements of Total Electron Content (TEC) were sometimes observed in the Asian sector during the recovery phase of geomagnetic storms (e.g., Lei (J Geophys Res Space Phys 123: 3217–3232, 2018), Li (J Geophys Res Space Phys 125: e2020JA028238, 2020). However, they focused only on the dayside ionosphere, and no dedicated studies have been performed to investigate the nighttime ionospheric behavior during such kinds of storm recovery phases. In this study, we focused on two geomagnetic storms that happened on 7–8 September 2017 and 25–26 August 2018, which showed the prominent daytime TEC enhancements in the Asian sector during their recovery phases, to explore the nighttime large-scale ionospheric responses as well as the small-scale Equatorial Plasma Irregularities (EPIs). It is found that during the September 2017 storm recovery phase, the nighttime ionosphere in the American sector is largely depressed, which is similar to the daytime ionospheric response in the same longitude sector; while in the Asian sector, only a small TEC increase is observed at nighttime, which is much weaker than the prominent daytime TEC enhancement in this longitude sector. During the recovery phase of the August 2018 storm, a slight TEC increase is observed on the night side at all longitudes, which is also weaker than the prominent daytime TEC enhancement. For the small-scale EPIs, they are enhanced and extended to higher latitudes during the main phase of both storms. However, during the recovery phases of the first storm, the EPIs are largely enhanced and suppressed in the Asian and American sectors, respectively, while no prominent nighttime EPIs are observed during the second storm recovery phase. The clear north–south asymmetry of equatorial ionization anomaly crests during the second storm should be responsible for the suppression of EPIs during this storm. In addition, our results also suggest that the dusk side ionospheric response could be affected by the daytime ionospheric plasma density/TEC variations during the recovery phase of geomagnetic storms, which further modulates the vertical plasma drift and plasma gradient. As a result, the growth rate of post-sunset EPIs will be enhanced or inhibited.

Characteristics of Ionospheric Storm on October 13, 2016 at the Greenwich Meridian

AbstractAn ionospheric positive disturbance during a moderate geomagnetic storm on October 13, 2016 was studied with total electron content (TEC) from a chain of GNSS receivers and F2 layer peak parameters from two ionosondes at the Greenwich meridian. During a slow decrease of Dst, a large enhancement of the daytime TEC was observed from high to middle latitudes successively while there was small disturbance observed in low latitudes. The occurrence of TEC peak delayed with decreasing latitude. Similarly, positive disturbance of hmF2 and foF2 was recorded and their onsets were earlier in high latitude than middle latitude. These observations indicated that the ionospheric positive disturbance propagated equatorward which denied the dominant effect of prompt penetration eastward electric field (PPEF). Using the latitudinal and temporal variations of TEC peaks, the propagation velocity of the disturbance was estimated and found to decrease with decreasing latitude. Two available Fabry‐Perot interferometers (FPI) at the meridian W of the Greenwich showed that nighttime equatorward wind in middle latitude increased significantly after the storm sudden commencement (SSC), while no change of that was observed in low latitude. The coincidence of the directional and latitudinal features between the ionospheric disturbance and the wind observation suggested the impact of the positive storm was most likely the equatorward wind surge which raised the ionosphere to higher altitude where lower chemical loss led to increase of electron density in the daytime.