Ionospheric climatology at Africa EIA trough stations during descending phase of sunspot cycle 22

Journal of Atmospheric and Solar-Terrestrial Physics 172 (2018) 83–99

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Ionospheric climatology at Africa EIA trough stations during descending phase of sunspot cycle 22 B.O. Adebesin a, *, A.B. Rabiu b, O.S. Bolaji c, d, J.O. Adeniyi a, C. Amory-Mazaudier e, f a Space Weather Group, Environment and Technology Research Cluster, Department of Physical Sciences, Landmark University, P.M.B 1001, Omu-Aran, Kwara State, Nigeria b Centre for Atmospheric Research, National Space Research and Development Agency, Anyigba, Nigeria c Department of Physics, University of Lagos, Nigeria d Department of Physics, University of Tasmania, Australia e Sorbonne Universites, UPMC Univ. Paris 06, UMR 7648, Laboratoire de Physique des Plasmas, F-75005, Paris, France f T/ICT4D, ICTP, International Centre for Theoretical Physics, Strada Costiera, 11, I - 34151, Trieste, Italy

A R T I C L E I N F O

A B S T R A C T

Keywords: Equatorial ionosphere Vertical plasma drifts Sunspot cycle Electron density Fountain effect

The African equatorial ionospheric climatology during the descending phase of sunspot-cycle 22 (spanning 1992–1996) was investigated using 3 ionosondes located at Dakar (14.70 N, 342.60 E), Ouagadougou (12.420 N, 358.60 E), and Korhogo (9.510 N, 354.40 E). The variations in the virtual height of the F-layer (h’F), maximum electron density (NmF2), vertical plasma drift (Vp) and zonal electric field (Ey) were presented. Significant decrease in the NmF2 amplitude compared to h’F in all of the stations during the descending period is obvious. While NmF2 magnitude maximizes/minimizes during the E-seasons/J-season, h’F attained highest/lowest altitude in J-season/D-season for all stations. D-season anomaly was evident in NmF2 at all stations. For any season, the intensity (Ibt) of NmF2 noon-bite-out is highest at Dakar owning to fountain effect and maximizes in March-E season. Stations across the EIA trough show nearly coherence ionospheric climatology characteristics whose difference is of latitudinal origin. Hemispheric dependence in NmF2 is obvious, with difference more significant during high-solar activity and closes with decreasing solar activity. The variability in the plasma drift during the entire phase is suggested to emanate from solar flux variations, and additionally from enhanced leakage of electric fields from high-to low-latitudes. Existing African regional model of evening/nightttime pre-reversal plasma drift/ sunspot number (PREpeak/R) relationship compares well with experimental observations at all stations with slight over-estimation. The correlation/root-mean-square-deviation (RMSdev) pair between the model and observed Vp during the descending phase recorded 94.9%/0.756, 92.4%/1.526, and 79.1%/3.612 at Korhogo, Ouagadougou and Dakar respectively. The Ey/h’F and Ey/NmF2 relationships suggest that zonal electric field is more active in the lifting of h’F and suppression of NmF2 during high- and moderate-solar activities when compared with lowsolar activity. This is the first work to show higher bite-out at the equatorial northern-station (Dakar) than southern-station (Korhogo) using ionosonde data.

1. Introduction 1.1. Background The F-region is one of the most important regions in the ionosphere for long distance high frequency communication. As a result of this advantage, it is necessary to have a better understanding of the electrodynamics regarding the ionospheric F-region. Some of the possible causes of variation in the ionospheric F-layer include (i) electrodynamics (i.e.

fountain effect at low latitudes, penetration of magnetospheric electric fields, plasma convection), (ii) solar ionizing variation (i.e. seasonal variation of Sun's activities, solar cycle variations), (iii) solar wind and geomagnetic activity (i.e. manifestation of geomagnetic storms, particle precipitation and Joule heating), and (iv) neutral atmosphere (i.e. lower atmosphere weather coupled through mesopause, gravity waves) (Rishbeth and Mendillo, 2001). All of these have significant effect on the F-region vertical plasma drift, which is a very important ionospheric tool for investigating ionospheric electrodynamics in the equatorial and low

* Corresponding author. E-mail addresses: [email protected], [email protected] (B.O. Adebesin). https://doi.org/10.1016/j.jastp.2018.03.009 Received 24 October 2017; Received in revised form 23 January 2018; Accepted 15 March 2018 Available online 16 March 2018 1364-6826/© 2018 Elsevier Ltd. All rights reserved.

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Journal of Atmospheric and Solar-Terrestrial Physics 172 (2018) 83–99

enhancement (PRE) during the equinox compared with the solstice season. Adebesin et al., (2013a and 2013b) used digisonde data for 2010, a year of low solar activity to obtain Vp over Ilorin (8.500 N, 4.680 E; dip lat. 2.95
). The work of Adebesin et al., (2013a) is the first to report vertical plasma drift in Ilorin. They reported higher magnitudes in peak PRE during equinox (18.1 ms-1) than in solstice (14.7 ms-1). In addition, they reported 4.2, 9.0, 10.7, and 13.3 ms-1 regarding the magnitudes of the downward peak reversal in June solstice, March equinox, September equinox, and December solstice, respectively. Adebesin et al., (2013b) compared the nighttime Vp over Ilorin with that of the Incoherent Scatter Radar (ISR) observation at Jicamarca. They reported higher PRE magnitude during equinoctial months in Jicamarca, and during December solstice and equinoxes in Ilorin. Adeniyi et al., (2014a) used digisonde inferred plasma drift to quantify the equatorial electrojet current over Ilorin ionosphere, the first of such studies in the African sector. They found higher daytime (06:00–16:00 LT) correlation between the electrojet current strength and Vp. They also observed good agreement between Vp and a magnetic proxy parameter described by E ¼ [d(ΔH)/dt]max in the morning hours, indicating the east-west electric field in the EEJ. As far as modelling efforts in comparison with the observations of vertical plasma drift is concern in the equatorial African sector, Adebesin et al., (2015a) used the longest time-span (1966–1998 that covered three sunspot cycles 20, 21, and 22) of plasma drift at Ouagadougou (geomag. Lat. 0.59) to deduce a regional empirical relationship/model between the vertical drift velocity and the sunspot number (R). This regional empirical relationship/model between Vp and R as revealed by the works of Adebesin et al., (2015a) showed stronger potential of deducing Vp magnitude at only evening/nighhtime period in the equatorial ionization anomaly (EIA) trough of African sector. This is possible because the corresponding R value is archived at www.sidc.be/silso/datafiles. Also, both the NmF2 and h’F data employed alongside Vp are used to describe both the qualitative and quantitative variability of the ionospheric climatology at the three stations in the present study. The F-layer virtual (h’F) or real (hmF2) height had been found to control the inhibition/generation of equatorial plasma instability at the bottomside region (Adebesin et al., 2015a). In the same way, NmF2 variability can be affected by temperature and composition changes, direct dynamic forcing, and E-region dynamo electric fields that produces F-region vertical drifts (Mendillo et al., 2002).

latitude region. For example, the F-region vertical plasma drift could change the ionospheric height (Scherliess and Fejer, 1999; Sreeja et al., 2009; Adebesin et al., 2015a). Prabhakaran Nayar and Sreehari (2004) reported that vertical plasma drift at evening times in the equatorial F-region is majorly controlled by the zonal electric field. They further suggested that the zonal electric field, which was partly generated from the vertical polarization field arising from the enhanced thermospheric winds could couple the E-region conductivity. At the geomagnetic equator, the magnetic and electric fields are perpendicular and their influence on ionospheric plasma is the fountain effect, a phenomenon by which ionospheric plasma is lifted to higher altitudes. Relevant to the present work are the first two causes – electrodynamics and solar cycle variations. Consequently, the descending phase of sunspot cycle 22 will be investigated with regards to some F-layer ionospheric measured parameters over three dip-equatorial stations in the African sector (Fig. 1). The ionospheric parameters include the Flayer virtual height or height of reflection (h’F), the maximum plasma (electron) density (NmF2), the inferred vertical plasma drift (Vp), and the estimated zonal electric field (Ey). The study is necessitated because regional F-region equatorial Vp model had been developed and implemented in the Brazilian, Peruvian and Indian sectors (Scherliess and Fejer, 1999; Fejer et al., 2008a; Luhr et al., 2008; Kil et al., 2009). However, little information on plasma drift observations had been documented in the African sector. This is due to non-availability of adequate data and modelling techniques. Some investigations carried out in the equatorial region over the African sector regarding vertical plasma drift include the works of Oyekola and Oluwafemi (2007), Oyekola and Kolawole (2010), Adebesin et al., (2013a; b), and Adeniyi et al., (2014a). Oyekola and Oluwafemi (2007) had investigated Vp using ionosonde data from Ibadan (7.4
N, 3.9
E; dip 6
S) and reported daytime/nighttime saturation threshold magnitude for certain level of F10.7 solar radio flux index. They obtained absolute mean value of 15 ms-1/10 ms-1 respectively for quiet/disturbed magnetic activity conditions. Oyekola and Kolawole (2010) obtained Vp from Ouagadougou (12.40 N, 358.50 E, dip angle 5.90 N) ionosonde data covering the year 1989. They compared their results with IRI-2007 model and found that their observed magnitudes are over estimated by the model. They further reported higher magnitude in the pre-reversal

1.2. Present work This work is aimed at (i) evaluating the variations of NmF2, h’F, and Vp during the descending phase of sunspot cycle 22; (ii) investigating the variability of the ionospheric parameters across the three stations under study; (iii) validating the response of vertical plasma drift to the variability in the international sunspot number, especially during PRE period, over each station, and comparing with the evening/nighttime regional drift model developed by Adebesin et al., (2015a); (iv) observing the response of the zonal electric field relative to the PREpeak period at all stations, (v) making a comparison between the PREpeak magnitude in this work and that of other previous works in Africa and other sectors of the world, and (vi) discussing the patterns of behaviour of ionospheric variability. All of these ionospheric parameters are analysed diurnally, seasonally, annually, and sunspot-cyclic at the three stations. This is because local time, seasons, sunspot cycle, and geographical locations are vital to ionospheric variation owing to changes in the components of solar radiation and other chemical processes (Sardar et al., 2012). To the best of our knowledge, this is the first work that would consider such investigation during a descending phase and at three different equatorial stations in the African sector. To this end, section 2 describes the dataset and method of presentation. The results obtained and discussion were highlighted in section 3. Section 4 presents the response of the annual zonal electric field at the three stations, as well as the global representation of EIA. The vertical drift PREpeak/sunspot number relationship

Fig. 1. Map showing the selected ionospheric stations in the African sector. The near-horizontal black line is the magnetic equator while the pair of nearhorizontal red lines are the magnetic latitude at 20
north and south. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 84

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Journal of Atmospheric and Solar-Terrestrial Physics 172 (2018) 83–99

was emphasized and verified for validation in section 5. Section 6 summarises the major findings.

where do e2 represents the plasma diffusion coefficient and increases with increase in the normalized height z: The do is a constant and Iis the geomagnetic dip. According to Iheonu and Oyekola (2006), D is restrained by the almost horizontal geomagnetic fields at equatorial latitudes. Therefore, dN=dz will approximately be zero. The value of sin2 I

2. Data and processing The study involves three ionosonde stations located at the magnetic equator of Africa. Data availability informs the choice of these stations, and their geomagnetic/geographic coordinates were depicted in Table 1. The three stations are located within the equatorial electrojet belt as revealed by their geomagnetic coordinates (see Fig. 1). The IPS-42 ionosondes, from where the data used in this work are archived were installed during the International Equatorial Electrojet Year (IEEY). The data were provided by the Ecole Nationale Superieure de Telecommunications de Bretagne. Study period spans 1992–1996, marking the descending phase of sunspot cycle 22. The respective annual international sunspot number (R) for 1992, 1993, 1994, 1995, and 1996 are 133.0, 76.1, 44.9, 25.1, and 11.6. The measured ionospheric parameters are the F-layer virtual height (h’F), the derived maximum electron density (NmF2), the inferred vertical plasma drift (Vp), and the estimated zonal electric field (Ey). The NmF2 was obtained from the F2-layer critical frequency (foF2) using the relation: foF2ðHzÞ ¼ 9:0 

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi NmF2ðm3 Þ

is generally around 0.0078 at equatorial region. Hence, the input of diffusion effect at equatorial latitude in the daytime is assumed small. Ignoring the D term in equation (3), it reduces to dN ¼ q  LðNÞ  M dt

The implication of equation (5) is that around the geomagnetic equator, contribution from diffusion term is apparently low. Hence, it will be regarded small in the treatment of vertical drift obtained using ground-based ionosonde measurements. In the same vein, the importance of neutral winds is less observed until around the evening-time PRE period. It therefore follows that starting from early daytime period to late evening hours, the dN relation in equation (5) is somewhat small and can dt be discarded; just as the production term qis neglected. Hence, these is what causes the daytime limitation to the accuracy of vertical drift obtained from ionosonde inferred measurements. The nighttime ionospheric variations is controlled by two major processes – the ionospheric electrodynamics and the recombination processes (Chen et al., 2008). However, the former of the two processes according to the authors is being driven by (i) the actions of neutral winds and electric fields which empowers the ion drift, (ii) the influx and outflow of field-aligned plasma due to the changing height of the peak density, (iii) diffusion processes, and (iv) thermal expansion and contraction. Quantity M which also represents the transport effects due to electric fields and thermospheric meridional neutral winds is given by:

(1)

The local time (LT) concept is used throughout the study. Due to the fact that there is no direct measurement of vertical drift (Vp) in Africa, we deduced the inferred Vp using the relation in equation (2) Vp ¼ dðh'FÞ=dt

(2)

The Vp data inferred for both DAR and KOR are new, while that of OUA had earlier been presented in part by Adebesin et al., (2015a) covering 32 years (1966–1998), being the longest time-span of Vp investigation as far as African sector is concerned. For all of the ionospheric parameters considered, the monthly median values are obtained from the daily values. The annual averages are estimated using the monthly median values. It is important to note that while h’F and NmF2 were considered for the entire 24 h, Vp was considered only for the evening/nighttime period spanning 16:00–06:00 LT. This is to avoid some daytime constraints associated with Vp deduced from h’F as highlighted in the works of Iheonu and Oyekola (2006), Adeniyi et al., (2014b) and Adebesin et al., (2015b). These constraints include (i) taking the diffusion term of the continuity equation to be low in the treatment of ionization transport in the F2-region at daytime, (ii) discarding the time rate of change of NmF2 (dN/dt), and (iii) neglecting the production term q. The continuity equation is given in equation (3) as dN ¼ q  LðNÞ  M þ D dt

  M ¼ div NVp

(3)

  2 d N dN N þ 3 þ dz2 dz2 2

(6)

By implication, quantity M characterises the mechanisms responsible for hemispheric transportation of plasma that initiates EIA (Bolaji et al., 2017). While the lower ionosphere is characterised by the production and loss of ions without the transport process, the F-layer ionosphere and its top-side are controlled by all parameters in equation (3) (Bolaji et al., 2017 and the references therein]. Adeniyi et al., (2014b) had suggested that d(h’F)/dt obtained from ionogram data provides an apparent drift measurable with E x B drift around the evening/sunset hours. Also, the works of Bertoni et al., (2006) and Adeniyi et al., (2014b) had given credence to vertical drifts inferred from ionosonde height profile data as near accurate being that they align well with the experimental Vp from ISR observation especially during PRE and nighttime periods. The most reliable being the Incoherent Scatter Radar (ISR) drift observation at Jicamarca, as it has the capacity of direct measuring of the ionospheric electron density, plasma temperature and the line-of-sight plasma drift across a wide range of height (Zhang and Holt, 2007). The only shortcoming to the Jicamarca IS Radar is that it makes observations in just about 3–4 times in a month (Yue et al., 2008). For instance during the low solar activity year 2010, observations using the Jicamarca ISR were only available for about 11% of the total number of days (Adeniyi et al., 2014b). Besides, Risbeth (1981) had submitted that though the magnitude of 300 km and beyond for ionosonde inferred drift may be a necessary condition for the triggering of Rayleigh–Taylor (R–T)

where N is the maximum electron density, q the production term, L(N) the loss term, M the divergent movement term, and D the diffusion term. At daytime, the quantity M which contains the plasma drift velocity is considered. The parameter D is define as D ¼ do e2 sin2 I

(5)

(4)

Table 1 List of ionospheric stations with Geomagnetic and Geographic coordinates. Station Name

Dakar Ouagadougou Korhogo

Station code

DAR OUA KOR

Geomagnetic

Geographic

Lat. (0N)

Long. (0E)

Lat. (0N)

Long. (0E)

2.32 0.59 1.26

57.97 71.46 67.38

14.73 12.42 9.51

342.64 358.60 354.39

85

Magnetic inclination (degrees)

Data availability

Data used

þ5.53 þ1.45 2.25

1971–1997 1966–1998 1992–2002

1992–1996 1992–1996 1992–1996

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Journal of Atmospheric and Solar-Terrestrial Physics 172 (2018) 83–99

(7) reduces to equation (8) given as:

instability, but may not be a sufficient condition. Another general limitation at obtaining Vp from ionosonde data of h’F is the 300 km minimum threshold value for h’F which according to Bittencourt and Abdu (1981) must be met before drift velocities inferred from ground-based measurements can equate reasonably well with radar observations. There are however occasional periods when h’F is less than 300 km around the 16:00–06:00 LT window, during which time Vp was determined in the present work. This seems to be a constraint. Consequently, the Vp presented here may be considered to be an apparent representation of the actual vertical E x B drift since chemical correction is not performed. This can still be allowed in the absence of direct radar measurement. It is also worth pointing out that some previous works, such as Uemoto et al., (2010) and Abdu et al., (1981) obtained Vp from ground-based measurement for h’F < 300 km at some instances of their works (without carrying out chemical correction), and yet with well-established results comparable with ISR drift observations. This was extensively highlighted and discussed in the review work by Adebesin (2016). Adebesin (2016) had discussed the sufficiency and exactness of the 300 km threshold value of the F-layer height from the point of view of previous outstanding results, coupled with some present observations related to some key issues, and new direction to follow. Scali et al., (1997) had also explained the reason why the plasma obtained from ionosonde measurements differ from that of the ISR. The authors claimed that while the ionosonde Doppler shift echo is a product of the rate of change of time of the electrical distance to the point of reflection, the ISR Doppler shift echo measures directly the line-of-sight of Vp. Therefore there is bound to be magnitude difference in the two techniques. According to Woodman et al., (2006), vertical drifts obtained from ionosonde measurements (h’F or hmF2) is determined by the apparent vertical velocities of the height of reflection. Consequently, the determined velocity may include both the E x B vertical drift and the effect of the change in the electron density profile shape emanating from photochemical process. At daytime, equilibrium position is reached in the ionospheric region as a combined effect of electric dynamics and photochemistry; such that the shape of the electron density profile do not show a considerable variation. Hence the drift change pattern will be small. However, at nighttime (sunset, early sunrise, and evening periods), the drift is downward making the recombination process to dominate, while the direction and magnitudes of the zonal electric field may vary greatly. This interruption in the equilibrium condition will allow for the dynamic process to become dominated at most of these evening/midnight local times. Yue et al., (2008) had attributed this process to the reason why ionosonde inferred drifts aligned relatively well with ISR observations during such local periods. This further buttress the reason for investigating the drift pattern during the nighttime period. The only likely limitation to the method of obtaining drift velocity from F-layer height magnitude less than 300 km is the underestimation of the real E x B drift magnitude [e. g. Bittencourt and Abdu, 1981). While h’F is used in inferring the ionospheric vertical plasma drifts, studying the pattern of h’F also assist in identifying the limitations involved. h’F variations were plotted to reveal the true picture of our study, which some works may not want to reveal. More importantly, the variation in h’F pattern indicates the perturbation of electric field (Prabhakaran Nayar et al., 2009)]. Further, the plot of h’F in addition to that of plasma drift during and after the PREpeak period had the potential of allowing us to investigate the inhibition of scintillation and/or spread-F occurrence as revealed by Adebesin et al., (2015a). The zonal electric field (Ey) magnitude was approximately estimated from the product of the vertical plasma drift (Vp) and the geomagnetic field intensity (B) using the expression in equation (7). ! ! Vp ¼ E y  B B2

!.! Vp ¼ Ey B

(8)

yielding the expression for obtaining Ey as ! ! E y ¼ B ⋅Vp

(9)

The geomagnetic field can be accessed for any ionospheric location from the International Geomagnetic Reference Field (IGRF) calculator at http://www.ngdc.noaa.gov/geomag-web/#igrfwmm. For the purpose of this work, the IGRF-12 model (see IGRF-the 12th edition, 2015) is employed. The IGRF is a series of mathematical models of the Earth's main field and its annual rate of change. The variables fed into the model calculator are the geographic latitude and longitude of the desired station, its elevation/height above sea level and year of interest. For DAR, OUA, and KOR, the value of the geomagnetic field (B) are 30,072 nT, 33, 028 nT, and 31,875 nT respectively based on the IGRF-12 model for the epoch year 1992.5. This was repeated for the epoch periods 1993.5 through 1996.5. For the entire descending phase period (1992–1996), the geomagnetic field change per year at DAR, OUA, and KOR are 5.3 nT/year, 12.0 nT/year, and 8.2 nT/year respectively. These changes per year from 1992 to 1996 are small enough such that the annual averages are calculated. Thereafter, the magnitude of the zonal electric field is deduced from equation (9). It is worth noting that the magnitude of Ey was taken only during the period of maximum PREpeak at each station for each single year of study. The corresponding h’F and NmF2 magnitudes at these periods were thereafter recorded and presented (as would be seen later in section 4). Geomagnetic field around the equator had been found to vary between 25,000 to above 35,000 nT (Maynard et al., 1995). So our estimated value of B from the IGRF calculator which falls within this range is justified. For the seasonal variability, the monthly median values were further grouped into E-Season or Equinox (March, April, September, October), JSeason or June Solstice (May, June, July, August), and D-Season or December Solstice (November, December, January, February) according to Lloyd classification (Lloyd, 1861, Liu et al., 2010; Rabiu et al., 2012). The E-Season is further divided into March E-Season (March and April) and September E-Season (September and October). The international sunspot number (R) values used were obtained from http://www.sidc. be/silso/datafiles. R, a solar index, was introduced into the study because the relationship between ionospheric features and solar activity indices had been found to improve ionospheric models (Perna and Pezzopane, 2016). 3. Observations and discussion 3.1. Annual response of h’F, NmF2, and Vp Fig. 2(a–c) presents the annual ionospheric height profile (h’F), maximum electron density (NmF2), and vertical plasma drift (Vp) for the five successive years (1992–1996) constituting the descending phase of sunspot cycle 22. For the h’F observation, the highest magnitude observed was in the early year (1992) of the descending phase, and decreases towards 1996, though with almost equal magnitude from 1994 to 1996. We also observe that the evening time h’F peak magnitude at DAR showed up at about ~4 h after the peak in OUA and KOR for all the years with an exception in 1996. The NmF2 observation in Fig. 2(b) revealed a clear decrease in magnitude with decreasing sunspot activity from 1992 through 1996. This clear decrease in magnitude characteristic is however less noticeable for the h’F observation. This is further supported by the trend-line plot across NmF2 in Fig. 2(b) that we compared with the h’F trend-line in Fig. 2(a) at the three stations. The implication of the trendlines for all of the stations in Fig. 2(a), which is insignificant sunspot activity is shown with the almost horizontal line during the descending phase. h’F is largest around 19:00 LT and decreases towards the post-

(7)

However, around the magnetic equator, both Ey and B are horizontal and perpendicular [e.g. Grodji et al., 2017), and consequently equation 86

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Fig. 2. Annual response of the ionospheric (a) height profile (h’F) (b) maximum electron density (NmF2) and (c) vertical plasma drift (Vp) during the descending phase. Dotted arrows shown in (a) and (b) indicates the direction of trend-line for each of the stations considered.

noon peaks ranges from 0.73 to 1.42, 0.73–1.64, and 1.03–1.95  1012 em3 in similar order of stations. Meanwhile, NmF2 at DAR during both peak periods emerges at about 4 h after that observed at other stations. Generally for the entire descending phase period, the slight depression in the daytime NmF2 occurred around 12:00 LT at OUA and KOR while it was observed around 14:00 LT at DAR. The NmF2 observation at OUA

evening and the early morning hours. The NmF2 plot revealed two peaks – the pre-noon and the post-noon peaks. The pre-noon peak is however lower than the post-noon peak for all years and at all stations except OUA in 1992. Quantitatively, the prenoon NmF2 peak magnitudes ranges from 0.60 to 1.40, 0.60–1.72, and 1.03–1.95  1012 em3 at KOR, OUA, and DAR respectively. The post-

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controls the process forming the ‘bite-out’ characteristics. When the vertical E x B drift is downward, no ‘bite-out’ is formed. The post-noon peaks also emanate from the continuous production of plasma few hours before sunset, at which time production of ionization is still taking place, and hence the formation of the post-noon peak. Hence, this work is the first to show higher bite out at the equatorial northern station (DAR) than equatorial southern station (KOR) using ionosonde data in the African sector. The apparent reduction trend in NmF2 amplitude as the sunspot cycle decreases during the descending phase (evident by the respective downward trend-lines at all stations) could be explained on the basis of decrease of ionizing radiation coupled with the corresponding changes in the thermospheric parameters. Mikhailov et al., (2000) had however observed that the downward flux maximum value and the recombination efficiency decreases by lower multiplicative factors as we move from sunspot maximum to sunspot minimum conditions. This is what explains the decrease in NmF2 peak with decreasing sunspot activity. The decrease in the magnitudes of h’F and NmF2 with decreasing solar activity supports the results obtained by Radicella and Adeniyi (1999) using Ouagadougou ionosonde data. Ouattara et al., (2012) had also reported high correlation between foF2 and sunspot number R, implying higher amplitude of foF2 for higher magnitude of R and vice-versa. They also reported post-sunset foF2 peak higher than the pre-sunset peak during the descending phase at the two stations. This is in agreement with the present result regarding NmF2 pattern. Vp (Fig. 2(c)) was considered only for the evening/nighttime period spanning 16:00–06:00 LT due to the daytime constraint earlier mentioned in section 2. We observe that the evening time pre-reversal enhancement (PRE) peak (PREpeak) magnitude decreases with decreasing phase of sunspot activity. Two other characteristics of the Vp signatures are (i) Vp PREpeak magnitude at DAR appeared 1–2 h after that of OUA and KOR (ii) Vp reduces with increases in NmF2 magnitude (Fig. 2b and c). Vp has been found to control the electron gradient in the bottomside F-region after dusk time (Sreeja et al., 2009). Fig. 4 depicts the average standard deviation plot of h’F, NmF2, and Vp for the entire descending phase period at KOR (left panel), OUA (middle panel), and DAR (right panel). Evidently, one observes the greatest dispersion of data about the mean around the PRE peak period (1900 LT for KOR and OUA, and around 2000 LT for DAR) in the h’F and Vp plots. This is not surprising as the drift values were generated from the ionosonde h’F profile. For the NmF2 observation, the least spread of data about the mean was yet recorded around the PRE peak period across all stations just as indicated for the h’F and Vp plots. The Vp characteristics shown in Fig. 2(c) is further elaborated by the bar chart plot in Fig. 5. Here, the direction of movement of Vp (arrows AE) in attaining PREpeak is shown for each of the years (a –e) constituting the descending phase at the entire stations. The 1992 PREpeak magnitude is very obvious (indicated by the longer-upward pointing arrow A). This

and KOR are closer in magnitude except for the 1992 observation when the magnitude was higher at OUA. The slight depression in the daytime (or noon-bite-out characteristics) variation of NmF2 at OUA and KOR are similar. Higher value of NmF2 found in 1992 at OUA compared to KOR is due to latitudinal distribution of plasma initiated by ‘fountain effect’. This is also responsible for the higher magnitude in NmF2 at DAR compared to OUA and KOR throughout the entire descending phase of the solar activity. The fountain effect results from plasma movement (mainly atomic oxygen and electrons) by the action of the E x B drift, diffusion, and winds. Because of the action of E x B drift, the plasma moves perpendicular to the magnetic field lines. DAR (Geomag. Lat. 2.32 oN, dip: þ5.53) is at the northern hemisphere of the EIA trough, KOR (Geomag. Lat. 1.26 oN, dip: 2.25) is at the southern hemisphere of the trough, and OUA is in the dip of the trough (Geomag. Lat. 0.59 oN, dip: þ1.45). Hence the low, moderate and high reduction in noon bite out at KOR, OUA and DAR respectively. Apart that the noon-bite out occurred earlier at OUA due to its closeness to the geomagnetic equator, more plasma are drifted into KOR, which is closer to the geomagnetic equator along the plasma distributions compared to DAR. Longitudinal difference is because they are not on the same longitude, but all these characteristics are the typical behaviour of latitudinal distribution of plasma. Ouattara et al., (2012) used Ouagadougou - OUA data (1966–1998) and data from Dakar – DAR (1971–1997) and reported significant/non-significant foF2 variability at DAR/OUA during the decreasing solar cycles. This observation is adduced to the equatorial fountain effect pointed out in the present work, establishing that less plasma flow from the equator dip station OUA to DAR. Appleton (1946) and Rush et al., (1969) established EIA as plasma redistribution across the hemispheres, thereby generating a peak (crest) plasma in each hemisphere of the equatorial/low latitudes and reduced plasma (trough) at the geomagnetic equator. Fig. 3 further revealed convincing evidence when the correlation coefficient of 0.974 is established for the OUA/KOR NmF2 relationship, and that of OUA/DAR is 0.836. Again the higher correlation obtained between OUA and KOR that supported more flow of plasma to KOR is an indicator revealing that the geomagnetic distance of KOR from OUA is closer compared to DAR. We also observed that as the solar activity period decreases from 1992 through 1996, NmF2 variation difference between OUA and DAR closes up. The observed sharp rise in NmF2 during the daytime (pre-noon peak) which occurred between 09:00–12:00 LT at all stations during the entire years is linked to the increased intensity of the Sun's radiation. The solar intensity influenced the F2-layer to attain a dynamic steadiness around local noon and in association with the E x B drift creates plasma drifting up and away from the equator causing the highest daytime plasma depletion around local noon (Radicella and Adeniyi, 1999; Adebesin et al., 2013a). After the noon when the Sun intensity wane, the E x B drift persist and initiates reduction. In summary, it is the action of the vertical E x B plasma drift that maximizes before and around noontime that

Fig. 3. NmF2 Correlation plot of (a) OUA versus KOR and (b) OUA versus DAR. 88

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Fig. 4. Standard deviation plot of h’F, NmF2, and Vp for the entire descending phase period at KOR (left panel), OUA (middle panel), and DAR (right panel).

profile (at 05:00 LT) is also obvious at the two stations. However, the h’F variability pattern over DAR was different from the other two stations. The difference is that when h’F at OUA and KOR reached their respective peak around 19:00 LT, h’F at DAR is increasing. Also, when h’F variations at OUA and KOR are reducing around 00:00 LT, h’F at DAR reached its peak value of 297 km. Another obvious difference is the slight increase during pre-sunrise hours (05:00 LT) observed at OUA and KOR, that is absent at DAR. Mendillo et al., (2002) had attributed the nighttime h’F behaviour to the thermal effects and equatorward winds. The NmF2 plot (in Fig. 6(b)) revealed higher magnitude of plasma at DAR compared to the other two stations. We found that NmF2 pattern at both OUA and KOR closes, including pre-sunrise magnitude intensification around 04:30 LT. This pre-sunrise intensification in NmF2 around 04:30 LT is absent in DAR. Generally, there is a significant reduction in NmF2 from 16:00 LT to 04:00 LT at all stations. The bar-chart plot indicating the response of the plasma drift velocity is shown in Fig. 6(c). Bar-chart is used to depict simultaneous magnitude and direction of Vp. We found that there is a significant growth in the drift pattern at all stations at 16:00 LT which later peak (PREpeak) at 18:00 LT in OUA and KOR, and at 19:00 LT in DAR. The minimum of the downward excursion found at OUA and KOR started at 20:00 LT until 5:00 LT; whereas the observation at DAR revealed a downward excursion

is followed by a shorter-upward pointing arrow B in 1993, indicating decrease in the PREpeak magnitude at all stations. Our observations in 1994 where arrow C is horizontal is an indication that there is appreciable PREpeak magnitude, which is getting weak and reduced to ~6.4 m/ s as year 1995 is closer. In 1995, the shortest-upward pointing arrow D that reached ~6.5 m/s around 1700 LT changed its direction pointing downward after 1700 LT. In 1996, downward pointing arrow E that begins with Vp of ~6.1 m/s is obvious. Hence Fig. 5 had revealed the PREpeak magnitude and its direction of propagation during the descending phase period. The PREpeak amplitude had been reported to decrease towards periods of low solar activity because of the corresponding decrease in the equatorial zonal wind and gradient (Fejer et al., 1999).

3.2. Response of h’F, NmF2, and Vp during the entire descending phase The average ionospheric climatology parameters used for the entire descending phase are shown in Fig. 6(a–c). We observed that the evening time h’F response at both OUA and KOR peaks at 19:00 LT with a magnitude of 296 and 302 km respectively. For all other times, there are still good relationship in terms of the trend between the two stations with an exception for around 21:00 LT to midnight hours where an altitude difference of ~19 km was observed. The pre-sunrise lifting of the height

Fig. 5. Yearly clustered column bar chart for the vertical plasma drift plot highlighted in Fig. 2c for DAR, OUA and KOR. Black dashed arrow indicates direction of PRE peak magnitude between 16:00 and 18:00 LT. 89

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associated with high latitude electrodynamic disturbances. They were believed to have resulted from the combined effects of relatively short-lived prompt electric fields driven by the solar wind magnetosphere dynamo and longer lasting ionospheric disturbance dynamo activities driven by enhanced energy and momentum deposition from the high latitude ionosphere. Fig. 7 clearly highlights the column bar-chart of Vp for the entire years over KOR (upper panel), OUA (middle panel) and DAR (lower panel). The shaded portion on each panel indicates the time of maximum PRE magnitude. The shaded portions indicate that PREpeak at OUA and KOR occurred at 18:00 LT while that of DAR was seen at 19:00 LT as earlier revealed in Fig. 6. However, the shaded portions for each station revealed a decrease in the PREpeak magnitude as one moves down the descending phase from 1992 to 1996. This also points to the fact that PREpeak is solar activity dependent. The Vp downward reversal peak value also revealed solar dependence. Vp maximum downward peaks appear at KOR at 21:00 LT and at OUA at 20:00 LT. The observation at DAR however do not show a consistent time at which the maximum downward magnitude appears because it varies between 01:00 LT and 03:00 LT. 3.3. Seasonal observations As earlier stated in section 2, the seasons were divided into four viz: March E-season (or March Equinox - March and April), September ESeason (or September Equinox – September and October), J-season (June solstice - May to August), and D-season (December solstice- November to February). The average seasonal observations of the parameters at DAR, OUA, and KOR were depicted in Fig. 8 spanning the entire 24 h for h’F

Fig. 6. Response of (a) h’F (b) NmF2 and (c) Vp averaged over the entire five years constituting the descending phase of sunspot cycle 22. Plot covers only the evening/nighttime observations.

starting from 01:00 LT to 06:00 LT. The downward excursion minimum peak value occurred at 21:00 LT for OUA and KOR, and at 03:00 LT for DAR. We observed higher (at DAR)/lower (at OUA and KOR) magnitudes of NmF2 that resulted into corresponding lower/higher plasma drift velocities. While prompt penetration electric field arising from the action of solar wind magnetosphere and electric field from disturbance wind dynamo had been found to be responsible for the vertical drift variability during disturbed periods (Abdu et al., 2009; Santos et al., 2012; Adekoya and Adebesin, 2015); Gravity waves and coupling processes arising from upward propagating planetary waves are dominant during quiet conditions (Abdu et al., 2006). However, during episodes of the combined magnetically quiet and disturbed conditions (like the case in the present study), the major source of ionospheric vertical drift variability is the solar flux variations [e.g. Abdu et al., 2010), which has been noted as a key factor in the long-term variation of the ionosphere around the magnetic equator. Aside from these, the plasma drift variability (Fig. 6(c)) from one station to another during the entire descending phase may have been additionally generated by enhanced leakage of high latitude electric fields to lower latitudes, as well as from large perturbations in the global wind system (Adebesin et al., 2013b). This is because Fejer (2002) had earlier highlighted the various modulating factors of the low-latitude evening-time prompt penetration electric fields, and submitted that the steady state leakage of high latitude electric fields into the low latitude at evening time played a prominent role. Besides, Fejer et al., (2008b) had explained that the variability are

Fig. 7. Vertical plasma drift bar-chart variation for KOR (upper panel), OUA (middle panel) and DAR (lower panel) between 16:00 and 06:00 LT. The shaded portion on each panel indicates time of maximum PRE magnitude. 90

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KOR. Ouattara et al., (2012) had reported seasonal foF2 peak pattern appearing in the E-seasons at OUA and DAR. The D-season variations where NmF2 pre-noon peaks are higher than the post-noon peaks revealed the D-season (i.e. December/winter) anomaly (Perna et al., 2015) which is opposite to the J-season observation. The D-season anomaly is majorly considered when its NmF2 magnitude is greater than that of the J-season at the same point in time over the same Earth's surface point. This is irrespective of the solar insolation in D-season compared with the J-season (Pavlov and Pavlova, 2009). Rishbeth et al., (2000) and Mikhailov and Perrone (2011) attributed the D-season anomaly to the variation in neutral composition. Refer to the work of Pavlov and Pavlova, (2005) for more on D-season anomaly. Ouattara et al., (2009) using Ouagadougou data spanning 1966–1998 and covering three solar cycles had presented foF2 (or NmF2) climatology among other parameters investigated in the West African sub-sector. The authors reported that the quiet time foF2 revealed the D-season and semi-annual anomalies with peaks in March/April and October. This observation is consistent with the NmF2 result obtained in the present study. They further reported that foF2 amplitude is solar cycle

and NmF2 observations while Vp was investigated only for the evening/ nighttime period. The h’F observations revealed that for all stations, Jseason recorded the highest height profile, and followed by March Eseason. The peak values ranges from 316 to 325 km (between 19:00–23:00 LT) for March E-Season, 325–349 km (between 00:00–01:00 LT) for J-season, 289–294 km (between 19:00–00:00 LT) for September E-season, and 254–314 km (between 18:00–19:00 LT) for D-season. While DAR recorded the highest altitude in J-season, it recorded the lowest in the other seasons. Qualitatively, NmF2 pattern in panel (b) showed that DAR recorded the highest plasma magnitude, followed by OUA, and the least in KOR during the entire seasons. This is a seasonal evidence of the fountain effect which builds up later to increase NmF2 magnitude at DAR. As earlier mentioned, DAR is far from OUA compared to KOR, so the plasma drifting from the dip equatorial station (OUA) takes few hours before reaching DAR. Two NmF2 peaks were noticeable – the pre-noon and the post-noon peaks. The NmF2 pre-noon peak is lower than the post noon peak during both E-seasons (March and September) and J-season. In the D-season, the NmF2 pre-noon peak is higher than the post noon peak at all of the stations with an exception at

Fig. 8. Average seasonal observations for h’F, NmF2, and Vp for the entire descending phase period over DAR, OUA and KOR. 91

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Journal of Atmospheric and Solar-Terrestrial Physics 172 (2018) 83–99

8.0–15.0 m/s, and 7.5–12.0 m/s for the E-seasons, D-season, and J-Season respectively (i.e. from the results of Adebesin et al., 2015a, b, and the present result). The maximum projected limit for each season (i.e. 20.0, 15.0, and 12.0 m/s) can be enhanced either during the magnetic quiet or magnetic disturbed activity conditions (Grodji et al., 2017; Oyekola, 2009; Oyekola and Oluwafemi, 2007), but higher during the quiet magnetic condition than the disturbed condition. The low-latitude electric fields and currents during quiet-time had been found to show large departure characteristics compared to during/after enhanced geomagnetic activity conditions (Fejer et al., 2008a). Quiet geomagnetic activity conditions equate reasonably well with the ground state of the thermosphere having low rate of atomic oxygen (Mikhailov et al., 2007). Further, the minimum projected limit for each season (i.e. 8.0, 8.0, and 7.5 m/s for the E, D-, and J-seasons) may be further supressed during low solar activity conditions (Adebesin et al., 2015b). We also observed that during the quiet or disturbed magnetic activity, PREpeak amplitude in J-season compared reasonably well to (or in some cases higher than) those observed in E and D-seasons (Grodji et al., 2017 during quiet magnetic activity condition; Oyekola and Kolawole, 2010 during quiet condition; Oyekola and Oluwafemi, 2007 during quiet condition as well as disturbed condition). The comparison is a future study. However, during the combined quiet and disturbed magnetic activity condition, the PREpeak amplitude in J-season on the average is smaller than those of other seasons. The better coherence in variability of all parameters used (h’F, NmF2, Vp) between OUA and KOR (Figs. 2, 4a and 6 and 8) was attributed to the shorter distance between the two stations (about 320 km) when compared with OUA and DAR (about 1747 km). Latitudinal difference is therefore suggested to be at play.

dependent. It is worth pointing out that while the present study investigates the African equatorial climatology, it also provides avenue on a second look for validating previous results from related investigation in terms of similarities and differences. We also observed that the noon-time bite out characteristic in NmF2 at DAR occurred about 2 h after similar occurrence in both OUA and KOR for the entire seasons. For instance while it was observed at 11: 00 LT at OUA and KOR in the E and D-seasons, the corresponding observation was seen at 13:00 LT in DAR. The noon-time bite out was however not evident at DAR in the J-season. Further, between 18:00–06:00 LT, similar magnitude and pattern of NmF2 was observed at OUA and KOR with 97.7% correlation. For other periods (06:00–17:00 LT) same pattern was observed but with different magnitudes equalling 93.2% correlation between the two stations. In summary, NmF2 was more pronounced during the E-seasons (March and September), followed closely in the D-season and the least during J-season. According to Jayachandran et al., (1997), the local wind of the neutral atmosphere modulated by the EIA and the asymmetry in the ionization hemispheric distribution (which increases E-region Pederson conductivity as a consequence of EIA) may also be another reason for recording higher NmF2 peak magnitude during the E-seasons. The peaks of NmF2 variations at DAR occurred later after the peaks at OUA and KOR in all of the seasons, similar to the annual observation earlier reported in section 3.2. The Vp response indicated on the third panel was observed between 16:00–06:00 LT. The drift pattern revealed highest activity during the March E-season with PREpeak/maximum downward excursion magnitudes of 15.1/-7.7, 12.5/-6.3, and 9.8 ms-1/-5.8 ms-1 at OUA, KOR, and DAR respectively. This is closely followed by the D-season observation when the PREpeak/maximum downward excursion magnitudes are 10.8/6.3, 12.0/-10.3, and 7.9 ms-1/5.7 ms-1 in similar order of stations. While Vp response during both E and D-seasons reflected the typical pattern that could be obtained from ISR observations (Bertoni et al., 2006); the observational pattern during J-season, which also recorded the lowest value of PREpeak magnitude for all stations is slightly composite. This is because it does not immediately reflect the PRE peak. For these (J-season) observation, PREpeak/maximum downward excursion magnitudes are 7.4/-5.8, 7.4/-10.8, and 7.9/-8.0 ms1for OUA, KOR, and DAR. The PREpeak magnitude during J-season appeared few hours (between 19:00–22:00) after that in the E and D-seasons (between 18:00–19:00 LT). Adebesin et al., (2015a) attributed this form of PREpeak appearance in time lag between the J-season and other seasons to decrease in the equatorial zonal wind and conductivity gradient. Our observations also showed that plasma drift velocity decreases at the instance of NmF2 increase at both the E and D-seasons. This is the role of the post-sunset peak in upward plasma drift which often serves as an indicator to impulsive and rapid electron depletion over the equatorial ionosphere. This characteristic is however not significant during J-season. Furthermore, vertical E x B drift velocity had been reported to primarily account for seasonal differences in NmF2 at the magnetic equator (Anderson and Matsushita, 1974). The significant pre-sunrise enhancement of plasma drift seen in our results during the E and D-seasons around 05:00 LT had also been observed by Prabhakaran Nayar et al. (2009) using Trivandrum radar and digisonde data in the Indian sector. They ascribed it to the zonal electric field generated from polarization field caused by thermospheric wind and steep conductivity gradients in the E-region.

3.3.2. Comparison of seasonal PREpeak magnitude observations in the present study with past results obtained from the American and Indian sectors Fejer et al., (2008a) used the Ionospheric Plasma Electrodynamics Probe Instrument (IPEI) aboard the ROCSAT-1 satellite in an attempt to develop a quiet time global empirical model for the equatorial F-region vertical plasma drifts. They had considered four longitudinal sectors representative of 0
–1500 E (African-Indian sector), 1 50
–2100 E (Pacific), 2100–3000 E (Western American) and 3 00
–3600 E (Brazilian). They reported that the evening time PRE peak was highest during the equinox and December solstice and near the American sector compared to other longitudes. The simulations (using the Magnetosphere-Thermosphere-Ionosphere-Electrodynamics General Circulation Model, MTIEGCM) and observations to study the longitudinal variation of the PRE during the equinox by Vichare and Richmond (2005) revealed that their PRE peaks are larger in the America-Atlantic sector than in the Indian sector, and is attributed to the longitudinal variation of the geomagnetic field strength. Table 3 presents a comparison of PREpeak magnitude from other sectors outside Africa with their corresponding equipment/method, station, and solar activity conditions. The table revealed that the highest pre-reversal drift peaks occur in equinoxes (for all the American sector cases) except in Kodaikanal (Indian), where they are largest during December solstice. We also observed that the PREpeak magnitude during the equinoxes ranges between 26 m/s and 45 m/s (apart from the one reported by Adebesin et al., 2015b, during a period of low solar activity which is lower). A comparison between Tables 2 and 3 (i.e. African sector versus other sectors) revealed that PREpeak magnitude is generally lower in the African sector than in other sectors. For the equinox observation in Table 2, PREpeak ranges between 10 m/s and 40 m/s. This is also true for D-season and J-season in which the PREpeak magnitude is lesser in the African sector than in the American sector.

3.3.1. Comparison of seasonal PREpeak magnitude observations in the present study with previous results obtained for African sector A comparison between the maximum PRE magnitude of drift pattern obtained in this work with other previous studies done in the African sector are presented in Table 2, This is done to show the general quantitative seasonal morphology of Vp in the African sector. From the previous and the present studies, we can deduce that on the average, in the African sector, and during the combined magnetic quiet and disturbed condition, PREpeak magnitude ranges from approximately 8.0–20.0 m/s,

4. Annual response of the zonal electric field at all stations during PREpeak period The zonal electric field had been found to play a vital role in the electron density distribution over the ionospheric equatorial and low92

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Table 2 Various PREpeak magnitudes obtained from previous studies in the African sector in comparison with the present work. Reference

Study period

Equipment/ method used

Station name

E-season/equinox (m/s) March-E

September-E

Grodji et al., (2017)

1993 (moderate solar activity) 1994 (moderate) 1966–1998 (sunspot cycles, SC 20–22)

IPS-4.2 (h’F mean)

Korhogo

40.0

IPS-4.2 (h’F mean)

Ouagadougou SC 20 (1966–1976) Ouagadougou SC 21 (1976–1986) Ouagadougou SC 22 (1986–1996) Ouagadougou 1966–1998 Ilorin

Adebesin et al., (2015a)

Adebesin et al., (2015b)

Oyekola and Kolawole (2010) Oyekola (2009) Oyekola and Oluwafemi (2007) Present study

a b

2010 (low sunspot activity

1989 (high sunspot activity) 1958 (high sunspot activity) 1958 (solar maximum) and 1964 (solar minimum) 1992–1996 (descending phase sunspot cycle 22)

DPS-4.2 (hmF2 mean) DPS-4.2 (h’F mean) Ionosonde (hmF2 mean) Ionosonde (h’F median) Ionosonde (h’F mean) IPS-4.2 (h’F median)

D-season/Dec. solstice (m/s)

J-season/June solstice (m/s)

Magnetic activity condition

30.0

30.2

31.3

Quiet

22.1 18.5

30.1 16.2

28.4 10.7

28.3 10.7

19.5

19.0

13.1

11.2

19.0

17.9

14.6

10.0

18.1

15.7

12.8

12.0

9.2

3.8

8.0

0.3

Ilorin

12.7

12.8

13.3

2.2

Ouagadougou

17.0a

14.0

16.0

Combined disturbed and quiet conditions Quiet

Ibadan

33.6a

26.7

26.9

Quiet

Ibadan

33.0

28.0 23.0 10.8 12.0 7.9

42.0 23.0 7.4 7.4 7.9

Quiet Disturbed Combined disturbed and quiet conditions

28.0 18.0 10.9 8.6 5.7

b

Ouagadougou Korhogo Dakar

15.1 10.9 12.5 8.6 9.8

Combined disturbed and quiet conditions

Averaged over the entire E-season months. Insignificant value.

Table 3 PREpeak magnitudes obtained from previous works from other sectors outside Africa. Reference

Study period

Equipment/ method used

Station name

E-season/ equinox (m/s) March-E SeptemberE

D-season/Dec. solstice (m/s)

J-season/June solstice (m/s)

Magnetic activity condition

Adebesin et al., (2015b)

2010 (low solar activity)

Incoherent Scatter Radar (ISR) Digisonde (h’F) and GPS Receivers ISR AE-E Satellite HF Sounder

Jicamarca (American) Jicamarca (American) Jicamarca (American) Kodaikanal (Indian) Jicamarca (American) Peru (Peruvian)

13.6

6.4

a

b

28.0

14.0

Combined disturbed and quiet conditions –

45.0b

38.0

35.0

–

26.3b

38.0

25.0

Quiet

b

38.0

17.0

38.0b

26.0

a

Combined disturbed and quiet conditions –

Lee et al. (2005) Scherliess and Fejer (1999) Empirical model Ramesh and Sastri (1995)

Apr. 1999–Mar. 2000 (high solar activity 1968–1992 1977–1979 High solar activity

Fejer et al., (1999)

1968–1998

ISR

Huang (1974)

Mar. 1968–Dec. 1969

Scatter radar

a b

32.0

50.0

9.6

Insignificant value. Equinoctial month average values (comprising of March and September equinoxes).

latitudes. Fig. 9 highlights the annual response of the zonal electric field Ey (Fig. 9(a)) relative to the corresponding observations of NmF2 (Fig. 9(b)) and h’F (Fig. 9(c)) at all stations. Ey magnitude was taken during the maximum PREpeak period for each station, and the corresponding h’F and NmF2 magnitudes were recorded and presented. Both Fig. 9a and c revealed that the variation in Ey is proportional to the variation in h’F at all stations in 1992, 1993 and 1994. For instance, comparing panels (a) and (c), the station with the maximum/minimum Ey magnitude in 1992 in panel (a) revealed the maximum/minimum magnitude of h’F in 1992 in panel (c), and same for 1993 and 1994. The pattern is however different during the low solar activity years of 1995 and 1996. The stations that revealed this differing pattern are KOR in 1995 and DAR in 1996. In 1995, h’F at KOR is very large compared to h’F observations at OUA and DAR. Similarly in 1996, h’F at DAR is larger than that observed at KOR and OUA. Generally, the observation revealed that the emergence of the enhanced zonal electric field results in a

corresponding rapid uplift of the equatorial F-layer profile. The zonal electric field is responsible for the production of the vertical plasma drift motion. From panels (a) and (b), Ey and NmF2 plots revealed reversed or inverse relationship at all stations between 1992 and 1994. The station with the maximum/minimum Ey magnitude presents the minimum/ maximum value of NmF2. The pattern was also different in 1995 (at KOR) and 1996 (at DAR). NmF2 magnitude which is larger at the stations close to the equator (KOR and OUA) than at DAR in 1996, as one would have expect NmF2 to be larger in amplitude at DAR. This is because a year of low solar activity (1995 and 1996) leads to low ionization meaning low plasma is available form drift along the EIA crest. This quantity of low plasma will not be enough to drift to higher latitude and can easily get to KOR before DAR. The observations between the Ey/h’F and Ey/NmF2 relationships therefore suggest that the zonal electric field is more active in lifting the ionospheric height profile and supressing the magnitude of 93

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Fig. 9. Average annual response of (a) zonal electric field (b) peak electron density, and (c) height profile at all stations during the maximum PREpeak period; (d) depicts the average entire descending phase magnitude for Ey, NmF2, and h’F at DAR, OUA, and KOR.

showed that total electron content (TEC) magnitude, which is a function of NmF2, is higher in the anomaly crest stations than at equatorial locations; and is in agreement with the present observation. Jayachandran et al., (1997) had suggested that a well-developed EIA is a necessary and sufficient condition for the generation of irregularities in the ionospheric region. The growth rate of this irregularities/instabilities significantly depends on the density gradient which is clearer in the nighttime equatorial F-region. NmF2 was also observed to show hemispheric dependency from Fig. 2(b). From Fig. 2(b), while the variation of NmF2 at DAR (northern hemisphere) was above that of OUA (dip equator), the one at KOR (southern hemisphere) was below that of OUA. However, these hemispheric difference in NmF2 is more significant during the high solar activity year (1992) and closes up with decrease in solar activity. This emphasize the significance of the subsolar point in relation to the magnetic equator as one of the major reasons for NmF2 variations, aside daytime meridonial wind. From Fig. 8(b), higher NmF2 magnitudes were observed in equinoxes than in solstices. This is because the intensity and the duration of solar radiation incident on the earth's surface depends largely on the position of the sun relative to the earth; which subsequently influences the rate of ionization in the ionosphere. On a broader perspective, Bolaji et al., (2017) using TEC data from 27 GPS receivers and 3 ground-based magnetometers to characterise EIA morphology had reported simultaneous observations of EIA crests in both hemispheres of Africa-Middle East, and whose morphology is different from that observed over Asia. Adeniyi and Joshua (2014) had shown that the likelihood of the occurrence of a noon-bite out is reduced with increase in geomagnetic latitude. They further submitted that beyond about 22 0N geomagnetic latitude, the noon-bite out effect is hardly visible. Generally, the strength of the equatorial plasma fountain and thermospheric neutral winds have been suggested to be the most essential processes affecting the formation of EIA (Rishbeth, 2000; Lin et al., 2007). On a global longitudinal variation structure of EIA, the three longitudinal dependencies of EIA occurrence according to Walker (1981) are (i) displacement of the magnetic and geographic equators, (ii) equatorial electric and magnetic fields variations, and (iii) magnetic declination angle at the magnetic equator. Unfortunately, these three factors, according to Sagawa et al., (2005) could not explain all aspects of the observed longitudinal structure. More importantly, the type in which a

the peak electron density during the high (1992) and moderate (1993–1994) solar activities when compared with the low solar activity years (1995–1996). However, Fig. 9(d) presents the bar chart of the average Ey, NmF2, and h’F at DAR, OUA, and KOR during the descending phase period. Magnitudes of Ey and NmF2 were simultaneously presented on the primary y-axis (LHS) while h’F was presented on the secondary y-axis (RHS). Fig. 9(d) revealed that the zonal electric field at OUA (Geomag. Lat. 0.590 N), recorded the highest magnitude of 0.34 mV/m and minimum NmF2 magnitude of 0.92 1012 em3. As we move progressively away from the dip equator towards KOR (Geomag. Lat. 1.26 0N), the magnitude of Ey reduces to 0.32 mV/m with an increased NmF2 magnitude of 0.96 1012 em3 compared to that obtained at OUA. A further move towards DAR (Geomag. Lat. 2.32 0N) revealed the minimum magnitude of Ey (0.22 mV/m) with a corresponding maximum value of NmF2 (1.15  1012 em3). This further situate the influence of fountain effect at these stations. Ey during the post-sunset period drives the F-region altitude up creating the fountain effect in the evening quadrant of the day; thus revealing a general direct relationship between Ey and h’F across all stations. It may therefore be suggested that the altitude dependence of the F-layer bottomside is a function of dip latitude. 4.1. Global reports on EIA and their comparison with the present study The plasmas which are lifted to higher altitudes through the action of fountain effect diffuses along the magnetic field lines through the activity of gravitational and pressure gradient forces. These action consequently result in the emergence of a double-humped (crests) latitudinal distribution of ionization regarded as the EIA (having the depleted ionization region in-between the crests as the trough). For a strong and welldeveloped EIA, NmF2 will exhibit remarkable variability with local time, altitude, and latitude. For the three EIA stations in the present study, NmF2 variability at different latitudinal locations was established with the NmF2 observation at DAR having higher magnitude due to fountain effect and peaks appearing about 3–4 h after that observed at KOR and OUA which are closer to the dip equator. This is because as the strength of the electric field increases, the additional plasmas lifted to higher latitudes over the equator get transported to farther latitudes away from the equator. The result presented by Venkatesh et al., (2015) 94

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seemingly small longitude difference (about 40
) could cause a large EIA change. This led to the assertion that forcing from below (i.e. tide and planetary wave) may be the other factor determining the longitudinal characteristics of EIA development. Further, Thuillier et al., (1976) attributed the longitudinal variations of EIA to field-aligned plasma emanating from the action of neutral wind in the F-layer. The field-aligned plasma transport modifies the development of the EIA so that the asymmetry of two crests increases with increasing plasma transport velocity (Hanson and Moffet, 1966). The nighttime global characteristics of EIA presented by Sagawa et al., (2005) revealed that other forcing like non-migrating tide originating from the lower atmosphere (having wave number 4 feature) may be responsible for the longitudinal variation of EIA. The wave number 4 is determined as the peak-to-peak or dip-to-dip separation of approximately 90
longitude (one-fourth of 360
) characteristics. The authors further submitted that the EIA development is significantly suppressed mostly in the region around 60
longitudes. If to go by these, then the variation in NmF2 variation across the three stations in the present study is expected to be of latitudinal origin. This is because the present station longitudinal coverage is between 58
and 71
, which is within the range reported by Sagawa et al. (2005) regarding inactive EIA activity.

Table 5 Annual variation in NmF2 noon bite-out intensity for different years of the descending phase of the SC-22 during the pre- and post-noon episodes.

The intensity of the noon-time bite-out of NmF2 across the three stations of investigation were estimated both annually and seasonally in order to properly situate the effect of the EIA on the stations. The intensity (Ibt) is measured using the relationship in equation (10) [e.g. Joshua et al., 2014) (10)

where Mp is the maximum NmF2 (pre-noon or post-noon) peak magnitude before or after the noon bite-out, and Md is the minimum depression during the bite-out. Table 4 revealed that for any season, the intensity (Ibt) of NmF2 noon-bite-out is highest at DAR owning to fountain effect and maximizes in March-E season. However, Table 5 shows that the intensity reduces with decreasing solar activity during both pre-noon and post-noon periods.

D-season (1012 em3)

J-season (1012 em3)

Pre-noon: DAR OUA KOR Post-noon: DAR OUA KOR

2.84

2.43

2.72

a

2.51 1.93 3.08

1.90 1.65 2.86

2.23 1.89 2.61

2.52 2.04

2.12 1.83

2.18 1.94

a

1995 (1012 em3)

1996 (1012 em3)

Pre-noon: DAR OUA KOR Post-noon: DAR OUA KOR

3.28

2.60

2.01

1.74

1.47

3.16 2.64 3.64

2.21 2.14 2.96

1.74 1.73 2.32

1.56 1.45 2.01

1.27 1.26 1.75

3.06 2.66

2.32 2.18

2.04 1.79

1.66 1.50

1.44 1.43

PREpeak ¼ 0:158 ðRÞ þ 1:495

(11)

The model had earlier been used to deduce the annual PREpeak value for the Ilorin ionosphere during year 2010 (a year of low solar activity) and the results obtained are similar to that of the observed annual magnitude with 100% accuracy [see Adebesin et al., 2015a). Consequently, since the three stations under consideration in the present study are around the African EIA trough, it is necessary to further validate the submission of Adebesin et al., (2015a) in this work by observing the plasma drift variation with R during the descending phase. Fig. 11 depicts the annual magnitude pattern of PREpeak. The modelled pattern (MODEL) is obtained from equation (11) and the respective annual PREpeak pattern observations at DAK, OUA, and KOR that was obtained by averaging 12 months PREpeak values (constituting each year) over each station. The ‘MEAN’ on the horizontal axis is the overall median of the entire years (1992–1996) during the descending phase. The Figure revealed that there is a better relationship between the modelled PREpeak and the observed, especially for the low solar activity years 1994–1996. We observed that the performance between the modelled and the observed Vp increases with decreasing sunspot activity.

Table 4 Variation in NmF2 noon bite-out intensity for different seasons during the preand post-noon episodes. September E-season (1012 em3)

1994 (1012 em3)

A regional vertical plasma drift model for the equatorial ionization anomaly (EIA) trough in the African sector during the PRE peak period had earlier been implemented by Adebesin et al., (2015a). The model projected a good relationship between the monthly/seasonal/annual evening time PRE peak magnitude and the international sunspot number (R). This is because for any given monthly/seasonal/annual value of R, the corresponding PRE peak magnitude can be deduced to the nearest accurate value. Ouagadougou ionosonde data spanning 1966–1998 had been used to infer the drift pattern for the analysis. The relation as highlighted by the authors is given as:

According to Perna and Pezzopane (2016), improvement in ionospheric models can be achieved primarily by studying the relationship between ionospheric characteristics and some solar activity indices. Consequently, the sunspot number relationship with PREpeak is featured in this section. Fig. 10 highlights the correlation relationship of the average monthly variation of PREpeak with the international sunspot number (R) during the entire descending phase for each of the stations considered (a-c). The h’F data (and hence Vp) is not available for 1992 at

March E-season (1012 em3)

1993 (1012 em3)

5.1. Validation of the African equatorial regional drift model obtained from past observation

5. Variation observations of descending phase monthly PREpeak mean value with sunspot number at all stations

Event/Station

1992 (1012 em3)

KOR. Other areas of gaps in plot in Fig. 10(a–c) are periods when data are unavailable. The PREpeak/R have 79%, 81%, and 69% correlation at DAR, OUA, and KOR respectively. The fourth panel (Fig. 10(d)) is a monthly average bar-chart of PRE during the entire descending phase period in all of the stations. This monthly average value was obtained from the hourly values of PREpeak for similar months spanning 1992–1996. Also, we obtained the monthly median value (i.e. average median for January are the individual median PREpeak magnitudes for January 1992–1996). At OUA and DAR, the highest/lowest PREpeak magnitude was obtained in March/June, and highest/lowest PREpeak was found in November/June at KOR. Further, the average highest value of PREpeak was obtained in March at OUA and DAR, while it appeared in November at KOR and the lowest in June for all stations. It is also worth mentioning that we found semi-annual asymmetry in the PREpeak drift variation in Fig. 10(d). The first peak occurred in March while the other peak occurred in October/November at all stations. See Rishbeth et al., (2000) for more on semi-annual asymmetry.

4.2. Intensity of the noon-time bite-out

Ibt ¼ Mp þ Md

Event/Station

1.89 1.47 a

1.95 1.53

Not available as noon bite-out is missing. 95

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Fig. 11. Variation pattern in the modelled annual PREpeak versus observational pattern over DAK, OUA, and KOR. The ‘MEAN’ on the horizontal axis depicts the entire descending phase PREpeak magnitude.

h  2 i1=2 RMSdev ¼ sum model PREpeak observed PREpeak =N

(12)

N, being the total number of data points considered, revealed values of 3.612, 1.526, and 0.756 respectively for DAR, OUA, and KOR. This points to the fact that the Adebesin et al., (2015a) regional model performed best at KOR, followed by OUA, and least in DAR. This assertion is further supported with the error analysis of the relationship between the modelled and the observed PREpeak drift presented in Table 7. The error value was obtained from the relation in equation (13) Errorvalue ¼ absolute value of ½observedvalue ðjÞ  modelledvalue =modelledvalue (13) where j represent the respective PREpeak value at KOR, OUA and DAR respectively. The bolded value across each row is the least error value for each annual observation. The lower the value, the better the relationship between the observed and the modelled parameter. The error value ranges between 18.6 and 30.5%, 1.0–7.5%, and 2.4–11.7% for DAR, OUA, and KOR respectively. For the entire descending phase, the lowest error value was observed at KOR, and followed by OUA. The fact that the model performed best at KOR when compared to OUA (from which station the data for the model equation was generated) implies that the climatology characteristics is about the model and not where the data for the model was obtained. If it were to be about the data station, one would have expected that OUA should perform best. In essence, we suggest that the station in the African region where the model data was generated is insignificant, but only need to be in the EIA region.

Fig. 10. (a–c) Monthly variation of plasma drift during PRE with sunspot number for the entire descending phase years. Inset is the corresponding correlation coefficient between the two parameters for each station. (d) Monthly average PRE variation bar-chart for the three stations.

Furthermore, Table 6 highlights the variation pattern in the annual/ entire descending phase PREpeak magnitude between the model and the observed Vp. Also, Table 6 shows the percentage difference/agreement between the model and the observed drifts. The ‘MEAN’ represents the sunspot average during descending phase. We found evidence that both the annual and the sunspot descending phase of PREpeak revealed better relationship between the model and observations for the three stations. However, the annual percentage relationship between the model and the observed was better at OUA and KOR (ranging from 85 to 99%) compared to OUA and DAR (70–81%). Also, during the sunspot descending phase (bolded values), the relationship revealed averaged percentage values of 79.1, 92.4, and 94.9% for DAR, OUA, and KOR respectively. We also employed the root-mean-square deviation (RMSdev) statistics. Here RMSdev represents the sample standard deviation of the differences between the modelled and the observed values at each of the three locations. RMSdev has been found to be a useful tool in studies related to climatology and forecasting in the verification of experimental results (Adebesin et al., 2014). Thus, the RMSdev between the modelled and the observed plasma during PRE peak magnitude is defined by the expression in equation (12)

6. Summary Ionospheric data were used to study ionospheric climatology at OUA, KOR, and DAR, three stations at the EIA trough in Africa. Our study during the descending phase of sunspot cycle 22 revealed dramatic and significant local time, seasonal, annual, phase, latitudinal, and longitudinal variation in h’F, NmF2, Vp, and Ey. Variability in the drift velocity pattern may have increased from the action of solar flux variations and additionally through the enhanced outflow of aurora region electric fields into the low-latitudes and from large perturbations in the global wind system. This work considered vertical drifts as a vital factor responsible for the dynamics of F2 region ionosphere, as the presence of a strong PRE will always trigger irregularities. We summarize our results as follows:

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Table 6 Variation pattern in the annual PREpeak magnitude between the model and observed plasma drift (‘MEAN’ indicates the average for the descending phase 1992–1996). YEAR

1992 1993 1994 1995 1996 MEAN

Annual Model PREpeak (m/s) 22.51 13.52 8.59 5.46 3.33 10.68

Annual Observed PREpeak magnitude (m/s)

Diff. between Model and Observed (m/s)

% Difference between Model and Observed

% Agreement in magnitude between Model and Observed

DAK

OUA

KOR

DAK

OUA

KOR

DAK

OUA

KOR

DAK

OUA

KOR

16.07 9.40 6.28 6.48 3.96 8.44

19.33 13.01 7.62 6.04 3.36 9.87

21.97 12.64 7.34 5.74 2.94 10.13

6.44 4.12 2.31 1.02 0.63 2.24

3.18 0.51 0.97 0.58 0.03 0.81

0.54 0.88 1.25 0.28 0.39 0.55

28.6 30.5 26.9 18.7 18.6 20.9

14.1 3.8 11.3 10.6 0.9 7.6

2.4 6.5 14.6 5.0 11.7 5.1

71.4 69.5 73.1 81.3 81.4 79.1

85.9 96.2 88.7 89.4 99.1 92.4

97.6 93.5 85.4 95.0 88.2 94.9

peak however shows clear sunspot activity and seasonal dependence at all stations; 6. Past works in the African sector and the present study revealed that PREpeak amplitude in J-season is well comparable to/or higher in some instances to those observed during the E and D-seasons during episodes of either quiet or active magnetic activities. However, during the combined quiet and disturbed magnetic activity, PREpeak amplitude in J-season on the average is smaller when compared to other seasons. A comparison of the evening time PREpeak revealed that PREpeak magnitude is generally lower in the African sector than in other sectors, thus exhibiting longitude dependency. It maximizes in the equinoxes, and followed by D-season. This seasonal and longitudinal dependence of the evening drifts suggests the importance of ionospheric conductivity in the nighttime equatorial ionosphere electrodynamics. 7. The observations from the Ey/h’F and Ey/NmF2 relationships suggest that Ey is more active in lifting the ionospheric height profile and supressing the amplitude of the peak electron density during the high (1992) and moderate (1993–1994) solar activities when compared with the low solar activity years (1995–1996); 8. The hemispheric difference in NmF2 is more significant during high solar activity year and reduces with decreasing solar activity. For any season, the intensity (Ibt) of NmF2 noon-bite-out is highest at DAR and maximizes in March-E season.

Table 7 Error in the annual PREpeak magnitude between the model and observed plasma drift. Year

1992 1993 1994 1995 1996 MEAN value of Descending Phase

Error between Model and observed DAR

OUA

KOR

0.2861 0.3047 0.2689 0.1868 0.1892 0.2101

0.1413 0.0377 0.1129 0.1062 0.0090 0.0758

0.0240 0.0651 0.1455 0.0513 0.1171 0.0521

1. Better coherence in variability of h’F, NmF2, and Vp was obtained between OUA and KOR (which are closer in distance  320 km) when compared with OUA and DAR (1747 km). The coherence is attributed to latitudinal distribution of plasma by equatorial fountain effect. Seasonal and annual signatures of equatorial fountain effects were obvious. The magnitude difference in the variation of NmF2 between OUA and KOR reduces with decreasing solar activity; 2. Though the present study presents an empirical investigation into the climatology of the EIA region in the African sector, it also provides opportunity for validating previous results, in terms of similarities and differences. This work is the first to show higher bite out at the equatorial northern station (DAR) than equatorial southern station (KOR) in the African sector; 3. A decrease in the vertical plasma drift results in the NmF2 increase. The decrease in magnitude trend during the solar activity descending phase is more obvious for the NmF2 response than for h’F observation at all stations. While NmF2 magnitude maximizes/minimizes during the E-seasons/J-season, h’F attained highest/lowest altitude during the J-season/D-season for all stations; 4. The PREpeak/sunspot number (R) regional model developed by Adebesin et al., (2015a) for the evening/nighttime period in the African sector inferred from ground-based ionosonde measurement performed well with the observed data at all stations, with a little over-estimation by the modelled equation. The entire descending phase recorded 94.9, 92.4, and 79.1% correlation for the model versus observed drift pattern and root mean square deviation of 0.756, 1.526, and 3.612 at KOR, OUA and DAR respectively. The error value between the modelled and observed drift pattern ranges from 18.6 to 30.5%, 1.0–7.5%, and 2.4–11.7% for DAR, OUA, and KOR respectively. For the entire descending phase, the lowest error value was observed at KOR, and followed by OUA. The performance agreement between the modelled and the observed vertical plasma drift PREpeak magnitude increases with decreasing sunspot activity; 5. Semi-annual asymmetry in the monthly annual response of electron density was observed at all stations with the first peak in March and the second in October/November months. D-season anomaly was also observed at all stations. Two distinct peaks – the pre-noon and the post-noon peaks were observed for the NmF2, and can occur for any season and sunspot activity level. The amplitude of the post-noon

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