Morphology of the equatorial ionization anomaly in Africa and Middle East due to a sudden stratospheric warming event

Journal of Atmospheric and Solar-Terrestrial Physics 184 (2019) 37–56

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Morphology of the equatorial ionization anomaly in Africa and Middle East due to a sudden stratospheric warming event

T

O.S. Bolajia,b,∗, E.O. Oyeyemia, O.E. Jimoha, A. Fujimotoc, P.H. Dohertye, O.P. Owolabid, J.O. Adeniyid, E.O. Falayif, E. Abeg, R.O. Kakaa, A. Kotoyeh a

Department of Physics, University of Lagos, Akoka, Lagos, Nigeria Department of Mathematics and Physics, University of Tasmania, Australia c International Centre for Space Weather Science and Education (ICSWSE), Kyushu University, Fukuoka, Japan d Department of Physics, University of Ilorin, Ilorin, Nigeria e Institute for Scientific Research, Boston College, Chestnut Hill, MA, USA f Department of Physics, Tai Solarin University of Education, Ijagun, Ogun, Nigeria g Department of Physics, Federal University, Oye-Ekiti, Ekiti, Nigeria h Abraham Adesanya Polytechnic, Ijebu-Igbo, Ogun, Nigeria b

ARTICLE INFO

ABSTRACT

Keywords: Total electron content Equatorial ionization anomaly Latitudinal twin EIA crests Planetary waves Sudden stratospheric warmings

Using total electron content (TEC) data deduced from 18 Global Positioning System (GPS) receivers in Africa and Middle East, we investigated the morphology of the equatorial ionization anomaly (EIA) and its underlying variations before, during and after the 2009 sudden stratospheric warming (SSW) event. A southern EIA crest stronger than the northern EIA crest was observed for most of the days before the SSW event, while the EIA troughs were significantly obliterated after these SSW induced phases. In addition to the observed marked depletion of the hemispheric EIA crests during the SSW peak phase, we observed a terdiurnal variation straddling the northern EIA crests. This background terdiurnal signature is suggested to be partly responsible for the transport of more plasma to the northern hemisphere at the expense of southern hemisphere during the SSW peak phase. The consequences are higher pre-noon and post noon crests in the northern hemisphere compared to a single crest in the southern hemisphere. Contrary to previous modeling and experimental reports that the reductions in ionospheric TEC are due to semidiurnal variations resulting from the SSW peak phase, our results show that a terdiurnal variation was responsible for reducing the EEJ strength and TEC at the E-region and F2region's topside, respectively. At the southern middle latitudes, an underlying diurnal variation was seen to initiate an increment in TEC during the SSW descending phase.

1. Introduction Sudden stratospheric warming (SSW) is a meteorological event that connects the lower and the upper atmosphere. Although, SSW occurs predominantly in the northern winter hemisphere (O’Neill, 2003), the whole atmosphere of the Earth responds to these large-scale meteorological events (Fuller-Rowell et al., 2011). During the phenomenon, stronger planetary waves (PWs) amplitudes that last for several days are generated from the troposphere (Matsuno, 1970). Typically, the activity of these planetary waves leads to dramatic changes in the high – latitude stratospheric temperature and wind. In effects, the temperature of the polar stratosphere could surge by several tens of degrees, while the high – latitude stratospheric wind decelerates (minor warming) or reverses direction (major warming). The meridional circulation changes ∗

associated with this event can modulate the stratospheric, mesospheric zonal winds and connects the mesosphere-lower thermosphere (MLT) region with the low and middle latitude ionosphere. These connections include changes in vertical drifts/equatorial electrojet (EEJ) and lunar tides (Fejer et al., 2010; Park et al., 2012; Yamazaki et al., 2012a), modulation of E-layer ionospheric currents (Vineeth et al., 2007; Yamazaki et al., 2012b; Bolaji et al., 2016), changes in total electron content (TEC) data (Fejer et al., 2010; Goncharenko et al., 2010a, 2010b; 2013; Paes et al., 2014) and perturbations in atomic oxygen ([O]), molecular nitrogen ([N2]) and peak electron density of F2 layer (NmF2) (Yamazaki and Richmond, 2013; Pedatella et al., 2016). The underlying variation responsible for most of these aforementioned SSW related changes was disclosed by Goncharenko et al. (2010a) and Paes et al. (2014). Both studies established that GPS TEC

Corresponding author. Department of Physics, University of Lagos, Akoka, Lagos, Nigeria. E-mail address: [email protected] (O.S. Bolaji).

https://doi.org/10.1016/j.jastp.2019.01.006 Received 20 July 2018; Received in revised form 9 January 2019; Accepted 10 January 2019 Available online 23 January 2019 1364-6826/ © 2019 Published by Elsevier Ltd.

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variability over South American sector is driven by a phase shift of semidiurnal signature. In addition to the efforts made in South American sector, contributions of SSW effects over African equatorial and low latitude was made by de Jesus et al. (2017). Key to their results is also the presence of semidiurnal perturbations in the behavior of TEC over the low latitude in these regions. In the case of vertical ion drift during SSW events, Chau et al. (2010) and Fejer et al. (2010) reported that this enhanced semidiurnal variation of electric field initiates redistribution of plasma from equatorial trough to higher latitudes. Furthermore, Bolaji et al. (2016) revealed that there were significant hemispheric reductions in the solar quiet (Sq) currents over Africa due to SSW effects, in the 2009 SSW peak phase. Meyer (1999), Liu and Roble (2002) and Sridharan et al. (2009) suggested that the modulation of upward propagating semidiurnal and diurnal tides during SSW peak period could be responsible for these significant reductions in the magnitude of hemispheric Sq current at the E-region. This can subsequently modulate F-region and its top-side since the E-region is reasonably coupled the F-region and its top-side through the equatorial ionization anomaly (EIA) phenomenon. An example of such reduction in ionospheric electron density (NmF2) at the Fregion during SSW events can be found in the work of Pedatella et al. (2016). Using the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM), they unraveled the underlying variations responsible for the NmF2 reduction during the stratospheric temperature peak. While neglecting the effects of gravity waves, their findings emphasized on the role of enhanced 12-h semidiurnal westward propagating tidal wave with zonal wave number 2 (SW2) and planetary waves at reducing the NmF2. Yamazaki and Richmond (2013) had earlier suggested that upward-propagating diurnal and semidiurnal tides induces mean circulation in the lower thermosphere, which in turn, reduces the atomic oxygen and coupled the ionosphere by eddy diffusion. Other findings from their work include reduction of NmF2 magnitude and ionospheric [O]/[N2] ratio with a corresponding production/loss of atomic oxygen ions [O+]. Other modeling experiments done by Fuller-Rowell et al. (2010, 2011) and Wang et al. (2011) that unveiled the underlying variations responsible for changes due to the SSW events indicated that an increasing variability of the propagating SW2 tide possibly modulated ionospheric electrodynamics prior to and after the peak in the stratospheric temperature. In addition, they revealed that the nonlinear wave-to-wave interaction of the 8-h terdiurnal westward propagating tide wave with zonal wave number 3 (TW3), SW2 and the 24-h diurnal westward propagating tidal wave with zonal number 1 (DW1) can lead to significant growth in the TW3 at the expense of SW2 and DW1 during the peak in the stratospheric temperature. This indicates that increasing TW3 magnitude in the lower thermosphere can change the local time variation of the Sq current, NmF2 and GPS TEC during the peak in the stratospheric temperature. It is worthwhile to note that these aforementioned EIA studies during SSW were investigated predominantly in South American sector, while previous studies of Bolaji et al. (2017) that focus solely on the African sector examined the EIA morphology during non SSW periods. Hence, the effects of SSW on the EIA signatures in Africa region have not been fully clarified. Also, there is a lack of observational evidence during the 2009 SSW event, which clearly demonstrates that the TW3 variation obviously modulates the GPS TEC when the stratospheric temperature reaches its peak. In this study, TEC data obtained from 18 GPS stations over Africa and the Middle East were used to profile the EIA during the 2009 SSW event. Also, the underlying variations responsible for the EIA responses during different phases of the 2009 SSW event were examined. The implications of these underlying variations with regard to observed changes in the GPS TEC during different phases of the SSW event were also investigated.

Fig. 1a. Map of Africa and Middle East showing the location of the eighteen GPS stations investigated.

2. Materials and methodology GPS data offer an efficient opportunity to estimate TEC values with greater spatial and temporal coverage (Bolaji et al., 2012) than ionosondes. Location of the GPS receivers denoted with station codes in Africa and Middle East are shown in Fig. 1a. Table 1 depicts their geographical and geomagnetic coordinates. Data obtained from 18 GPS stations located between geomagnetic latitudes 35.56oN and 41.09oS in the region under study are employed to investigate and characterize the EIA from TEC variability for the period before, during and after the 2009 SSW event. This time frame spans between January and March, 2009. We selected such geomagnetic latitude range and time frame in order to explain the effects of SSW on the latitudinal profiling of EIA crests over this equatorial low-middle latitude region. According to the World Data Center (WDC) catalogue available via http://wdc.kugi.kyoto-u.ac.jp/, (World Data Centre) the year 2009 was a low solar active year with annual sunspot number of 3.1 being characterized by insignificant geomagnetic disturbances (Bolaji et al., 2012, 2013). As shown by Fig. 1b, records provided by the Space Physics Data Facility of the National Aeronautics and Space Administration shows the behavior of the solar activity indicators from January to March, 2009. This data can be obtained via http://omniweb. gsfc.nasa.gov/form/dx1.html. While, the F10.7 flux values (stem lines in red) is in the range from ∼66 to ∼72 solar flux units (sfu, where 1 sfuis 1 × 10−22 Wm−2 Hz−1), the geomagnetic Kp index (bars in black) is in the range from 0 to ∼3.0. We noticed that the Kp index, on any of the days was low, the largest being 3.0 on February 14 and March 12. The corresponding F10.7 flux (stem lines in red) for these days was ∼72 sfu. In addition, the International Sunspot Number data of the National Oceanic and Atmospheric Administration (NOAA) available via https://www.ngdc.noaa.gov/stp/solar/solardataservices.html reveal that 76 days out of the 90 days are without sunspots. For example, between January 13 and 30 (Fig. 1b), when the SSW event was 38

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Table 1 Station codes, geographic and geomagnetic coordinates of the stations investigated. Stations

Country

Station Codes

Geographic Coordinates Latitude

BAKU ERDEMLI KUWAIT AL WAJH NAMAS JIZAN SHEB-MENSHEB ASMARA MEK’ELE NAZRET YAMOUSSOUKRO MT. BAKER ENTEBBE TANZANIA TUKUYU VACOAS METEO RICHARDSBAY SUTHERLAND

AZERBAIJAN TURKEY KUWAIT SAUDI ARABIA SAUDI ARABIA SAUDI ARABIA ERITREA ERITREA ETHIOPIA ETHIOPIA COTE D’IVOIRE UGANDA UGANDA TANZANIA TANZANIA MAURITIUS SOUTH AFRICA SOUTH AFRICA

0

BAKU MERS KUWT ALWJ NAMA JIZN SHEB ASMA DAKE NAZR YKRO BAKC EBBE TANZ TUKC VACS RBAY SUTH

40.37 N 36.560N 29.320N 26.450N 19.210N 16.690N 15.820N 15.330N 13.480N 8.540N 6.810N 0.360N 0.050N 8.570S 9.250S 20.290S 28.800S 32.390S

S (E ) =

Longitude

Latitude

0

0

49.81 E 34.250E 47.970E 36.370E 42.040E 42.100E 39.030E 38.930E 39.460E 39.260E 5.280W 29.900E 32.460E 39.290E 33.650E 57.490E 32.100E 20.660E

1 = 1 cos (Z )

35.56 N 30.710N 23.480N 18.710N 11.460N 9.020N 7.320N 6.780N 4.750N 0.290S 2.900S 9.240S 9.520S 18.550S 19.510S 30.320S 38.660S 41.090S

RE cos (E ) RE + hs

2

Longitude 121.870E 105.940E 119.690E 107.780E 114.050E 114.180E 110.580E 110.490E 110.050E 110.980E 77.260E 101.520E 104.120E 110.570E 104.820E 125.530E 97.950E 84.600E

0.5

(2)

Where RE is the Earth radius measured in kilometer (km) and hs is the height of the ionosphere from the Earth surface, which is approximately equal to 350 km. Regarding the VTEC accuracy, we employed records with a minimal elevation angle of 20° to eliminate ionospheric effects due to multipath. The VTEC data are further subjected to a two sigma (2σ) iteration to achieve a GPS positioning accuracy at the 95% confidence level, which is equivalent of 1.96 standard deviations (∼2σ). The resulting values were obtained by averaging all of the VTEC values over all pseudorandom numbers (PRNs) on a day. Equations (1) and (2) were implemented using the GPS TEC software by Seemala and Valladares (2011). Following this estimation, the GPS STEC data for the 18 stations under investigation were computed based on the averaged 1-min VTEC data, hereafter refer to as TEC. Hourly TEC values were estimated based on the averaged 1-min VTEC data and employed to investigate the day-to-day variability of EIA in each month, from January to March, 2009. Based on the method of Bolaji et al. (2016), the daily mean variations of zonal mean air temperature were grouped into six phases (Fig. 2a): (1) the SSW pre-condition phase (1–17 January), (2) the SSW ascending phase (18–21 January), (3) SSW peak phase (22–24 January), (4) the SSW descending phase (25–31 January), (5) the after SSW phase (1–25 February) and (6) the no SSW phase (26 February – 31 March). The SSW pre-condition phase is related to days before the stratospheric zonal mean air temperature increases by more than 25 K within a week, the SSW ascending phase depicts days when the stratospheric zonal mean air temperature started increasing more than 25 K within a week before it gets to its peak, the SSW peak phase indicates days when the stratospheric zonal mean air temperature increment is around the peak value, the SSW descending phase represents days when the stratospheric zonal mean air temperature is reducing after the SSW peak phase, the after SSW phase is when the stratospheric zonal mean air temperature recovered from the surge of the strongest temperature and the no SSW phase is when the stratospheric zonal mean air temperature is varying almost similar to the SSW pre-condition phase. Fig. 2b shows the zonal mean zonal wind, which is another stratospheric parameter indicating the severity of the SSW event. It reduces suddenly during the SSW ascending phase, gets to zero during the SSW peak phase, reverses to negative values during the SSW descending phase, starts recovering from negative values after the SSW phase and

Fig. 1b. Geomagnetic indices, Kp (bar plots in black) and solar flux, F10.7 (stem plots in red) for January–March, 2009.

significantly active, January 19 is the only day with an increased sunspot number of magnitude7. This indicates that the contribution of solar and geomagnetic activity to ionospheric perturbations during the 2009 SSW event is not significant. Bolaji et al. (2012, 2013) reported that the GPS receiver records contained slant total electron content (STEC), which are polluted with satellite differential delay, bS (satellite bias), receiver differential delay, bR (receiver bias), and the receiver inter-channel bias (bRX). The unbiased STEC records were obtained subtracting all of these biases from the records of biased STEC and then converted to the vertical total electron content (VTEC) as follows: VTEC = [STEC - (bS + bR + bRX)] / S(E)

Geomagnetic Coordinates

(1)

where STEC is the uncorrected slant TEC measured by the GPS receiver, S(E) is the obliquity factor with zenith angle, Z, at the ionospheric pierce point (IPP), E is the elevation angle of a satellite in degrees and VTEC is the vertical TEC at the IPP. The S(E) was defined by Mannuci et al. (1993) and Bolaji et al. (2012, 2013) as follows:

39

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Fig. 2a. Stratospheric zonal mean air temperature for January–March, 2009 (red double arrow indicating period of each phase).

recovers fully to positive values during the no SSW phase. The daily mean values of stratospheric zonal mean air temperature (Fig. 2a) and zonal mean zonal wind (Fig. 2b) at 60oN and 10 hPa (approximately 32 km) between January and March, 2009, were obtained based on the NCEP-NCAR (the National Centers for Atmospheric Prediction and the National Center for Atmospheric Research) reanalysis of Kalnay et al. (1996). The NCEP-NCAR reanalysis data are publicly available online via http://www.esrl.noaa.gov/psd/at the NOAA, Earth System Research Laboratory, Physical Sciences Division, Boulder, Colorado. The VTEC values for all of the days in each phase are averaged and six phases of SSW conditions are deduced for each of the station under investigation. To examine the underlying variations that are responsible prior to, during and after the SSW event, we employed the technique of Goncharenko et al. (2010b) to deduce the TEC difference by subtracting the daily mean TEC values during the SSW pre-condition phase from the daily TEC values beginning from the SSW ascending to the no-SSW phase. The estimated difference in TEC (TECDIFF) is mathematically

expressed as follows: TECDIFF = TECASD-NoSSW – TECMEAN-PreSSW,

(3)

where TECASD-NoSSW is the daily TEC from January 18 to March 31, 2009, and the TECMEAN-PreSSW is the TEC value averaged in the time period from 1 to 17 of January, 2009. 3. Results As described above in Section 2.0, TEC values estimated with the use of GPS STEC records are computed for the 18 African and Middle East GPS stations (Fig. 1) from January to March 2009. Local times (LT) corresponding to day-to-day latitudinal TEC variations are shown in Fig. 3a–c. The y-axis depicts the geomagnetic latitude of the stations on the right-hand side, the station codes on the left-hand side and both are plotted against the LT on the x-axis. The color bar beside the contour plots on the right hand-side shows the magnitudes of TEC at both

Fig. 2b. Stratospheric zonal mean zonal wind for January–March, 2009 (red double arrow indicating period of each phase). 40

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Fig. 3a. (i): Day-to day variability of EIA over Africa and Middle East from 01 to 10 January 2009. (ii): Day-to day variability of EIA over Africa and Middle East from 11 to 20 January 2009. (iii): Day-to day variability of EIA over Africa and Middle East from 21 to 31 January 2009.

Fig. 3a. (continued) 41

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Fig. 3a. (continued)

hemispheres. The white background regions in the contour plots indicate the missing data points. Fig. 4 shows the variability of TEC for each of the 2009 SSW phases in Africa and Middle East. Similar two-

dimensional plots in Fig. 5a–c depict the day-to-day TECDIFF. Fig. 6a shows TECDIFF during the SSW phases that unveil the physical underlying variations of latitudinal TEC from January 18 to March 31, 2009.

Fig. 3b. (i): Day-to day variability of EIA over Africa and Middle East from 1 to 9 February 2009. (ii): Day-to day variability of EIA over Africa and Middle East from 10 to 18 February 2009. (iii): Day-to day variability of EIA over Africa and Middle East from 19 to 28 February 2009. 42

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Fig. 3b. (continued)

Fig. 3b. (continued) 43

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Fig. 3c. (i): Day-to day variability of EIA over Africa and Middle East from 01 to 10 March 2009. (ii): Day-to day variability of EIA over Africa and Middle East from 11 to 20 March 2009. (iii): Day-to day variability of EIA over Africa and Middle East from 21 to 31 March 2009.

Fig. 3c. (continued) 44

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Fig. 3c. (continued)

Fig. 4. Variations of TEC on all hours across Africa and Middle East latitudes prior to, during and after the SSW event in 2009.

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Fig. 5a. (i): Day-to-day difference in TEC variations over Africa and Middle East from 17 to 23 January 2009.(ii): Day-to-day difference in TEC variations over Africa and Middle East from 24 to 31 January 2009.

Fig. 5a. (continued)

TEC at SHEB (∼8oN) are seen to dominate most of the days between January and March. It is very important to note that these TEC reductions at ∼8oN that produced these latitudinal twin EIA crests shown in Fig. 3a–c and Fig. 4 are smaller in magnitude compared to the other observed TEC values. In addition, the TEC value at ∼8° of the northern crests is associated with missing data points on almost all of the days. We therefore conclude that such persistent discontinuity in the TEC values at SHEB (∼8oN) inhibiting the formation of a single northern crest is not realistic for irradiance in the extreme ultra-violet (EUV) wavelength range and neutral atmospheric parameter variations. We finally concluded that since the same derivation method of calculating TEC values was employed for all the 18 GPS stations and all our TEC

Fig. 7a–j shows the line plots of TECDIFF during the SSW phases at the northern and southern crests. Fig. 8a–e and 8f-j depicts the EEJ strength and its corresponding underlying variations, respectively, during the SSW phases. The TEC is measured in TECU (1016 electron m−2), while the EEJ strength is measured in nano-Tesla (nT). 3.1. EIA as day-to-day latitudinal TEC variations between January and March, 2009 The expected single northern crests in the day-to-day latitudinal variations of TEC are rare (Fig. 3a–c). Instead, two northern crests (one at ASMA (∼7oN) and the other at JIZN (∼9oN)), separated by reduced 46

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Fig. 5b. (i): Day-to-day difference in TEC variations over Africa and Middle East from 1 to 9 February 2009.(ii): Day-to-day difference in TEC variations over Africa and Middle East from 10 to 18 February 2009. (iii): Day-to-day difference in TEC variations over Africa and Middle East from 19 to 28 February 2009.

Fig. 5b. (continued)

outputs are realistic with an exception at SHEB, then, the latitudinal twin EIA crests apparently seen in Fig. 3a–c and 4 are due to errors in data obtained from the GPS receiver at SHEB. In January (Fig. 3a i-iii) and February (Fig. 3b i-iii), most of the southern daily EIA crests straddling TANZ (∼19oS) and TUKC (∼20oS) were higher than the northern EIA crests straddling ∼7oN and 9oN. In March (Fig. 3c i-iii), most of the daily southern EIA crests did not straddle between ∼19oS and ∼20oS. The southern EIA crests in March straddling over YKRO, BAKC, EBBE and TANZ on most of the days were characterized by corresponding TEC values in the range from ∼18 to ∼31 TECU. An exception was found on March 13 at about 1200 LT with a moderate TEC values at ∼19oS (∼24 TECU) and ∼20oS (∼18 TECU)

that also extended into the southern middle latitude. The EIA trough, which is expected at ∼0° geomagnetic latitude (GL) according to the existing theory of the EIA (Appleton, 1946), was not visible between 6 and 11 January. However, such trough was visible between 8 and 11 February. It is obvious from our results as shown in Fig. 3a and b that during the days when the EIA trough was not visible at the geomagnetic equator, the transported plasma straddled the EIA trough. This indicates that the plasma obliterated the EIA trough and relocated it in the direction of transporting plasma (e.g. January 7). In regards to the characterization of EIA trough in March, it is interesting to report that there are no EIA troughs visible at 0° GL on any of the days. 47

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Fig. 5b. (continued)

Fig. 5c. (i)Day-to-day difference in TEC variations over Africa and Middle East from 1 to 10 March 2009. (ii): Day-to-day difference in TEC variations over Africa and Middle East from 11 to 20 March 2009.(iii): Day-to-day difference in TEC variations over Africa and Middle East from 21 to 31 March 2009.

At the southern middle latitude in January (Fig. 3a i-iii), higher TEC values at RBAY (∼39oS) were observed during all of the days compared with SUTH (∼41oS) and VACS (∼30oS). The highest TEC value of ∼27 TECU was seen at ∼39oS on January 3 at about 1400 LT. Similar to observations in January, the TEC values at RBAY in February (Fig. 3b iiii) during all of the investigated days are higher compared to the other southern middle latitude stations (VACS and SUTH). The highest TEC magnitude seen at RBAY on February 14 at about noon was ∼30 TECU. The highest TEC value in March at the southern middle latitude (∼39oS) was ∼27 TECU. From January to March, an increase in the TEC magnitudes in the range from ∼7 to ∼13 TECU were found in the northern middle latitude at MERS (∼31oN).

3.2. Latitudinal TEC variability during SSW phases Apart from the data error due to GPS receiver at SHEB that caused persistent discontinuity in the TEC value at ∼8oN (SHEB) and created the unrealistic latitudinal twin EIA crests during all of the SSW phases (Fig. 4), EIA crests during the SSW ascending and peak phase are characterized by pre-noon and post noon peaks. Regarding the comparison of the EIA crests magnitudes at both hemispheres shown in Fig. 4, we found that the magnitudes of the southern EIA crest during the SSW pre-condition is higher than that of the northern crest. In the case of ascending phase, the EIA crests at both magnitude are the same (∼24 TECU). However, the magnitude of the northern EIA crest at ∼7oN is higher than its southern EIA crest during the SSW peak, after the SSW and the no SSW phases. These differences in the magnitudes of 48

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Fig. 5c. (continued)

Fig. 5c. (continued)

TEC and latitudinal positions of EIA crests at both hemispheres indicate that the northern and southern EIA crests are strongly non-symmetrical. In the SSW descending phase, we found equal magnitude of the EIA crest (∼24 TECU) at ∼7oN and ∼20oS, which depicts symmetry in the TEC value at both EIA crests. Reductions in the TEC value were found straddling NAZR and YKRO (near to the geomagnetic equator) during the SSW ascending, peak and descending phases (hereafter referred to as the SSW-induced phase) compared with those ones during other SSW phases. Also, we observed that the northern and southern crest values during SSW peak phase are lower compared that of the SSW ascending and descending phases. In addition to obliteration of EIA trough, the highest EIA crest in the northern hemisphere straddling ∼7oN and ∼9oN during the no SSW phase persisted for a longer period compared to other SSW phases. In the southern middle latitude, we found that the TEC magnitude is highest at RBAY in all of the SSW phases compared to VACS and SUTH.

3.3. TECDIFF variations in the day-to-day and SSW phases from January 18 to March 31 Typically, the pattern of diurnal TEC variation is characterized by high and low TEC values during a 24-h period, with maximum values around noon hours. While the pattern of semidiurnal TEC variation is characterized by two maxima and one minimum TEC values during the 24-h period, a terdiurnal TEC variation typically experience three maxima and two minima during such window. Interestingly, we found these diurnal and sub-diurnal variations in our observed TECDIFF between January and March, 2009 (Fig. 5a–c). During this period, the occurrence of diurnal variations are higher than that of semidiurnal and terdiurnal variations. Of high interest are the semidiurnal and terdiurnal variations. The semi-diurnal variations are obvious in January around ∼7oN and ∼11oN on day 20, 24, 26, 27, 30 and 31 (Fig. 5a i-ii). For example on January 24, a semidiurnal signature is characterized by 49

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Fig. 6. Difference in TEC on all hours across Africa and Middle East latitudes prior to, during and after the SSW event in 2009.

Fig. 7. Underlying variations modulating the TEC during SSW phases at the northern and southern crests.

southern EIA crest (∼19oS - ∼20oS), we found semidiurnal signatures on February 8, 9, and 14 (Fig. 5b i-ii). The semidiurnal signature on February 8 (Fig. 5b i) at ∼9oN is characterized by maximal (∼8 TECU), minimal (∼-5 TECU) and moderate (∼2 TECU) TEC values around 0800 LT, 1500 LT and 1700 LT, respectively. On the same February 8 at ∼19oS, moderate (∼7 TECU), minimal (∼-10 TECU) and maximal

maximal (∼11 TECU), minimal (∼-2 TECU) and moderate (∼4 TECU) TEC values at ∼11oN around 1000 LT, 1500 LT and 1800 LT, respectively (Fig. 5a ii). For the same day, in the southern hemisphere, we found maximal (∼9 TECU), minimal (∼3 TECU) and moderate (∼4 TECU) TEC values at ∼20oS around 1100 LT, 1500 LT and 1800 LT, respectively. For the TEC straddling ∼5oN and ∼11oN and the 50

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Fig. 8. EEJ strength and its corresponding underlying variation during the 2009 SSW phases.

(∼8 TECU) TEC values were observed around 0900 LT, 1300 LT and 1600 LT, respectively. Also in March, we observe semidiurnal variation on day 4, 13, 21 and 22 at ∼20oS (Fig. 5c i-iii). For example, moderate (∼1.4 TECU), minimal (∼-11 TECU) and maximal (∼5 TECU) TEC values were observed on March 4 (Fig. 5c i) around 0700 LT, 1400 LT and 1700 LT, respectively. It is interesting to observe that the SSW phases (Fig. 6) also reveal the underlying variability patterns that were shown in Fig. 5a–c. For example, a conspicuous semidiurnal variation was observed straddling ∼19oS and ∼20oS in all the SSW phases, with an exception in the SSW peak phase. This semidiurnal signature seen during the SSW ascending phase is characterized by moderate (∼2.3 TECU), minimal (∼-4.6 TECU) and maximal (∼3.8 TECU) TEC values around 1100 LT, 1400 LT and 1700 LT, respectively. In the SSW descending phase, such signature is characterized by moderate (∼1.7 TECU), minimal (∼-6.5 TECU) and maximal (∼2.1 TECU) TEC values around1000 LT, 1400 LT and 1800 LT, respectively. After the SSW phase, this signature experienced moderate (∼1.7 TECU), minimal (∼-7.9 TECU) and maximal (∼2.2 TECU) TEC values around 0700 LT, 1400 LT and 1700 LT, respectively. In the northern hemisphere, we also found semidiurnal variations straddling ∼9oN and ∼11oN in all the SSW phases with exceptions during the SSW peak and the no SSW phases. Remarkably, the exception in the SSW peak phase unveils significant terdiurnal variations in the northern hemisphere. Such patterns are seen to straddle ∼5oN and ∼11oN, but with a moderate pattern in the southern hemisphere around EBBE (∼10oS). For example, a terdiurnal signature at ∼7oN was characterized by TEC values of ∼3.8 TECU, ∼ −2.1 TECU, ∼1 TECU, ∼ −1.2 TECU and ∼2.1 TECU around 0800 LT, 1100 LT, 1200 LT, 1300 LT and 1700 LT, respectively. To explain the underlying mechanisms that are responsible for the

observed semidiurnal and terdiurnal TEC signatures during the 2009 SSW phases, we plot these sub-diurnal variations in Fig. 7 for the specific northern and southern crest stations. With this in mind, we also plot in Fig. 8, the EEJ strength during these SSW episodes. By comparing Fig. 7 with Fig. 8, we found that the significant reduction in EEJ strength (Fig. 8b) was driven by a terdiurnal variation (Fig. 8 g) in the SSW phase. In this view for the SSW descending phase, the marked increase in EEJ strength with its associated counter electrojet, CEJ (Fig. 8c) was seen to be accompanied by a semidiurnal variation (Fig. 8h). These features will be discuss in details in section 4.2.2. 4. Discussion 4.1. Plasma transport and latitudinal extent of EIA crest On the electrodynamics of ionospheric plasma transport, it is worthy to mention that the EIA phenomenon is basically the vertical uplift of ionization density to higher latitudes of the hemispheres and the subsequent diffusion of these bands of plasma down magnetic field lines (Appleton, 1946; Moffet and Hanson, 1965). Such upwelling process is typically driven by the eastward dynamo electric fields generated by interaction of neutral winds in the lower thermosphere with the day time ionospheric E-layer. Trans-equatorial thermospheric wind, which is highest at the EIA trough (Raghavarao et al., 1998) could play an additional role in contributing to EIA asymmetries (Hanson and Moffett, 1966) and create different conditions in the two hemispheres. One of such conditions is that the downward diffusion of plasma along magnetic field lines by vertical E×B drift will be assisted by neutral wind blowing in the same direction of the plasma diffusion. In the other hemisphere, the plasma diffusion along magnetic field lines 51

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will be inhibited anytime the neutral wind is blowing against its direction (Anderson and Roble, 1981). Also important to the electrodynamics of ionospheric plasma transport is if more intense solar irradiance around the subsolar point is at one of the hemispheres, that hemisphere will be characterized by larger ionization density compared to the other hemisphere. Our results on most of the days between January 1 and February 15 (mid-February) revealed that the latitudinal extent of EIA crest is between ∼10oN and ∼20oS (Fig. 3a and b). The magnitudes of most of these EIA crests at ∼20oS are higher than those in the northern crests straddling ∼7oN and ∼9oN. These are not in agreement with the experimental works of Pedatella and Forbes (2010), Goncharenko et al. (2013), Paes et al. (2014) and simulation experiments by Pedatella and Liu (2013). In their works, they reported a higher magnitude of ionization density at the northern crest compared to that one at the southern crest. In addition, they found that the location of the northern EIA crest is at higher latitude compared to that of the southern EIA crest. They attributed their findings to strong seasonal trans-equatorial thermospheric wind, which is active in January and February, assisting the vertical E×B drift to significantly uplift ionospheric plasma equatorward during southern summer and poleward during northern winter. Further experimental observations of trans-equatorial thermospheric wind in the African low latitude by Mohamed Kaab et al. (2017) and Fasil Tesemaet al. (2017) confirmed that early evening meridional winds from January to February exhibit stronger poleward wind in winter periods compared with those ones during equinoctial months. By implication, if the strong trans-equatorial thermospheric winds are active, larger ionospheric plasma is expected at higher latitudes in the northern hemisphere compared to the southern hemisphere. However, it is easier to decipher from our results that the flow direction of the trans-equatorial thermospheric wind, which is caused by the interhemispheric wind flow from summer to winter, is not active on most of the days between January and mid-February (Fig. 3a and b). Rather, stronger southern EIA crests at higher latitudes than the northern crests straddling ∼7oN and ∼9oN are visible around ∼20oS. We therefore suggest that these EIA asymmetries in TEC distribution from January to mid-February are partly due to larger ionization density as a result of more intense solar radiation around the sub-solar point, which is located in the southern summer hemisphere. From February 16 to March 2 (Fig. 3bii-3ci), the trans-equatorial thermospheric wind is suspected to be slightly active and the sub-solar point location, which was formerly located in the southern summer hemisphere from January to mid-February appears to have shifted equator ward. Such sub-solar location of southern EIA crests was observed to be straddling ∼5oS and ∼15oS, particularly in March. Also, the transport of larger ionospheric plasma to the northern hemisphere and away from ∼20oS towards the geomagnetic equator is more evident in March. Based on these features, the trans-equatorial themospheric winds were active in March and contrast research efforts of Mohamed Kaab et al. (2017) and Fasil Tesema et al. (2017). Hence, the trans-equatorial thermospheric wind in March, which is weak in magnitude, is active with regards to the vertical E×B drift assisting to increase TEC values near northern geomagnetic equator and further away the latitudinal extent of ∼10oN. Also, significant increments in the TEC magnitudes at the low latitudes in the northern hemisphere have been attributed to closeness of the sub-solar point to the magnetic equator in equinoctial months (Wu et al., 2004; Rama Rao et al., 2006; Lee et al., 2010; and Bolaji et al., 2012). In effects, the slight active (16 February to 2 March) and active (3–31 March) seasonal trans-equatorial thermospheric wind leads to the transport of ionospheric plasma towards the geomagnetic equator from the southern hemisphere (Fig. 3b and c). This plasma transport into the northern hemisphere, which is significant in March, is responsible for the obliteration of EIA trough around 0oGL. This is why the obliteration of EIA trough is low, moderate and high in January, February and March, respectively.

4.2. EIA morphology and underlying variations during SSW phases 4.2.1. SSW ascending phase Despite the persistent discontinuity in TEC values at SHEB (∼8oN), which had been clarified in subsection 3.1, we found northern pre-noon and post noon EIA crests in the SSW ascending and peak phase. This feature is consistent with the pre-noon and post-noon crests that were previously reported by Venkatesh et al. (2015) and Bolaji et al. (2017), when there is no SSW. They attributed the cause of pre-noon EIA crests to stronger eastward daytime electric field near noon hours, while the post-noon EIA crests are explained in terms of the overwhelming influence of photoionization over the weak eastward electric field after noon hours. As mentioned in subsections 3.1 and 3.2, such northern pre-noon and post noon crests are seen in the SSW ascending and peak phase (Fig. 4). Contrary to the physical mechanisms that triggered these crests when there is no SSW, a careful observation based on Fig. 2a, reveals that as the stratospheric temperature is approaching its peak in the SSW ascending phase, semidiurnal variations straddling ∼5oN and ∼11oN are seen to control the northern EIA crests (Figs. 6 and 7a). This indicates that during the SSW ascending phase, a semidiurnal variation is responsible for modulating the expected single northern crest to the twin EIA crests. This result is in agreement with model simulations by Fuller-Rowell et al. (2010) and Wang et al. (2011) and experimental observations by Goncharenko et al. (2010a) and Paes et al. (2014). With emphasis on zonal wave number 2 (SW2), their studies further confirm that the semidiurnal westward propagating tidal wave is mainly responsible for modulating TEC values prior to the SSW peak phase. 4.2.2. SSW peak phase As the peak in stratospheric temperature is reached in the SSW peak phase (Fig. 2a), interesting underlying terdiurnal variations were straddling ∼5oN and ∼11oN (Figs. 6 and 7b). These northern terdiurnal signatures straddling DAKE, ASMA, SHEB and JIZN in the SSW peak phase (Fig. 7b) imply that terdiurnal tides exist in the low latitude ionosphere. This is the first experimental confirmation of model simulations done by Fuller-Rowell et al. (2010, 2011) and Wang et al. (2011) on 2009 SSW event. Their studies demonstrated that the terdiurnal westward propagating tidal wave with zonal wave number 3 (TW3) dominates the SW2 and the diurnal westward propagating tidal wave with zonal wave number 1 (DW1) in the SSW peak phase. Apart from these observed terdiurnal signatures, we do not have any proof to unveil that they are either migrating tides or not. Therefore, we will not refer to our observed diurnal, semidiurnal and terdiurnal signature as DW1, SW2 and TW3, in that order. Instead, each signature will simply be referred to as diurnal, semidiurnal and terdiurnal variation as far as this investigation is concerned. The first direct influence of the dominating terdiurnal variation in the SSW peak phase (Figs. 6 and 7b) is that it probably modulated the expected single northern crest to pre-noon and post noon crests (Fig. 4). Since our latitudinal TEC distributions (Fig. 6) were characterized by diurnal, semidiurnal and terdiurnal variations, it is obvious that they can interact with each other and all of these observed scenarios are possible. A similar nonlinear interaction of the migrating diurnal and semidiurnal tides, which serves as one of the possible platforms that might produce terdiurnal tides in the thermosphere had been earlier proposed by Glass and Fellous (1975), Manson and Meek (1986) and Teitelbaum et al. (1989). Also, in addition to the suggestion of Mayr et al. (1979) and the nonlinear wave-to-wave interaction theory of Pedlosky (1986), Fuller-Rowell et al. (2010, 2011) and Wang et al. (2011) have used models revealing a significant increase in the TW3 at the expense of SW2 and DW1 tidal waves during the period of the peak in stratospheric temperature, which coincides with our SSW peak phase (from 22 to 24 January). The results from their models demonstrated that the SW2 is mainly donating energy to TW3 during the SSW ascending phase. They also showed that the SSW peak phase is characterized by changes in TW3 and SW2 magnitudes, which significantly 52

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increase and decrease, respectively. Also, while the TW3 is increasing and the SW2 is decreasing, the change rate of the DW1 during the SSW peak phase was fluctuating around zero value indicating the possibility of wave-to-wave interactions between these three waves. Apart from generating pre-noon and post noon crests, the second likely implication of the dominating terdiurnal variation at the cost of semidiurnal variation subsequent decrease during the SSW peak phase is increase in the magnitude of the northern pre-noon and post noon crests straddling ∼5oN and ∼11oN when compared to that of a single crest in the southern hemisphere straddling ∼19oS and ∼20oS (Fig. 4). Experimental (Pedatella and Forbes, 2010; Goncharenko et al., 2013) and modeling results (Pedatella and Liu, 2013) have revealed similar higher northern crests compared to the southern crests during SSW events. Therefore, in addition to the physical mechanism responsible for plasma transport in the low latitude during the 2009 SSW event, we suspect that the background terdiurnal, semidiurnal and diurnal variations can be partly involved. We had earlier reported this during the SSW peak phase when the terdiurnal signature appears stronger around the northern crest (straddling ∼5oN and ∼11oN) compared to the semidiurnal signature around the southern crest (straddling ∼19oN and ∼20oN). There is also a trace of terdiurnal variation at EBBE (∼10oS, along the southern crest) possibly indicating that the reducing semidiurnal variation at the southern crest (Fig. 6) is likely donating energy to increase the terdiurnal signature along the southern crest. All of these scenarios during the SSW peak phase can be responsible for changing a higher southern crest during the SSW pre-condition to a higher northern crest during SSW peak phase. It can also be responsible for changing same value of crests seen during ascending phases to a higher northern crest during SSW peak phase (Fig. 4). Although, the diurnal variation is not clearer during the SSW peak phase (Fig. 6), this is an experimental evidence that the interaction of semidiurnal and diurnal variations can trigger significant terdiurnal variation the lower atmosphere. A significant increase in the TW3 tide magnitude can be associated with the vertical E×B drift and may be related to the plasma transport in the ionosphere at low latitudes according to Forbes (1995). This association can possibly take place to alter the background upper atmosphere and to subsequently modulate the plasma density. Thus, a terdiurnal signature should also be strong near the EIA crest, given that both the semidiurnal and diurnal signatures are characterized by highest TEC values at either of the northern or southern EIA crests. Our observations in Figs. 6 and 7 panel a-j obviously support this hypothesis. This is because a strong terdiurnal, stronger semidiurnal and the strongest diurnal tidal variations are conspicuous in our results near northern crest during the SSW peak, the descending and the no SSW phases, respectively. The third and last implication of the northern terdiurnal variation during the SSW peak phase (Figs. 6a and 7b) is reduction in the overall TEC value at the hemispheric crests (Fig. 4) when compared to those ones during other SSW phases. We interpreted these terdiurnal variations (Fig. 7b) to be related to upward propagating tides characterized by a third harmonic component in their wind composition (TW3 tide). Also, it is important to recall that being a third harmonic in the wind composition, the TW3 tide was often found to be of less magnitude than those ones of simultaneously measured DW1 and SW2 tides. Now, imagine a terdiurnal signature having a higher magnitude than that of semidiurnal signature (Figs. 6 and 7b). Then, one should expect a significant reduction in the TEC magnitude at the hemispheric crests (Fig. 4) during the SSW peak phase compared to other SSW phases. From a global point of view, Bolaji et al. (2016) found similar hemispheric reductions in the solar quiet (Sq ) magnitude during the 2009 SSW peak phase, which they attributed to decrease in the upward propagating tides from the middle atmosphere. For the purpose of explaining the sub-diurnal variations from an EEJ perspective, we revisited the horizontal magnetic field intensity data over Africa in January 2009. We computed the EEJ strength (Fig. 8a–e) by pairing Addis Ababa (AAB, an EEJ site) with Nairobi (NAB, an off-EEJ site). Note that

the EEJ is a proxy of the zonal electric field at the E-layer. We found that the hemispheric reductions in the Sq magnitude during the SSW peak phase (Bolaji et al., 2016) are also repeatable in the EEJ strength (Fig. 8b). In addition, the corresponding underlying background variation in Fig. 8g during the SSW peak phase revealed a terdiurnal variation. This is seen to be largely responsible for the reductions in EEJ strength. Using the circulation changes that were generated from the coupled Thermosphere Ionosphere Mesosphere Electrodynamics-General Circulation Model/Community Climate Model-3(TIME-GCM/CCM3) at Mesosphere Lower Thermosphere Ionosphere (MLTI) heights, Liu and Roble (2002), Sridharan et al. (2009) and Meyer (1999) explained how the stratospheric zonal mean zonal wind could modulate the nonlinear interaction of planetary waves (quasi 16-day wave) and the prevailing tidal wind components (diurnal, semidiurnal, terdiurnal and quarter diurnal) during SSW events. This mechanism, which generates a family of secondary waves, also modulates tidal amplitudes at the periods of a planetary wave (Beard et al., 1999; Mitchell et al., 1999). These modulated tidal amplitudes enhanced Sq /EEJ strength and decreased Sq /EEJ strength during the 2006 SSW event, when the phase of the quasi 16-day wave is positive and negative, respectively (Vineeth et al., 2007). While the origin of this quasi 16-day planetary wave (PW) was previously found over high and middle latitude ionosphere (Pancheva et al., 2009), recent investigations on the 2009 SSW event by Goncharenko et al. (2010), Patra et al. (2014), Yadav et al.(2017) have also confirmed significant increase in the quasi 16-day PW activity over the low latitude ionosphere. In addition, increase in lunar semidiurnal wave activity is also an offshoot of the 2009 SSW event (Xiong et al., 2013; Pedatella et al., 2014; Yadav et al., 2017). However, our focus, as far as this 2009 SSW investigation is concern is not on the perturbations of the new and full moon phases. When the phase of quasi 16-day wave was negative in the SSW peak phase, it is seen that the aforementioned mechanism is supported by the significant reduction in EEJ strength (Fig. 8b) and hemispheric Sq values (Bolaji et al., 2016). This is because, the reduction in upward propagating tide that is responsible for reducing the Sq /EEJ strength can be easily traced to the nonlinear interaction of quasi 16 day wave. This wave is seen to be visible in the EEJ strength (Fig. 8g), where it is characterized with negative phase and thermospheric terdiurnal variation. Now, on linking the hemispheric reduction in our present investigation of GPS TEC with that one seen in Sq current (Bolaji et al., 2016), EEJ strength (Fig. 8 panel b) and NmF2 (Pedatella et al., 2016), we strongly relied on the fact that all of these reductions occurred during 2009 SSW peak phase (January 22-24). Another fundamental fact concerning the 2009 SSW is that while the thermospheric tidal components are interacting nonlinearly with the quasi 16-day wave when its phase was negative, the generated terdiurnal variation (reduced upward propagating tide), which is visible at the E-region reasonably coupled to the F-region and its top-side through the EIA phenomenon (already discussed in subsection 4.1). Based on these facts, similar reductions seen in our GPS TEC at both EIA crests during the SSW peak phase (Fig. 4) further confirm that the effect of strong forcing due to the major 2009 SSW event at ∼100 km (E-layer) and ∼500 km (F-layer) is evident at the F-layer top-side of the ionosphere above Africa and the Middle East. Contrary to our observed terdiurnal variation (Figs. 7b and 8), which is responsible for reducing hemispheric crests (Fig. 4) and EEJ strength (Fig. 8b), respectively, an enhanced SW2 was suggested as the tidal component that modulates the F-region during SSW peak phase (Yamazaki and Richmond, 2013). This enhanced SW2 tide was also established by Pedatella et al. (2016) in their TIE-GCM model simulations as the leading tidal component responsible for reducing the NmF2 during the SSW peak phase. These scenarios of SSW peak phase imply that the terdiurnal signature at the E-region was discontinued at the Fregion (Pedatella et al., 2016), but get imprinted on the TEC magnitudes at the top side of the F-region. This discontinuity in the terdiurnal 53

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signature at the F-region when satellite data and models were used will be investigated as a future topic. In summary, we posit that while the semidiurnal variation is responsible for the NmF2 reduction at the Flayer during the SSW peak phase, the terdiurnal variation is responsible for the reduction in EEJ strength at the E-region and GPS TEC at the Fregion topside.

4.2.5. Modulation of middle latitude ionosphere during SSW phases Similar to low latitudes, the latitudinal TEC variability at the middle latitudes indicates an increment in TEC magnitude at MERS (∼31oN) and RBAY (∼39oS) during the SSW descending phase compared to other SSW phases (Fig. 4). On comparing Figs. 4 and 6a in the SSW descending phase, it is seen that such enhanced TEC value at ∼31oN and ∼39oS is due to semidiurnal and diurnal signatures, respectively. This middle-latitude semidiurnal signature around ∼31oN is in good agreement with the research efforts of Chau et al. (2010), Goncharenko et al. (2010b), Fuller-Rowell et al. (2010), Liu et al. (2010), Wang et al. (2011), Jin et al. (2012), Pedatella and Liu (2013) and Paes et al. (2014). These indicate that SW2 is responsible for the increase in TEC values during the SSW descending phase. However, at RBAY (Fig. 6), we observed that a significant underlying diurnal variation in the southern middle latitude was responsible for significant increase in TEC magnitude during SSW ascending phase (Fig. 4) when compared to other SSW induced phases. The reduction found in TEC value at middle latitudes (∼31oN) for SSW ascending and peak phase is due to the aforementioned terdiurnal variations (Fig. 6). At ∼39oS, a weak semidiurnal variation (Fig. 6) was responsible for reduction in TEC values during SSW peak phase (Fig. 4), and the 6-h (quarter diurnal) variation (Fig. 6) was responsible for significant reduction in TEC value during SSW ascending phase at RBAY.

4.2.3. SSW descending phase As the zonal mean zonal wind was westward during SSW descending phase (Fig. 2b), the semidiurnal variation dominated at almost all latitudes and got enhanced at the hemispheric crests (Fig. 6). One of the implications is that the northern crest during SSW descending phase is higher than that one of the SSW pre-condition, ascending and peak phase (Fig. 4), while it is only higher than that of the SSW peak phase in the southern hemisphere. These semidiurnal signatures modulating the SSW descending phase shared similar characteristics with the results of investigations made by Chau et al. (2010), Goncharenko et al. (2010b), Paes et al. (2014) and Laskar et al. (2014) and the modeling results of Fuller-Rowell et al. (2010), Wang et al. (2011) and Pedatella and Liu (2013). They reported that the SW2 is the dominating tidal component that can be responsible for an increment in TEC value during SSW descending phase. Similar semidiurnal signature was revealed during the SSW descending phase in Fig. 8h as the dominating atmospheric tide responsible for increment in the EEJ strength (Fig. 8c). Therefore, our results indicate that the semidiurnal variation modulating the EEJ strength at the E-layer during the SSW descending phase was coupled to the top side of F-layer.

4.3. Modulation of pre-reversal enhancement in TEC during different SSW phases

4.2.4. After the SSW and no SSW phase After the SSW event, when the zonal mean zonal wind is recovering from negative values (Fig. 2b), we suspect an increase in the effect of trans-equatorial thermospheric winds (Batista et al., 2011; de Paula et al., 2015; Bolaji et al., 2017) that transport plasma poleward through equator ward at low latitudes. This feature is seen in Fig. 4 when higher northern crest compared to the southern crest was sustained after the SSW phase. Also, at almost all geomagnetic latitudes (Fig. 6), TECDIFF after the SSW phase was characterized by semidiurnal variations. This semidiurnal variation was also seen in the EEJ strength (Fig. 8i), depicting that it can possibly originates from the E-region. For the no SSW phase, when the stratospheric zonal mean zonal wind recovers fully from negative values, indicating that the 2009 SSW event had subsided (Fig. 2b), we noticed that significant increments in TEC magnitudes at the northern hemisphere were sustained compared to the southern hemisphere. Other obvious features are moderate TEC values extending to ∼19oN, the EIA crest straddling ∼7oN and ∼9oN over longer periods (1000 LT – 1500 LT), obliteration of the EIA trough and the southern EIA crest at ∼20oS. Focusing on Fig. 6, the response of the background variations to these features revealed that from ∼15oS to 0° GL, a significant increase in TEC magnitudes characterized by diurnal variations is more evident between NAZR (∼0.30oS) and EBBE (∼10oS) during evening hours. These diurnal variations extend into the afternoon hours between ∼0.30oS and ∼9oN and stabilize near noon and afternoon hours between NAMA (∼11oN) and KUWT (∼23oN). The origin of this increase in TEC at the northern EIA crest after the SSW and no SSW phases that triggers all of these consequences can be traced to seasonal and semiannual effects, possibly not related to the SSW event. The seasonal effect may be due to the active seasonal transequatorial neutral wind blowing from the southern (warmer) summer hemisphere to the northern (less warm) winter hemisphere (Mohamed Kaab et al., 2017; Fasil Tesema et al., 2017). It could also be an offshoot of the sub-solar point around the northern equator in equinoctial months (Wu et al., 2004; Rama Rao et al., 2006; Lee et al., 2010; Bolaji et al., 2012). In addition, these hemispheric asymmetries after SSW and no SSW phases may be due to higher seasonal [O]/[N2] ratio in the northern hemisphere (Rishbeth et al., 2000) and the semiannual TEC variation (Bolaji et al., 2012).

Fig. 4 shows a consistent pre-reversal enhancement (PRE) at YKRO (EIA trough, ∼3oS) compared to other equatorial station (NAZR). We posit that the occurrence of PRE at YKRO at the expense of NAZR could be due to longitudinal difference. This is because the station YKRO is not only close to the geomagnetic equator, but it is also at the farthest west of the stations within Africa that were examined in this work (Fig. 1a). Typically, the PRE is visible at low latitude when the reversal of eastward to westward electric fields at about sunset is accompanied by a strengthened eastward electric field. The PRE, which is low during the SSW peak phase, moderate during the SSW descending phase and high during the SSW ascending phase is an indicator of scintillation, spread-F and plasma bubble at the low latitudes (Basu et al., 1988; Whalen, 2004; Gupta et al., 2002; Retterer and Gentile, 2009; Bolaji et al., 2012). As a topic of the future, we will investigate in detail, the characteristics of the PRE at the EIA trough during SSW. 5. Conclusions In this work, we have investigated the EIA morphology in Africa and Middle East during the 2009 SSW event based on TEC data obtained from 18 GPS receivers. On most days in January, the southern crests are higher than those of the northern crests. As a result of consistent plasma transport into the northern hemisphere from the southern hemisphere, the latitudinal day-to-day EIA trough at 0° GL in March was obliterated. For the first time, we observed experimental terdiurnal variations in ionospheric TEC, which originated from the E-region during the SSW peak phase and modulates the expected single northern crest to prenoon and post noon crests. The terdiurnal signature was also responsible for transport of more plasma to the northern hemisphere during the SSW peak phase. Contrary to previous report by Pedatella et al. (2016) that the reductions in ionospheric electron densities at F2region are due to semidiurnal tides resulting from the SSW peak phase, our results show that a terdiurnal variation was responsible for reducing the EEJ strength and TEC at the E-region and F2-region's topside, respectively. At the southern middle latitudes, an underlying diurnal variation was responsible for the TEC increment during the SSW descending phase. 54

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Conflicts of interest

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The author(s) declare(s) that they have no competing interests. Authors’ contributions Author with the initials OSB developed the data analysis, methodology, interpreted the results and wrote the manuscript. EOO, OPO, JOA, ABR and EOF participated and contributed to the writing of the manuscript. OEJ, AF, ROK and OOO participated in the data analysis and plots. PHD provided technical support for data analysis. EA provided GPS data and plots. Acknowledgements The total electron content data from the Global Positioning System (GPS) receivers used in this article are freely available online in the University-Governed Consortium (UNAVCO) website via http://www. unavco.org/data/gps-gnss/data-access-methods/dai2/app/dai2.html. The availability of these materials is based on GPS data services provided by the UNAVCO facility with support from the National Science Foundation and the National Aeronautics and Space Administration under the NSF Cooperative Agreement Number EAR-0735156. One of the authors, BOS acknowledges and appreciates the support of all the hosts of GPS receivers in Africa for keeping records of these data and making them available for this study. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jastp.2019.01.006. References Anderson, Roble, 1981. Neutral wind effects on the equatorial F-region ionosphere. J. Atmos. Sol. Terr. Phys. 43, 835–843. https://doi.org/10.1016/0021-9169(81) 90061-1. Appleton, E.V., 1946. Two anomalies in the ionosphere. Nature 157, 691–693. https:// doi.org/10.1038/157691a0. Basu, S., Mackrnzie, E.M., Basu, S., 1988. Ionospheric constraints on VHF/UHF communication links during solar maximum and minimum periods. Radio Sci. 23, 363–378. https://doi.org/10.1029/RS023i003p00363. Batista, I.S., Diogo, E.M., Souza, J.R., Abdu, M.A., Bailey, G.J., 2011. Equatorial Ionization Anomaly: the Role of Thermospheric Wind and the Effect of the Geomagnetic Field Secular Variation, Aeronomy of the Earth's Atmosphere and Ionosphere. Springer Dordrecht Heidelberg, London New York, pp. 317–328. https:// doi.org/10.1007/978-94-007-0326-1_23. Beard, A.G., et al., 1999. Non-linear interactions between tides and planetary waves resulting in periodic tidal variability. J. Atmos. Sol. Terr. Phys. 61, 363–376. Bolaji, O.S., Adeniyi, J.O., Radicella, S.M., Doherty, P.H., 2012. Variability of total electron content over an equatorial West African station during low solar activity. Radio Sci. 47, RS1001. https://doi.org/10.1029/2011RS004812. Bolaji, O.S., Adeniyi, J.O., Adimula, I.A., Radicella, S.M., Doherty, P.H., 2013. Total electron content and magnetic field intensity over Ilorin, Nigeria. J. Atmos. Sol. Terr. Phys. 98, 1–11. https://doi.org/10.1016/j.jastp.2013.02.011. Bolaji, O.S., Oyeyemi, E.O., Owolabi, O.P., Yamazaki, Y., Rabiu, A.B., Okoh, D., Fujimoto, A., Amory-Mazaudier, C., Yoshikawa, A., 2016. Solar quiet currents responses in the African sector due to a 2009 sudden stratospheric warming. J. Geophys. Res. Space Phys. 121. https://doi.org/10.1002/2016JA022857. Bolaji, O., Owolabi, O., Falayi, E., Jimoh, E., Kotoye, A., Odeyemi, O., Rabiu, B., Doherty, P., Yizengaw, E., Yamazaki, Y., Adeniyi, J., Kaka, R., Onanuga, K., 2017. Observations of equatorial ionization anomaly over Africa and Middle East during a year of deep minimum. Ann. Geophys. 35, 123–132. https://doi.org/10.5194/angeo35-123-2017. Chau, J.L., Aponte, N.A., Cabassa, E., Sulzer, M.P., Goncharenko, L.P., Gonzalez, S.A., 2010. Quiet time ionospheric variability over Arecibo during sudden stratospheric warming events. J. Geophys. Res. 115https://doi.org/10.1029/2010JA015378. A00G06. de Jesus, R., Batista, I.S., de Abreu, A.J., Fagundes, P.R., Venkatesh, K., Denardini, C.M., 2017. Observed effects in the equatorial andlow-latitude ionosphere in the South American and African sectors during the 2012 minor sudden stratospheric warming. J. Atmos. Sol. Terr. Phys. 157–158, 78–89. https://doi.org/10.1016/j.jastp.2017.04. 003. de Paula, E.R., Jonah, O.F., Moraes, A.O., Kherani, E.A., Fejer, B.G., Abdu, M.A., Muella, M.T.A.H., Batista, I.S., Dutra, S.L.G., Paes, R.R., 2015. Low-latitude scintillation

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