Equatorial Ionization Anomaly Structure Research Articles

Responses of the Nigerian low-latitude ionosphere to geomagnetic storms of the ascending and maximum phases of solar cycle 24

This study investigates the responses of the Nigerian ionosphere to thirteen geomagnetic storms that occurred during the ascending and maximum phases of the solar cycle 24. The Total Electron Content (TEC) data obtained from the Nigerian Global Navigation Satellite System (GNSS) network (NIGNET) were used.The ionosphere over Nigeria recorded marked diurnal TEC variations around 1100–1700 LT, with the most significant variations between 1400 and 1600 LT time period, and minimum variation at 0600 LT. Another cogent piece of information from this study is that the values of TEC and ionospheric irregularities at all the five GNSS stations under investigation consistently matched one another. The implication of this is that the values of TEC and irregularities, at both quiet time and storm time, at any location in Nigeria could be used as the representative values at any other locations in the country, particularly in areas where there are no GNSS systems. The equinoctial maxima and June solstice minima effects were clearly observed in our data. Furthermore, ionospheric irregularities occurrences also followed semiannual patterns with two peaks in April and October, and the least occurrences in June, and generally localized around 2000–0100 LT. At daytime during geomagnetic storms in the equatorial/low-latitude regions, Prompt Penetration Electric Field (PPEF) intensified forward plasma fountain effect, while at nighttime, the reversed plasma fountain effect returns plasma backward to cause the Equatorial Ionization Anomaly (EIA) crests to contract, or totally coalesce the EIA structure to form a single strip of plasma density around the equator.

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

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

Responses of the African-European equatorial-, low-, mid-, and high-latitude ionosphere to geomagnetic storms of 2013, 2015 St Patrick’s Days, 1 June 2013, and 7 October 2015

This study investigates ionospheric responses to 2013 and 2015 St. Patrick’s Days (CME-driven), 1 June 2013 and 7 October 2015 (CIR-driven) geomagnetic storms over the African-European equatorial-, low-, mid-, and high-latitude regions. We constructed the Equatorial Ionization Anomaly (EIA) of Total Electron Content (TEC) and ionospheric irregularities using data derived from Global Navigation Satellite System (GNSS) receivers along 30°–45°E meridian lines. TEC intensities and EIA crests’ locations were significantly influenced by geomagnetic storms. Asymmetries in EIA crests’ locations, attributable to neutral wind effect were generally observed, consistently higher in the northern hemisphere than the southern hemisphere. Of all the four geomagnetic storms, storm of 17 March 2015 was the most intense and most impactful on the ionosphere. Generally, the two CME-driven storms were more geoeffective than the two CIR-driven storms. The phases of responses of the ionosphere to storms were dependent on geomagnetic storms’ onset time. Daytime onset time produced eastward PPEF which intensified forward plasma fountain to cause pole-ward expansion of the African-European EIA structure, while night-time westward PPEF reversed plasma fountain to cause equator-ward shrinking of the EIA structure. At the equatorial/low-latitude regions, ionospheric irregularities were influenced by PPEF orientation, and they were confined to night-time and early morning hours. At high latitudes, ionospheric irregularities were related to auroral activities, and they spread across both daytime and night-time hours at storm-time period.

Seasonal and Interhemispheric Effects on the Diurnal Evolution of EIA: Assessed by IGS TEC and IRI-2016 over Peruvian and Indian Sectors

The global total electron content (TEC) map in 2013, retrieved from the International Global Navigation Satellite Systems (GNSS) Service (IGS), and the International Reference Ionosphere (IRI-2016) model are used to monitor the diurnal evolution of the equatorial ionization anomaly (EIA). The statistics are conducted during geomagnetic quiet periods in the Peruvian and Indian sectors, where the equatorial electrojet (EEJ) data and reliable TEC are available. The EEJ is used as a proxy to determine whether the EIA structure is fully developed. Most of the previous studies focused on the period in which the EIA is well developed, while the period before EIA emergence is usually neglected. To characterize dynamics accounting for the full development of EIA, we defined and statistically analyzed the onset, first emergence, and the peaks of the northern crest and southern crest based on the proposed crest-to-trough difference (CTD) profiles. These time points extracted from IGS TEC show typical annual cycles in the Indian sector which can be summarized as winter hemispheric priority, i.e., the development of EIA in the winter hemisphere is ahead of that in the summer hemisphere. However, these same time points show abnormal semiannual cycles in the Peruvian sector, that is, EIA develops earlier during two equinoxes/solstices in the northern/southern hemisphere. We suggest that the onset of EIA is a consequence of the equilibrium between sunlight ionization and ambipolar diffusion. However, the latter term is not considered in modeling the topside ionosphere in IRI-2016, which results in a poor capacity in IRI to describe the diurnal evolution of EIA. Meridional neutral wind’s modulation on the ambipolar diffusion can explain the annual cycle observed in the Indian sector, while the semiannual variation seen in the Peruvian sector might be due to additional competing effects induced by the F region height changes.

Atmospheric and ionospheric disturbances associated with the M > 6 earthquakes in the East Asian sector: A case study of two consecutive earthquakes in Taiwan

The phenomenon of Lithosphere-Ionosphere coupling is a popular topic for researchers for the last two decades. This study aims to analyze the seismo-ionospheric anomalies (SIAs) in East Asia induced by the M > 6 earthquakes (EQs) from Total Electron Content (TEC) of permanent IGS stations operating around the vicinity of the corresponding epicenters by considering a case study of two successive EQs from Taiwan. Our results showed remarkable TEC enhancements one day before both impending EQs during quiet geomagnetic activity (Kp less than 3, Dst ≥ −20), where the variations were found one day before each EQ during UT = 02–08 (LT = 10–16) hours in the form of differential TEC (dTEC). On the other hand, V-shaped gradual decreases in temporal atmospheric parameters took place one day before the M 6.1 EQ that gradually decreased till the M 6.4 EQ day before turning back to normal. The anomaly maps of geopotential height and surface air temperature showed intense decreases and the consequent OLR enhancements right along the edge of the Philippine Sea plate in the northwest direction. Furthermore, we observed a distinctive phenomenon of equatorial ionization anomaly (EIA) enhancement in the latitudinal GIM-TEC profiles near the epicentral longitude, where the EIA structure modified the shape as a double crust structure with a trough near to the equator that testifies the generation of the seismogenic electric field. Our multi-precursory study is another contribution in the acknowledgment of LAI coupling (Lithosphere, Atmosphere, Ionosphere) hypothesis in the prediction of the future EQs.

Temporal evolution of the EIA along 95°E as obtained from GNSS TEC measurements and SAMI3 model

The total electron content (TEC) derived from GNSS measurements at a trans-hemispheric meridional chain of ground stations around 95°E longitude are used to study the quiet time inter-hemispheric structure and dynamics of the equatorial ionization anomaly (EIA) during the period March 2015 to February 2016. The stations are Dibrugarh (27.5°N, 95°E, 43° dip), Kohima (25.6°N, 94.1°E, 39° dip), Aizawl (23.7°N, 92.8°E, 36° dip), Port Blair (11.63°N, 92.71°E, 9° dip) and Cocos Islands (12.2°S, 96.8°E, 43° dip). The observation shows that the northern crest of the EIA lies in the south of 23°N (Aizawl) in all seasons but recedes further south towards the equator during December solstice. The largest poleward expansion of the northern (southern) EIA is observed in the March equinox (December solstice). The equinoctial and hemispherical asymmetry of TEC is noted. The winter anomaly is observed in the northern hemisphere but not in the southern hemisphere. The highest midday TEC over any station is observed in the March equinox. The TEC in southern summer (December solstice) is significantly higher than that in the northern summer (June solstice). The observed northern EIA contracts equatorward in the postsunset period of solstice but the southern EIA persists late into the midnight in the December solstice. The asymmetry may be attributed to the different geographic location of the magnetically conjugate stations. The SAMI3 simulations broadly capture the EIA structure and the inter-hemispheric asymmetry during solstices. The difference between observations and the SAMI3 is higher in March equinox and December solstice. The higher E × B vertical drift in the 90–100°E sector and the large geographic-geomagnetic offset in observing stations may have contributed to the observed differences.

Equatorial ionization anomaly in the low‐latitude topside ionosphere: Local time evolution and longitudinal difference

AbstractAlthough the equatorial ionization anomaly (EIA) has been widely studied, it was seldom investigated from observations for the topside ionosphere. In this paper, the climatology characteristics of the latitudinal structure of topside ion density (Ni) were investigated in detail using ROCSAT‐1 and DMSP Ni observations. EIA double‐peak structure may exist at 600 km, depending on longitude, local time, season, and solar activity, while it is not a prevalent characteristic at 840 km even at solar maximum sunset. Local time evolution of the EIA at 600 km was presented. The double‐peak structure begins to appear at noontime, being later than the appearance of the EIA in F2 peak region. The pronounced EIA induced by the strong prereversal enhancement at solar maximum begins to appear at 19:00 LT and can last to premidnight; EIA crest‐to‐trough ratio is largest (lowest) at March equinox (June solstice) and reaches a maximum at 20:00 LT in all seasons. EIA structure shows evident longitudinal difference. Pronounced EIA exists around about 100°E at 13:00 LT at the two equinoxes and June solstice, while it exists at more extensive longitudes (about 90°E to 240°E) at December solstice. The trans‐equator plasma transport induced by neutral winds can weaken the double‐peak structure in the topside ionosphere. The longitudinal difference in the EIA structure at 600 km is related to the longitudinal variations of equatorial upward plasma drift and geomagnetic declination.

Occurrence of the dayside three‐peak density structure in the F2 and the topside ionosphere

AbstractIn 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.

SBAS operation in the Indian sector under simultaneous scintillation on different GNSS links

Scintillation observations of transionospheric signals from two GEOs (55°E and 140°E) and from the satellite constellations of GPS and GLONASS are carried out simultaneously during the equinoctial months of high solar activity years (2011–2012) from Raja Peary Mohan College (RPMC) center situated near the equatorial ionization anomaly (EIA) crest to study the vulnerability conditions of SBAS operation. Scintillations in excess of ~10dB are recorded simultaneously in the two GEO links in ~8% of the observing days with an average duration of ~28min. Under this condition the availability of GPS and GLONASS also falls below the optimum number (4) necessary for navigation solution, rendering the SBAS operation doubtful. GPS is found to be more susceptible to exhibit receivers׳ loss of lock than the GLONASS. The study reveals that two GEO links with pierce point longitude separation of ~10° are not suitable for maintaining faithful SBAS opeartion around the EIA crest. Occurrence of GEO scintillations is found to be associated with the strength and duration of the emerging post-sunset EIA structure.

A study on ionospheric scintillation near the EIA crest in relation to equatorial electrodynamics

AbstractEquatorial 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.

Statistical study of the F3 layer characteristics retrieved from Intercosmos-19 satellite data

This paper presents a statistical study of the equatorial F3 layer characteristics for the 1979–1981 period of high solar activity using the Intercosmos-19 (IK-19) topside sounding large data set (about 3600 passes across the equator). We revealed diurnal, seasonal, latitudinal and longitudinal variations in the F3 layer occurrence probability (P), and diurnal variations in maximum height of the F3 layer (hmF3). Three local peaks in P appear in diurnal variations for all seasons at: (1) 12:00–14:00 LT; (2) 17:00–18:00 LT; (3) 20:00–22:00 LT. The occurrence of these peaks is associated with diurnal variations in electromagnetic vertical plasma drift at the vicinity of geomagnetic equator. The F3 layer height increases from ∼450km in the early morning hours to 600–750km in the afternoon and reaches the maximum height of ∼800km at 21:00 LT. The latitudinal distribution of F3 layer has a maximum at the geomagnetic equator and it is approximately symmetric with respect to the geomagnetic equator regardless of the season. The F3 layer occurs most frequently in winter (45%), less frequently in summer (43%) and least frequently (37%) in the equinoxes. The F3 layer occurs more often in spring than in autumn. The longitudinal variation in P depends on longitudinal structure of equatorial ionization anomaly that principally determined by the variation in vertical plasma drift at the geomagnetic equator.

On the relationship between the postmidnight thermospheric equatorial mass anomaly and equatorial ionization anomaly under geomagnetic quiet conditions

The equatorial mass anomaly (EMA) in the thermosphere and equatorial ionization anomaly (EIA) in the ionosphere are two interesting phenomena in low-equatorial latitude regions. Previous studies have shown that the EMA appears between 1000 and 2000 local time (LT) and its location of trough is aligned with dip equator, indicating the plausible role of the EIA structure in the development of EMA. In this report, we conducted a statistical study of the occurrence of postmidnight EMA and EIA on the basis of the CHAMP in situ measurements during 2002-2008. Our results revealed that clear EMA and EIA structures are sometimes visible in the postmidnight sector (0100-0600 LT) during geomagnetic quiet periods (Kp < 3). The postmidnight EMA is not necessarily accompanied by the EIA signature in both case study and statistics sense being distinct from the daytime situation. In addition, the occurrence rates of postmidnight EMA and EIA display contrasting behavior with respect to their local time, longitudinal and solar activity dependences. The highest occurrence rate for EMA is 8% at around 0300 LT, while the occurrence rate of the EIA decreases gradually from about 30% at around 2300 LT to similar to 5% at 0600 LT. Longitudinal occurrence of postmidnight EIA presents a wave-like pattern; however, no salient feature appears for the longitudinal occurrence of EMA. Postmidnight EMA is more likely to occur at lower solar activity, whereas an opposite trend presents in the EIA. On the basis of above results, our findings imply that a simple EIA-EMA cause-effect relationship does not hold in the postmidnight sector.

Global, seasonal, and local time variations of ion density structure at the low‐latitude ionosphere and their relationship to the postsunset equatorial irregularity occurrences

Seasonal and longitudinal (s/l) variations of ion density structure near 600 km altitude in the low‐latitude ionosphere, observed by ROCSAT‐1 for two similar high solar activity years of 2000 and 2002, are examined in five different local time (LT) sectors. The gross features in the s/l variations of density structure during the daytime are very similar to each other in these 2 years. A reproducibility of the daytime density structure can thus be assumed, including the shape of equatorial ionization anomaly (EIA), for similar solar flux input. The nighttime structure during the solstice shows a shape‐preserving decay in the latitudinal variation and maintains similar density level for these 2 years. In contrast, the nighttime structure in the equinox indicates noticeable differences in the 2 years. A strong global transport effect that exists in the solstice can be identified as the cause of such difference in density structure between the solstice and the equinox. The model results from both the 2006 International Reference Ionosphere model and the thermosphere ionosphere electrodynamics general circulation model cannot reproduce the global, seasonal, and local time (LT) variations of the low‐latitude ionospheric structure observed by ROCSAT. The observed longitudinal variation of the crest‐to‐trough ratio of the average background EIA structure in the 1800–1900 LT sector fails to correlate with the longitudinal variation of the postsunset equatorial irregularity occurrences in a season. There is also no correlation found between the seasonal variations of the crest‐to‐trough ratio and the irregularity occurrence at almost all longitudes, except at the longitudes of South America‐Atlantic sector where some correlation seems to exist.

Characteristics of the ionospheric total electron content of the equatorial ionization anomaly in the Asian-Australian region during 1996–2004

Abstract. Ionospheric total electron content (TEC) of the equatorial ionization anomaly (EIA) is studied by analyzing dual-frequency signals of the Global Position System (GPS) acquired from a network of receivers around the Asian-Australian region during 1996–2004. The latitude, occurrence time, strength of the most developed EIA crest, and crest-to-trough ratio (CTR) for both the noon and post-sunset sector obtained from a daily TEC contour map have been used to study the solar cycle variations of EIA in the Asian-Australian region. The results reveal that semiannual and seasonal variations were the dominant factor that controls the morphology of the EIA structure which can be identified in the past studies (e.g. Wu et al., 2008). It is also found that the latitude and local time position of the anomaly crest show a hemispheric asymmetry because (a) The northern crest of EIA is expanded during the equinox indicating a weak semiannual variation while the southern crest is inhibited during June–August presenting a strong seasonal variation, and (b) The local time of the northern crest appears ~1.3 h earlier than that of the southern crest in June while showing no difference at December. Solar activity dependence is more evident in the EIA crest region than in the EIA trough region and least in the post-sunset sector at equinox. A seasonal linear relationship is derived between the post-sunset CTR and solar flux, which should be caused by the solar-dependant equatorial E×B vertical drift.

FORMOSAT‐3/COSMIC observations of seasonal and longitudinal variations of equatorial ionization anomaly and its interhemispheric asymmetry during the solar minimum period

Using an extremely valuable global data set from Formosa Satellite (FORMOSAT‐3)/Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation experiment, a comprehensive study has been carried out on the seasonal and longitudinal variations of equatorial ionization anomaly (EIA) and the temporal variation in the hemispheric asymmetry of EIA during the low solar activity period from November 2006 to October 2007. The interesting result observed from this investigation is the local‐time‐dependent variation in the hemispheric asymmetry of EIA. During the solstices, it has been consistently observed that the EIA crest in the winter hemisphere appears stronger than that in the summer hemisphere during morning to noon hours. In contrast to this, during noon to early afternoon hours, the ionization in the winter EIA crest decreases rapidly, and the crest in the summer hemisphere becomes more intensified than that in the winter hemisphere. Further, this transition of stronger EIA crest from winter hemisphere to summer hemisphere occurs around 1200–1300 LT during the December solstice months and is delayed by a couple of hours (seen around ∼1400 LT) during June solstice months. The causative neutral and electrodynamical mechanisms are discussed in light of relative contributions from the field‐aligned plasma transport due to transequatorial interhemispheric neutral wind, the strength of the equatorial fountain process, and the ion drag effects during different local times and seasons. The results from the Sami2 is Another Model of the Ionosphere (SAMI2) model simulation also exhibit similar local‐time‐dependent variation in the hemispheric asymmetry of EIA, which further supports our argument. Also, it was observed that the large magnetic declination of the field lines and the four‐peaked longitudinal structure of EIA can significantly modulate the interhemispheric asymmetry of EIA even during the equinoxes.

Solar activity dependence of the electron density in the equatorial anomaly regions observed by CHAMP

We have investigated the solar activity dependence of the electron density at equatorial and low latitudes using 6 years (a) of measurements between 1 August 2000 and 1 August 2006 from CHAMP and compared it with the international reference ionosphere (IRI) model. The solar activity dependence observed by CHAMP at 400 km altitude exhibits significant variation with latitude, season, and local time. First, the electron density in the crest regions of the equatorial ionization anomaly (EIA) grows roughly linearly from solar minimum to solar maximum, with a higher growth rate than that in the EIA trough region. Second, the solar activity dependence in the EIA crest regions varies strongly with season. The growth rate of the electron density with increasing solar activity around equinoxes is about 1.5 to 2 times greater than that near solstices. Third, the solar activity dependence of the EIA structure varies significantly with local time. In the noon sector the crest‐to‐trough ratio (CTR) obtained at 400 km altitude varies within only a small range between 1.14 and 1.43 from solar minimum to solar maximum. In the postsunset local time sector, however, the CTR grows remarkably with solar activity level, reaching values of above 3.9 at solar maximum. These differences are attributed to the different solar activity dependence of the vertical plasma drift in corresponding local time sectors. The IRI model was found to reproduce the equatorial electron density well near 400 km in the noon sector at all solar activity levels. However, it significantly overestimates it in the postsunset to premidnight sector at high solar activity levels. The major cause for this overestimation has been found to be the IRI's inadequate representation of the F2 layer maximum height (hmF2) in this sector, while the IRI's lack of equatorial spread F seems to play only a minor role.

Motions of the equatorial ionization anomaly crests imaged by FORMOSAT‐3/COSMIC

The equatorial ionization anomaly (EIA) structures and evolutions are imaged using radio occultation observation of the newly launched FORMOSAT‐3/COSMIC (F3/C) satellite constellation. Three‐dimensional ionospheric images provide unprecedented detail of the EIA structure globally. This paper presents images of the EIA structures during July–August 2006 and discusses the development and subsidence of the EIA. Clear seasonal asymmetries in both ionospheric electron density and layer height are observed. Two‐dimensional (cross section) maps at a meridian provide dynamic variations and motions of the northern and southern EIA crests. Results suggest that in addition to the asymmetric neutral composition effect, interactions between the summer‐to‐winter (transequatorial) neutral winds and strength of the equatorial plasma fountain effect play important roles in producing asymmetric development of the EIA crests as imaged by the F3/C.