1. Department of Physics, Lagos State University, Ojo, Lagos, Nigeria. @
COMPARISON OF DIURNAL AND SEASONAL VARIABILITY OF THE CRITICAL FREQUENCY OF THE F2 LAYER OVER ILORIN, JICAMARCA AND OKINAWA DURING MINIMUM SOLAR ACTIVITY BY
Onori E.O., Somoye E.O., Ometan O. O., Ogwala A., Ogungbe A.S., Adeniji-Adele R.A., Ogabi C.O., Adejo O. A.., Oluyo K.S., Sode A.T.
(LASU Ionospheric and Radio Propagation Research Group)
Department of Physics, Lagos State University, Ojo, Lagos, Nigeria. @
Faculty of Science Conference
on Cutting Edge Scientific Research: A Gateway to National Development
10th - 14th October , 2017

2. OUTLINE
INTRODUCTION
AIM
DATA SOURCE
METHODOLOGY
RESULTS AND DISCUSSION
CONCLUSION

3. INTRODUCTION
Figure 1: The Ionosphere as Reflecting Mirror ( Courtesy Encyclopaedia Britannica, Inc).

4. Appleton-Hartree equation (Klobuchar, 1996)
INTRODUCTION CONTD.
Appleton-Hartree equation (Klobuchar, 1996)
(1)
Where Ne, E and m are electron density of ionosphere, charge and mass of the electrons, εo is the free space permittivity, θ = the angle of the ray with respect to the Earth’s magnetic field, ƒ is the plasma frequency and ƒH is the gyrofrequency.

5. Ionospheric Layers : D, E, and F
INTRODUCTION CONTD.
Ionospheric Layers : D, E, and F
Figure 2: Layers of The Ionosphere (Courtesy Windows of The Universe).

6. INTRODUCTION CONTD.
Critical frequency(fo): The critical frequency is the highest frequency of an
ionospheric layer that reflects radio wave at vertical incidence. The critical
frequency of F2 layer is denoted as foF2
It is the most widely used parameter in describing the state of the ionosphere. It is directly proportional to the amount of F2 region ionization.
The state of the ionosphere is determined by the foF2 values. It is influenced by solar activity, geomagnetic activity and the neutral atmosphere effects.

7. INTRODUCTION CONTD.
Ionospheric variability: is the deviation from climatological means of the ionosphere. It can occur on hourly, daily, seasonal, annual and solar cycle scales. (Zhang and Holt, 2008)
It has great impact on trans - ionospheric radio communication and navigation systems (Zhang et al. 2004, Basu et al. 2002).
AIM
The aim of this study is to compare the variability of the critical frequency of F2 layer (foF2), on diurnal, seasonal and semiannual scales in three equatorial ionospheric station from different sectors.

8. DATA SOURCE
F2-layer critical frequency (foF2) hourly values are obtained from these three ionospheric stations shown in table 1
Table 1: Station location and Coordinates
Station
Coordinates
Sector
Ilorin, Nigeria
(Lat. 8.47oN, Long.4.6oE, dip 4.1oS)
African
Jicamarca, Peru
(Lat. 11.9oS, Long. 76.8oW,dip1oS)
American
Okinawa, Japan
(Lat. 26.3oN, Long.127.8oE, dip 36.8oN)
Asian
Space Physics Interactive Data Resource (SPIDR) website (

9. DATA PROCESSING STEPS Analysis METHODOLOGY CONTD.
Evaluating Variability (VR)
CalculatingStandard deviation
Computing monthly Mean hourly values.
Tabulating raw data on hourly, daily and monthly
DATA PROCESSING STEPS
Analysis

10. Table 2. Grouping of data into different seasons
METHODOLOGY
Diurnal and seasonal effects are investigated by grouping data into seasons and finding the average of the months that fall in a particular season. Each season contains three months defined as follows
Table 2. Grouping of data into different seasons
SEASONS
MONTHS
DECEMBER SOLSTICE
November, December and January
MARCH EQUINOX
February, March and April
JUNE SOLSTICE
May, June and July
SEPTEMBER EQUINOX
August, September and October
Adeniyi et al. (2007) and Somoye et al. (2013)
March equinox was not considered for all stations due to lack of available data at Ilorin station for that season.

11. Relative variability (VR) is given (2)
METHODOLOGY CONTD.
The relative variability (VR) is obtained by using the monthly mean, µ and the
standard deviation, σ at each hour, adopting the paradigm of Forbes et al., 2000;
Rishbeth and Mendillo, 2001; Bilitza et al., 2004 and Adebesin et al., 2014,
Relative variability (VR) is given
(2)
For the diurnal and seasonal evaluation of the coefficient of foF2 variability; the means (µ) of the foF2 values for the 3 months that make up the season and their corresponding standard deviations (σ) were used, although, 2 months of data were used for the June solstice and 1 month data for the December solstice owing to data limitation.
Similarly, for the semiannual evaluation; the semiannual means (µ) of the foF2 values for all the months of the year under consideration and their corresponding standard deviations (σ) were used.

12. variability at some other ionospheric stations .
METHODOLOGY CONTD.
Akala et  al., (2011), Somoye et  al., (2011, 2013), Onori et al., 2015 and
Somoye et  al., 2017 had made use of equation 2 in studying the ionospheric
variability at some other ionospheric stations .
Zhang et al. (2004) and Ezquer et al. (2004)
VR (%) = (Interquartile range/median) x 100 that is
(3)
Relative variability is often expressed as a percentage. A lower percentage
indicates that the data set is less varied while a higher percentage indicates
data set is more varied.

13. RESULTS AND DISCUSSION
Variability Pattern in foF2 (Diurnal and Seasonal Variation)
Figure 2: Diurnal variation of foF2 VR for all season during a year of minimum solar activity (2010) at Jicamarca
Figure 3: Diurnal variation of foF2 VR for all season during a year of minimum solar activity (2010) at Okinawa
Figure 1: Diurnal variation of foF2 VR for all season during a year of minimum solar activity (2010) at Ilorin

14. RESULTS AND DISCUSSION
Semiannual Variation of foF2 VR
Figure 4: Diurnal variation of foF2 VR values derived from semiannual mean of foF2 values for all seasons during a year of minimum solar activity (2010) at Ilorin, Jicamarca and Okinawa

15. RESULTS AND DISCUSSION
On a general consideration, all the plots in Figures 1, 2 and 3 reveal the same diurnal features during the entire three seasons.
foF2 VR is observed to be high during nighttime and low during the daytime. However, in Okinawa station, a noticeable difference in foF2 variability pattern was observed on comparison with the patterns observed at Ilorin and Jicamarca stations.
This may likely be due to the fact that Okinawa is latitudinally located above the northern crest of the equatorial anomaly (Balan and Bailey, 1995; Bala et al., 2009).
During the daytime (0600 – 1800 LT) the variability VR was observed to be lowest about (6 - 16%) for Ilorin, (6 – 13%) for Jicamarca and (8 – 25%) for Okinawa.

16. RESULTS AND DISCUSSION
At nighttime, the variability had increased to about ( %) at Ilorin, ( %) at Jicamarca and about (25 – 39%) at Okinawa.
The plots are characterized by two peaks, the Post-midnight peak with a magnitude of about ( %), ( %) and (22 – 39%) respectively at Ilorin, Jicamarca and Okinawa were observed between 0000 and 0500 LT for all seasons.
The second peak (the pre-midnight peak) with a value of about ( %), ( %) and ( %) were observed between 1800 and 2300 LT at Ilorin, Jicamarca and Okinawa respectively for all seasons.
The two foF2 variability peaks observed are attributed to sudden electron density gradients triggered by the onset and turn-off of solar ionization, as well as the superimposition of spread-F on the background electron density (Chou and Lee, 2008; Akala et al., 2010; Adebesin et al., 2014). In addition the vertical E x B plasma drift is also responsible for these observed peaks.

17. RESULTS AND DISCUSSION
Chou and Lee (2008) explained the observed increase in the foF2 VR with decreasing solar activity in terms of low reference value and ion loss. At night the ionospheric density was largely dependent on the recombination rate, which could be affected by the gas composition and the magnetic meridional winds.
The daytime minimum foF2 VR of between 6 and 16% observed during this minimum solar activity period is consistent with the result of Bilitza et al. (2004) who recorded a value range of 5–15% over two African equatorial stations of Ouagadougou and Korhogo (lat. 9.3◦N, long. 5.4◦W, dip 0.67◦S)
Oladipo et al., (2008) recorded 3–10% for the same daytime period while investigating another equatorial station in the African region, during low solar activity periods.

18. RESULTS AND DISCUSSION
Figure 4 clearly revealed that relative variability foF2 at Ilorin is highest, followed by that at Okinawa and least at Jicamarca.
Furthermore, the study revealed that Okinawa data are characterized by higher absolute values of foF2 in comparison with data from Ilorin and Jicamarca.
It is clear from the study that Okinawa data show unusual features that required further study. Thus the analysis of a large volume of data from Okinawa station will be necessary to ascertain clearer characteristics of its ionospheric foF2 variability.

19. RESULTS AND DISCUSSION Diurnal and Seasonal Variation of Mean foF2
Figure 6: Diurnal variation of mean foF2 for all season during a year of minimum solar activity (2010) at Jicamarca
Figure 7: Diurnal variation of mean foF2 for all season during a year of minimum solar activity
(2010) at Okinawa
Figure 5: Diurnal variation of mean foF2 for all season during a year of minimum solar activity (2010) at Ilorin

20. RESULTS AND DISCUSSION
Semiannual Variation of Mean foF2
Figure 8: Diurnal variation of mean foF2 values derived from semiannual mean of foF2 values for all seasons during a year of minimum solar activity (2010) at Ilorin, Jicamarca and Okinawa

21. RESULTS AND DISCUSSION
Both the observed pre-noon and afternoon foF2 peaks in the F2-layer and the variability are due to variations of vertical E × B plasma drifts (e.g., Fejer et al. 1995; Fejer 1997; Adebesin et al. 2013).
For this minimum solar activity period, (Figures 5, 6 and 7), the post-noon peak otherwise known as the afternoon peak is slightly greater than the pre-noon peak for the three stations, though that of Jicamarca seems nearly equal in appearance.
Okinawa station shows a contrary observation during the nighttime i.e. it was lowest in December solstice and highest during June solstice.
This may likely be due to the fact that Okinawa is latitudinally located above the northern crest of the equatorial anomaly (Balan and Bailey, 1995; Bala et al., 2009).

22. RESULTS AND DISCUSSION
Observations have shown that foF2 increases after sunrise; the increase being more prominent at lower and equatorial latitudes. Maximum is reached in the early afternoon and there is a rapid decrease shortly after sunset.
During the nighttime, the critical frequency is lowest in June solstice (winter) and highest during the December solstice and September equinox for Ilorin and Jicamarca stations, but that of Jicamarca is more distinct.
Liu et al reported that the F2 layer critical frequency during nighttime is lowest in June solstice and highest during the equinoctial months.

23. RESULTS AND DISCUSSION
Chou and Lee (2000) reported that the mean foF2 maximum is controlled by enhanced eastward electric fields (EEF) and neutral winds, and this may be supported by diffusion of super fountain plasma from the equator towards Okinawa (a little above the crest of equatorial anomaly).
Radicella and Adeniyi (1999) reported similar observation for Ouagadougou (12.4 oN, 1.5 oE; dip 5.9◦) an equatorial station, during a low solar activity year.

24. RESULTS AND DISCUSSION
Meanwhile at Jicamarca station, both the pre-noon and post-noon peaks recorded an approximate average value of 8.0 MHz. These results for low solar activity period (2010) are closely in agreement with the results reported by Adebesin et al., 2014 using average values for low solar activity years of 2009 and 2010.
For all four plots (Fig. 5, 6, and 8), we observed the noon bite-out profile characterized by the presence of the strength counter electrojet (SCE), a phenomenon associated with the magnetic equatorial region (Faynot and Villa, 1979; Doua et al., 2013).
The noon bite-out feature occurred at Ilorin and Jicamarca stations around 1200 LT and earlier at Okinawa around 1000 LT. The lowest minimum 7.0 MHz, 6.3 MHz and 7.0 MHz at Ilorin, Jicamarca and Okinawa respectively were observed during June solstice.

25. CONCLUSION
Diurnal analysis revealed that the critical frequency of the F2 layer is more
prone to variability (VR) during the nighttime than the day time at these
stations with two characteristics peaks (post-midnight peak and pre-midnight
peak).
The peaks at Ilorin ( %, %) are observed to be higher in values
than those at Jicamarca ( %, %) and Okinawa (22 – 39%, 15 –
30%).
Seasonally, December solstice maximum was noticed at Ilorin.
Semiannual analysis showed that foF2 VR is highest at Ilorin, followed by
Okinawa and least by Jicamarca station during the nighttime.

26. three stations, with Jicamarca having highest values for both peaks.
CONCLUSION
Diurnal curves of Mean foF2 revealed pre-noon and post-noon peaks at all
three stations, with Jicamarca having highest values for both peaks.
Noon bite - out lowest minimum for the three stations were observed during
June solstice around 1200 LT.
Overall conclusion, foF2 variability is highest at Ilorin, followed by
Okinawa and lowest at Jicamarca while for F2 layer ionization the reverse
is the case.

27. Thank You
For Listening

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