Equatorial Ionization Anomaly Crest Region Research Articles

Dynamic Processes Associated With Prominent Ionospheric Variations in a Narrow Longitudinal Zone Near the EIA Crest Region as Revealed by ICON Satellite and Ground Beidou Receiver Observations Over South Asia

AbstractNeutral winds and plasma motions from the Ionospheric Connection Explorer mission were used to investigate dynamic processes associated with large regional total electron content (TEC) differences in a narrow longitudinal zone exceeding ∼45 TECU obtained from dense Beidou geostationary receivers at low latitudes over South Asia. The TEC east‒west asymmetry was defined to quantify this difference, which exceeded ∼5 TECU per 1° longitude distance and occurred around the equatorial ionization anomaly (EIA) crest on 31 March 2021 under geomagnetically quiescent conditions. Moreover, the magnetic meridional ion velocity exhibited a large longitude difference around the EIA crest during periods with strong ionospheric east‒west asymmetry. Furthermore, the longitude difference variation was consistent with the TEC east‒west asymmetry variation during 11–21 March 2020 and 27 March–4 April 2021. No significant correlations between TEC east‒west asymmetry and longitude differences under conditions with neutral winds and magnetic zonal ion drifts were observed in either case. The different vertical plasma drifts could contribute to ionospheric changes with a zonal range of a few hundred kilometers.

Wavenumber-4 Structure in COSMIC-2 Observations: Vertical Plane Perspective

High-rate radio occultation (RO) in COSMIC-2 (FORMOSAT7) enables us to investigate the finer details of the ionosphere owing to the measurements being made at a significantly high spatiotemporal resolution, which was unthinkable a decade ago. In the vertical plane, local-time ionospheric wavenumber-4 (WN4) structures display tilted phase-fronts over the equatorial ionization anomaly (EIA) belt. The longitudinal extent of a tilted WN4 phase-front approximates the zonal wavelength of nonmigrating DE3 tide in the local-time frame of reference, i.e., ~900. The WN4-filtered (residual) component indicates a greater tilt (when visible), with a larger longitudinal extent of a wavenumber structure in the vertical plane. The WN4 structure over the EIA crest region is found to be out of phase (in phase) with respect to that over the EIA trough region during daytime (nighttime), which also depended on the altitude under consideration. Intriguingly, above 400 km, the WN4 structures in the EIA crest and trough regions are seen to be in phase with each other at all local times. The phenomenon of the “longitudinal co-location” of WN4 over the EIA crest and trough regions at altitudes above ~400 km at all local times remains unexplained. Results also highlight that the formation of WN4 is governed by a complex interplay of direct forcing of nonmigrating tides and the zonal electric field whose characteristics within the EIA belt vary drastically with latitude and altitude under consideration.

Solar Flux Dependence of Post‐Sunset Enhancement in Vertical Total Electron Content Over the Crest Region of Equatorial Ionization Anomaly

AbstractBased on 10 years' of vertical total electron content (VTEC) data in solar cycle 24 from Ahmedabad (23.0°N, 72.6°E, dip angle 35.2°), a station under the crest region of equatorial ionization anomaly (EIA), it is shown that both the integrated residual and total post‐sunset enhancements in VTEC are conspicuous during Equinox and December solstice when solar flux level exceeds 110 solar flux unit (sfu) with the exception of the year 2012–2013. The post‐sunset enhancements are absent in June solstice at this local time even if the solar flux level exceeds 110 sfu. The integrated residual and total VTEC enhancements during post‐sunset hours are found to be linearly correlated with the solar flux level in Equinox and December solstice. It is noted that a parabolic fit works better for the integrated total VTEC enhancement during December solstice suggesting a possible saturation of VTEC enhancements at high solar flux levels. Based on these observations and Thermosphere Ionosphere Electrodynamics‐General circulation model (TIE‐GCM) outputs, it is argued that the pre‐reversal enhancement (PRE) in the equatorial F region zonal electric field works in tandem with latitudinal gradient in the F region plasma density to determine the degree of VTEC enhancement over the EIA crest region during post‐sunset hours. These results highlight the solar flux dependence of the post‐sunset enhancement of VTEC over the crest region and show that sudden stratospheric warming events in 2012–2013 suppressed these enhancements in December solstice even if solar flux levels exceeded 110 sfu.

Responses of the Ionosphere and Neutral Winds in the Mesosphere and Lower Thermosphere in the Asian‐Australian Sector to the 2019 Southern Hemisphere Sudden Stratospheric Warming

AbstractIn this study, the neutral winds from meteor radars and the total electron content (TEC) from Beidou Geostationary (GEO) satellites were utilized to study the responses of the mesosphere and lower thermosphere and the ionosphere in the Asian‐Australian sector to the recent Sudden Stratospheric Warming (SSW) occurred in the Antarctic polar region during September 2019. It was demonstrated that the ionospheric TEC increased significantly, especially around the equatorial ionization anomaly (EIA) region during this SSW. A pronounced semidiurnal signature in the deviation TEC was observed near the EIA crest region, which was more significant in the southern hemisphere. Meanwhile, the enhancement period of the amplitudes of solar and lunar semidiurnal tides in winds in the mesosphere was coincided with that in TEC. The solar semidiurnal amplitudes increased from about 18 to 35 m/s in the zonal wind and from 15 to 25 m/s in the meridional wind at Mengcheng (33.4°N, 116.5°E, MLAT 27.4°N), and from about 15–38 m/s in the zonal wind at Kunming (25.6°N, 103.6°E, MLAT 19.3°N). The lunar semidiurnal amplitudes increased from 2 to 6 m/s in the zonal wind and from about 3–9 m/s in the meridional wind at Mengcheng, and from about 3–7 m/s in the meridional wind at Kunming, respectively. These indicated that the semidiurnal signature in TEC during the 2019 southern hemisphere SSW could be associated with the SSW induced by the atmospheric semidiurnal tide variations in the mesosphere.

Interaction Between an EMSTID and an EPB in the EIA Crest Region Over China

AbstractFew observations investigated the interaction between an electrical medium‐scale traveling ionospheric disturbance (EMSTID) and an equatorial plasma bubble (EPB). This paper presents another interaction between a southwestward propagating EMSTID and an eastward drifting EPB in the equatorial ionization anomaly (EIA) crest region of China. When the EMSTID and the EPB touched each other, several depletions of the EMSTID (EPB) showed the eastward (westward) velocity disturbances of the EPB (EMSTID) depletions. Besides, phase elongations of the EPB depletions contrarotated as the EMSTID propagated southwestward. However, of important finding is that the interaction of the EMSTID and the EPB could have polarized one depletion of the postmidnight EPB that should have become a fossilized bubble. Inside that polarized EPB depletion were meter‐scale irregularities that caused activated radar echoes and enhanced ranged spread F (RSF). The interaction occurred in descending ionosphere and the lower density regions got filled up with an enhanced density plasma. We propose that the EMSTID and the EPB could have electrically coupled with each other, causing an enhanced polarization electric field (PEF) that polarized that EPB depletion; the E × B gradient drift instability (Kelley, 1989) could have caused the meter‐scale irregularities when that enhanced PEF was imposed on that reactivated EPB depletion surrounded by that enhanced density plasma. This study provides observational evidence that how an electrical couple of EMSTID and EPB events can activate a postmidnight EPB depletion that should become a fossilized structure.

Evidence for the Significant Differences in Response Times of Equatorial Ionization Anomaly Crest Corresponding to Plasma Fountains During Daytime and Post‐Sunset Hours

AbstractBased on 10 years' (2010–2019) of vertical total electron content (VTEC) data from Ahmedabad (23.0°N, 72.6°E, dip angle 35.2°) and campaign based OI 630.0 nm airglow intensity measurements from Mt. Abu (24.6°N, 72.7°E, dip angle 38.0°), it is shown that plasma density over the equatorial ionization anomaly (EIA) crest region increases in varying degrees during post‐sunset hours (2000–2100 LT) in magnetically quiet periods. The post‐sunset peak in VTEC precedes the corresponding peak in airglow intensity. By comparing post‐sunset VTEC enhancements with ionosonde observations from Tirunelveli (8.7°N, 77.7°E, dip angle 1.7°), it is shown that pre‐reversal enhancement (PRE) of the zonal electric field causes these enhancements over the EIA crest region. These observations are supported by TEC measurements by GAGAN (GPS Aided Geo Augmented navigation), the Indian Satellite‐based Augmentation System (SBAS). Comparison of average VTEC variations with global empirical model drifts reveals that the post‐sunset enhancements in VTEC occurs ∼1.7 h after the PRE and are significant only during December solstice and equinoctial months in high solar activity years similar to seasonal variations in PRE amplitudes. This time delay (response time of EIA crest) is almost half compared to the average response time (3–4 h) associated with the daytime fountain. Based on the latitudinal gradient in SBAS‐TEC, it is proposed that the PRE drives plasma from 5°N to 10°N magnetic latitudes to the EIA crest region leading to shorter response time. These results show the important role of the PRE in conditioning the EIA crest region.

Latitudinal Dependence of Daytime Electron Density Bite‐Out in the Ionospheric F2‐Layer

AbstractDaytime bite‐out in diurnal variations of the F2‐layer electron density often occurs in the equatorial ionosphere, while it was less reported at mid‐latitudes. In this paper, the climatology of daytime bite‐out was investigated using ionosonde measurements in the East Asia sector. The bite‐out can take place at both low‐ and mid‐latitudes and mainly in summer months; and it significantly depends on latitudes. The bite‐out appears as foF2 stays stable or slightly decreases for hours in early morning and then enhances till late afternoon at equatorial ionization anomaly (EIA) crest latitudes. It becomes more significant and durative at mid‐latitudes; foF2 increases and reaches a peak in the morning and then continuously declines till afternoon. In particular, the diurnal variation pattern of foF2 is nearly opposite to that of photoionization at the higher‐latitude station Yakutsk at solar maximum. Moreover, the bite‐out is more remarkable at solar minimum (maximum) than at solar maximum (minimum) at low‐latitudes (mid‐latitudes). The trans‐equator plasma outflow caused by neutral winds and the upward thermal diffusion can result in the bite‐out in the EIA crest region, while the subsequent field‐aligned plasma influx induced by the fountain effect ceases it. At mid‐latitudes, the diurnal variation magnitudes of the thermospheric composition and photoionization production rate are also important factors determining bite‐out degree in addition to neutral wind‐induced downward transport and the upward thermal diffusion.

Variability of NmF2 during solar minima at the Equatorial Ionization Anomaly crest region

The variability of NmF2 during solar minima of solar cycle 21–22, 22–23 and 23–24 has been studied for two low latitude stations, Karachi (Pakistan) and Okinawa (Japan), as both stations are situated at the Equatorial Ionization Anomaly (EIA) crest region. Diurnal, seasonal and annual behaviour of variability has been investigated using relative standard deviation approach. At both the stations, a direct relation of variability (CV) with Smoothed Sunspot Numbers (SSN) is noted which is opposite to the results found by many authors who have worked on different solar activity periods. It is suggested that the E-layer is responsible for this direct relation as an increase in the E-layer ionization with decrease in SSN for solar minimum has also been noted for Karachi in our previous work (Talha et al., 2019). The interaction of tides and waves with the E-layer might be responsible for the increase in ionization of the E-layer and the decrease in variability in the F-layer. Also, the night-time variability is observed to be higher than the daytime variability during minima as expected due to the low mean NmF2 or large deviation during night-time. Very prominent post-midnight peaks and not very well shaped post-sunset peaks are also noticed at both stations. Besides the mentioned peaks, a small increase in variability during mid-day is noticed which might be due to the flow of fountain plasma from the equator to the EIA crest region. Seasonal comparison of variability showed high variability during daytime in winter and during night-time in equinox while lowest variability is noted in summer. Lowest variability at post-midnight is noted in winter instead of summer which might be due to the complex interaction of waves and tides with the lower ionosphere. Small difference in variability between both stations is also noticed due to difference in their longitude. Geomagnetic field configuration at both stations is different which in turn causes change in the electron density and also difference in variability at both stations.

Morphological Characteristics of Equatorial Ionization Anomaly Crest Over Nanning Region

AbstractUsing the ionospheric total electron content (TEC) and the location of the equatorial ionization anomaly (EIA) crest derived from GPS data observed at Nanning (22.84°N, 108.33°E, dip latitude ~ 12.14°N), China, a GPS station situated near the daytime EIA crest region, the local time, seasonal, and solar cycle variations of EIA crests from 2006 to 2015 was investigated. The TEC and the magnetic latitude (MLAT) location of EIA crest experience an ascendant and descendant process with the daytime evolution of EIA crest. Also, this diurnal variation of EIA crest depends on season and solar cycle. The TEC and MLAT location of EIA crest is stronger and higher in equinoctial months than that in solistial months showing a semiannual variation. The TEC and MLAT location of EIA crest increase with the solar activity. The local time, seasonal, and solar cycle variations of EIA crest is consistent with that of the strength of equatorial electrojet (EEJ). Comparatively, the correlation of the EEJ strength with the EIA location is better than that with the TEC of EIA crest.

Simulations of the equatorial thermosphere anomaly: Geomagnetic activity modulation

AbstractThe modulation of geomagnetic activity on the equatorial thermosphere anomaly (ETA) in thermospheric temperature under the high solar activity condition is investigated using the Thermosphere Ionosphere Electrodynamics General Circulation Model simulations. The model simulations during the geomagnetically disturbed interval, when the north‐south component of the interplanetary magnetic field (Bz) oscillates between southward and northward directions, are analyzed and also compared with those under the quiet time condition. Our results show that ionospheric electron densities increase greatly in the equatorial ionization anomaly (EIA) crest region and decrease around the magnetic equator during the storm time, resulting from the enhanced eastward electric fields. The impact of both the direct heat deposition at high latitudes and the modulation of the storm time enhanced EIA crests on the ETA are subsequently studied. The increased plasma densities over the EIA crest region enhance the field‐aligned ion drag that accelerates the poleward meridional winds and consequently their associated adiabatic cooling effect. This process alone produces a deeper temperature trough over the magnetic equator as a result of the enhanced divergence of meridional winds. Moreover, the enhanced plasma‐neutral collisional heating at higher latitudes associated with the ionospheric positive storm effect causes a weak increase of the ETA crests. On the other hand, strong changes of the neutral temperature are mainly confined to higher latitudes. Nevertheless, the changes of the ETA purely due to the increased plasma density are overwhelmed by those associated with the storm time heat deposition, which is the major cause of an overall elevated temperature in both the ETA crests and trough during the geomagnetically active period. Associated with the enhanced neutral temperature at high latitudes due to the heat deposition, the ETA crest‐trough differences become larger under the minor geomagnetic activity condition than under the quiet time condition. However, when geomagnetic activity is further elevated, the ETA crests tend to be masked by high temperatures at middle and high latitudes.

Quiet time variability of the GPS TEC and EEJ strength over Indian region associated with major sudden stratospheric warming events during 2005/2006

This paper describes the quiet time variabilities of the ionospheric total electron content (TEC) derived from the signals from Global Positioning Satellite System (GPS) recorded at several stations in India along with simultaneous observations of equatorial electrojet (EEJ) strength obtained from geomagnetic field variations during January–March 2006 when sudden stratospheric warming (SSW) events occurred. Analysis of the observations presented here confirms that strong correlation exists among the variabilities in EEJ strength and GPS TEC observations. Investigations suggest that there exist large‐scale wave like structures with periodicity of quasi 16‐day wave in the TEC observations near the equatorial ionization anomaly (EIA) crest quite similar to that of EEJ strength. Our observations also indicate the existence of morning enhancement and evening reduction of TEC and EEJ strength and vice versa during SSW events similar to that reported elsewhere. Using these observations, it is suggested that the quiet time variabilities seen in the GPS TEC over EIA could be caused due to the nonlinear interaction of upward propagating planetary waves (PWs) with atmospheric tides. Presence of similar periods in the EEJ strength and TEC observations near the EIA crest region, supports the view that the large‐scale wave like structures seen in TEC near the EIA crest are associated with PWs that are modifying the primary eastward electric field in the equatorial E region and hence the EEJ strength through non linear interactions with atmospheric tides.

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.

Diurnal and seasonal variation of F2-layer ionospheric parameters at equatorial ionization anomaly crest region and their comparison with IRI-2001

Diurnal and seasonal variations of critical frequency of ionospheric F2-region ‘foF2’ and the height of peak density ‘hmF2’ are studied using modern digital ionosonde observations of equatorial ionization anomaly (EIA) crest region, Bhopal (23.2°N, 77.6°E, dip 18.5°N), during solar minimum period 2007. Median values of these parameters are obtained at each hour using manually scaled data during different seasons and compared with the International Reference Ionosphere-2001 model predictions. The observations suggest that on seasonal basis, the highest values of foF2 are observed during equinox months, whereas highest values of hmF2 are obtained in summer and lowest values of both foF2 and hmF2 are observed during winter. The observed median and IRI predicted values of foF2 and hmF2 are analyzed with upper and lower bound of inter-quartile range (IQR) and it is find out that the observed median values are well inside the inter-quartile range during the period of 2007. Comparison of the recorded foF2 and hmF2 values with the IRI-2001 output reveals that IRI predicted values exhibit better agreement with hmF2 as compared to foF2. In general, the IRI model predictions show some agreement with the observations during the year 2007. Therefore it is still necessary to implement improvements in order to obtain better predictions for EIA regions.

Temporal and spatial variations in TEC using simultaneous measurements from the Indian GPS network of receivers during the low solar activity period of 2004–2005

Abstract. With the recent increase in the satellite-based navigation applications, the ionospheric total electron content (TEC) and the L-band scintillation measurements have gained significant importance. In this paper we present the temporal and spatial variations in TEC derived from the simultaneous and continuous measurements made, for the first time, using the Indian GPS network of 18 receivers located from the equator to the northern crest of the equatorial ionization anomaly (EIA) region and beyond, covering a geomagnetic latitude range of 1° S to 24° N, using a 16-month period of data for the low sunspot activity (LSSA) years of March 2004 to June 2005. The diurnal variation in TEC at the EIA region shows its steep increase and reaches its maximum value between 13:00 and 16:00 LT, while at the equator the peak is broad and occurs around 16:00 LT. A short-lived day minimum occurs between 05:00 to 06:00 LT at all the stations from the equator to the EIA crest region. Beyond the crest region the day maximum values decrease with the increase in latitude, while the day minimum in TEC is flat during most of the nighttime hours, i.e. from 22:00 to 06:00 LT, a feature similar to that observed in the mid-latitudes. Further, the diurnal variation in TEC show a minimum to maximum variation of about 5 to 50 TEC units, respectively, at the equator and about 5 to 90 TEC units at the EIA crest region, which correspond to range delay variations of about 1 to 8 m at the equator to about 1 to 15 m at the crest region, at the GPS L1 frequency of 1.575 GHz. The day-to-day variability is also significant at all the stations, particularly during the daytime hours, with maximum variations at the EIA crest regions. Further, similar variations are also noticed in the corresponding equatorial electrojet (EEJ) strength, which is known to be one of the major contributors for the observed day-to-day variability in TEC. The seasonal variation in TEC maximizes during the equinox months followed by winter and is minimum during the summer months, a feature similar to that observed in the integrated equatorial electrojet (IEEJ) strength for the corresponding seasons. In the Indian sector, the EIA crest is found to occur in the latitude zone of 15° to 25° N geographic latitudes (5° to 15° N geomagnetic latitudes). The EIA also maximizes during equinoxes followed by winter and is not significant in the summer months in the LSSA period, 2004–2005. These studies also reveal that both the location of the EIA crest and its peak value in TEC are linearly related to the IEEJ strength and increase with the increase in IEEJ.

Long period oscillations in the equatorial ionization anomaly correlated with the neutral wind in the mesosphere

Long period oscillations of plasma in the equatorial ionization anomaly (EIA) are analyzed with the data from the EIA crest-region station (Okinawa) during 19771̄990. The oscillations have periods at 2 2.5. 3 4, 5 7 and 10–18 days, and make an appearance in all seasons. The relative amplitudes, however, are large in summer and even larger in winter. There is also a slight variation in the seasonal distribution for different periods. The 2-day wave is stronger in summer and the near 2.5 day wave in winter. The summer amplitude peak of the longer period oscillations tends to split and shift to the beginning and end of the summer. All these characteristics are consistent with observational results of neutral wind oscillations in the mesosphere. In the year-by-year variations, it is revealed that the EIA long period oscillations are negatively correlated with solar activity, and the longer the period the more obvious is the negative correlation. We also show that the occurrence of the 2-2.5 day wave and the period of the 10–16 day wave arc modulated by the equatorial QBO. In the east phase of the QBO, it is easier to see the EIA long period oscillations.