Positive Ionospheric Storm Research Articles

East–West Difference in the Ionospheric Response During the Recovery Phase of May 2024 Super Geomagnetic Storm Over the East Asian

AbstractIn this study, we offer an extensive examination of the F‐region ionospheric disturbances during the May 2024 superstorm, focusing primarily on the middle‐low latitude regions of East Asia. Our analysis is grounded in a wealth of data sources including Total Electron Content (TEC), ionospheric parameters NmF2 and hmF2, Electron Density Profile (EDP) retrieved from Radio Occultation (RO) data, and ∑[O]/[N2] from the Global Ultraviolet Imager (GUVI), among others, complemented by model simulations. The observed negative ionospheric storm effect, characterized by a significant and long‐lasting reduction in electron density across the entire China, commenced immediately following the sudden storm commencement (SSC) on 10 May and continued through the main and early recovery phase of the storm on 11 May. On 11–12 May, positive ionospheric storm impacts were initially observed in a restricted geographical area from the post‐midnight to sunrise, first manifesting over the eastern regions of China and then shifting to the central regions. Subsequently, a pronounced negative storm effect persisted throughout the later stages of recovery phase. In contrast, the western regions of China experienced a positive storm effect on 12 May followed by a comparatively mild negative storm phase. This persistent extensive zonal gradient in electron density across the East Asian region resembles the scenarios depicted in prior superstorms attributed to the thermospheric circulation patterns. The disparity in the ionospheric response from east to west in this area is probably a common feature during superstorms, potentially resulting from an arch‐shaped structure of elevated ∑[O]/[N2].

Ionospheric response of the March 2023 geomagnetic storm over European latitudes

The present study describes a spatial and temporal analysis of the ionospheric variability over the European sector during a major storm event from 23 to 24 March 2023. The analysis is mainly based on ionospheric characteristics such as foF2, hmF2 and Vertical Total Electron Content (VTEC) over ten Digisondes and co-located GNSS receivers. The event started at 12:00UT on 23 March as a positive ionospheric storm followed by a negative ionospheric storm to 04:00 UT on the next day SYM-H<-183nT. A notable latitude dependence of the ionospheric response is more prominent over the northern mid-latitude region attributed to the equatorward displacement of the Midlatitude Ionospheric Trough (MIT). To verify this hypothesis, Swarm latitudinal electron density profiles from in-situ Langmuir probe measurements over Europe were considered. These profiles demonstrate a clear MIT displacement from ∼65°N on 22 March to ∼55°N on 23 March with a subsequent recovery around ∼65°N on 25 March.

Ground and Space-based response of the ionosphere during the geomagnetic storm of 02–06 November 2021 over the low-latitudes across different longitudes

The strong geomagnetic storm of class G3 that occurred on 03 November 2021 (Dst min = −118 nT, Kp = 8) was the first of this class recorded during the ascending phase of solar cycle 25. In this study, we examined the response of the ionosphere during the geomagnetic storm period from 02–06 November 2021 using the Total Electron Content (TEC) observations from a chain of Global Navigation Satellite System (GNSS) receiver stations in five longitudinal sectors i.e, Asian, East-African, West-African, South-American, and Pacific-west sectors, in the low-latitudes and space-based Swarm satellite electron density, Ne and TEC measurements. The Rate of Change of TEC Index (ROTI) was derived from the ground and space based TEC measurements to examine the occurrence of ionospheric irregularities. The Rate of change of electron density index (RODI) was also derived from Swarm Ne measurements to identify the occurrence of ionospheric irregularities at Swarm altitudes. A positive ionospheric storm effect was observed in all the longitudinal sectors considered in this study. A clear hemispherical asymmetry, with higher VTEC in the northern hemisphere was observed. The determining factors for ionospheric responses to this storm are; local time of the storm’s onset, local time of storm’s minimum SYM-H, and changes in thermospheric O/N2 ratio. These geomagnetic storm effects were discussed in terms of the Prompt Penetration Electric Field (PPEF) storm induced wind lifting effect and Disturbance Dynamo Electric Field (DDEF). The geomagnetic storm inhibited the occurrence of ionospheric irregularities in all the sectors, during the main phase, except for three IGS receiver stations in South American sector. Overall, a longitudinal dependence of the enhancement/inhibition of ionospheric irregularities was observed. The generation of post-sunset irregularities was attributed to the local time occurrence of maximum ring current, PPEF and DDEF.

Effects of Strong Geomagnetic Storms on the Ionosphere and Degradation of Precise Point Positioning Accuracy during the 25th Solar Cycle Rising Phase: A Case Study

Approaching the peak year of the 25th solar activity cycle, the frequency of strong geomagnetic storms is gradually increasing, which seriously affects the navigation and positioning performance of GNSS. Based on the globally distributed GNSS station data and FORMOSAT-7/COSMIC-2 occultation data, this paper explores for the first time the effects of the G4-class geomagnetic storm that occurred on 23–24 April 2023 on the global ionosphere, especially the ionospheric equatorial anomalies and F-layer perturbations. It reveals the precise point positioning (PPP) accuracy degradation during a geomagnetic storm. The results show that the ionospheric rate of total electron content index (ROTI) and near high latitude GNSS phase scintillations index have varying levels of perturbation during geomagnetic storms, with the maximum ROTI and phase scintillations index exceeding 0.5 TECU/min and 0.8, respectively. The equatorial ionization anomaly (EIA) shows an enhanced state (positive ionospheric storms) during geomagnetic storms, and the cause of this phenomenon is most likely the equatorward neutral wind. The variation of the S4 index of the FORMOSAT-7/COSMIC-2 satellite reveals the uplift of the F-layer during geomagnetic storms. During geomagnetic storms, the PPP accuracy degrades most seriously at high latitudes, the maximum MAE exceeds 2.3 m, and the RMS in the three-dimensional (3D) direction exceeds 2.0 m. These investigations can provide case support for space weather and GNSS studies of the impact of geomagnetic storms during peak solar activity years.

Variability of Ionosphere Over Indian Longitudes to a Variety of Space Weather Events During December 2006

AbstractThis paper highlights the impact of intense solar events over India during 3–20 December 2006. Ionospheric effects of a major solar flare (X9) on December 5 (10:35 UT) have been investigated by using dayside and nightside magnetometer data, dayside ionosondes, and dayside GPS vTEC observations. On the next day, a stream of fast solar wind hits the magnetosphere, causing a HILDCAA (High Intensity Long Duration Continuous Auroral Activity) preceded by moderate geomagnetic storm. The origin and characteristics of a positive ionospheric storm which occurred over Tirunelveli (TIR, geomagnetic latitude: −0.18°N) in the recovery phase of storm due to simultaneous presence of enhanced O/N2 and WEJ or weakened EEJ during the HILDCAA (7th and 8th of December) is investigated. Subsequently, on December 14, the most powerful CME since the Halloween event impacts the Earth, and three SSCs are recorded on December 14, 16, and 18. The variability of the ionosphere over the Indian longitude sector due to these intense space weather fluctuations is presented by utilizing the magnetometers, ionosonde, GPS vTEC, and satellite‐based observations in the same region. This study reports the influence of prompt penetration of the magnetospheric convection electric field and the disturbance dynamo on several key ionospheric and magnetic parameters within the Indian longitude sector.

Assessing the Effects of a Minor CIR‐HSS Geomagnetic Storm on the Brazilian Low‐Latitude Ionosphere: Ground and Space‐Based Observations

AbstractThis paper investigates the effects of a minor G1 Co‐rotating Interaction Region (CIR)/High‐Speed Stream (HSS)‐driven geomagnetic storm that occurred on (13–14 October 2018), during deep solar minimum. We used simultaneous observations from multiple instruments, namely; ground‐based Global Navigation Satellite Systems (GNSS) receivers, a Digisonde, ground magnetometers, and space‐based observations from the National Aeronautics and Space Administration Global‐scale Observations of Limb and Disk (GOLD) and SWARM missions. This study presents a detailed picture of the low‐latitude ionosphere response over the Brazilian sector during a minor storm. Our results showed that the minor CIR/HSS‐driven storm caused a positive ionospheric storm of over ∼20 TECU in magnitude. For the first time, periodic post‐sunset irregularities and Equatorial Plasma Bubbles, equatorial plasma bubbles, were analyzed using GOLD FUV OI 135.6 nm emission, Total Electron Content (TEC) maps, Rate of TEC index, ROTI, and TEC gradients. Fluctuations in the interplanetary magnetic field Bz and changes in the thermospheric column density ratio (∑O/N2) are discussed as the main sources of ionospheric changes during the storm. This paper highlights the importance of monitoring and understanding the impact of Sun‐Earth interactions and provides insight into the behavior of the low‐latitude ionosphere during minor geomagnetic storms.

Multiple Longitude Sector Storm‐Enhanced Density (SED) and Long‐Lasting Subauroral Polarization Stream (SAPS) During the 26–28 February 2023 Geomagnetic Storm

AbstractThis paper conducts a multi‐instrument analysis and data assimilation study of midlatitude ionospheric disturbances over the European and North American longitude sectors during a strong geomagnetic storm on 26–28 February 2023. The study uses a set of ground‐based (GNSS receivers, ionosondes) observations, space‐borne (DMSP, GOLD) measurements, and a new TEC‐based ionospheric data assimilation system (TIDAS). We observed a series of distinct storm‐time features with regard to storm‐enhanced density (SED) and subauroral polarization stream (SAPS) as follows: (a) Under multiple ring current intensifications, the storm‐time subauroral ionosphere produced long‐lasting duskside SAPS for ∼36 hr along with considerable dawnside SAPS for several hours. (b) Associated with long‐lived SAPS, strong SED occurred consecutively in the European longitude sector near local noon during a positive ionospheric storm and later in the North American longitude sector near local dusk during a negative ionospheric storm. (c) The 3‐D morphology of SED in multiple longitude sectors was reconstructed using TIDAS data assimilation technique with fine‐scale details, which revealed a narrow ionospheric plasma channel with electron density enhancement and layer uplift.

Geomagnetic storm-induced perturbations over Indian equatorial ionosphere

This study reveals the geomagnetic storm-induced ionospheric perturbations observed over the near-equatorial Indian stations - Cochin and Changanacherry in 2021. The 205 MHz VHF radar at Cochin University of Science and Technology (CUSAT) detected the storm-induced E- and F- region echoes during the moderate (28th February − 6th March 2021) and weak (19th − 25th March 2021) storm events. The fast-fluctuating northward interplanetary magnetic field (IMF BZ) and inhibition of the equatorial ionization anomaly (EIA) observed on 1st March indicate the suppression of daytime eastward electric field by the westward electric field associated with the counter electrojet (CEJ). Concurrent equatorward shift in the EIA crest and minor positive storm enhancement in the vertical total electron content were observed. The CEJ could have helped in the accumulation of photo-generated plasma at the near-equatorial region by pushing the daytime F-layer downward. During the moderate storm phase, post-sunset E-region echoes were detected during 1st − 3rd March, attributed to the disturbance dynamo electric field (DDEF). Nevertheless, during the main phase of the weak storm, a poleward shifting of the EIA crest occurred on 20th March, indicating the presence of a prompt penetration electric field (PPEF), which enhanced the upwardE×Bdrift. A major positive ionospheric storm enhancement that lasted for 7 h occurred on 20th March due to the DDEF. Bottom-type F-region echoes were observed over Cochin during the recovery phase of the weak storm. This work highlights the observation of equatorward and poleward shifting of the EIA using a 205 MHz radar.

Ionospheric and magnetic signatures of extreme space weather events of 17 March and 23 June 2015 over the African sector

We have investigated responses of the African sector of the ionosphere to 17 March and 23 June 2015 severe geomagnetic storms. The vertical total electron content (TEC) measurements from Global Navigation Satellite System (GNSS) stations, which are located across the African continent, have been used to estimate the deviation of TEC (ΔTEC). The observed ionospheric response to these events was different at different locations. In the main phase of the 17 march 2015 storm, a significant positive ionospheric storm (ΔTEC maximum) occurred at Meli (245.8% enhancement) and Adis (105% enhancement) stations in the post-sunset period. On the other hand, during midnight negative ionospheric storms (ΔTEC minimum: ∼ 46%–68% depletion ) were observed at Dakr, Nklg, Wind, and Haro stations. The effects of 23 June 2015 storm are more pronounced during the initial phase. In the recovery phase of 23 June storm, negative storm effects observed at the eastern dip latitude and western equatorial stations. In contrast, at the northern and southern mid-latitude stations, we observed positive storm effect during the night periods. Night time TEC enhancement at mid-latitude stations probably caused by the vertical drift of the ionospheric plasma. We have also studied the magnetic signatures of the severe storms of 17 March and 23 June 2015 by analyzing their impacts on the horizontal (H-component) of the Earth’s magnetic field. By using a moving average filter, we have separated the effect of the magnetic disturbance (DP2) and the disturbance dynamo (Ddyn). The H-component analysis shows that there was maximum oscillation of DP2 and Ddyn currents occurrence when the IMF-Bz directed southward. In the main and recovery phases of the storm, this strong current oscillation is the driver for the development of positive ionospheric effect.

Observational evidence of thermospheric wind and composition changes and the resulting ionospheric disturbances in the European sector during extreme geomagnetic storms

On November 1st and 2nd, 2021, four Halo coronal mass ejections were ejected from the Sun, releasing billions of tons of high-energy particles into interplanetary space. These were directed towards the Earth and reached our planet on November 3rd and 4th, 2021, generating the first G3-level extreme geomagnetic storm since the beginning of the 25th solar cycle. In this study, we investigate the thermospheric and ionospheric responses in the European sector to a G3-level storm using various observational data from Fabry-Perot interferometer, Ionospheric Connection Explorer/Michelson Interferometer for Global High-resolution Thermospheric Imaging (ICON/MIGHTI), and Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI). The results show positive ionospheric storms in the middle and low latitudes of Europe which may be associated with the equatorward and westward neutral winds induced by heating in the polar region. In contrast, negative storms were detected at high latitudes in association with the increase in thermospheric density (upwelling). These two antithetical responses were confirmed by using European ionosonde and total electron contents (TEC) observation chains distributed over a wide range of latitudes. Finally, we, for the first time, attempt to identify the imaginary boundary line between the two responses.

Driver of the Positive Ionospheric Storm over the South American Sector during 4 November 2021 Geomagnetic Storm

During geomagnetic storms, ionospheric storms can be driven by several mechanisms. Observations performed using ground- and space-based instruments were used to reveal the driver of the positive ionospheric storm over the South American sector during the 4 November 2021 geomagnetic storm. The positive storm appeared from 10:30 UT to 18:00 UT and covered the region from 40°S to 20°N. The maximum magnitudes of TEC (Total Electron Content) enhancement and relative TEC enhancement were about 20 TECU and 100%, respectively. Defense Meteorological Satellite Program (DMSP) also observed a significant electron density increase over South America and the eastern Pacific Ocean. In the meantime, about 50% ∑O/N2 enhancement was observed by the Global-scale Observations of the Limb and Disk (GOLD) satellite at low latitudes. Ionosonde observations (AS00Q and CAJ2M) registered an ~80 km uplift in F2 peak height (HmF2) and a prominent F2 peak electron density (NmF2) increase ~3 h after the uplift. A prominent enhancement in the cross-polar cap potential (CPCP) in the southern hemisphere was also observed by Super Dual Auroral Radar Network (SuperDARN) one hour earlier than the HmF2 uplift. Measurements of the Ionospheric Connection Explorer satellite (ICON) showed that the outward E×B drift was enhanced significantly and that the horizontal ion drift was poleward. According to the ICON ion drift observations, the HmF2 uplift was caused by an electric field rather than equatorward neutral wind. We propose that the enhanced eastward electric field dominated the positive ionospheric storm and that the thermospheric composition variation may have also contributed.

Physical effects of geocosmic storms. Part 3

A review of the results of observing the effects of the most interesting ionospheric and magnetic storms that took place in 2018–2019 is presented. Ionospheric and magnetic storms are considered as components of geocosmic storms. In accordance with the systemic paradigm of L.F. Chernogor, a geocosmic storm is a complex of processes caused by a solar storm, reduced to synergistically interacting storms in a magnetic field (magnetic storm), ionosphere (ionospheric storm), atmosphere (atmospheric storm) and electric field atmospheric-ionospheric-magnetospheric origin (electric storm). Using the analysis of specific magnetic and ionospheric storms as an example, it is shown that, in addition to general patterns, each storm has its own individual characteristics. The expediency of a comprehensive study of the manifestations of each new geocosmic storm has been confirmed. A wide variety of ionospheric and magnetic storms have been confirmed. A weak magnetic storm is not necessarily accompanied by a weak ionospheric storm, while a strong one is accompanied by a strong one. An ionospheric storm can be single-phase (positive or negative) or multi-phase, when the phases alternate. Storms can last from several hours to many days. Storms are accompanied by both aperiodic and quasi-periodic variations. The latter are caused by atmospheric gravity waves (periods 10–180 min) and infrasound (period less than 5 min). Of all the components of a geocosmic storm, except for academic ones, ionospheric storms are of the greatest practical interest. It is these storms that limit the potential characteristics of radio systems for various purposes (telecommunications, radio navigation, radar, remote radio sounding, radio astronomy). It is shown that ionospheric storms are accompanied by the appearance of multipath, broadening of the Doppler spectra (and even their collapse), significant aperiodic and quasi-periodic variations in the Doppler frequency shift, the amplitude of radio signals, and the trajectories of radio waves in the decameter range. In particular, the height of reflection of radio waves changed during ionospheric storms by 50–180 km. The electron concentration during moderate negative ionospheric storms decreased up to 2 times, and during moderate positive ionospheric storms it increased up to 3 times. Under the action of atmospheric gravity waves, the relative amplitude of quasi-periodic variations in the electron concentration reached 50%, and due to low-frequency infrasound, it did not exceed 0.7%.

Ionospheric response to the 23–31 August 2018 geomagnetic storm in the Europe-African longitude sector using multi-instrument observations

This study presents ionospheric responses of the mid and low-latitude region in the Europe-African longitude sector (along 30°±10°E) to the intense geomagnetic storm of 23–31 August 2018 (SYM-Hmin = −207 nT) using the Global Ionospheric Map (GIM) and Global Positioning System (GPS) receivers data, the satellite data (SWARM, Defense Meteorological Satellite Program (DMSP), Global Ultraviolet Imager on board the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (GUVI/TIMED)), and Prompt Penetration Equatorial Electric Field model (PPEFM). The percentage deviation in total electron content (TEC) denoted by ΔTEC (%) was used to observe the ionospheric storm effects. The rate of change of TEC index (ROTI) derived from GPS-TEC and the rate of change of plasma density index (RODI) obtained from SWARM satellites were utilized to quantify the occurrence of ionospheric irregularities. Results obtained from GPS receivers and GIM data revealed a large increase in TEC (positive ionospheric storm effect) in the equatorial and low-latitude region of Africa, and a decrease in TEC (negative ionospheric storm effect) over the midlatitude region of Europe and Africa during the storm recovery phase. The decrease in [O]/[N2] ratio is the possible cause for the observed negative ionospheric storm effect. Hemispheric asymmetry were noticed over Europe-African longitude sector during the storm main and recovery phases. The occurrence of ionospheric irregularities over the low-latitude region of Africa in the premidnight and post-midnight was suppressed (ROTI<0.4 TECU/min). This could be related to the local time at which the minimum SYM-H occurred which corresponded to daytime over Europe-African longitude sector. This, on the other hand, may not support the development of conducive environment for the generation of ionospheric irregularities. However, significant fluctuation in the plasma density was noticed by the SWARM-C on 23, 25, 30, and 31 August 2018 over equatorial and low-latitude region of Africa during the post-midnight period.

The global response of terrestrial ionosphere to the December 2015 space weather event

This paper investigates the ionospheric storm of December 19–21, 2015, which was initiated by two successive CME eruptions that caused a G3 space weather event. We used the in situ electron density (Ne) and electron temperature (Te) and the Total Electron Content (TEC) measurements from SWARM-A satellite, as well as the O/N2 observations from TIMED/GUVI to study the ionospheric impact. The observations reveal the longitudinal and hemispherical differences in the ionospheric response to the storm event. A positive ionospheric storm was observed over the American, African and Asian regions on 20 December, and the next day showed a negative storm. Both these exhibited hemispheric differences. A positive storm was observed over the East Pacific region on 21 December. It is seen that the net effect of both the disturbance dynamo electric field and composition differences become important in explaining the observed variability in topside ionospheric densities. In addition, we also discuss the Te variations that occurred as a consequence of the space weather event.

Behaviors of Ionospheric Topside Ion Density, Ion Temperature, and Electron Temperature During the 20 November 2003 Superstorm

AbstractWe identified a few new storm‐time ionospheric phenomena by analyzing disturbances in topside ion density, electron temperature, and ion temperature at ∼840 km altitude measured by the Defense Meteorological Satellite Program satellites during the 20 November 2003 magnetic storm. The storm‐time ion density enhancements showed different features at different local times. Longitudinal structures in the enhanced ion density occurred in the morning sector and extended from equatorial regions to middle latitudes. Ion density increase due to enhanced fountain effect was observed in the evening sector and lasted for ∼18 hr. A positive ionospheric storm occurred during the late recovery phase of the storm and was associated with increased atomic oxygen to molecular nitrogen column density ratio. Electron temperature at subauroral latitudes reached 8000 K during the storm, ∼4000 K higher than the quiet‐time temperature. The subauroral temperature enhancement lasted for 2–3 days. Simultaneous enhancements in the ion density, electron temperature, and ion temperature from subauroral to equatorial latitudes occurred in the night‐time ionosphere and lasted for ∼18 hr. A negative correlation between ion density and electron/ion temperature variations occurred in the dusk sector for ∼12 hr. An enhanced ion temperature crest in the winter hemisphere during the magnetic storm lasted for 2 days. A decrease in the ion temperature crest was also observed with an increase of the ion density. These new features in the ionospheric density and temperature, together with the results from previous studies, provide a more comprehensive scenario of the ionospheric response to the superstorm.

Physical effects of geocosmic storms

A review of the results of observing the effects of the most interesting ionospheric and magnetic storms that took place in 2018–2019 is presented. Ionospheric and magnetic storms are considered as components of geocosmic storms. In accordance with the systemic paradigm of L.F. Chernogor, a geocosmic storm is a complex of processes caused by a solar storm, reduced to synergistically interacting storms in a magnetic field (magnetic storm), ionosphere (ionospheric storm), atmosphere (atmospheric storm) and electric field atmospheric-ionospheric-magnetospheric origin (electric storm). Using the analysis of specific magnetic and ionospheric storms as an example, it is shown that, in addition to general patterns, each storm has its own individual characteristics. The expediency of a comprehensive study of the manifestations of each new geocosmic storm has been confirmed. A wide variety of ionospheric and magnetic storms have been confirmed. A weak magnetic storm is not necessarily accompanied by a weak ionospheric storm, while a strong one is accompanied by a strong one. An ionospheric storm can be single-phase (positive or negative) or multi-phase, when the phases alternate. Storms can last from several hours to many days. Storms are accompanied by both aperiodic and quasi-periodic variations. The latter are caused by atmospheric gravity waves (periods ~10–180 min) and infrasound (period less than 5 min). Of all the components of a geocosmic storm, except for academic ones, ionospheric storms are of the greatest practical interest. It is these storms that limit the potential characteristics of radio systems for various purposes (telecommunications, radio navigation, radar, remote radio sounding, radio astronomy). It is shown that ionospheric storms are accompanied by the appearance of multipath, broadening of the Doppler spectra (and even their collapse), significant aperiodic and quasi-periodic variations in the Doppler frequency shift, the amplitude of radio signals, and the trajectories of radio waves in the decameter range. In particular, the height of reflection of radio waves changed during ionospheric storms by 50–180 km. The electron concentration during moderate negative ionospheric storms decreased up to 2 times, and during moderate positive ionospheric storms it increased up to 3 times. Under the action of atmospheric gravity waves, the relative amplitude of quasi-periodic variations in the electron concentration reached 50%, and due to low-frequency infrasound, it did not exceed 0.7%.

Ionospheric Reconstruction Using GNSS Signals Around an Anomaly Crest Location in Indian Longitude Sector

AbstractThe present study aims to reconstruct the vertical ionospheric electron density (IED) profile using computerized ionospheric tomography (CIT) techniques, over the northern crest of the equatorial ionization anomaly (EIA) which is one of the most geo‐physically sensitive regions of Indian longitude sector. Total electron content (TEC) measurements obtained from GNSS signals from receivers located at Kolkata (22.57°N, 88.36°E), Purulia (23.33°N, 86.36°E), and Siliguri (26.73°N, 88.39°E) and International GNSS Service (IGS) stations of Port Blair (11.64°N, 92.71°E), Bangalore (13.02°N, 77.57°E), Hyderabad (17.42°N, 78.55°E), Lucknow (26.91°N, 80.96°E) and Lhasa (29.66°N, 91.10°E) have been incorporated into the tomography algorithm for ionospheric reconstructions. Among the different iterative techniques used for retrieving IED profiles on a 2‐dimensional altitude versus latitude plane fixed at a certain longitude, simultaneous iterative reconstruction technique (SIRT) was shown to be the optimum choice based on the representation of the EIA phenomenon and reconstruction accuracy. A comparative assessment between two ionospheric models IRI2016 and PIM1.4 was performed, with PIM based initial guess for inversions resulting in lower reconstruction errors. The 2D CIT algorithm was then used to reconstruct the hourly vertical IED profiles to study the impact of the severe G4‐class St. Patrick's Day storm on 17 March 2015 and reconstructed electron densities showed a large increase over the values obtained from the geomagnetically quiet day of 10 March 2015, indicating a positive ionospheric storm phase. Ionospheric response to the G1‐class geomagnetic disturbance on 5 December 2017 and the average F‐region IED values have also been compared with a geomagnetically quiet day of 3 December 2017.

Geomagnetic Storm Effect on F2-Region Ionosphere during 2012 at Low- and Mid-Latitude-Latitude Stations in the Southern Hemisphere

The ionospheric effects of six intense geomagnetic storms with Dst index ≤ −100 nT that occurred in 2012 were studied at a low-latitude station, Darwin (Geomagnetic coordinates, 21.96° S, 202.84° E), a low-mid-latitude station, Townsville (28.95° S, 220.72° E), and a mid-latitude station, Canberra (45.65° S, 226.30° E), in the Australian Region, by analyzing the storm–time variations in the critical frequency of the F2-region (foF2). Out of six storms, a storm of 23–24 April did not produce any ionospheric effect. The storms of 30 September–3 October (minimum Dst = −122 nT) and 7–10 October (minimum Dst = −109 nT) are presented as case studies and the same analysis was done for the other four storms. The storm of 30 September–3 October, during its main phase, produced a positive ionospheric storm at all three stations with a maximum percentage increase in foF2 (∆foF2%) of 45.3% at Canberra whereas during the recovery phase it produced a negative ionospheric storm at all three stations with a maximum ∆foF2% of −63.5% at Canberra associated with a decrease in virtual height of the F-layer (h’F). The storm of 7–10 October produced a strong long-duration negative ionospheric storm associated with an increase in h’F during its recovery phase at all three stations with a maximum ∆foF2% of −65.1% at Townsville. The negative ionospheric storms with comparatively longer duration were more pronounced in comparison to positive storms and occurred only during the recovery phase of storms. The storm main phase showed positive ionospheric storms for two storms (14–15 July and 30 September–3 October) and other three storms did not produce any ionospheric storm at the low-latitude station indicating prompt penetrating electric fields (PPEFs) associated with these storms did not propagate to the low latitude. The positive ionospheric storms during the main phase are accounted to PPEFs affecting ionospheric equatorial E × B drifts and traveling ionospheric disturbances due to joule heating at the high latitudes. The ionospheric effects during the recovery phase are accounted to the disturbance dynamo electric fields and overshielding electric field affecting E × B drifts and the storm-induced circulation from high latitudes toward low latitudes leading to changes in the natural gas composition [O/N2] ratio.

The Feature of Ionospheric Mid-Latitude Trough during Geomagnetic Storms Derived from GPS Total Electron Content (TEC) Data

This study aims to investigate the features of the ionospheric mid-latitude trough over North America by using the MIT total electron content data obtained during three geomagnetic storms that occurred in August 2018, September 2017, and March 2015. The mid-latitude trough position sharply moves equatorward from the quiet-time subauroral latitude to mid-latitude with the decrease in SYM-H during geomagnetic storms. We find that the ionospheric behavior of TEC around the mid-latitude trough position displays three kinds of ionospheric storm effect: negative ionospheric storm effect, unchanged ionospheric behavior, and positive ionospheric storm effect. These ionospheric storm effects around the mid-latitude trough position are not always produced by the mid-latitude trough. The ionospheric storm effects produced by the mid-latitude trough are limited in the narrow mid-latitude trough regions, and are transmitted to other regions with the movement of the mid-latitude trough.

Ionospheric GPS-TEC responses from equatorial region to the EIA crest in the South American sector under intense space weather conditions

We present and discuss the ionospheric F-region observations from equator to the equatorial ionization anomaly (EIA) regions over the South American sector during an intense space weather event occurred between 27 and May 29, 2017. During this geomagnetic storm, the symmetric-H (SYM-H) reached a minimum of −142 nT at ∼0700 UT on May 28, 2017. For this investigation, we analyze the vertical total electron content (VTEC) observations from a chain of nearly 120 Global Positioning System (GPS) stations. Magnetometer measurements obtained at two stations in the low latitude regions are also presented. The observations do not indicate prompt penetration electric field (PPEF) effects in the VTEC variations. Magnetometer's observations over Cuiabá (CBA) and Cachoeira Paulista (CXP) in central west and south parts of Brazil, respectively, have shown a strong cross-correlation with SYM-H in the period between 3 and 48 h. The results also show positive ionospheric storm phase during the recovery phase on May 28, 2017. Positive effect during the recovery phase of the geomagnetic storm is possibly associated with effects of disturbances winds. During the recovery phase, a strong intensification of the EIA took place, possibly related to an additional ionization effect. The VTEC values show differences between the west and east sectors. This indicates that the EIA crest is stronger in the east sector than in the west sector, possibly due to the combination of disturbance wind effects and geomagnetic field geometry where in the east sector the field lines are more inclined.