Responses of the African and American Equatorial Ionization Anomaly (EIA) to 2014 Arctic SSW Events

Aside from the influence of forcing from above on the ionosphere during space weather, forcing from below also have significant influence on the ionosphere. This study investigates responses of the equatorial ionization anomaly (EIA) in the African and American longitudinal sectors to the combined effects of 2014 Sudden Stratospheric Warming (SSW) events and geomagnetic storms that coexisted with them. The study locations cover ±40° geomagnetic latitudes in both sectors. A multiinstrument approach with models was adopted. During the SSW events, a hemispherical asymmetry in TEC distribution was observed, with higher plasma ionization in the Northern Hemisphere (NH). Generally, in both sectors, EIA crests locations shifted to higher latitudes during peak phases of SSW, except in the SH of the African sector, where crests locations shifted to lower latitudes. Reversal of stratospheric zonal mean wind direction supported reversed fountain effect. TEC responded positively to SSW peak phases and daytime or nighttime orientation of Prompt Penetration Electric Field (PPEF) and PPEF strength played major role on TEC responses to storms. PPEF values were generally weak, but comparatively higher in the American sector. TEC were predominant in the American sector than the African sector due to the comparative higher electrodynamics over the American sector. EIA crests were generally located at higher latitudes on the days of SSW peaks than on the days of geomagnetic storms, except in the NH of the American sector. In both sectors, geomagnetic storms modified ionospheric irregularities by weakening or enhancing them, while the major SSW event weakened irregularities.


Introduction
Previous studies have revealed that the variability in the ionosphere during Sudden Stratospheric Warming (SSW) events is connected to the coupling of the lower and upper atmosphere (Bessarab et al., 2012;Bolaji et al., 2019;de Jesus, Batista, Jonah, et al., 2017;Goncharenko et al., 2013). Understanding of this connection between the lower and upper atmosphere has remained a major concern for the space science community. SSWs are large-scale meteorological events characterized by the sudden breakdown of the stratospheric polar vortex arising from the dynamic forcing of the upward propagating planetary waves originating from the lower atmosphere . The accepted mechanism for their occurrence was attributed to the nonlinear interaction of the vertically propagating planetary waves with zonal wind flow (Matsuno, 1971). SSW events can be classified into major or minor warming. Both major and minor warmings are characterized by the abrupt rise in Abstract Aside from the influence of forcing from above on the ionosphere during space weather, forcing from below also have significant influence on the ionosphere. This study investigates responses of the equatorial ionization anomaly (EIA) in the African and American longitudinal sectors to the combined effects of 2014 Sudden Stratospheric Warming (SSW) events and geomagnetic storms that coexisted with them. The study locations cover ±40° geomagnetic latitudes in both sectors. A multiinstrument approach with models was adopted. During the SSW events, a hemispherical asymmetry in TEC distribution was observed, with higher plasma ionization in the Northern Hemisphere (NH). Generally, in both sectors, EIA crests locations shifted to higher latitudes during peak phases of SSW, except in the SH of the African sector, where crests locations shifted to lower latitudes. Reversal of stratospheric zonal mean wind direction supported reversed fountain effect. TEC responded positively to SSW peak phases and daytime or nighttime orientation of Prompt Penetration Electric Field (PPEF) and PPEF strength played major role on TEC responses to storms. PPEF values were generally weak, but comparatively higher in the American sector. TEC were predominant in the American sector than the African sector due to the comparative higher electrodynamics over the American sector. EIA crests were generally located at higher latitudes on the days of SSW peaks than on the days of geomagnetic storms, except in the NH of the American sector. In both sectors, geomagnetic storms modified ionospheric irregularities by weakening or enhancing them, while the major SSW event weakened irregularities.

Plain Language Summary
We investigated the responses of the EIA in the African and American sectors to the combined effects of 2014 SSW events and the geomagnetic storms that coexisted with them. We chose the study locations to cover ±40° geomagnetic latitudes in both sectors. We adopted a multiinstrument approach with models. Our results showed that the EIA crests locations in the African sector shifted from lower latitudes on pre-SSW days to higher latitudes on minor SSW peak days in the NH. In the SH, these crests locations shifted from higher latitudes on pre-SSW days to lower latitudes on SSW peak days, except on the day of the major SSW peak when the crest location shifted from lower latitude on pre-SSW days to higher latitude on SSW peak days. In the American sector, EIA crest locations shifted from lower latitudes on pre-SSW days to higher latitudes on the SSW peak days in both hemispheres. Reversal of stratospheric zonal mean wind direction supported reversed fountain effect. Both the strength and orientation of PPEF played major roles on TEC responses to storms. Furthermore, occurrence of geomagnetic storm modified ionospheric irregularities. Weakening of irregularities was noticed during days of the major warming in the American sector.
IDOLOR ET AL. stratospheric temperature at 90°N and 10 hPa (Andrews et al., 1987;Chau et al., 2012). Major SSW warming is accompanied by wind reversal from westerly to easterly, while the minor warming is characterized by the deceleration of the zonal wind without any wind reversal (Siddiqui et al., 2015;Vieira et al., 2017).
Earlier studies have shown different trends of SSW-associated ionospheric variability; that is, the semidiurnal pattern in vertical plasma drift with morning enhancement and afternoon reduction was reported by Chau et al. (2009Chau et al. ( , 2011. Similar pattern of morning TEC enhancement and afternoon reduction in the American sector for different SSW events have also been reported Fagundes et al., 2015;Paes et al., 2014;Vieira et al., 2017). Additionally, some of these past investigations reported different longitudinal responses (Anderson & Araujo-Pradere, 2010;Fejer et al., 2010;Sridharan et al., 2009;Vineeth et al., 2009), and a corresponding F-region reduction in electron density (NmF2) during the SSW events (Pedatella et al., 2016). De Paula et al. (2015 reported weakening of ionospheric scintillations at a single crest station in the Brazilian sector for three major (winter 2002, 2003, and 2013) SSW events. Bolaji et al. (2016) reported hemispheric asymmetry in solar quiet ( Sq) E current during the peak phase of 2009 SSW event in the African sector. Bolaji et al. (2019) also demonstrated the weakening of TEC and equatorial electrojet (EEJ) current intensity for the same peak phase of the 2009 SSW event in the African sector.
It is worthy to note that some of the SSW events earlier reported by some of the earlier workers had simultaneous occurrences of geomagnetic storms (e.g., de Jesus, Batista, Jonah, et al., 2017;Goncharenko et al., 2013;Vieira et al., 2017). Geomagnetic storm is a major component of space weather. Space weather is a combination of impacts of time varying conditions of all physical processes, originating from the Sun through the interplanetary medium to the Earth (Akala & Adewusi, 2020;Poppe & Jorden, 2006). Occurrence of extreme space weather events can significantly degrade the performance and reliability of modern space-based and ground-based technological systems, e.g., Global Navigation Satellite System (GNSS) services, space shuttles, assets monitoring, power grids, among many others (Akala & Adewusi, 2020), with attendant huge socio-economic consequences (Eastwood et al., 2017;NRC, 2008;Oughton et al., 2017). Furthermore, space weather events are injurious to the health of astronauts on missions (Lanzerotti, 2001). Unfortunately, previous authors did not attempt to isolate the distinct individual contributions, arising from SSW forcing and geomagnetic storm forcing to ionospheric changes. For instance, de Jesus, Batista, Jonah, et al. (2017) investigated the ionospheric response of a cascade of SSW events from February 2 to April 10, 2014 in the Brazilian sector. Their study focused on only the distinct features of the SSW events, without attention to the possible influences of the geomagnetic storms that coexisted with the SSW events on ionospheric conditions. According to the authors, these SSW features include, increase in the afternoon and nighttime vertical TEC and weakening of ionospheric irregularities. Generally, a combined effect of SSW forcing and geomagnetic storm forcing is expected to cause more ionospheric effects and in turn, more negative impacts on space-based and ground-based critical infrastructures. To this end, concerted efforts are required to distinctly characterize these two geophysical phenomena in order to isolate the contributions of each of them to ionospheric dynamics.
Furthermore, most of the previous studies on SSW effects on the ionosphere focused more on the American and Asian sectors, with little still known of the ionospheric responses of the African sector to SSW events. Goncharenko et al. (2013) reported enhancements of the equatorial ionization anomaly (EIA) crests with a noticeable hemispheric asymmetry in the American sector during 2013 SSW event. The authors linked the observed ionospheric disturbances to anomalous variations in equatorial vertical ion drift. On the other hand, Maute et al. (2015) showed that the typical phase shift signature of the daytime maximum vertical drift in the American sector was conspicuously absent in the African sector during the peak phase of 2013 SSW event. The physical source of this longitudinal difference raised curiosity. Consequently, the need to unravel the ionospheric electrodynamics that is responsible for the observed longitudinal difference is sacrosanct.  reported TEC perturbations in the American and African sectors during the 2012 minor SSW warming event. However, these authors restricted the electrodynamics aspect of their study to the American sector only. The authors were not able to draw any firm conclusion on the influence of ionospheric electrodynamics on the African ionospheric variations during the SSW event that they investigated.

10.1029/2021SW002812
3 of 26 Arising from the above identified gaps in some of the previous research efforts on SSW events, this study investigates the responses of TEC, vertical plasma drift, and ionospheric irregularities over the African and American longitudinal sectors to SSW events of 2014 (a year of high solar activity) and the geomagnetic storms that jointly occurred with the SSW events. The underlying background mechanisms responsible for the observed changes in the electrodynamics of the vertical drift and the EIA are also examined. The results from this study will be useful in beefing-up the understanding of the space science community on the responses of the global ionosphere to SSW events, particularly the SSW events that are associated with high solar activity and geomagnetic activity, with a view to improving existing ionospheric models.

Data and Methods of Analysis
The world map indicating the locations of Global Positioning System (GPS) stations used in this study is presented in Figure 1. In both the African and American sectors, the latitudes of the stations were within the range of ±40° geomagnetic latitudes. For this reason, the locations of the GPS stations in the Northern Hemisphere (NH) for the African sector were extended to Ukraine. Tables 1 and 2 lists the GPS stations with their station codes, geographic and geomagnetic coordinates in both sectors. The stratospheric data (temperature at 90°N and 10 hPa; and the zonal wind at 60°N and 10 hPa) from January 1 to April 30, 2014 that were used in this study were obtained from National Oceanic and Atmospheric Administration (NOAA) website (http://www.esrl.noaa.gov/psd/). Also using all the available stratospheric data from the NOAA satellites from 1979 to the current SSW year investigated, the 36 years historical mean was generated. To probe the solar activity and geomagnetic activity of the ionosphere, Kp, F10.7 cm flux, and Dst index were used for the days investigated. Dst and Kp indices were obtained from the website of the World Data Center for Geomagnetism Kyoto (http://wdc.kugi.kyoto-u.ac.jp/kp/index.html), while the F10.7 cm flux was obtained from the website of the National Aeronautics and Space Administration (NASA) space physics data facility (http://omniweb.gsfc.nasa.gov/form/dx1.html). All days with Kp ≤ 3 (ƩKp < 24) were classified as quiet days, while we adopted the geomagnetic storm criteria of a minimum Dst of: −100 nT  E Dst  E −50 nT for moderate storm, and −200 nT  E Dst  E −100 nT for major storm (Gonzalez et al., 1994;Loewe & Prolss, 1997) to categorize the five geomagnetic storms that occurred within the period of the   (Table 3). We estimated the Prompt Penetration Electric Field (PPEF) for February 18-28, and April 11-15, using the real-time electric field model for geomagnetism developed by the Cooperative Institute for Research in Environmental Sciences (CIRES) of the University of Colorado at Boulder (http://geomag.colorado.edu/real-time-model-of-the-ionospheric-electric-fields.html). This model utilizes a frequency-dependent transfer function to determine the prompt penetration of the interplanetary electric field response to the equatorial ionosphere (Manoj & Maus, 2012;Manoj et al., 2008Manoj et al., , 2013. We used the GOPI TEC processing software (Seemala & Delay, 2010) to extract the TEC data from the GPS observable data. The GPS data were obtained from the University NAVSTAR Consortium, UNAVCO (www. unavco.org), Système d'Observation du Niveau des Eaux Littorales, SONEL (www.sonel.org), African Geodetic Reference Frame, AFREF (www.afrefdata.org), and International Global Navigation Satellite Systems Service (IGS) (www.igs.org). The difference in TEC procedure by Goncharenko et al. (2010b) was adopted to evaluate background variations during the SSW event. The computed hourly mean for each station was subtracted from the days during the SSW event and expressed mathematically as: where TEC Sd is the hourly mean TEC data for the days of the SSW events from February 2 to April 30, 2014 and TECq m is the hourly mean TEC data for the pre-SSW days (with ƩKp < 24) from January 1 to 31, 2014 for each station, with the exception of January 2, 2014 which had ƩKp = 26. Additionally, we noted that the second minor SSW (M-SSW-2) event overlapped with the geomagnetic activity of February 19, 20, and 21, 2014. The implication of the overlap of these two geophysical conditions is that the combined effect of the two geophysical phenomena is expected to have effect on the modification of the ionosphere. Table 4 lists the geomagnetic conditions of February 7-9 and 19-22, 2014. From Table 4, except for February 7, the geomagnetic storm indices for all other days did not meet the minimum condition for a quiet day. It is important to also stress that days; February 7-9 define the days of M-SSW-1 peak phase. By implication, the ionospheric effect of February 7 is solely SSW-induced. In order to filter the SSW-induced ionospheric effects from the geomagnetic storm-induced ionospheric effects during the period of overlap of M-SSW-2 and geomagnetic activity, we subtracted the SSW-induced ionospheric effects of February 7, 2014 from those of February 19, 20, and 21, 2014.  To explore the effects of 2014 SSW event on the ionospheric irregularities, the Rate of change of TEC (ROT) expresses in units of TEC/min was computed and the 5 min standard deviation of the ROT to obtain ROT Index (ROTI) (Pi et al., 1997). ROTI is expressed mathematically as: In addition, the 30-min resolution of ROTI was estimated to eliminate noise spikes (Oladipo et al., 2014). The 30-min resolution of ROTI hereafter referred to as ROTI ave was obtained by computing the ROTI average value for all satellites in view at a particular location over 30-min interval. The occurrence of irregularities was classified into three distinct threshold levels using the ROTI ave < 0.4 to indicate the absence of irregularities, 0.4 < ROTIave < 0.8 indicates the presence of moderate irregularities and ROTI ave > 0.8 to indicates the presence of severe irregularities (Bolaji et al., 2020;Oladipo & Schuler, 2013).
The EEJ current measured in nano-Tesla (nT) for the African and American sectors for the 2014 SSW event was obtained from two pairs of groundbased magnetometers located at the equatorial and low-latitude regions (Anderson et al., 2002Yizengaw et al., 2014). The magnetic field data for the Addis Ababa station (geomagnetic coordinates: 0.9°N, 110.5°E) located at the equatorial region were obtained from the International Real-time Magnetic Observatory Network, INTERMAGNET (www.intermagnet.org), while the data for the Nairobi station (10.76°S, 108.51°E) were obtained from Magnetic Data Acquisition System (MAGDAS) (http://mag-das2.serc.kyushu-u.ac.jp/). In the American sector, both the equatorial and low-latitude magnetometers data at Jicamarca (0.8°N, 5.7°W) and Plura (6.8°N, 9.4°W) were obtained from the Low Ionospheric Sensor Network (LISN) (http://lisn.igp.gob.pe/data/). The EEJ current was processed using the procedure outlined by Rabiu et al. (2017). The EEJ current was calculated using the expression below: where ΔH eq is the horizontal magnetic field intensity for a station located at equatorial region and ΔH off-eq is the horizontal magnetic field intensity for a station located outside the equatorial regions. Also, the empirical relationship established by  was adopted to estimate the vertical drift. This relationship is given by the expression below (Anderson et al., 2002Kassamba et al., 2020;Yizengaw et al., 2011Yizengaw et al., , 2014: where VD is the estimated vertical drift, Yr is the year, DOY is the day of the year, F d is the daily F10.7 cm solar flux, F a is the 81-day average value of the adjusted F10.7 cm solar flux, Ap and Kp are the daily and 3 hourly geomagnetic indices, LT is the local time in hours at the magnetometer stations, and ΔH is the difference in the horizontal magnetic field intensity for each pair of magnetometer stations. It should be noted that the EEJ current obtained from equation (3)     provides detailed explanations on this model. Furthermore, the magnetic field data derived from the Absolute Scalar Magnetometer (ASM) on board the SWARM satellites was used to calculate the EEJ current intensity The EEJ current intensity was estimated from the scalar magnetic field data using the current inversion technique outlined by Aiken et al. (2013Aiken et al. ( , 2015. Only satellites with orbital crossing at the geographic equator closest to those of the ground-based magnetometer stations for the period of the March 3-17, 2014 were considered in the current study. The EEJ current depicting the height-integrated current density profile was measured in milli Ampere per meter (mA/m) (Aiken et al., 2015).

Results
Figure 2a shows stratospheric zonal mean air temperature at 90°N, stratospheric historic zonal mean air temperature at 90°N, stratospheric zonal mean zonal wind at 60°N, stratospheric historic zonal mean zonal wind at 60°N, all at 10 hPa, and the daily Dst variations during the period of the pre-SSW and SSW events (January 1 to April 30, 2014). Conventionally, winter months (December-February) are the months of SSW occurrences, but SSW features could extend to equinoctial months of March and April (de Jesus, Batista, Jonah, et al., 2017), which we also observed in the current results ( Figure 2a). Conse quently, we extended our analysis to cover the months of March and April because of the observed abrupt rise in stratospheric temperature within a few days and the zonal wind reversal from easterly to westerly direction in March, with the historical mean temperature and temperature intersecting in April. In Figure 2, three minor and one major SSW events were marked in magenta dash lines. The first minor SSW (M-SSW-1) event occurred on February 2-12 with a corresponding temperature peak (240 K) on February 8. On February 7 and 9, the temperature values were 236 K for each day. The second minor SSW (M-SSW-2) event occurred on February 17-25, with a peak temperature (232 K) on February 21. On February 19, 20, and 22, the temperature values were 224, 231, and 227 K, respectively. The third minor SSW (M-SSW-3) event occurred between February 27 and March 10 with observed dual peak temperature recorded on 3rd (237 K) and 6th (237 K) of March, respectively. The major SSW event occurred between March 14 and April 20 with a corresponding observed peak temperature (253 K) on March 16, 2014. The first three SSW events occurred during the gradual deceleration of the stratospheric zonal wind, while the fourth event was associated with reversal of the zonal wind on March 25 to indicate the occurrence of a major warming. Series of successive minor warming events usually precede major warming event. O'Neill (2003) and Paes et al. (2014) ascribed these trends to instances of complete breakdown of the weakened polar vortex during winter solstices. The 36-year stratospheric mean temperature and wind prior to the current year of study is shown by the solid blue and red lines in Figure 2. The Dst plot shows five geomagnetic storms during the duration of the SSW period with their properties listed in Table 3. The first geomagnetic storm of February 19 is a dual peak storm, with a second peak occurring on February 20. Figure 2b shows -3biv shows day-to-day plots of EIA TEC for the period of January-April 2014 for the American sector. Weak TEC (∼45 TECU) enhancement was also observed for days in January (Figure 3bi). The daily TEC variation of the SSW events February-April 2014 showed more TEC enhancements within the range of ∼70-98 TECU. Figure 3biii shows that on March 3, more severe TEC enhancement in the range 70-80 TECU were observed in the NH, while the SH recorded lower TEC enhancement within the range of 60-68 TECU. On March 15-18, higher TEC values were observed in the NH compared to the lower TEC values in the SH. Reduction in TEC at the higher latitudes and TEC intensification toward the equatorial region (reverse fountain effect) was associated with the major wind reversal on March 25. The remaining days investigated in March and April showed more TEC enhancements across both hemispheres. Table 5 shows the variations of EIA crests locations in the African and American sectors during 2014 pre-SSW and extremely quiet geomagnetic days, SSW peak days, and geomagnetic storm days. Four most geomagnetically  in Figures 4ai and 4aii. Most days in March showed semidiurnal TEC enhancement feature across both hemispheres in contrast to just a few days of semidiurnal pattern observed in February and April. In the American sector, a few days of semidiurnal variation pattern ( Figure 4bi) were observed in February with TEC enhancement coinciding with February 27 geomagnetic storm activity. We also observed diurnal TEC patterns for most days in March with semidiurnal pattern visible for a few days. Reduction in TEC was clearly seen for some days between March 16 and 20, 2014, spanning across both hemispheres and coinciding with days of the major SSW warming. In addition, semidiurnal variation patterns were also observed in April (Figure 4biii), and days of April were characterized mostly by more TEC enhancements in NH than the SH. To show the terdiurnal patterns clearly, with reference to the terdiurnal pattern of March 3 as the representative of other terdiurnal patterns. Figures 5a and 5b Table 5 Variations

of EIA Crests Locations in the African and American Sectors During 2014 Pre-SSW and Extremely Quiet Geomagnetic Activity Days, SSW Peak Days, Four Most Quiet Geomagnetic Days of February and April, 2014, and Geomagnetic Storm Days
American sectors to the geomagnetic storms under investigation using the mean of the geomagnetically quiet days in February as a reference. Only the February storms are considered because visual inspection in Figure 4 shows clearly that TEC responded negatively to the April 12 storm in both sectors. From Figure 6, TEC responded positively to the February 23 and 27 moderate geomagnetic storms in both sectors. TEC responded positively to the February 19 major geomagnetic storm, and negatively to February 20 storms in the African sector (Figure 6a). In the American sector, TEC responded negatively to the February 19 and 20 storms (Figure 6b).  Figure 7a, the response to TEC at the EIA crests regions were positive during the occurrence of the major GS (February 19) at 15:00-21:00 UT (18:00-24:00 LT) in the African sector. The moderate storm of February 20 also showed a negative TEC response within the EIA crests regions at 08:00-20:00 UT (11:00-23:00 LT). Figure 7b shows a negative TEC response within the EIA crests regions in the American sector at 20:00-04:00 UT (15:00-24:00 LT) to the major storm. The moderate storm also showed a positive TEC response (∼8 TECU) within the EIA crest regions at 08:00-20:00 UT (03:00-15:00 LT). Furthermore, on February 21, the African sector showed a positive EIA TEC response at 08:00-21:00 UT (11:00-24:00 LT), while American sector also revealed a positive TEC response at 06:00-24:00 UT (01:00-19:00 LT).
Figures 8ai-8aiv and 8bi-8biv show the latitudinal variations of ionospheric irregularities over the African and American sectors. Figure 8ai shows both moderate and severe irregularities observed between 18 and   sectors (∼0.1 mV/m) but increased to almost 0.2 mV/m on the day of the recovery phase of the moderate storm (April 13). Manoj et al. (2008Manoj et al. ( , 2013 reported that interplanetary electric field is positively correlated with the zonal equatorial electric field on the local dayside and negatively correlated on the local nightside. Interplanetary electric field on the other hand is known to be related to PPEF (Tsurutani et al., 2008). Previously, Astafyeva et al. (2016) reported that positive values of interplanetary electric field imply eastward orientation of the zonal electric field during daytime and westward orientation during nighttime (Arowolo et al., 2021).   the next day to 108.2 nT at noon. A similar pattern of increase in EEJ intensity from 102.2 to 102.6 nT was recorded on March 6-7. However, we observed lower EEJ value on March 15 in comparison to EEJ value on March 16, coinciding with the day of peak stratospheric temperature of the major SSW event. Figures 11a and 11b show the eastward height-integrated current profile of the EEJ current for the African and American sectors obtained from the SWARM satellite with equatorial orbital crossing closest to those of the ground-based magnetometer station. As shown in Figure 11a, in the African sector, the day-to-day EEJ current from March 3 to 17 showed a trend pattern consistent with those of the EEJ values obtained by ground-based magnetometer. The EEJ current on March 3 was 5.7 mA/m, decreased to 4.5 mA/m on March 4. Also, from March 6 to 7, EEJ current profile showed a similar significant decrease in magnitude from 16.9 to 8.5 mA/m with the EEJ current profile from March 16 to 17 showing slight decrease from 14.1 to 13.6 mA/m. However, an increase in magnitude from 12.0 to 17.5 mA/m was observed on March 10-11. Figure 11b shows the day-to-day EEJ eastward current for the American sector. Our results indicate an increase in EEJ current from 9.9 to 17.7 mA/m on March 3-4, coinciding with the minor SSW event. A similar increment from 21.4 to 42.8 mA/m was recorded on March 6-7, coinciding with the minor SSW event. However, on March 16-17, a significant decrease in EEJ current from 48.4 to 18.3 mA/m was observed for the American sector.

Discussion
As shown in Figure 3, in both longitudinal sectors, well-developed two peak EIA structures were consistently observed. The daily EIA profile showed variability in TEC and crests locations in both sectors ( Figure 4). Overall, in terms of TEC intensity in both sectors, TEC generally responded positively to the SSW peak days, except on February 8 when a negative response was observed. However, in terms of the EIA crests locations, in both sectors, EIA crest locations generally shifted from lower latitude locations on pre-SSW to higher latitude locations on minor SSW peak days, except on February 8 (Figure 3 and Table 5). Generally, in terms of hemispherical distributions of TEC, a clear asymmetry in TEC in both longitudinal sectors was observed. Higher plasma ionization was consistently observed in the NH than in the SH in both sectors. Fagundes et al., (2015) reported the same hemispheric trend in TEC distribution over the Brazilian sector during the 2009 SSW event. Similarly, the hemispheric asymmetry in TEC distribution was also reported in the Asian sector . Laskar and Pallamraju (2014) reported that the hemispheric asymmetry in ionospheric data may be due to the interaction of SSW-induced meridional wind and transequatorial neutral wind.
On March 25, 2014, on the day of the reversal of stratospheric zonal mean wind direction, both longitudinal sectors showed equator-ward movement of plasma (reverse fountain effect). As a typical day of an equinoctial month, one would naturally expect an enhancement in EIA TEC (equinoctial effect), but on the contrary, on March 25, 2014 (a day of reversal of stratospheric zonal mean wind direction), showing clearly SSW effect on the EIA as contrary to the expected equinoctial effect of TEC enhancement in equinox. The SSW effect that caused the equator-ward shift in EIA crest from higher to lower latitudes may be attributed to the significant increase in both planetary and gravity waves arising from decelerating stratospheric zonal mean wind ( have shown that increase in upward propagating waves arising from the thermosphere plays a major role in the modification of the ionospheric wind systems, creating tidal components which modulate the dynamo electric field. Pancheva and Mukhtarov (2011) explained that equator-ward plasma movement arising from the modified current system during SSW events is driven by equator-ward winds of lower thermospheric origin due to thermospheric heating. Furthermore, Fagundes et al. (2015) attributed the physical mechanism that is responsible for ionospheric variations to the changes in neutral gas composition resulting from changes in thermospheric composition during SSW event. Generally, EIA crests in both hemispheres shifted to higher latitudes on days of SSW peaks than on the days of geomagnetic storms, except for the NH of the American sector (Table 5).
In this study, except for the February 19 and 20 geomagnetic storms that overlapped with the SSW peak days, other geomagnetic storms during the days of investigation did not directly overlap with the days of the peak temperatures ( Figure 2). As listed in Table 5, we used four most geomagnetically quiet days of Comparatively, these days were also within the region of low stratospheric temperature ( Figure 2). Furthermore, we used the quiet days of February to assess the level of storm-induced ionospheric variability by the four geomagnetic storms of February and the quiet days of April to assess the level of storm-induced ionospheric variability by the April geomagnetic storm.
As listed in Table 3, the local time of the onset of all the five geomagnetic storms occurred during daytime in the African sector and during nighttime in the American sector, with the exception of the major geomagnetic storms of February 19 whose local onset time was daytime. The February 20 moderate storm is the second peak of the February 19 major storm. In the African sector, the local time of the main phase of the major and moderate geomagnetic storms of February 19, 20, and April 12 was during daytime, and during nighttime for moderate geomagnetic storms February 23 and 27. However, in the American sector, the local time of the main phase of the major and moderate geomagnetic storms of February 19, 27, and April 12 were during nighttime, and during daytime for moderate geomagnetic storms of February 20 and 23. Recently, Arowolo et al. (2021) concluded that the daytime or nighttime orientation of PPEF plays major role in ionospheric responses to geomagnetic storms. This is also very evident in our results, particularly in the American sector. From our result, in the African sector, with the reference to the quiet days of the month of occurrence of each geomagnetic storm, TEC responded positively to the February 19, 23, and 27 geomagnetic storms, and negatively to February 20 and April 12 geomagnetic storms. In the American sector, the responses of TEC to February 19, 20 and April 12 geomagnetic storms were negative and positive to the 23 and 27 geomagnetic storms. Negative ionospheric responses are caused by a decrease in atomic oxygen density; a decrease in oxygen ion concentration and an increase in the molecular nitrogen density, leading to an increase in the loss rate to cause a decrease in the ionization density in the F-region (Mosna et al., 2021). On the other hand, positive ionospheric responses are caused by an increase in atomic oxygen density; an increase in oxygen ion concentration and a decrease in the molecular nitrogen density, leading to an increase in the production rate to cause an increase in the ionization density in the F-region (Mikhailov et al., 1994;Mosna et al., 2021).  and Arowolo et al. (2021) had earlier explained that during storm time, daytime eastward PPEF supports forward fountain to cause EIA TEC enhancement and poleward movement of the EIA crests from its quiet time location. On the other hand, storm time nighttime westward PPEF supports reversed fountain to cause EIA TEC reduction and equator-ward movement of the EIA crests from its nominal quiet time and daytime location. However, we observed that the PPEF values during the periods of the geomagnetic storms were quite low (<0.3 mV/m). Consequently, the associated storm time fountain effects were weak. On the overall, although generally low, PPEF values were comparatively higher in the American sector than in the African sector, and could have been responsible for more enhancements in TEC in the American sector than the African sector during the period of occurrences of the five geomagnetic storms.
Variations of ionospheric irregularities over the African and American sectors during the period of investigation are shown in Figure 8.  (Akala et al., 2013;Akala, Awoyele, & Doherty, 2016;Akala, Idolor, et al., 2016). Our results for the African sector ( Figure 8aii) coinciding with the days of major geomagnetic storm depicted a suppression of irregularities on February 19 and moderate occurrence of irregularities on February 20. The moderate geomagnetic storm of February 23 caused moderate irregularities, while the storm of February 27 caused more irregularities across both hemispheres. The days of the major and moderate geomagnetic storms of February 19 and 20 in the American sector ( Figure 8bii) shows an enhanced occurrence of ionospheric irregularities on February 19-20, and a suppression of irregularities during the moderate geomagnetic storm of February 23 and 27. However, aside from the geomagnetic storm days in the American sector (Figure 8biii), the effects of the major SSW event were clearly observed with suppression of irregularities in March as against the expected equinoctial effects of enhanced irregularities in March. The observed weakening of ionospheric irregularities is consistent with past studies of ionospheric irregularities in the Brazilian sector (de Jesus, Batista, Jonah, et al., 2017;De Paula et al., 2015). The weakening of irregularities may be ascribed to the decrease in zonal neutral winds arising from the interaction of the meridional and transequatorial winds (De Paula et al., 2015;Oyedokun et al., 2020). The interaction of these wind systems provides conducive platform for regulating the growth of ionospheric irregularities (Laskar & Pallamraju, 2014). In addition, during the period of SSW events, similar report by  revealed that quasi stationary planetary waves played crucial role in the preconditioning of ionospheric irregularities.
Comparatively, from this study, TEC were generally predominant in the American sector than the African sector. From our results, both the SWARM-derived and ground-based EEJ data showed higher intensity in the American sector than the African sector during the cascade of minor and major SSW events (Figures 10  and 11). The computed EEJ is related to vertical electric fields (Anderson et al., 2002 which are in turn responsible for the strength of the fountain effect (Chakraborty & Hajra, 2008). Our estimated VD drift ( Figure 12) also showed higher variations over the American sector than over the African sector during the cascade of minor and major SSW events. These results of longitudinal differences in vertical drift electric field are consistent with the inferred vertical plasma drift during 2013 SSW event by Maute et al. (2015) which was attributed to the difference in neutral wind flow patterns and geomagnetic field strength. The equatorial vertical drift generally influences changes in the electron density and the EIA (de Jesus, Batista, . Previously,  and  had reported higher TEC variability during days of surge in the stratospheric temperature and the subsequent movement of the EIA crest regions to higher latitudes in the American sector. Generally, the longitudinal differences in the equatorial/low-latitude ionospheric electrodynamics in the African and American sectors are attributable to geometric effects arising from the differences between the geomagnetic field lines and geographic latitude lines in both sectors (England, 2012;Goncharenko et al., 2020). There are clear offsets between geographic latitudes and geomagnetic latitudes in both sectors. For instance, the geomagnetic equator line is in the geographic NH in the African sector and in the geographic SH in the American sector. These offsets are more in the American sector than in the African sector. Furthermore, there are also geometric configurations related to the offsets, and they are more conspicuous in the American sector than in the African sector. Unlike the African equatorial/low-latitude region, at the American equatorial/low-latitude region, there are clear geometric effects arising from the protuberances of the geomagnetic field lines.

Conclusions
We investigated the responses of the equatorial/low-latitude ionosphere over the African and American sectors to: (a) minor and major winter arctic SSW events (January-April, 2014) and (b) geomagnetic storms that occurred during the same period of the SSW events. The conclusions from this study are: 1. In addition to the roles played by geomagnetic storms and other solar events in space weather, lower atmospheric couplings also play significant roles, particularly in the ionospheric part of space weather (ionospheric weather) (Mosna et al., 2021). With reference to the pre-SSW quiet days, TEC intensity responded positively to the SSW peak days in both longitudinal sectors, except on February 8 in the African sector, when a negative TEC response was recorded. In terms of the EIA crests locations, in both sectors, EIA crests locations shifted from lower latitudes on pre-SSW days to higher latitudes on minor SSW peak days in the northern hemisphere, except on February 8 in the African sector. In the southern hemisphere, the EIA crests locations shifted from higher latitudes on pre-SSW quiet days to lower latitudes on SSW peak days in the African sector, while in the American sector, EIA crests locations shifted from lower latitudes on pre-SSW quiet days to higher latitudes on SSW peak days in the southern hemisphere. In addition, the reversal of stratospheric zonal mean direction supported reversed fountain effect 2. From our results, aside from the days of the geomagnetic storms of February 19 and 20, 2014, the days of other geomagnetic storms that coexisted with the SSW events did not directly overlap with the days of the peak temperatures of the SSW events. For the days when SSW events overlapped with geomagnetic storms (M-SSW-2), after isolating the contributions by SSW forcing on ionospheric effects in the African sector, EIA TEC responded positively to the major geomagnetic storm of February 19, and negatively to the moderate storm of February 20. In the American sector, EIA TEC responded negatively to the major geomagnetic storm and positively to the moderate storm of February 20. For the periods of M-SSW-1, M-SSW-3, and major SSW, over both sectors, ionospheric effects arising from SSW-induced forcing and geomagnetic storm-induced forcing were clearly distinct 3. A clear asymmetry in hemispherical distributions of TEC was observed in both longitudinal sectors.
Higher plasma ionization and higher latitudinal locations of the EIA crests were generally recorded in the NH than SH in both longitudinal sectors, although, there were cases where the EIA crests were located at higher latitudes in the SH than the NH in the African sector. In both longitudinal sectors, we observed that EIA crests were located at higher latitudes on the days of SSW peaks than on the days of the geomagnetic storms, implying that ionospheric effects were more influenced by SSW forcing than geomagnetic storm forcing during the period of study. From our results, the geomagnetic storms investigated were generally characterized with low PPEF values with attendant weak storm time effects on the ionosphere. Daytime or nighttime orientation of PPEF plays major role in ionospheric responses to geomagnetic storms 4. Over the African sector, the major geomagnetic storm completely suppressed ionospheric irregularities, while the moderate storm of February 20 and 23 caused moderate irregularities. The moderate storm of February 27 caused enhancement in irregularities, while the April 12 moderate storm caused suppression of irregularities. Over the American sector, the major storms of February 19 caused enhancements in irregularities and a weakening of irregularities during the moderate storm of February 20. The moderate storms of February 23, 27, and April 12 caused suppression of ionospheric irregularities. The major SSW forcing weakens irregularities on March 15 and 21-23 5. TEC generally recorded higher intensity in the American sector than in the African sector. From the PPEF, EEJ, and inferred vertical drift data, ionospheric electrodynamics over the American sector during the period of investigation was higher than that of the African sector (Figures 10-12). As presented by Goncharenko et al. (2020), the longitudinal differences in ionospheric electrodynamics can be attributed to the geometric effects arising from the differences in geomagnetic field configurations in both sectors 6. The results from this study will expand the understanding of the space/atmospheric science community on the responses of African and American equatorial/low-latitude ionosphere to a combined effect of geomagnetic storm forcing and SSW forcing during period of high solar activity, with a view to isolating the role(s) of each individual forcing on ionospheric changes. Furthermore, these results will also support the development of future models that could predict the occurrences of SSW-induced and geomagnetic storm-induced ionospheric disturbances, with a view to mitigating the adverse impacts of such ionospheric disturbances on technological systems

Data Availability Statement
The stratospheric data used in this study were downloaded from the website of the National Oceanic and Atmospheric Administration (NOAA) (http://www.esrl.noaa.gov/psd/). The Kp and Dst index was downloaded from the World Data Center for Geomagnetism Kyoto (http://wdc.kugi.kyoto-u.ac.jp/kp/index. html). The F10.7 solar flux index was downloaded from the website of the National Aeronautics and Space Administration (NASA) space physics data facility (http://omniweb.gsfc.nasa.gov/form/dx1.html) while the 81-day adjusted F10.7 cm solar flux was obtained from celestrak public access website (https://celestrak. com/SpaceData/). The GPS data used in this analysis were obtained from the following listed organizations; International Global Navigation Satellite Systems Service (IGS) (www.igs.org); University NAVSTAR