Comparative analysis of nocturnal vertical plasma drift velocities inferred from ground-based ionosonde measurements of hmF2 and h′F

Journal of Atmospheric and Solar-Terrestrial Physics 122 (2015) 97–107

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Comparative analysis of nocturnal vertical plasma drift velocities inferred from ground-based ionosonde measurements of hmF2 and h′F B.O. Adebesin a,b,n,1, J.O. Adeniyi a, I.A. Adimula a, O.A. Oladipo a, A.O. Olawepo a, B.W. Reinisch c a

Department of Physics, University of Ilorin, Ilorin, Nigeria Department of Physical Sciences, Landmark University, P.M.B 1001, Omu-Aran, Kwara State, Nigeria c Lowell Digisonde International, LLC, 175 Cabot Street, Suite 200, Lowell, MA 01854, USA b

art ic l e i nf o

a b s t r a c t

Article history: Received 9 January 2014 Received in revised form 29 August 2014 Accepted 19 November 2014 Available online 23 November 2014

Variations in the evening/nighttime ionosonde vertical plasma drift velocities inferred from the time rate of change of both the base of the F-layer height (Vz(h′F)) and height of the peak electron density (Vz(hmF2)) from an equatorial station were compared for better description of the E
B drifts. For better interpretation, both results were compared with the Incoherent Scatter (IS) radar observations (Vz(ISR)) which is taken to be the most accurate method of measuring drift, and therefore the data of reference level. An equinoctial maximum and June solstice minimum in post-sunset pre-reversal enhancement (PRE) was observed for Vz(hmF2), Vz(ISR), and Vz(h′F). The percentage correlation between VzhmF2 and Vzh′F ranges within 55–70%. While PRE for Vz(hmF2) peaked at 19 LT for all seasons, Vz(h′F) peaked at 18 LT for September equinox and December solstice, and start earlier. The nighttime downward reversal peak magnitudes for Vz(hmF2) and Vz(h′F) are respectively within the range of  4 to  14 and  2 to  14 m/s; whereas Vz(ISR) ranges within  12 and 34 m/s; and the peak time was reached earlier with the ionosonde observations than for the ISR. The PRE peak magnitude for Vz(hmF2), Vz(h′F) and Vz(ISR) varies between 3–14, 2–14, and 4–14 m/s for the entire seasons. Our results revealed higher drift correlation coefficients in both Vz(hmF2) vs. Vz(ISR) (0.983) and Vz(h′F) vs. Vz(ISR) (0.833) relationships during the equinoxes between 16–20 LT, at which time the F-layer altitude is higher than the 300 km threshold value; and lower for solstice period (0.326 and 0.410 in similar order). A better linear relationship between Vz(hmF2) and Vz(h′F2) was observed during the reversal (19–21 LT) phase period. PRE velocity was shown to be seasonal and solar activity dependent. Both VzhmF2 and Vzh′F compares almost equally with the ISR measurement. However, the PRE peak magnitude for the drift inferred using h′F2 is closer to the corresponding ISR magnitude during the equinoxes; whereas the drift inferred from hmF2 best represent the ISR magnitude for solstices. We established that both VzhmF2 and Vzh′F are governed by the same mechanism at nighttime, and as such any of them can be used to infer vertical drift as long as the 300 km threshold value condition is considered, otherwise chemical correction may be required for the F-layer uplift. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Vertical plasma drift Incoherent Scatter Radar F-layer Pre-reversal enhancement Ionosonde

1. Introduction The equatorial E
B drift velocities (Vz) have been found to be important key parameters for many ionospheric models, as they help describe vertical plasma motion near the magnetic equator, especially in the evening sector, and as major drivers for the n Corresponding author at: Landmark University, Department of Physical Sciences, Omu-Aran, Kwara State, Nigeria. E-mail addresses: [email protected], [email protected] (B.O. Adebesin). 1 Present address: Department of Physical Sciences, Landmark University, P.M.B 1001, Omu-Aran, Kwara State, Nigeria.

http://dx.doi.org/10.1016/j.jastp.2014.11.007 1364-6826/& 2014 Elsevier Ltd. All rights reserved.

generation of Equatorial Spread-F (Martinis et al., 2005) and plasma bubbles (Fejer et al., 1999). Vertical drift velocity refers to how fast the F-layer height is moving in the vertical direction with time. While there are space-borne measurements techniques of plasma drifts, and the installation of Incoherent Scatter Radar (ISR) at some selected ionospheric stations (e.g. Jicamarca), there are still some stations where such facilities are not available. A good example of these is in the African sector. For such locations, the use of ground-based ionosondes measurement is common. A modern ionosonde according to Reinisch et al. (1998, and the reference therein) operates in the same way as a High Frequency (HF) Doppler radar systems, which measures echo amplitude and phase, angle of arrival, and Doppler spectrum as a function of

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range. Digital operations control, and real time signal processing helps to operate such ionosondes in diverse modes supporting different purposes like routine ionospheric sounding, channel probing, real-time frequency management, and research of ionospheric structure and dynamics. Till date, only Jicamarca Radio Observatory (JRO) has been able to provide routine ground-based measurements of plasma drifts in the equatorial region (Rodrigues et al., 2013), despite the relevance of equatorial electric field in ionospheric modeling. Measurements of both the vertical and zonal drifts have been made at JRO using ISR technique for over 47 years. Equatorial ISR measurements are available only at Jicamarca, and the observations from this station have been the pivot for vertical drift modeling (Woodman et al., 2006; Fejer, 1997). Woodman et al. (2006) had carried out a comparison analysis of the ISR and digisonde observation of the drift velocities over Jicamarca. They reported that the observations from both methods of instrumentation are only equivalent around a large evening-time PRE during equinox. However, the relationship in terms of magnitude and direction between the two is poor in June solstice. The estimation of the “apparent” vertical plasma drift from Ground-based ionosonde measurements has been carried out by computing the time rate of change of height, either by using the the base of the F-layer height, h′F (e.g. Araujo-Pradere et al., 2010; Uemoto et al., 2010; Kelley et al., 2009; Lee et al., 2005), or the F2layer height of the peak electron density, hmF2 (Adeniyi et al., 2014a; Adebesin et al., 2013a, 2013b; Obrou et al., 2003; Anderson et al., 2002). Both hmF2 and h′F methods of inferring drift are well documented and with good results. Recently, Adeniyi et al. (2014b) had made comparison analysis into the drift inferred from Ilorin, relative to the corresponding observation of the ISR observation at Jicamarca as well as the International Reference Ionosphere (IRI) model data. Their investigation spans the same period (2010) and location as in the present work. Consequently, this work is set to investigate comparatively if there exist any difference between the drift velocities obtained using both h′F (Vz(h′F)) and hmF2 (Vz(hmF2)) parameters from the same station, and to what extent such variability occurs, by using the Jicamarca ISR measurements as our data of reference level (and not a repetition of the work of Adeniyi et al. (2014b)). Bearing in mind that this kind of comparative investigation is yet to be carried out in the African sector (unless otherwise argued), our study location is therefore Ilorin (Geogr. Lat. 8.5°N, Long. 4.5°E, dip 7.9°), an equatorial station in the African sector. Owing to some constrains that can characterize the accuracy of the ground-based inferred daytime observations (as will be seen in the next section), this work had concentrated on the evening/nighttime periods (16– 06 LT) only. Further, due to the underlying ionization in the vicinity of the E-region, there is a considerable group retardation during the daytime; whereas this retardation is relatively undersized as a result of small electron content in the vicinity of the E-region at nighttime.

2. Drifts from ionosonde measurements and constraints The limitation of drift inferred from the time rate of change of height is best explained with the continuity equation as highlighted in a recent work by Adeniyi et al. (2014b). For this equation, the plasma (electron) density N is related to the production term (q), loss term (L(N)) and divergent term of ionization → (M = div(NV )) by

dN (ht) = q − L(N) − M dt

(1)

→ Parameter V from the divergent term, M, is the plasma drift velocity. At altitudes above the F2 peak, electron density is mainly influenced by diffusion, and this is the height at which the drift is more significant. At altitudes below the F2 peak, the electron density in the daytime would have roughly the equilibrium value it would have outside diffusion. According to Iheonu and Oyekola (2006) and Adeniyi et al. (2014b), the main quantities that describe the transport processes acting on the different layers of the ionosphere are the neutral winds, electric fields, and diffusion. Consequently, Eq. (1) implies that around the magnetic equator, the input from diffusion is visibly irrelevant, and is subsequently neglected in the treatment of vertical drift inferred from groundbased ionosondes. Moreso, the contribution of neutral winds is not so obvious until during the evening-time enhancement (PRE). This implies an early daytime to late evening hours drift that is majorly controlled by the electric field. Additionally, the expression on the left hand side of Eq. (1), for the same time span is neglected because of its insignificant effect; just as qis assumed ineffective, and hence discarded. This statement affirms a daytime limitation to the accuracy of vertical drift obtained from ionosonde inferred measurements. Consequently, the dh/dt inferred from ionosondes/digisondes yields an apparent drift velocity which may depict the characteristic of the vertical E
B drift, more importantly during the evening hours at which time plasma recombination is assumed insignificant. Refer to the work by Nicolls et al. (2006) for more explanation on the chemistry of recombination effects. It has been suggested by Bittencourt and Abdu (1981) that the vertical drifts obtained by digisonde measurements from ionosonde stations only match the E
B drift if F-layer is above 300 km and can be equivalent to ISR measurements. It is believed that at this height, transport process will dominate over recombination effects. Risbeth (1981) and Adebesin et al. (2013a) had considered this condition of Bittencourt and Abdu as necessary, but not be sufficient. On a positive note, Bertoni et al. (2006) had compared ionospheric drift data from digisonde (DPS-4) and ISR measurements during the campaign periods of 7–11 October, 2002 and 19– 23 March, 2003 at Jicamarca and reported that between 17–21 LT, and starting from around 02–08 LT, both sets of observation responds almost equally, but deviates at other hours. This work had since then provided a remarkable success to the use of inferred drift from digisonde height parameters to match those observed with Incoherent Scatter Radar, especially during the nighttime period, at which time convection dominates over production and recombination (see also Woodman et al. (2006)).

3. Data and methodology Three sets of data during the extreme prolonged solar minima activity period of 2010 are used in this work. First is the hourly data of the height of the peak electron density of the F2-layer (hmF2), and is obtained from the quarter-hourly monthly mean value of the DPS-4.2 digisonde located at the Ionospheric Observatory of the University of Ilorin, an African equatorial station, using the CARP program (Refer to the work of Adeniyi et al. (2014a) for full analysis of the method involved). Second is the data from the base of the F-layer height (h′F), and were obtained directly from the Standard Archiving Output (SAO) data format from the digisonde over the same location and year as in the first dataset. For both set of data, the monthly hourly mean values of the vertical plasma drift velocities (Vz) were inferred by computing the time rate of change of both height parameters differently. For instance, Vz(h′F) ¼d(h′F)/dt and Vz(hmF2) ¼d(hmF2)/dt. Further, both set of data were available between March and December

B.O. Adebesin et al. / Journal of Atmospheric and Solar-Terrestrial Physics 122 (2015) 97–107

2010, and therefore their seasonal hourly averages are: March equinox (March and April), June Solstice (May–July), September Equinox (August–October), and December Solstice (November and December). The third dataset is the Incoherent Scatter Radar (ISR) direct measurement obtained from Jicamarca (Georg. Lat. 11.95°S, Long. 76.87°W), an equatorial station in the Peruvian sector. The Jicamarca ISR measurement technique is the most reliable in computing vertical plasma drift velocities (though characterized with high power transmitters, not allowing for a better time coverage) and comparing with other ionosonde stations data (e.g. Woodman et al., 2006), and is obtained from the Madrigal database at Jicamarca Radio Observatory website address http://jro-db.igp.gob.pe/ madrigal/ for the year 2010. However, 2010 at this station is characterized by limited datasets. Data were only available for 41 days between March and December, 2010, constituting about 13.5% of the expected data. The seasonal hourly data spread is as follows: March equinox (March and April, 10 days data), June Solstice (May–July, 14 days data), September Equinox (August– October, 12 days data), and December Solstice (November and December, 5 days data). Data were not available for September equinox between 16–17 LT, as well as June solstice between 00– 06 LT (see also Adebesin et al. (2013a)). Comparing the observed Vz response (inferred from both hmF2 and h′F) with IRI model in this case may not necessarily be helpful, since the model only presents an average picture and does not also represent all longitudes. It is also worthy to mention that bulk of the data fed into the IRI-2007 global model are observations from the Jicamarca ISR observations (see Scherliess and Fejer (1999)). Detailed morphological characteristic of vertical plasma drift for the same period and location had been presented earlier by Adebesin et al. (2013b) using the time rate of change of hmF2 to infer the drift. Further, Adeniyi et al. (2014b) had compared the drift result obtained by Adebesin et al. (2013b) with that of Jicamarca ISR and IRI model as earlier mentioned in Section 1 of this work. In this respect, the present work is aimed at, and limited to a comparison observation between the drift inferred from hmF2 (already determined by Adebesin et al. (2013b)) and that of h′F (now to be determined) during the evening/nighttime period (i.e. 16–06 LT). The essence of introducing the Jicamarca ISR observation into this work is to use it as a data of reference level, in other to see how both Vz(h′F) and Vz(hmF2) responds relative to it (the ISR observation), and not for direct comparison.

4. Results 4.1. Seasonal variation in hmF2 and h′F pattern over Ilorin The seasonal average variations of h′F and hmF2 for the entire 24 h are highlighted in Fig. 1a and b respectively. The reason for showing the entire 24 h in this section is to see a clearer/extended picture of how each parameter varies generally. For the h′F observation, there are two peaks – the daytime peak and the postsunset peak. The daytime peak values are 386, 347, 335 and 311 km respectively for June solstice, March equinox, September equinox, and December solstice; whereas the post-sunset peak values are 251 (at 20 LT), 302 (at 20 LT), 298 (at 19 LT) and 322 km (at 19 LT) in similar order. The daytime peak magnitudes occurred at local noontime (except for June solstice that peaked around 13 LT). The hmF2 observations also recorded two peaks – the prenoon and the post-sunset peaks. The pre-noon peak ranges within 321–370 km. The highest was during December solstice, and peaked least in June solstice. The post-noon peak magnitudes are 404, 372, and 371 km for December solstice, September equinox, and March equinox respectively. There is no significant peak observation for June solstice during the post-sunset period of the hmF2 observation. The seasonal hourly average values of the observed hmF2 data over Ilorin in comparison with the most recent international reference ionosphere (IRI-2012) model between 16–06 LT revealed a better correlation between the two, suggesting reliable digisonde measurement at Ilorin (Fig. 2), and hence, consistent ionospheric data. However, the following characteristics were observed from the figure: (i) while the ionosonde observation revealed a postsunset increase in Ilorin hmF2 for all seasons (except June solstice), the feature was missing on the IRI-model, except for March equinox (ii) the daytime observation revealed significant over-estimation of the model above Ilorin data, except briefly during the March equinox (iii) a fairer fit was observed between the two within 20–06 LT. It can therefore be emphasized that the h′F data used is as well reliable, as same data-set source is used together with hmF2. The h′F data reliability cannot be ascertained using the model observation because h′F is not represented on the IRI interface. 4.2. Variation in drift pattern inferred from hmF2 and h′F over Ilorin The seasonal average vertical plasma drift pattern inferred from both hmF2 (Vz(hmF2)) and h′F (Vz(h′F2)) over Ilorin ionosphere are shown in Fig. 3. The figure revealed the following characteristics: (i) while Vz(hmF2) peaked at 19 LT for all seasons, Vz(h′F) peaked at 18 LT for September equinox and December solstice (ii) the evening time PRE for the Vz(h′F) observation started earlier around

March Equinox

h'F (km)

360

400

Sept. Equinox

360

Dec. Solstice

320 280

JUN. SOLSTICE SEPT. EQUINOX

DEC. SOLSTICE

320 280 240

240 200

MAR. EQUINOX

June Solstice

hmF2 (km)

400

99

200 6

8

10 12 14 16 18 20 22

LT

0

2

4

6

6

8

10 12 14 16 18 20 22

0

2

4

LT

Fig. 1. Seasonal average variation of (a) the base of the F-layer height (b) height of the peak electron density, over Ilorin for March–December, 2010.

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March Equinox

June

400

400

350

350

300

300

250

250

Solstice

hmF2 (km)

hmF2 (ILORIN)

200 16

18

20

22

0

2

4

6

September Equinox

200 16

400

400

350

350

300

300

250

250

18

20

22

hmF2 (IRI-2012)

2

0

4

6

4

6

December Solstice

200

200 16

18

20

22

0

2

16

6

4

18

20

22

2

0

LT Fig. 2. Comparison plot between Ilorin and IRI-2012 hmF2 observations for 2010 for (a) March equinox (b) June solstice (c) September equinox (d) December solstice. Observation is for 16–06 LT. The short vertical line (with head and foot) depicts the standard deviation about the mean.

16 LT for all seasons, while that of Vz(hmF2) started at the same time during the equinoxes, and 1 h later for the solstice period (iii) the magnitude of the PRE startup (i.e. around 16 LT) for the Vz(h′F) observation is significantly lower than the corresponding value of Vz(hmF2)) for the entire seasons, but more pronounced in March equinox and June solstice (iv) between the nighttime and the pre-

June Solstice

March Equinox d(hmF2)/dt

15

sunrise hours (23–06 LT) there is a good agreement between the two (correlation coefficient R ¼0.64). In order to quantitatively investigate the difference/similarities between the two variables (Vz(hmF2) and Vz(h′F)) further, the evening/nighttime observations was divided into three significant time phases. This are (i) 16–19 LT (spanning the interval at which

d(h'F)/dt

15

0 -5 16 17 18 19 20 21 22 23 0

1

2

3

4

5

6

-10

-15 -20

5

0 -5 16 17 18 19 20 21 22 23 0

2

3

4

5

1

2

3

4

5

6

-15 -25

LT

Sept. Equinox

LT

Dec. Solstice

15

15

10

10

5

5

0 -5 16 17 18 19 20 21 22 23 0 -10

1

2

3

4

5

6

0 -5 16 17 18 19 20 21 22 23 0 -10

-15

-15

-20

-20

-25

1

-10 -20

-25

DRIFT (m/s)

DRIFT (m/s)

10

5

DRIFT (m/s)

DRIFT (m/s)

10

-25

LT

LT

Fig. 3. Seasonal comparison of Ilorin nighttime average vertical drift (Vz) variation obtained from hmF2 and h′F.

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Table 1 Equinoctial and solsticial magnitude variations of Vz(hmF2) and Vz(h′F) for different nighttime event periods covering March–December, 2010. Season

Equinoxes Solstices

16–19 LT

19–21 LT Vz(h′F)

Vz(hmF2)

Vz(h′F)

Vz(hmF2)

Vz(h′F)

12.6–4.5 7.8–4.5

12.7–18.2 13.3–19.0

12.6–10.7 7.8–13.3

12.7–12.9 2.2–5.0

2.5–8.9 2.1–8.6

5.7–12.9 3.2–10.2

the evening time pre-reversal enhancement for both observations started and reach their respective peaks), (ii) 19–21 LT (signifying the period during which the plasma reversal starts and attain its downward peak magnitude), and (iii) 21–06 LT (the remaining nighttime observation period). This similarities/differences were observed during the equinoctial (March and September equinoxes) and solsticial (June and December solstices) periods spanning March–December, 2010. The observation was presented in Table 1. The two values shown for each pair of observation represents the minimum and maximum drift magnitude for a particular period of observation. For instance, at 16–19 LT, the Vz(hmF2) magnitude during the equinoxes spans between 12.6 and  4.5 m/s. Here, 12.6 m/s is the maximum upward peak value, while  4.5 is the minimum downward peak magnitude for this interval (i.e. 16– 19 LT). Three significant inferences could be drawn from the table. For the three phases of observation (16–19, 19–21, and 21–06 LT), (i) the difference in the drift magnitude between the maximum and the minimum peak values during the solstices is higher/ broader for Vz(h′F) than for the corresponding Vz(hmF2) observation except for the 19–21 LT phase (ii) the difference in the drift magnitude during the equinoxes is higher/broader for Vz(h′F) than for the corresponding Vz(hmF2) observation (iii) in general, the magnitude of the maximum/minimum value variation is higher during the equinoxes than at solstices whether for Vz(hmF2) or Vz(h′F). For this third inference, it is believed that the stronger vertical E
B drift during the equinoxes shifts the plasma to elevated heights at which time recombination effect is almost insignificant, thereby allowing the plasma to exhibit longer lifetime and consequently resulting in a higher magnitude during the equinoxes (Chen et al., 2008; Adebesin et al., 2013a). Depicted in Table 2 are the respective correlation coefficients (R) of the linear relationship between Vz(hmF2) and Vz(h′F2) for different time span considered in Table 1 and observational periods. The bolded values are the periods when R is largest. Table 2 revealed higher correlation values mostly during the reversal period of all observational period except March equinox and June solstice. This seems to suggest that there is a better linear relationship between both Vz(hmF2) and Vz(h′F2) during the reversal period than other nighttime periods. The correlation was also observed to be higher during the entire equinoxes than the Table 2 Correlation coefficient for the linear fit of Vz(hmF2) vs. Vz(h′F2) for different observational period and local time (LT) span.

March equinox June solstice September equinox December solstice Entire equinoxes Entire solstices March–December, 2010.

Table 3 Seasonal correlation coefficient (R) and root mean square deviation (RMSdev) between drift inferred from hmF2 (Vz(hmF2)) and h′F (Vz(h′F)) over Ilorin for March– December, 2010.

21–06 LT

Vz(hmF2)

Observation period

101

Correlation coefficient (R) for Vz(hmF2) vs. Vz(h′F2) 16–19 LT

19–21 LT

21–06 LT

0.865 0.738 0.873 0.617 0.760 0.729 0.741

0.804 0.640 0.955 0.895 0.872 0.796 0.804

0.570 0.667 0.658 0.585 0.637 0.610 0.570

Bolded values depict highest correlation coefficients for each row of observations.

Season

R

RMSdev

March equinox June solstice September equinox December solstice March–December, 2010

0.622 0.547 0.690 0.655 0.640

6.201 5.793 4.514 4.723 5.361

solstices for the three considered time span. Seasonally and annually, R varies from 61–87%, 64–96%, and 57–66% for the 16–19, 19–21, and 21–06 LT events respectively. The correlation coefficients between the two parameters are yet considered for the entire evening/nighttime period (16–06 LT) and are listed in Table 3. There are better correlations between the two for the entire seasons. Similarly, the root mean square deviation (or the third moment about the mean) given by RMSdev ¼ sqrt[(Vz(hmF2) – Vz(h′F))2]/sqrt[n], where n is the number of observations, revealed that the best performance between the two is during September equinox and followed by December solstice, which also validates the higher correlation coefficients observed during this two seasons. On the average RMSdev ranges within 4.5– 6.5 for the entire seasons. Annually for the year 2010, R is approximately 0.64 and RMSdev E5.36 between Vz(hmF2) and Vz(h′F). 4.3. Vz(hmF2) and Vz(h′F) over Ilorin in comparison with a standard (ISR) measurement technique Though the available 41 days of data at Jicamarca are used for the respective classification of the seasonal average response (as earlier mentioned in Section 3) at this station between March and December, 2010; while Ilorin ionosonde data for the same period covers 263 days for the four seasons considered. It is therefore worth mentioning that the number of days considered for averaging the Jicamarca seasonal observation (which is not the same with that considered for Ilorin) is of no immense consequence, as it only serves as a reference level data to show how well both the Vz(hmF2) and Vz(h′F) observations compares and differs from each other. In this respect, the most vital argument is for the different dataset for obtaining both Vz(hmF2) and Vz(h′F) at Ilorin to be computed for equal number of days (since they are the varying parameters under investigation). Depicted in Fig. 4 are the average seasonal variations of the vertical plasma drift inferred from hmF2 and h′F over Ilorin, together with the ISR observations for the same period of observation during the evening/nighttime periods. Morphologically, the following features were observed from the figure: (i) there is a general downward trend in the three observations between 19– 06 LT for all season except for Vz(ISR) during December solstice in which there is a positive excursion between 21–23 LT (ii) there is little or insignificant PRE observation for the three techniques during June solstice (iii) there is a better linear agreement between the three techniques for the equinoctial seasons starting from 16– 20 LT than the solstice months. In general, the figure revealed better correlation pattern between the drift values inferred from both hmF2 (Vz(hmF2)) and h′F (Vz(h′F)) at Ilorin (as earlier discussed in Section 4.2), but deviates away from the ISR (Vz(ISR)) observations mostly in terms of magnitude. The observed drift deviation between the two stations could be attributed to a combination effect of both the E-region electric and magnetic fields. Adebesin et al. (2013a) had reported that the magnetic field lines over the ionosphere may be considered a good electrical conductor, suggesting that ionospheric magnetic conjugate points are electrically

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d(hmF2)/dt

March Equinox

d(h'F)/dt

June Solstice

Vz (ISR)

September Equinox

December Solstice

20

DRIFT (m/s)

10 0 -10 -20 -30 -40 16 18 20 22

0

2

4

6

16 18 20 22

0

2

4

6

16 18 20 22

0

2

4

6

16 18 20 22

0

2

4

6

LT Fig. 4. Vz(hmF2) and Vz(h′F) drift inferred from digisonde measurement over Ilorin in comparison with ISR measurement over Jicamarca for March equinox, June solstice, September equinox, and December solstice, 2010.

joined together, and therefore the field lines are assumed to be at virtual equi-potential points. If this is so, then the difference could have arisen from the fact that Jicamarca is not located at the actual conjugate point of Ilorin. In essence, the difference is due to their longitudinal separation, since for a specified value of zonal electric field, vertical plasma drift (given as Vz ¼E/B) would be different. This leaves us with the option of accepting meridional neutral winds (in some instances) as an additional factor for the observed differences. Another reason for the plasma variation from the two techniques (digisonde and ISR) used according to Scali et al. (1997) is that while the digisonde Doppler shift echo is a product of the rate of change of time of the electrical distance to the point of

[d(hmF2)/dt] -Vz (ISR)

reflection; the ISR Doppler echo measures directly the line-of-sight of Vz. Therefore, there will be a magnitude difference in the two techniques. Further, in other to achieve a better quantifiable estimate of the performances of both Vz(hmF2) and Vz(h′F) with a standard drift measurement technique, the drift from the ISR (Vz(ISR)) was used together with the ionosonde technique to present the seasonal deviation plots (Fig. 5). This is achieved by subtracting the average hourly ISR observation from each of the respective ionosonde drift observations for each season. For this analysis therefore, the ISR measurement is taken as the reference zero level. The inference from this is that any value above the reference level is a decrease of the ISR observation below the ionosonde value, while values

June Solstice

March Equinox

[d(h'F)/dt] -Vz (ISR)

30

20 10 0 16 17 18 19 20 21 22 23 0

1

2

3

4

5

6

DEVIATION (m/s)

DEVIATION (m/s)

30

20 10 0

16 17 18 19 20 21 22 23 0

2

3

4

5

6

2

3

4

5

6

-20

-20

LT

LT

September Equinox

December Solstice 30

20 10 0 16 17 18 19 20 21 22 23 0

1

2

3

4

5

6

DEVIATION (m/s)

30

DEVIATION (m/s)

1

-10

-10

20 10 0 16 17 18 19 20 21 22 23 0

1

-10

-10 -20

Vz (ISR) is the reference zero line Below zero line implies Vz (ISR) increase Above zero line implies Vz (ISR) decrease

LT

-20

LT

Fig. 5. Deviation pattern in the observations of drift inferred from hmF2 and h′F (over Ilorin) with that of the ISR measurement at Jicamarca spanning March–December, 2010.

B.O. Adebesin et al. / Journal of Atmospheric and Solar-Terrestrial Physics 122 (2015) 97–107

below the zero line imply an increase of the ISR observation over the ionosonde measurements. Evidently, between 21–06 LT, there is a reduction in the magnitude of the ionosonde drifts when compared with the ISR measurement during the equinoxes, with peaks around local midnight. Paucity of data could not allow for much comment during June solstice, but the few plotted points reveal a decrease of lower magnitude of the ionosonde measurements compared to that of the ISR observation between 18–23 LT. The deviation pattern during December solstice is somehow different, in that while there is depletion in the magnitude of the ionosonde measurements relative to the ISR observation between 16–19 LT and 00–06 LT, there appears an enhancement within 20– 23 LT. The deviation away from the ISR observation at peak values range within 9.5 and 26.5 m/s for Vz obtained from hmF2 data; and within 12.7 and 26.5 m/s for the Vz obtained using h′F2 data. Both the enhanced/decrease in the drift values obtained from Vz(hmF2) and Vz(h′F) show considerable closeness, signifying that both vary almost equally with the ISR observation. 4.4. Diurnal drift observation for 3 September, 2010 The diurnal variation of the ionosonde measurements over Ilorin in comparison with the ISR technique for 3 September, 2010 is presented in Fig. 6. A critical look into the 41 days of available data over Jicamarca within March-December 2010 reveal the 3 September event as the day with the best comprehensive set of data between 16–06 LT interval. This is the reason for its choice, coupled with the availability of corresponding ionospheric data over Ilorin for the same time interval. The top panel of the figure highlights the h′F and hmF2 observations. Similar pattern was h'F

hmF2

height (km)

420

380 340 300

260 220

180

16 17 18 19 20 21 22 23 Vz(ISR)

0

1

2

Vz (h'F)

3

4

5

6

5

6

5

6

Vz (hmF2)

30

drift (m/s)

20 10 0 -10 16 17 18 19 20 21 22 23

0

1

2

3

4

-20

-30 -40 Vz(h'F) -Vz(ISR)

Vz(hmF2) -Vz(ISR)

deviation (m/s)

40 30 20

10 0 -10 16 17 18 19 20 21 22 23

0

1

2

3

4

-20 -30

LT Fig. 6. Ionospheric variation of h′F and hmF2 over Ilorin (top panel), drift pattern in d(h′F)/dt and d(hmF2)/dt over Ilorin together with Jicamarca ISR observation (middle panel), and deviation pattern in the ionosonde observations with respect to the ISR measurement (bottom panel). All observations are for 3 September, 2010.

103

observed between 19–06 LT. Lower magnitude was recorded for the h′F observation between 16–18 LT. The middle panel illustrate Vz(hmF2) and Vz(h′F) with comparable trend and magnitude, but deviates from the ISR observation. For this event, the following characteristics are observed: (i) the PRE peak magnitude of the ionosonde measurements is well higher than that of the ISR (ii) the PRE peaked at 18 LT for all three techniques (iii) the downward reversal peaks at different time for the three observations, within an interval of 3 h to each other, with that of Vz(ISR) peaking last (iv) the respective correlation coefficients for the Vz(hmF2)/Vz(h′F), Vz(h′F)/Vz(ISR), and Vz(h′F)/Vz(ISR) relations are 0.869, 0.497, and 0.375. The lower magnitude obtained for the Vz(h′F),/Vz(ISR) and Vz(h′F)/Vz(ISR) relation in comparison with September equinox values of 0.843 and 0.530 respectively in similar order (Section 5) points to the fact that average seasonal observation is better for the representation of ionospheric variation as it involves larger database. The bottom panel revealed an almost equal deviation pattern in both the Vz(hmF2) and Vz(h′F) in response to the ISR observations. In general, there are consistent pattern in (i) the two ionosonde height observations, (ii) the two inferred drifts over Ilorin, and (iii) the deviation pattern in the Vz(hmF2) and Vz(h′F) with respect to Vz(ISR) for both the considered diurnal observation as well as the seasonal observations in Section 4.3.

5. Discussion The ISR offers a dominant procedure in the detection of ionospheric plasma drifts, especially at equatorial regions, and has been used extensively in the development of several empirical models. However, Scherliess and Fejer (1999) developed the first empirical global model using data from Jicamarca ISR measurements and AEE satellite observations which were later incorporated into the IRI-2007 model. Hence, ISR observations has turn out to be the most appropriate procedure for the treatment of ionospheric plasma parameter observations to obtain various aeronomy parameters, most especially in the F-region. For instance, much helpful and consistent information on neutral composition, temperature and thermospheric winds have been acquired using ISR investigation (e.g. Alcayde et al., 1974; Oliver, 1979; Litvin et al., 2000). Mikhailov and Lilensten (2004) had attributed the major approach of the use of IS radars to the application of the energy equation for Oþ ions to uncover neutral temperature and atomic oxygen concentration at the F2-region heights; as well as on the momentum equation for Oþ ions to find meridional thermospheric winds. In the absence of necessary equipment for the vertical drift radar measurement, ground-based ionosonde inferred drift are used, and is obtained from the time rate of change of the F-layer height, provided the lifting of the F-layer height is 300 km and above. It has been observed that the action of the trans-equatorial thermospheric neutral winds at F-region heights is one of the major factors (aside the dynamo action of atmospheric tides) that control the deposition of plasma at low latitudes (e.g. Rodrigues et al., 2013); as it has a hemispherical pushing effect on the plasmas, thereby creating asymmetries in the ionospheric plasma distribution. This is a very germane factor in the use of F2-region height in the determination of vertical plasma drift from groundbased ionosonde measurements. The typical Ilorin drift observation inferred from both hmF2 and h′F, as well as the ISR observations revealed that (i) the evening-time PRE peak magnitude for Vz(hmF2), Vz(h′F) and Vz(ISR) varies between 3–14, 2–14, and 4–14 m/s for the entire seasons. The PRE peak magnitude for VzhmF2 in June solstice is insignificant. (ii) the nighttime downward reversal peak magnitudes for Vz(hmF2)

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Table 4 Seasonal correlation coefficient (R) and root mean square deviation (RMSdev) for Vz(hmF2) vs. Vz(ISR) and Vz(h′F) vs. Vz(ISR) over Ilorin for March–December, 2010. Season

Correlation (R)

March equinox June solstice September equinox December solstice

RMSdev

Vz(hmF2) vs. Vz(ISR)

Vz(h′F2) vs. Vz(ISR)

Vz(hmF2) vs. Vz(ISR)

Vz(h′F2) vs. Vz(ISR)

0.652 0.241 0.530

0.525 0.396 0.843

16.90 5.39 10.80

17.09 9.73 11.34

0.989

0.446

11.92

12.11

and Vz(h′F) are respectively within the range of  4 to  14 and  2 to 14 m/s; whereas that of the Vz(ISR) ranges within  12 and 34 m/s. Depicted in Table 4 are the linear fit correlation relationship and the RMSdev between Vz(hmF2) vs. Vz(ISR) and Vz(h′F) vs. Vz(ISR) observations for the evening/nighttime periods. The bolded values represent the more correlated coefficient values (or the more quantifiable estimate of performance) for each set of corresponding data per season. With respect to the correlation (R), Vz(hmF2) vs. Vz(ISR) recorded higher correlation in March equinox and December solstice; whereas R was higher during June solstice (though very poor) and September equinox for the Vz(hmF2) vs. Vz(ISR) pair of observation. Note that there are fewer datasets for June solstice (no data for Vz(ISR) between 00–06 LT), so the values recorded may not be the most accurate representation. Despite data paucity, the observations recorded in Table 4 is consistent with the result obtained by Woodman et al. (2006) who reported that the observations from both methods of instrumentation (digisonde and ISR) in terms of magnitude and direction between the two is poor in June solstice. For the seasonal RMSdev, Vz(hmF2) vs. Vz(ISR) recorded the better fit for all seasons. Recall that the smaller the RMS deviation between two quantities, the better the relationship between them. Based on the RMSdev deduction, the drift values obtained from both the time rate of change of hmF2 and h′F seems to respond almost equally to the Jicamarca ISR measurements when their RMSdev values are rounded up to zero decimal place (except for June solstice). The relationship is further reduced into smaller observational time span/phases (i.e. 16–19, 19–21, and 21–06 LT earlier employed in Section 4.2) for more intra-hour variation in terms of correlation treatment (Table 5). The table revealed that for the 19–21 time span observation, higher correlations are recorded for the Vz(hmF2)/Vz(ISR) relation for all seasons (except June solstice) and the entire March–December, 2010 observation. Unavailability of Table 5 Seasonal correlation relation for Vz(hmF2) vs. Vz(ISR) and Vz(h′F2) vs. Vz(ISR) for different evening/nighttime observation time span. Season

March equinox June solstice September equinox December solstice March–December, 2010

16–19 LT

19–21 LT

21–06 LT

Vz(hmF2) vs. Vz(ISR)

Vz(h′F2) vs. Vz(ISR)

Vz(hmF2) vs. Vz(ISR)

Vz(h′F2) vs. Vz(ISR)

Vz(hmF2) vs. Vz(ISR)

Vz(h′F2) vs. Vz(ISR)

0.992 0.778 þþ

0.832 0.986 þþ

0.938 0.856 0.899

0.876 0.682 0.871

0.718 0.817 0.288

0.671 0.872 0.526

0.881

0.326

0.991

0.930

0.575

0.755

0.659

0.618

0.760

0.645

0.206

0.305

Bolded values depict highest correlation coefficients for each set of observations. þþ Data not available for the ISR measurement.

sufficient data during September solstice could not allow for comment during this season. For the downward reversal period of 19–21 LT, the Vz(hmF2)/Vz(ISR) relation recorded the highest correlation values for the entire seasons, and ranges within 85–99%. However the corresponding value of the Vz(h′F2)/Vz(ISR) relation ranges between 68 and 93%. For the remaining 21–06 LT observation, the highest magnitude was observed mostly for the Vz(h′F2)/Vz(ISR) pair. The deduction from this observations is that during the evening time plasma enhancement and reversal periods, the drift inferred from hmF2 measurements correlates better with that of the ISR measurements, whereas between 21–06 LT, the inferred drift from h′F observation performed better. 5.1. Some physical explanation At nighttime, an observed enhancement in the height (both hmF2 and h′F) immediately after sunset (Fig. 1) indicates that ions are moved to a state of lower recombination coefficient. The better linear agreement between the drift obtained from the three techniques starting from around 16–20 LT is as a result of the lifting of the height above 300 km around this period. This is true majorly for the equinoctial seasons and partly during December solstice (Fig. 4). During June solstice, there is no agreement between the ionosonde inferred drifts and the ISR observations (both in magnitude and pattern) majorly because both the ionosonde height magnitudes (hmF2 and h′F) around this period are lower than the 300 km threshold value. Recall from Bittencourt and Abdu (1981) that at nighttime period, when the F-layer is above 300 km, Vz from ionosonde observations can be determined to good accuracy, and can be equivalent to those obtained from ISR measurements of the F-region. It has also been suggested by Oyekola (2009, and the references therein) that for height range less than 300 km (i.e. when the F-layer is not high enough), the drift obtained from ionosonde observations underestimate the magnitude of plasma drift using ISR observations (similar to the situation in Fig. 4 between 21–06 LT, especially for the equinoctial seasons). Though it was reiterated by Bittencourt and Abdu (1981) that the electric field which manifests at these lower heights of the F-region still produce vertical E
B plasma drifts, but because of recombination processes, the apparent drift fades in strength; and consequently cannot match the E
B drift in magnitude. In essence, for F-layer height less than 300 km, it is assumed that chemical recombination could result in an apparent inferred vertical drift which needs to be corrected for, before it can match the E
B drifts, so as to be able to maintain realistic drift velocity values at such hours. However, ionospheric chemical correction is not our focus in this work. Refer to the work by Sumod et al. (2012), Torr and Torr (1979), and Nayar et al. (2009) for explicit analysis on the chemical correction process. The average seasonal RMSdev for Vz(hmF2) vs. Vz(ISR) and Vz(h′F) vs. Vz(ISR) between 16–20 LT and for other hours (21–06 LT) is presented in Table 6. From the table, the magnitude of the root mean square deviation for both height observations against Vz(ISR) between 16–20 LT is well lower than the corresponding Table 6 Seasonal root mean square deviation (RMSdev) for Vz(hmF2) vs. Vz(ISR) and Vz(h′F) vs. Vz(ISR) for 16–20 LT and 21–06 LT over Ilorin. Season

March equinox June solstice September equinox December solstice

Vz(hmF2) vs. Vz(ISR)

Vz(h′F2) vs. Vz(ISR)

16–20 LT

21–06 LT

16–20 LT

21–06 LT

2.28 6.07 4.05 5.94

20.64 5.02 12.91 13.98

8.20 13.95 6.51 10.12

21.26 6.67 13.10 12.99

High High India Peruvian Peruvian Kodaikanal Jicamarca Peru

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

Insignificant value.

nn

Q† Q†

nn

20.7 38.0 26.0

38.0 35.0 Peruvian Jicamarca

Scherliess and Fejer (1999) Empirical model n Ramesh and Satri (1995) n Fejer et al. (1991) Huang (1974)

Africa Africa Peruvian Africa Africa Africa Peruvian

Q†≡Quiet condition; D† ≡ Disturbed condition. nafter Oyekola (2009).

HF Sounder VHF radar Scatter radar 26.3 þ þ 50.0 þ þ 38.0 þ þ

45.0 þ þ

28.0/18.0 32.0 þ þ 33.0/nn 16.0 26.9 42.0/26.0 14.0

nn

Q† Q† Q†/D†

2.2 0.3

13.3 8.0 6.4 14.0 26.7 28.0/23.0 28.0

12.7 9.2 13.6

17.0 þ þ 33.6 þ þ

12.8 3.8 9.6

September (m/s) Solstice (m/s) Solstice (m/s) March

Ilorin Ilorin Jicamarca Ouagadougou Ibadan Ibadan Jicamarca

The precise understanding of the PRE variability can help improve our knowledge about equatorial Spread-F (ESF) onset period. Hence, the reason for laying emphases on the pre-reversal enhancement period. The evening time peak pre-reversal enhancement magnitudes for the three techniques of observation used in the present work (from Fig. 4) and those of other works from literatures are depicted in Table 8. The PRE is known to be controlled by thermospheric zonal wind as well as the longitudinal/local time gradient in the Pederson Hall conductivity across the terminator (Abdu et al., 1992; Adebesin et al., 2013c and the reference therein). From the present study, the average seasonal PRE peak values for Vz(hmF2) is lower than that of Vz(h′F) for the entire seasons. Most of the pre-reversal peaks observed from Table 8 reveal higher magnitudes during the equinoctial months, irrespective of the mode of data source (i.e. whether from hmF2 median, hmF2 mean, h′F mean, h′F median, or ISR measurements).

Present work Present work Present work Oyekola and Kolawole (2010) Oyekola (2009) Oyekola and Oluwafemi (2007) Lee et al. (2005)

5.2. Comparison with previous results especially during pre-reversal enhancement period

Data source

magnitudes for 21–06 LT, signifying a better relationship between the ionosonde and ISR observations for 16–20 LT (at which time hZ 300 km) than for the 21–06 LT. The better quantifiable estimate of each corresponding performances are the bolded values on the table. In general, the correlation coefficient (R) between 16– 20 LT for Vz(hmF2) vs. Vz(ISR) are 0.983 (for equinoxes) and 0.326 (for solstices). For the Vz(h′F) vs. Vz(ISR) relationship, R¼0.833 (during equinoxes) and 0.410 (during solstices). Further, linear model equations were obtained for (i) Vz(hmF2) vs. Vz(ISR), and (ii) Vz(h′F2) vs. Vz(ISR) separately for the equinox and solstice periods for the time duration of 16–20 LT (when hZ 300 km), 21–06 LT (when hr 300 km), and the entire 16– 06 LT. This is illustrated in Table 7. The bolded and the unbolded values on the table are the respective slopes and intercepts. The following characteristics were observed: (i) the magnitude of the slope was higher during the equinoxes than the solstices for all observations (ii) the slope magnitudes for the Vz(hmF2)/Vz(ISR) relation is higher than that of the corresponding Vz(h′F2) vs. Vz(ISR) for all time conditions and seasons (except for 21–06 LT during the equinoxes) (iii) the magnitude of the intercept was higher during the equinoxes than the solstices between 16–20 LT, while the reverse is the case for the 21–06 LT observations. For the entire evening/nighttime (i.e. 16–06 LT) period, the higher slope magnitudes during the respective equinoxes points to the fact that there is a better correlation between both ionosonde (Vz(hmF2) and Vz(h′F2)) observations and the ISR technique during equinoxes than during solstices. Higher magnitude (0.93) was also recorded for the Vz(hmF2) vs. Vz(ISR) than for the Vz(h′F2) vs. Vz(ISR) (0.73) relationship.

Equinoxes

Vz(ISR) ¼  0.26 Vz(h′F2)  6.98

December

Vz(ISR) ¼ 0.73 Vz(h′F2)  11.17

June

Vz(ISR) ¼  1.06 Vz(h′F2)  11.11

DPS-4.2 (h′F mean) DPS-4.2 (hmF2 mean) ISR Ionosonde (hmF2 mean) Ionosonde (h′F median) Ionosonde(h′F mean) Digisonde (h′F) and GPS receivers ISR/AE-E Satellite

Study period

Vz(ISR) ¼  0.14 Vz(h′F2)  17.30

25.0 17.0

Vz(ISR) ¼  0.20 Vz(h′F2)  2.96

March 1968–December 1969

2010 (March–December) 2010 (March–December) 2010 (March–December) 1989 (January–December) 1958 (January–December) 1957/1958 April 1999–March 2000 1968–1992/1977–1979

Vz(ISR) ¼ 0.48 Vz(h′F2) þ 1.02

Sector

16–20 LT Equinoxes Vz(ISR) ¼1.02 Vz(hmF2) þ 1.67 Solstices Vz(ISR) ¼0.31 Vz(hmF2)  2.65 21–06 LT Equinoxes Vz(ISR) ¼  0.54 Vz(hmF2)  18.83 Solstices Vz(ISR) ¼  0.65 Vz(hmF2)  11.15 16–06 LT Equinoxes Vz(ISR) ¼0.93 Vz(hmF2)  9.99 Solstices Vz(ISR) ¼  0.08 Vz(hmF2)  6.73

Vz(h′F2) vs. Vz(ISR)

Station

Vz(hmF2) vs. Vz(ISR)

Reference

Season

Table 8 Comparison between the peaks of pre-reversal velocity enhancement (PRE) for some ground-based ionosonde and IS radar observations including the present work.

Period

Solar activity

Table 7 Linear model equations for (i) Vz(hmF2) vs. Vz(ISR), and (ii) Vz(h′F2) vs. Vz(ISR) for the equinox and solstice periods for 16–20 LT and 21–06 LT.

105

Low F10.7 ¼ 81 Low F10.7 ¼ 81 Low F10.7 ¼81 High F10.7 ¼192 High F10.7 ¼ 208 High F10.7 ¼ 208 High

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The only exception is the result obtained by Oyekola and Oluwafemi (2007) under both quiet and disturbed conditions, in which case the highest PRE peak velocity was reached in June solstice, lowest in the equinoctial months and December solstice for the disturbed and quiet conditions respectively. The reason for the higher PRE magnitude observed for Vz(h′F) in the present study may have resulted from the fact that at nighttime, the D and E-region disappears, and there is only the F-region; and at that time, the rate of change of height of the base of the F-layer with time (d′h/dt) at such periods then tends to be more accurate; whereas, for the F-layer height of the peak electron density, the actual height may not be measured accurately since there is a merging of all layers. Bertoni et al. (2006 and the references therein), had reported that owning to the high conductivity along the magnetic field lines at daytime, E-region dynamo electric fields outside the magnetic equator diffuses into the F-region, thereby inhibiting the dynamo electric fields of the region. Consequently, at sunset, during which time the E-region conductivity (which maximizes at daytime) begins to decrease rapidly, the electric fields from both the E and F region combine such that vertical drift is considerably enhanced along the magnetic equator leading to the emergence of the PRE. The mechanisms for E–F coupling and their manifestation had been explained in detail by Cosgrove (2013). We also observed that the vertical drift PRE magnitudes obtained during low solar activity period from Table 8 are significantly lower than those obtained during high solar activities irrespective of the mode of data source. This suggests that vertical drift velocity is solar activity dependent (see also Sreeja et al. (2009)). Fejer et al. (1999) had earlier proposed that PRE amplitude and F region heights decrease toward period of solar minimum due to the corresponding decreases of the equatorial zonal wind and conductivity gradient, which invariably results in a small PRE. Therefore, the lower height (both hmF2 and h′F) recorded in the present work can be attributed to its characteristic year of extreme low solar activity. Radicella and Adeniyi (1999) had made a comparison study into hmF2 variation during high (1995) and low (1991) solar activity periods and reported higher values of hmF2 at high solar activity than those of low solar activity. Iheonu and Oyekola (2006) in a similar manner, had carried out comparison observation of Jicamarca ISR, AE-E, and h′F inferred drift over Ibadan (Ibadan I) during a period of high solar activity with another of Ibadan ionosonde inferred drift (Ibadan II) for low solar activity period. They recorded on the average, lower midday values (10 m/ s) of drift velocities during the low solar period (in equinox) compared with high solar activity velocity (20 m/s) on the average for Jicamarca, AE-E and Ibadan I. The differences observed in the nighttime drift magnitude (though with fairer pattern) obtained at Ilorin (using ionosonde technique) and Jicamarca (from ISR observation) apart from the differing method of instrumentation may still have arisen as a result of chemistry and divergent transport system (e.g. Adebesin et al., 2014). Another contributing factor for the differences in vertical drifts observed at both locations could be ascribed to the location of these stations with respect to the geographic equator. The equatorial electric fields are a result of the E/F region dynamos. The winds and tides driving the dynamos are known to vary with geographic latitude. Therefore, the neutral winds and tides over (and near) Ilorin and Jicamarca are expected to be distinct as a result of their latitudinal differences. The correlation coefficient (R) between the ISR drift observation and the inferred drift obtained from hmF2 over Ilorin between 17–21 LT is 0.7, and is consistent with the result obtained by Bertoni et al. (2006). The result by Lee et al. (2005) and Fejer et al. (1999) had shown that the largest and smallest drift velocities are during the equinox and

June solstice respectively.

6. Summary The equatorial F-region vertical drifts have been studied extensively using the ISR observations at Jicamarca Radio observatory (JIRO). HF Doppler radar had also been developed as a drift measuring technique at Trivandrum, India. Drift pattern had also been studied widely using the Atmospheric Explorer E (AE-E). Nonetheless is the use of Ground-based ionosonde measurements (especially in the Peruvian, Brazilian, and Indian sectors). All this different techniques according to Fejer et al. (1996) give conflicting results in the F-region evening vertical plasma drifts. However, not many studies on drifts have been carried out in the African sector of the equatorial ionosphere. Consequently, the major technique of drift measurement in the African sector is the ionosonde measurement. We have attempted to show the variation between drifts obtained from hmF2 and h′F from the same location and period in a typical African equatorial station, Ilorin. The typical Ilorin drift observation inferred from the time rate of change of both hmF2 (Vz(hmF2)) and h′F (Vz(h′F)) revealed that the difference in the magnitude of the maximum/minimum values are higher during the equinoxes than at solstices for both Vz(hmF2) and Vz(h′F) during the pre-reversal enhancement (16–19 LT), downward reversal (19–21 LT) and between 21–06 LT periods; and broader for the Vz(h′F) observation. Higher linear correlation (R) relationship between Vz(hmF2) and Vz(h′F) was recorded during the reversal period. R was higher during the entire equinoxes than the solstices for the three considered time phases. R varies from 61–87, 64–96, and 57–66% for the 16–19, 19–21, and 21–06 LT phases respectively. Average RMSdev between both method of analysis ranges within 4.5–6.5 for the entire seasons. Annually between Vz(hmF2) and Vz(h′F), R E0.64 and RMSdev E5.36 The following characteristics were also observed together with the Jicamarca ISR measurement technique: (i) a general downward trend in the three observations between 19–06 LT for all season except for Vz(ISR) during the December solstice; (ii) while Vz(hmF2) peaked at 19 LT for all seasons, Vz(h′F) peaked at 18 LT for September equinox and December solstice; (iii) the evening time PRE for the Vz(h′F) observation started earlier around 16 LT than for the other two techniques for all seasons; (iv) there is little or no PRE observation for the three techniques during June solstice; (v) the evening-time PRE peak magnitude for Vz(hmF2), Vz(h′F) and Vz(ISR) varies between 3–14, 2–14, and 4–14 m/s for the entire seasons; (vi) the nighttime downward reversal peak magnitudes for Vz(hmF2) and Vz(h′F) are respectively within the range of  4 to  14 and  2 to  14 m/s; whereas that of the Vz(ISR) ranges within  12 and  34 m/s. Further, our results revealed higher correlation for both Vz(hmF2) vs. Vz(ISR) ( R ¼0.983) and Vz(h′F) vs. Vz(ISR) (R¼ 0.833) relationships during the equinoxes between 16–20 LT, at which time the F-layer altitude is higher than the 300 km threshold value; and lower for solstice period (0.326 and 0.410 in similar order). An equinoctial maximum and June solstice minimum in postsunset PRE was established for Vz(hmF2), Vz(ISR), and Vz(h′F) On the average, higher peak pre-enhancement drift values were obtained for Vz(h′F) than for Vz(h′F) observation. It is established that vertical drift PRE velocity is seasonal and solar activity dependent. The downward peak reversal time was reached earlier with the

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ionosonde observations than that of the ISR. Both ionosonde procedures (hmF2 and h′F) compares almost equally with the ISR observation in magnitude and pattern. During the evening time plasma enhancement and reversal periods, the drift inferred from hmF2 measurements correlates better with that of the ISR measurements, whereas between 21–06 LT, the inferred drift from h′F observation performed better. However, during the PRE period, the peak magnitude for drift inferred using h′F2 is closer to the corresponding ISR magnitude during the equinoxes; whereas the drift inferred using hmF2 best represent the ISR magnitude for solstices. The linear model fit of the form y¼mx þc revealed better relationship between the ionosonde drifts during equinoxes than solstices.

Acknowledgment We would like to acknowledge the entire staff of the Department of Physics, University of Ilorin for the use of their Equatorial Ionospheric Observatory data, used in analyzing the ionosonde observations over Ilorin. The authors are also grateful to the provider of the Madrigal database at Jicamarca Radio Observatory obtained from the internet web address http://jro-db.igp.gob.pe/madrigal/ for the year 2010. One of the authors (B.O. Adebesin) would like to appreciate M. A. Abdu of the Divisão de Aeronomia, Instituto Nacional de Pesquisas Espaciais, Brazil for the personal communication on the ResearchGate platform, as well as for the exchange of some materials that are of relevance to this work. The constructive ideas suggested by the reviewers are well appreciated.

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