Equatorial vertical E × B drift velocities inferred from ionosonde measurements over Ouagadougou and the IRI-2007 vertical ion drift model

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Advances in Space Research 46 (2010) 604–612 www.elsevier.com/locate/asr

Equatorial vertical E  B drift velocities inferred from ionosonde measurements over Ouagadougou and the IRI-2007 vertical ion drift model O.S. Oyekola *, L.B. Kolawole Department of Physical sciences, College of Natural sciences, Redeemer’s University, Redemption City, Mowe, Ogun State, Nigeria Received 7 November 2009; received in revised form 31 March 2010; accepted 1 April 2010

Abstract F-region vertical plasma drift velocities were deduced from the hourly hmF2 values acquired from ionogram data over a near dip equatorial station Ouagadougou (12.4°N, 358.5°E, dip angle 5.9°N) in Africa. Our results are compared against the global empirical model of Scherliess and Fejer (1999) incorporated in the IRI model (IRI-2007) for 1600 to 0800 LT from 1 year of data during sunspot maximum year of 1989 (yearly average solar flux intensity, F10.7 = 192) corresponding to the peak phase of solar cycle 22, under magnetically quiet conditions. The drifts are entirely downward between 2000 and 0500 LT bin for both techniques and the root mean square error (RMSE) between the modeled and the ionosonde vertical plasma drifts during these periods is 3.80, 4.37, and 4.74 m/s for June solstice, December solstice and equinox, respectively. Ouagadougou average vertical drifts show evening prereversal enhancement (PRE) velocity peaks (VZP) of about 16, 14, and 17 m/s in June solstice, December solstice, and equinox, respectively, at 1900–2000 LT; whereas global empirical model average drifts indicate VZP of approximately 33 m/s (June solstice), 29 m/s (December solstice), and 50 m/s (equinox) at 1800 LT. We find very weak and positive correlation (+0.10376) between modeled VZP versus F10.7, while ionosonde VZP against F10.7 gives worst and opposite correlation (0.05799). The results also show that modeled VZP–Ap indicates good and positive correlation (+0.64289), but ionosonde VZP–Ap exhibits poor and negative correlation (0.22477). Ó 2010 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Equatorial ionosphere; Vertical drifts; Modeling and forecasting; Ionosonde technique; IRI-2007

1. Introduction A detailed understanding of equatorial vertical ion drifts can provide valuable information to numerous electrodynamical phenomena, particularly at nighttime equatorial and low-latitude ionosphere. In addition, ionospheric drifts at equatorial F-region altitudes in the African region are not sufficiently studied. Equatorial ionospheric vertical plasma drifts and zonal electric fields are controlled by complex E- and F-region electrodynamics processes, which are known to vary significantly with local time, season, solar cycle, and geomagnetic longitude (e.g., Fejer, 1997).

*

Corresponding author. Tel.: +234 7055565259. E-mail address: [email protected] (O.S. Oyekola).

A detailed understanding of this large variability during quiet and storm time is basic for improved prediction of the equatorial ionospheric climatology and weather (equatorial spread F and plasma bubbles) (Fejer et al., 1999), particularly in the nighttime equatorial F-region. The morphology of equatorial F-region plasma drifts has also been extensively examined using coherent and incoherent scatter radar observations at the Jicamarca Radio Observatory (Geographic: 12°S, 76.87°W; Magnetic dip 2°N) (e.g., Fejer et al., 1989, 1991; Fejer, 1997; Woodman, 1970; Woodman et al., 2006). These measurements have determined the dependence of the F-region plasma drifts on season, solar cycle, and magnetic activity. Their upward velocities of 20–25 m/s during the day and downward velocities of about the same magnitude at night were typical for high solar activity period. They also observed a rapid increase

0273-1177/$36.00 Ó 2010 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2010.04.003

O.S. Oyekola, L.B. Kolawole / Advances in Space Research 46 (2010) 604–612

in the upward velocity, commencing around the dusk and lasting between 1 and 2 h, which is a consistent feature appearing everyday with regularity, after which the velocities reverse to downward. Typical value of prereversal velocity enhancements may be as high as 40 m/s. The amplitude and duration of this prereversal peak in the upward velocities vary from one longitudinal region to another and with season, showing a remarkable dependence on magnetic declination (Batista et al., 1986). Fejer et al. (1979) showed that during geomagnetic storm conditions, vertical electrodynamics plasma drift are somewhat inhibited. Low-latitude vertical plasma drifts observations were made by the Ion Drifts Meter (IDM) probe on the board the low inclination (19.96°) Atmospheric Explorer-2 (AEE) satellite and the high inclination (90°) Dynamic Explorer-B (DE-2) (Coley et al., 1990; Coley and Heelis, 1989; Fejer et al., 1995). The results obtained from satellites are essentially identical with those obtained from Jicamarca radar measurements. They pointed out the occurrence of large longitudinal effects. In addition, zonal electric fields in the equatorial ionosphere were made by San Marco satellite during April–August 1988 (Maynard et al., 1995). Of recent, Fejer et al. (2008) presented quiet-time equatorial F-region vertical plasma drift model using ROCSAT-1 observations. They showed that satellite data were generally in good agreement with the model drift presented by Scherliess and Fejer (1999) especially during equinox. It is important to note, however, that the daytime satellite drifts observations are more reliable than the nighttime measurements (Scherliess and Fejer, 1999). Satellite usually provides good spatial but poor temporal coverage also. Scherliess and Fejer (1999) combined the AE-E data set of Fejer et al. (1995) with large Jicamarca incoherent scatter radar observations from 1968 to 1992 to present the first detailed global empirical model for the quiet F-region equatorial vertical drifts that take into account their diurnal, seasonal, solar cycle, and longitudinal variations. On the basis of this global equatorial vertical drift model, the authors suggested that future studies should incorporate data from ground-based observations other than Jicamarca, such as ionosonde measurements but should be fully validated first. In the equatorial region in African continent, there is a difficulty of paucity of data due to lack of ground-based observational instrumentations. This presents a serious problem in understanding the climatology of vertical ion drift that can provide important insight into many ionospheric phenomena of the region. The best one can do at the moment is to infer equatorial vertical drifts from available ground-based ionosonde. These ionosonde drifts have been comprehensively reported mainly in the evening and nighttime period (e.g., Oyekola et al., 2006, 2007, 2008; Oyekola, 2006, 2007; Oyekola and Oluwafemi, 2007, 2008). Except for the magnitudes of the drifts, the general characteristics of the average F-region vertical drifts are similar to that obtained from other ground-and spaced-

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based observational techniques. Although this technique largely underestimates the vertical plasma drifts when the F-region is not high enough due to the effects of plasma recombination (Bittencourt and Abdu, 1981; Fejer et al., 1989; Batista et al., 1990). Details of the limitations of deriving vertical plasma drifts from the ionosonde measurements have been given by Oyekola (2009a). The aim of this study is to compare and validate the vertical plasma motion inferred from F2-peak height determined from ionosonde observations at Ouagadougou, Burkina Faso (West African sector), with the predictions from the IRI-2007 vertical ion drift model at this latitude during a period of high solar activity year of 1989. 2. Data The database we have examined in this paper comprises of hourly values of monthly median of the F2-layer ionospheric propagation factor, M(3000)F2 obtained over a period of January to December 1989 at Ouagadougou, Burkina Faso, in West Africa, an ionospheric station near the dip equator. We only consider data for the 1600–0800 LT bin. In order to obtain the vertical drift velocities, we first transform M(3000)F2 to hmF2 using the formulae proposed by Shimazaki (1955). Although Shimazaki’s formula assumes the idealized case of radio waves reflected from a parabolic F2-layer above a spherical Earth: hmF2 = {1490/M3000}  176. This technique has been used by Goel et al. (1990) and Rishbeth et al. (2000). Hourly values of hmF2 were subsequently analyzed. In brief, three seasons were chosen: June solstice (May–August), December solstice (November–December, January–February), and equinox (March–April, September–October). The observed seasonally averaged vertical plasma drift velocities were estimated from the 4-month seasonal mean of hmF2 by inferring the time rate of change of hmF2, DhmF2/Dt. Detailed information of the procedure for measuring F-region vertical drifts from hmF2 can be found in our previous publications (e.g., Oyekola et al., 2007, 2008; Oyekola and Oluwafemi, 2008). The prereversal peaks in the upward velocities were deduced from the local time variation of vertical drift in each month. The corresponding 1-h monthly median values of global equatorial vertical drift model of Scherliess and Fejer were downloaded directly from web site: http://ccmc.gsfc.nasa.gov/modelweb/models/iri_vitmo.php. For the purpose of analysis, we also downloaded the monthly averaged values of 10.7 cm radio flux, F10.7, as proxy of the solar activity, and the corresponding Ap indices were also downloaded to characterize the geomagnetic activity. During these 12 months, the solar 10.7 cm radio flux varies considerably in the range 182–240 flux units with an average value of 192, a representative of high solar activity. Geomagnetic Ap index also varies appreciably with typical value between 11 and 36 with mean value of 19. Notice that the values of vertical drift obtained from the time rate of change of peak F2-layer height between the consecutive 1-h interval is a realistic representation of the

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true vertical drift velocities at F-layer altitudes (>300 km) (Bittencourt and Abdu, 1981) but during the period of lower F-layer heights (<300 km), the chemical recombination could cause an apparent vertical drift that needs to be corrected for, in order to obtain realistic drift velocity values at such hours. Also the heights between 150 and 300 km are sometimes contaminated by strong echoes from the electrojet irregularities that the most accurate measurements occur in the range between 300 and 450 km (Woodman, 1970). In the data to be presented in this paper, we have not carried out any correction, thus the drifts are to be considered as real F-region vertical drift since F-layer heights are near and above threshold value of 300 km during the hours under study. The scatter bars superposed on the ionosonde-deduced drift velocities in Figs. 1–3 are the standard error of the means, which provide estimation of uncertainty in the ionosonde vertical plasma drift. It is typically about 2 m/s for all seasons.

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3. Results In this section, we initially present diurnal and seasonal variations of the vertical drifts inferred from the ionosonde measurements at F-region altitudes over Ouagadougou during early evening and post-sunrise period (1600–0800 LT) and compare them with their respective IRI-2007 representations. Afterward, we compare the dependence of the evening prereversal velocity peak on solar and geomagnetic activity for both inferred and modeled drifts.

Fig. 2. Same as Fig. 1, but for December solstice (November–February) seasonal period.

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Local Time (hours) Fig. 1. Comparison of the average IRI-2007 model prediction for the quiet-time F-region equatorial vertical ion drifts in the African sector with ionosonde-inferred vertical E  B drift velocities during June solstice (May–August) for high solar activity year of 1989 under magnetically moderate conditions. Corresponding 4-month average of solar (Sa = 187) and geomagnetic (Ap = 17) indices are shown. The scatter bars denote the standard error of the means.

For the purpose of comparison, Fig. 1 shows the average F-region vertical drift variations derived from the ionosonde measurements plotted against those values predicted by the global empirical model of Scherliess and Fejer (1999) for June solstice high solar flux conditions. As can be seen, ionosonde drift remains downward (negative) between 1600 and 1800 LT and then increases rapidly from a value of about 4 m/s at 1800 LT to a maximum value of about 16 m/s at about 2000 LT. Then the drift turns downward again at 2030 LT, followed by a large oscillation of vertical drift until the drift reverses upward

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(positive) at 0500 LT. In contrast, the modeled drift rises gradually to a value near 33 m/s at 1800 LT. Thereafter, the drift reverses downward at about 1930 LT, followed by hour-to-hour variation till the drift reverses upward at 0600 LT. One may note that during June solstice the discrepancy is significant not only in magnitude but also in direction. Fig. 2 illustrates detail comparison between observed and modeled vertical drifts during December solstice. Fig. 2 also indicates many features of interest about the usual behavior of equatorial vertical ionospheric drifts at F-region heights, which we have previously discussed in Fig. 1. Specifically, we can see the following characteristics. First, ionosonde drift shows an evening prereversal velocity peak (PRE) of 14 m/s near 1900 LT and evening reversal time of about 2000 LT. IRI-2007 vertical drift model indicates PRE value of approximately 29 m/s at 1800 LT and an evening reversal time of 1900 LT. Clearly, the magnitude of the modeled PRE is roughly twice that of the observed PRE. Observe that the IRI-2007 model prediction of vertical drift also show PRE value that is twice that of ionosonde-derived PRE during June solstice (Fig. 1). Second, the magnitudes of vertical drifts deduced from the hourly hmF2 values differ drastically from their IRI-2007 representation. Further, the values of downward velocities from 2000 to 0600 LT interval for our observations are smaller by about a factor of 4 than the magnitudes of the modeled drifts. Three, the local time of PRE occurs later than the peak time predicted by the Scherliess and Fejer (1999) model. Comparing Fig. 2 with Fig. 1, we note instantly that the local time of occurrence of the evening peak velocity in the observational data is delayed with respect to its model representation. Fourth, we also note that the observed December solstice data appears to fluctuate moderately than the June solstice ionosonde drifts, which vary greatly. Fifth, the features of the evening and nighttime ionosonde-inferred vertical drifts variation in June solstice are considerably different, and the deviation from the IRI-2007 is more outstanding than they are in December solstice. Fig. 3 presents a comparison of F-layer vertical drifts estimated from ionosonde measurements with their IRI description during the equinox. In this season, the general characteristics are rather similar to those reported for both solstices (Figs. 1 and 2). Here, the value of evening prereversal velocity enhancement for Ouagadougou drift is about 17 m/s at 1900 LT and reversal time of 2000 LT. On the other hand, modeled PRE shows a magnitude of approximately 50 m/s at 1800 LT and a reversal time of 1900 LT. Actually, modeled PRE value is very much larger than the observed PRE magnitude by a factor of roughly 3. The pattern of behavior of vertical drift over Ouagadougou is somewhat consistent with that predicted by the Scherliess and Fejer (1999) model, especially between the PRE hours (1800–1900 LT) till around sunrise period (0500 LT). Further, the downward drift velocities from 2000 to 0500 LT for ionosonde drifts generally much lower in values than

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the predicted values. To qualify the difference, we find that ionosonde drifts during the equinox are smaller by a factor of 5 than its IRI representation when both are downward. From critical inspection of Figs. 1–3, the following points emerge: (1) IRI-2007 model representation of prereversal peak in the upward velocity is more or less double in the solstices, and triple during the equinox, as compared to the ionosonde PRE value. (2) The local time of occurrence of PRE is 1–2 h earlier in ionosonde than the times observed by the IRI. (3) Modeled reversal times near dusk occur by 1 h prior to the times seen at Ouagadougou. While the IRI reversal times near dawn occur 1/2–1 h later than the times observed by the ionosonde-inferred drift. (4) The root mean square (RMS) difference between the modeled and ionosonde drifts for 2000–0500 LT is 3.80, 4.37, and 4.74 m/s for June solstice, December solstice, and equinox, respectively. The RMS error is defined by sqrt{(E  B drift (IRI-2007) – E  B drift (ionosonde))2}/sqrt (N), where N is the number of data points used. Fig. 4 compares month-by-month value of empirical model PRE with that of ionosonde-inferred PRE. The modeled prereversal enhancement in the vertical drifts is noticeably larger than the ionosonde PRE in all the 12 months of high solar activity year of 1989. The IRI2007 model displays maximum during the equinox months (March, April, September, and October). For the ionosonde drifts, the maximum shows up in winter month (May) with a value near 25 m/s. In fact, the magnitude of peak velocity enhancement varies between 9–25 and 19– 50 m/s for ionosonde and IRI-2007 model, respectively, with monthly overall mean of about 17 and 40 m/s, in that order, for ionosonde and IRI. Thus, there exist substantial disparity between the PRE deduced from hourly hmF2 values and its IRI-2007 representation. The discrepancies between the ionosonde and modeled VZP are further checked with the solar and the geomagnetic effects by using simple linear fit to the data. In Fig. 5, the evening prereversal enhancements in vertical plasma drift, VZP values are plotted against F10.7 indices. The best-fit straight lines are also shown in Fig. 5. We find the following relationship for the IRI-2007: V ZP  0:0694ðF10:7Þ þ 24:0763;

ð1Þ

with correlation coefficient R of 0.1038, while for the ionosonde V ZP  0:0194ðF10:7Þ þ 20:8195;

ð2Þ

with R = 0.0579. Also it is apparent in Fig. 5, that evening peak velocities scatter considerably about the lines of fit. This implies that both the modeled and observed VZP may possibly not be influenced by solar variability. This conclusion should be regarded as tentative until more data are used for the analysis. It is further noticed that while the modeled VZP correlates positively with F10.7, observed VZP indicates anti-solar correlation with solar flux intensity. The ionosonde-inferred intercept on the VZP axis is about 13.5% lower than that estimated from the IRI model.

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Fig. 5. Prereversal velocity peaks as a function of solar flux for empirical model (left panel) and for ionosonde (right panel) during high solar activity year of 1989 for magnetically moderate conditions.

It should be pointed out that the seasonal variation in the PRE amplitude can more dominant during a given year than the variation with the monthly mean F10.7 during that year. Fig. 6 shows the behavior of VZP with geomagnetic Ap index during high solar flux for both the observed and the modeled, where again simple linear fits were used to

fit all the values. The fitted equation for the empirical model is given by: V ZP  1:1426ðApÞ þ 15:9952;

ð3Þ

with correlation coefficient R of 0.6429, whereas for the ionosonde we have

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V ZP  0:2001ðApÞ þ 20:8360;

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with R = 0.2248. These results evidently show that modeled VZP versus Ap indicates fairly good relationship and positive correlation, but link between the observed VZP against Ap exhibit poor relationship and negative correlation during high solar activity year of 1989. Here, ionosonde-derived vertical E  B drifts intercept on the VZP axis is approximately 23.2% greater than that deduced from the empirical model. Moreover, from Figs. 5 and 6 together with Eqs. (1)–(4), we note that the intercepts found in VZP versus F10.7 and VZP against Ap for both the observed and the IRI are positive. The values of the ionosonde intercepts are comparable, while that of IRI shows larger disparity. The gradients of the best-fit lines consistently fall below unity, except for the IRI VZP versus Ap plot. Consequently, we conclude that solar activity and geomagnetic variation appears not to have any relation with evening E  B vertical peak velocities using our limited data statistics. The reason for this lack of correlation is the mixing up of the more dominant seasonal dependence of the PRE with the dependence on F10.7. 4. Comparison with other results Comparisons between vertical drift velocities at F-region altitudes among different techniques have been reported in plentiful earlier studies (e.g., Fesen et al., 2000; Abdu et al., 2006; Anderson et al., 2004; Bertoni et al., 2006; Oyekola, 2006; Woodman et al., 2006). Fesen et al. (2002) using the National Centre for Atmospheric Research ther-

mosphere/ionosphere/electrodynamics general circulation model (TIEGCM) calculated the zonal and vertical plasma drifts for equinox, June, and December for low, medium, and high solar activity and compared the results with vertical plasma drifts at Jicamarca averaged over heights from 300 to 400 km. The authors pointed out that the main discrepancies were in the nighttime drifts, particularly, for the vertical velocities. Extensive comparative studies have been carried out in the South American equatorial sector using groundbased observations, as mentioned earlier. Abdu et al. (2006) determined F-layer vertical drift from the time rate of change of true height D(hF)/Dt from the Digisonde true height iso density lines over Sao Luis, for December and June during the year 2000 and compared them with the vertical drift model of Scherliess and Fejer (1999). That year was a period of high solar activity with F10.7 = 188.7 in June and 175 in December. They noted that the evening vertical drift velocity due to the PRE peaks around 1900 LT in December with the reversal taking place at 1930 LT, whereas in June the vertical drifts present a broader post-sunset peak around 2000 LT with reversal occurring around 2200 LT. Our ionosonde vertical drifts showed an evening prereversal velocity peak near 1900 LT and a reversal time of 2000 LT in December solstice, while they occurred at 2000 LT, and a reversal time of 2030 LT, respectively in June solstice. Clearly, good agreement can be seen in digisonde and ionsonde-inferred vertical drift results with respect to the PRE characteristics just mentioned above. They further reported that the vertical drift peak amplitude in the IRI was perceivably smaller than the observed values during both December and June. This result is in sharp contrast with our observations. We found that the predicted evening peak velocity is

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twice the values of the ionsonde PRE magnitude during the solstices. Further, Abdu et al. (2004, 2006) presented a comparison of the vertical drift variation in June over Sao Luis and Jicamarca together with their IRI representations. The vertical drifts for both station were obtained from the time rate of change of true height from the digisonde during moderate solar activity year of 1999 with F10.7 = 151.9. Their results showed that the local time of the evening vertical drift peak is considerably delayed over Sao Luis as compared to that over Jicamarca. Peak velocity occurred later than the peak time predicted by the IRI at both locations, the delay is clearer over Sao Luis than over Jicamarca. The amplitude of vertical drift over Sao Luis (35 m/s) is larger than that over Jicamarca (20 m/s). They concluded that the observed mean vertical drift differ significantly from their IRI representation in June at both locations, the difference stands out more over Sao Luis than over Jicamarca. These results differ from ours with respect to the fact that our deduced PRE values are always significantly smaller than the model results in contrast to the vertical drifts deduced from Digisondes over Jicamarca and Sao Luis that always tented to be higher than the IRI model values. Bertoni et al. (2006) using data acquired with a digisonde DPS-4 and an Incoherent Scatter Radar (ISR) both from Jicamarca Radio Observatory, Peru, presented a comparison of ionospheric vertical drifts velocities derived from these two instruments during campaign periods at Jicamarca (12°S, 77°W geographic, 2°N dip angle), namely, October 07–11, 2002, and March 19–23, 2003, representative of equinox period. They showed that around sunset and evening hours, between 1700 and 2100 LT, and during the post-midnight (between 0200 and 0300LT) until about 0800 LT, Digisonde and ISR-derived vertical velocities are usually quite comparable, but disagree at other hours. Woodman et al. (2006) pointed out that ISR and digisonde portable ionsonde (DSP) vertical drift velocities over Jicamarca are only comparable around a large prereversal enhancement on equinox day. In June solstice PRE was small and the discrepancies are significant both in magnitude and direction. At other times the agreement is poor. In contrast to this our ionosonde-deduced PRE values are always significantly smaller than the IRI model, which agrees with the ISR vertical drifts. However, F-region ionosonde vertical plasma drifts could be improved if a proper theoretical model is used to take into account the effects of production and recombination (Woodman et al. 2006). In a study carried out by Batista et al. (1986), comparing coincident incoherent scatter and ionosonde F-region vertical drifts, they showed that the reversal times after sunset derived by the two techniques are in good agreement, although the ionosonde generally underestimates the prereversal peak amplitude. Anderson et al. (2006) demonstrated that realistic, low latitude, daytime, vertical E  B drift velocities can be obtained from ground-based magnetometer in the Peruvian sector and that the average, quiet day E  B drift velocity versus local time curves are in excellent agreement

with the Scherliess–Fejer climatological curves in this sector. A critical look at the comparative data presented above undoubtedly indicates that the F-region vertical plasma drift variability is well studied at the South American equatorial region. On the other hand, experimental results of F-region vertical drifts from diverse probes are inadequate even non-existent in many cases in the African region of equatorial and low-latitude ionosphere. Our results are generally consistent, at least, with the features of quiet-time F-region evening and nighttime electrodynamics plasma drifts found in our previous ionosondeinferred drifts studies over Ibadan (6°S dip angle) based on measurements over 12-month period (e.g., Oyekola, 2006, 2009b; Oyekola et al; 2008). Indeed, we noticed that the evening velocity peaks presented by Oyekola (2009b) calculated as D(h0 F)/Dt for Ibadan were as twice that of PRE peaks estimated for Ouagadougou as D(hmF2)/Dt in the present study. Even when the two stations are almost equidistance from the dip equator (Ibadan is in the southern hemisphere while Ouagadougou is in the northern hemisphere). Already, the seasonal, longitudinal, and solar cycle variations of PRE peaks are well documented in the literature (e.g., Fejer et al., 1991, 2008; Batista et al; 1996; Scherliess and Fejer, 1999; Oyekola, 2009a). 5. Discussion and summary The most important contributor to F-region ionospheric vertical drift motions is the ionospheric zonal electric fields. The quiet-time low-latitude electrodynamics and electric field is believed to be generated by the E- and F-region neutral wind dynamos of the coupled ionosphere-thermosphere system (e.g., Rishbeth, 1971, 1981; Richmond et al; 1976; Stening, 1981; Heelis et al., 1974; Eccles, 1998) with exception of high latitude penetration fields during magnetic storms (Fejer and Scherliess, 1997). The relative efficiencies of the E- and F-region dynamos change significantly with season and solar activity and throughout the day (Fejer and Scherliess, 2001). Tidal winds in the MLT region play crucial roles in the ionospheric E- and F-region wind dynamo field (Takahashi et al., 2006). Using a coupled atmospheric/ionospheric model, Millward et al. (2001) showed that lower thermospheric tides could strongly modulate the dayside dynamo electric fields while having practically no effect on the PRE. The authors attributed the reason in part to the phase of the tidal forcing approaching zero at the terminators. We have presented a comparative analysis between IRI2007 model predicted vertical drifts and vertical ionization velocities derived from ionospheric measurements obtained with an ionosonde operated at Ouagadougou, Burkina Faso, a near magnetic equatorial station in African. Our results show that the IRI-2007 predictions of equatorial E  B drifts is largely an over estimate compared to the ionosonde drifts from early evening and near post-sunrise hours at all seasons during high solar flux conditions. Prereversal enhancement peak upward drifts in the IRI are

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persistently and appreciably larger in values than the observed values in each month. The local time of occurrence of evening peak velocities over Ouagadougou is considerably delayed with respect to the peak time predicted by the IRI. The behaviors of vertical drifts variation in June solstice are significantly different, and the deviation from the IRI prediction is more distinct, than they are in December solstice and equinox. Ionosonde reversal time near dusk and dawn occurs at about 2000 and 0530 h LT, respectively, while IRI vertical drift evening and morning reversal time occur at about 1900 and 0630 LT, respectively, except for June solstice. More specifically, IRI evening reversal time is about 1/2 or 1 h earlier than the reversal time seen in IRI. The relationship between VZP and solar F10.7 and geomagnetic Ap for both observed and modeled drift is complex as evident in the values of regression parameters. More analysis would be needed to study the phenomenon. Nonetheless, the differences that are noticed to exist will be valuable for advancement and update of the IRI model at equatorial latitudes, in particular, for the evening and nighttime longitudinal region considered in the present study, where there is dearth of observations. Indeed, the results here are less satisfactory but still reasonable. Finally, the discrepancy between ground-based vertical drift-inferred measurements and the global equatorial vertical ion drift model predictions are an exceptional challenge. Acknowledgements The authors earnestly thank the two referees for their work on this paper and the insightful comments and suggestions provided in the assessment. The suggested modifications helped improve the paper and make it clearer and easier to read. The author also acknowledge the kindness of the US National Oceanic and Atmospheric Administration (NOAA) for providing equatorial vertical plasma drift data from the website: http://nssdc.gsfc.gov/space/model/ models/iri.html. References Anderson, D., Angel, A., Chau, J., Veliz, O. Daytime vertical E  B drift velocities inferred from ground-based magnetometer observations at low latitudes. Space Weather 2, S11001, doi:10.1029/2004SW000095, 2004. Anderson, D., Anghel, A., Chau, J.L., Yumoto, K. Global, low-latitude, vertical E  B drift velocities inferred from daytime magnetometer observations. Space Weather 4, S08003, doi:10.1029/2005SW000193, 2006. Abdu, M.A., Batista, I.S., Reinisch, B.W., Carrasco, A.J. Equatorial Flayer height, evening prereversal electric field, and night E-layer density in the American sector: IRI validation with observations. Adv. Space Res. 34, 1953–1965, 2004. Abdu, M.A., Batista, I.S., Reinisch, B.W., Sobral, J.H.A., Carrasco, A.J. Equatorial F region evening vertical drift, and peak height, during southern winter months: a comparison of observational data with the IRI descriptions. Adv. Space Res. 37, 1007–1017, 2006. Batista, I.S., Abdu, M.A., Medrano, R.A. Magnetic activity effects on range type spread F and vertical plasma drifts at Fortaleza and

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