Abstract

The ionosphere displays a wide range of variations ranging from diurnal, seasonal, annual and solar cycle variation. In this paper we present a study of the comparison of diurnal, seasonal and semiannual variation of the variability (VR) of the critical frequency of the F2 layer (foF2) over Ilorin (Lat. 8.47oN, 4.6oE, dip 4.1oS) in the African sector, Jicamarca (Lat. 11.9oS, Long.76.8oW, dip 1oN) in the American sector and Okinawa (26.3oN, 127.8oE, dip 36.8oN) in the Asian sector during solar minimum period. These stations lie within the equatorial anomaly region of the ionosphere. 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 (20 - 43%; 16 - 25%) are observed to be higher in values than those at Jicamarca (17 - 27%; 15 - 22%) and Okinawa (22 – 39%; 15 - 30%). Seasonally, December solstice maximum was noticed at Ilorin. Semiannual analysis showed that foF2 VR is highest at Ilorin station, followed by Okinawa station and least by Jicamarca station during the nighttime. Diurnal curves of Mean foF2 revealed pre-noon and post-noon peaks at all stations, with Jicamarca having highest values for both peaks while the semiannual curves of mean foF2 values showed almost equal values for all stations. Overall, foF2 variability is highest at Ilorin and lowest at Jicamarca while for F2 layer ionization the reverse is the case.

Keywords: FoF2 variability, critical frequency, minimum solar activity, and F2 layer