Four‐Peak Structure on Equatorial Ionization Anomaly Crests During the May 2024 Geomagnetic Storms

Equatorial electrojet (EEJ) data, which are considered as a proxy index of equatorial electric field, are analyzed in conjunction with equatorial ionosonde, total electron content (TEC) and scintillation data near the equatorial ionization anomaly (EIA) crest for the equinoctial months of high solar activity years (2011–2012) to identify any precursor index of postsunset evolution of equatorial electron density irregularities and subsequent occurrence of scintillation near the northern EIA crest. Only geomagnetically quiet and normal electrojet days are considered. The diurnal profiles of EEJ on the scintillation days exhibit a secondary enhancement in the afternoon to presunset hours following diurnal peaks. A series of electrodynamical processes conducive for generation of irregularities emerge following secondary enhancement of EEJ. Latitudinal profile of TEC exhibits resurgence in EIA structure around the postsunset period. Diurnal TEC profile near the EIA crest resembles postsunset secondary enhancement on the days with afternoon enhancement in EEJ. Occurrence of equatorial spread F and postsunset scintillation near the EIA crest seems to follow the secondary enhancement events in EEJ. Both the magnitude and duration of enhanced EEJ are found to be important for postsunset intensification of EIA structure and subsequent occurrence of equatorial irregularities. A critical value combining the two may be considered an important precursor for postsunset occurrence of scintillation near the EIA crest. The results are validated using archived data for the years 1989–1990 and explained in terms of modulation effects of enhanced equatorial fountain.

The paper presents the effects of the storm‐time prompt penetration electric fields (PPEF) and traveling atmospheric disturbances (TADs) on the total electron content (TEC), foF2 and hmF2 in the American sector (north and south) during the geomagnetic storm on 23–24 April 2023. The data show a poleward shift of the Equatorial Ionization Anomaly (EIA) crests to 18°N and 20°S in the evening of 23 April (attributed to eastward PPEF) and the EIA crests remaining almost in the same latitudes after the PPEF reversed westward. The thermospheric neutral wind velocity, foF2, hmF2, and TEC variations show that TADs from the northern and southern high latitudes propagating equatorward and crossing the equator after midnight on 23 April. The meridional keograms of ΔTEC show the TAD structures in the north/south propagated with phase velocity 470/485 m/s, wave length 4,095/4,016 km and period 2.42/2.30 hr, respectively. The interactions of the TADs also appear to modify the wind velocities in low latitudes. The eastward PPEF and equatorward TADs also favored the development of a clear/not so clear F3 layer in northern/southern regions of the equator.

A coronal mass ejection erupted from the Sun on 21 April 2023 and created a G4 geomagnetic storm on 23 April. NASA's global‐scale observations of the limb and disk (GOLD) imager observed bright equatorial ionization anomaly (EIA) crests at ∼25° Mlat, ∼11° poleward from their average locations, computed by averaging the EIA crests during the previous geomagnetic quiet days (18–22 April) between ∼15°W and 5°W Glon. Reversed C‐shape equatorial plasma bubbles (EPBs) were observed reaching ∼±36° Mlat (∼40°N and ∼30°S Glat) with apex altitudes ∼4,000 km and large westward tilts of ∼52°. Using GOLD's observations EPBs zonal motions are derived. It is observed that the EPBs zonal velocities are eastward near the equator and westward at mid‐latitudes. Model‐predicted prompt penetration electric fields indicate that they may have affected the postsunset pre‐reversal enhancement at equatorial latitudes. Zonal ion drifts from a defense meteorological satellite program satellite suggest that westward neutral winds and perturbed westward ion drifts over mid‐latitudes contributed to the observed latitudinal shear in zonal drifts.

This paper investigates the response of the equatorial, and near equatorial, ionosphere to geomagnetic disturbances during the period November 8–10, 2004. Ionosonde data from Trivandrum (8.5°N 77°E and dip 0.5°N) and SHAR (13.5°N, 80.2°E, dip ~5.5°N), magnetic field data from Tirunelveli (8.7°N, 76.9°E, dip latitude 0.5°S) and Alibag (18.64°N, 72.87°E), and GUVI O/N2 data in the Indian longitude sector, are used for the study. The behavior of interplanetary parameters is also examined in conjunction with the ionospheric data. On 8 November, the EIA around noontime is not fully inhibited even though the electrojet strength an indicates inhibition of EIA due to a disturbance dynamo electric field effect. It is the enhanced O/N2 over TRV and SHAR, with a larger increase over SHAR, which results in a larger (than expected) value of the EIA proxy parameter. On 9 November, the comparable values of foF2 at TRV and SHAR around noon time is due to the combined effect of a weakened anomaly in the presence disturbance dynamo electric field effects leading to the EIA crest being near SHAR, and increased O/N2 values at TRV and SHAR with a larger increase at TRV. On 10 November, the very strong values of the EIA proxy-SHAR parameter is attributed to the combined effects of prompt penetration electric field related modulations of EIA, and significant O/N2 changes at the equatorial, and near equatorial, latitude. Thus, the study reveals the important role of storm-induced O/N2 changes, along with prompt penetration electric fields and disturbance dynamo electric fields in modulating the ionization distribution in the equatorial ionization anomaly (EIA) region during this period.

In this study we present for the first time the nonmigrating tidal spectrum for the electron density in the topside ionosphere on global scale for different seasons at both solar maximum and minimum conditions. The electron density observations from the CHAMP (CHAllenging Minisatellite Payload) satellite provide evidence for prominent nonmigrating tides at different latitude regions. At middle and high latitudes the most prominent diurnal tides are DE1, D0, and DW2, as well as semidiurnal tides SW1 and SW3 preferably during equinox seasons. DE1 is only found in the northern middle and high latitudes, while D0 and DW2 are much stronger in the Southern Hemisphere. These tides are believed to be driven by the thermospheric winds through ion‐neutral interactions. At equatorial and low‐latitude regions the most prominent diurnal tides are DE2 and DE3. DE2 is found to be present at low‐latitude regions throughout the whole year with larger amplitudes in the Southern Hemisphere, while DE3 shows largest amplitudes (symmetric above the dip equator) at the equatorial ionization anomaly (EIA) crest region during September equinox. A general antiphase behavior between the EIA crest and trough is observed for the tides DE3, DE2, DW2, and SW3. We consider this as strong evidence for the modulation of the EIA electron density by the E layer zonal electric field via the ion fountain effect. An exception is the tidal component D0, which exhibits antiphase variations between the two hemispheres. The phase value at the equatorial trough is halfway between those of the two hemispheres. Presently, we cannot give an explanation for it, and study concerning special model effort is further needed.

Assessment of IRI-2012, NeQuick-2 and IRI-Plas 2015 models with observed equatorial ionization anomaly in Africa during 2009 sudden stratospheric warming event

Scintillations of transionospheric satellite signals during geomagnetic storms can severely threaten navigation accuracy and the integrity of space assets. We analyze vertical Total Electron Content (vTEC) variations from the Global Navigation Satellite System (GNSS) at different latitudes around the world during the geomagnetic storms of June 2015 and August 2018. The resulting ionospheric perturbations at the low-and mid-latitudes are investigated in terms of the prompt penetration electric field (PPEF), the equatorial electrojet (EEJ), and the magnetic H component from INTERMAGNET stations near the equator. East and South-East Asia, Russia, and Oceania exhibited positive vTEC disturbances, while South American stations showed negative vTEC disturbances during both storms. We also analyzed the vTEC from the Swarm satellites and found similar results to the GNSS retrieved vTEC during different phases of both geomagnetic storms. Moreover, we observed that ionospheric plasma tended to increase rapidly during the afternoon in the main phase of the storms. At nighttime, the ionosphere depicted an opposite behavior under similar conditions. The equatorial ionization anomaly (EIA) crest expansion to mid and high latitudes is driven by PPEF during daytime at the main and recovery phases of the storms. The magnetic H component exhibits a longitudinal behavior along with the EEJ enhancement near the magnetic equator.

<strong class="journal-contentHeaderColor">Abstract.</strong> Scintillations of transionospheric satellite signals during geomagnetic storms can severely threaten navigation accuracy and the integrity of space assets. We analyze vertical Total Electron Content (vTEC) variations from the Global Navigation Satellite System (GNSS) at different latitudes around the world during the geomagnetic storms of June 2015 and August 2018. The resulting ionospheric perturbations at the low-and mid-latitudes are investigated in terms of the prompt penetration electric field (PPEF), the equatorial electrojet (EEJ), and the magnetic H component from INTERMAGNET stations near the equator. East and South-East Asia, Russia, and Oceania exhibited positive vTEC disturbances, while South American stations showed negative vTEC disturbances during both storms. We also analyzed the vTEC from the Swarm satellites and found similar results to the GNSS retrieved vTEC during different phases of both geomagnetic storms. Moreover, we observed that ionospheric plasma tended to increase rapidly during the afternoon in the main phase of the storms. At nighttime, the ionosphere depicted an opposite behavior under similar conditions. The equatorial ionization anomaly (EIA) crest expansion to mid and high latitudes is driven by PPEF during daytime at the main and recovery phases of the storms. The magnetic H component exhibits a longitudinal behavior along with the EEJ enhancement near the magnetic equator.

In this work, we discuss the occurrence of the dayside three‐peak electron density structure in the ionosphere. We first use a set of ground‐based and satellite‐borne instruments to demonstrate the development of a large‐amplitude electron density perturbation at the recovery phase of a moderate storm of 11 October 2008. The perturbation developed in the F2 and low topside ionospheric regions over the American sector; it was concentrated on the north from the equatorial ionization anomaly (EIA) but was clearly separated from it. At the F2 region height, the amplitude of the observed perturbation was comparable or even exceeded that of the EIA. Further analysis of the observational data together with the Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics model simulation results showed that a particular local combination of the thermospheric wind surges provided favorable conditions for the generation of the three‐peak EIA structure. We further proceed with a statistical study of occurrence of the three‐peak density structure in the ionosphere in general. Based on the analysis of 7 years of the in situ data from CHAMP satellite, we found that such three‐peak density structure occurs sufficiently often during geomagnetically quiet time. The third ionization peak develops in the afternoon hours in the summer hemisphere at solstice periods. Based on analysis of several quiet time events, we conclude that during geomagnetically quiet time, the prevailing summer‐to‐winter thermospheric circulation acts in similar manner as the storm‐time enhanced thermospheric winds, playing the decisive role in generation of the third ionization peak in the daytime ionosphere.

The responses of the low-latitude ionospheres to the April 2023 geomagnetic storm were investigated using observations from the Global Positioning System-Total Electron Content (GPS-TEC) over the American, African, and Asian–Australian sectors. The TEC distribution from the GPS-TEC receivers across the latitudes was utilized to delineate equatorial ionization anomaly (EIA) structures across the regions. During the main phase of the storm, American EIA crests shifted equatorward with a significant reduction in TEC magnitude (negative ionospheric disturbance), while African crests remained unchanged compared to the quiet period, accompanied with a slight enhancement in TEC magnitude (positive ionospheric disturbance). Conversely, the Asian–Australian EIA crests collapsed to form an extended southern crest with significant enhancement in the TEC magnitude (positive ionospheric disturbance). These longitudinal differences across the sectors depended on couples of physical processes such as storm-time prompt penetration electric field orientation, local storm-time occurrences, variation in inferred E × B drift, storm-time neutral wind, and variations in air composition. However, the geomagnetic storm overall effects on the American sector were ∼3% and ∼7% to the northern and southern crests, respectively. African sector observed ∼6% and 5% toward the northern and southern EIA crests, respectively, while the Asian–Australian sector, witnessed ∼32% effect to the southern EIA crest.

Responses of the African equatorial ionization anomaly (EIA) to some selected intense geomagnetic storms during the maximum phase of solar cycle 24

Investigation of the relationship between ionospheric scintillations and equatorial spread F (ESF) during adverse space weather conditions is of paramount significance for developing ionospheric scintillation mitigation strategies for Global Navigation Satellite System (GNSS) receivers. In this paper, we present correlation analysis between equatorial spread F (ESF) and L-band amplitude scintillations conducted at low-latitude Hyderabad station (17.47° N, 78.57° E) during the severe geomagnetic storm event of 17 March 2015 (St. Patrick's Day). Hyderabad location comes under the region between the magnetic equator and the northern crest of equatorial ionization anomaly (EIA). ESF information is derived from ionograms of ionosonde located at the Tata Institute of Fundamental Research (TIFR) Balloon Facility, Hyderabad, whereas amplitude scintillation index (S4) data have been obtained from the GPS receiver at Hyderabad (GAGAN network). Intense spread F (SSF) signatures were observed during post-sunset hours of 16 March (pre-storm day) and 17 March (storm day), 2015, where the spread F (SF) index is higher than 1 MHz. GPS satellite vehicle with pseudo-random noise code (PRN) 24 experienced strong scintillation (S4 = 1.06) at 18:53 UT on the pre-storm day of 16 March 2015. GPS PRN 30 satellite was affected severely on 17 March (storm day), when the observed S4 index is 0.99. Development of ESF and subsequent scintillations are due to the strong eastward prompt penetration electric fields over the Indian region. The inhibition of spread F and scintillations noticed on 18 March 2015 are because of disturbance dynamo electric fields and disturbance winds. The SSF occurrences were well correlated with scintillation index values at Hyderabad station, India. Variations of the virtual height of the F layer (h′F) and drift velocity (Vd) are related to the S4 values during the period of interest. The outcome of the preliminary results would be useful for development of ionospheric scintillation prediction algorithms for GPS users.

Equatorial ionospheric responses during two magnetic storms of moderate intensity are investigated, for the first time, by conjugate point observations in Brazil. The study focuses on storm‐induced changes in the evening prereversal vertical drift, thermospheric trans‐equatorial winds, spread F/plasma bubble irregularity development, electron density/plasma frequency heights, the EIA strength, and zonal plasma drifts. It is based on data obtained from five Digisondes operated in Brazil, three of them being part of a conjugate point equatorial experiment (COPEX) involving a dip equatorial and two magnetic conjugate sites at ±12°. The other two were operated at the equatorial ionization anomaly (EIA) trough and crest locations at nearby magnetic meridians. The results bring out, and clarify, many outstanding aspects of the strong influence of storm time electric fields on the equatorial ionosphere at different phases of the two long lasting storm sequences. During both storms prompt penetration electric fields dominated the ionospheric response features as compared to the disturbance wind dynamo effects that were not very conspicuous. An under‐shielding (over‐shielding) electric field occurring in the evening hours causes enhancement (suppression) of the prereversal vertical drift and post sunset spread F/plasma bubble generation. The same electric fields cause post sunset EIA enhancement and suppression, respectively. Post sunset (post midnight) spread F can develop from under‐shielding (over‐shielding) electric fields, while it can be disrupted by over‐shielding (under‐shielding) electric field. Trans‐equatorial winds are found to be ineffective to stabilize the post sunset F region against the destabilizing effect of strong prereversal vertical drift. Storm time westward plasma drifts are found to be driven by prompt penetration eastward electric fields (through their effect of inducing vertical Hall electric fields), rather than by a disturbance westward thermospheric wind during these storms.

[1] The response of the dayside equatorial F2 layer to the main phases of the 22 intense geomagnetic storms (Dst < −150 nT) in 1998–2008 is investigated using the digital ionosonde data from the equatorial stations in Brazilian, Indian, and Australian longitudes together with equatorial electrojet strength and IMF Bz; the storms include 15 superstorms (Dst < −200 nT). The observations show that there is a period during all MPs when the F2 layer peak rises (and falls) rapidly with large peak electron density (Nmax) reduction, the rise velocity strongly correlates with the intensity (Dst) of the storms, and the duration of the Nmax reduction corresponds to that of strong eastward electrojet when IMF Bz remains highly negative. The observations indicate the occurrence of strong eastward prompt penetration electric fields (PPEF) during the rapid F2 layer response. The PPEF drives the F2 layer peak rapidly upward, which reduces Nmax due to vertical expansion and diffusion. The results therefore suggest that the rapid F2 layer response (rapid rise (and fall) of peak height (hmax) with large Nmax reduction) observed by ionosondes can be used to detect the occurrence of the daytime eastward PPEF during intense geomagnetic storms irrespective of season and level of solar activity. The data also show two rare events of strong daytime westward electric fields due to disturbance dynamo and/or prompt penetration. The results are important when radars are not available to monitor the occurrence of the PPEF.

We use multiple instruments data to investigate the behavior of the equatorial and low‐latitude ionosphere during the geomagnetically active and quiet period of November 1–6, 2021. In this context, total electron content (TEC) data obtained from the Global Positioning System (GPS) receivers in the equatorial and low‐latitude regions of Asia, Africa, and America are used to assess variations in plasma density during the storm. The storm‐time ionization levels were found to vary significantly in the crests of the Equatorial Ionization Anomaly (EIA) region over the 3 longitudes. The Rate of Change TEC Index (ROTI) derived from GPS receiver measurements, is used to study the equatorial/low‐latitude ionospheric plasma irregularities at various longitudes under geomagnetically quiet and disturbed conditions. Observations showed longitudinal variations in the ionospheric irregularities under both quiet and disturbed conditions. Some days exhibit a decrease in the strength of the midnight plasma irregularities toward the East, that is, the irregularities are more pronounced in West America, less common in East America, and almost non‐existent in Africa and Asia. Our investigations show this storm prevented the occurrence of plasma irregularities at the equatorial/low‐latitude region in the American sector during the night following the main phase. In general, no significant storm effects were observed at the target locations in Africa and Asia. The existence of westward Prompt Penetration Electric Field (PPEF) and the Equatorial Electrojet (EEJ) during the main phase, from midnight to noon, is clearly related with the constriction of plasma diffusion and the consequent suppression of plasma irregularities. Thus, the longitudinal dependence for the generation of midnight plasma irregularities during this storm is mainly influenced by local time occurrence of maximum ring current, and the ionospheric electric fields.