Variability of foF2 in the African equatorial ionosphere A.O. Akala a, * , E.O. Oyeyemi a , E.O. Somoye b , A.B. Adeloye a , A.O. Adewale a a Department of Physics, University of Lagos, Akoka, Yaba, Lagos, Nigeria b Department of Physics, Lagos State University, Ojo, Lagos, Nigeria Received 25 August 2009; received in revised form 4 January 2010; accepted 4 January 2010 Abstract This paper presents the impact of diurnal, seasonal and solar activity effects on the variability of ionospheric foF2 in the African equa- torial latitude. Three African ionospheric stations; Dakar (14.8°N, 17.4°W, dip: 11.4°N), Ouagadougou (12.4°N, 1.5°W, dip: 2.8°N) and Djibouti (11.5°N, 42.8°E, dip: 7.2°N) were considered for the investigation. The overall aim is to provide African inputs that will be of assistance at improving existing forecasting models. The diurnal analysis revealed that the ionospheric critical frequency (foF2) is more susceptible to variability during the night-time than the day-time, with two peaks in the range; 18–38% during post-sunset hours and 35– 55% during post-midnight hours. The seasonal and solar activity analyses showed a post-sunset September Equinox maximum and June Solstice maximum of foF2 variability in all the stations for all seasons. At all the stations, foF2 variability was high for low solar activity year. Overall, we concluded that equatorial foF2 variability increases with decreasing solar activity during night-time. Ó 2010 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: African equatorial ionosphere; foF2 variability; Forecasting models 1. Introduction The ionosphere over equatorial latitudes is highly dynamic, and consequently poses serious threats to com- munication and navigation systems, especially during mag- netically disturbed days. In order to address these challenges, a proper understanding of ionospheric behav- iour over these zones (equatorial latitudes) and the inter- vening physical processes that lead to its continuous variability is important for effective forecasting (Abdu et al., 2004). An empirical model can provide reliable sim- ulation data for effective ionospheric study and forecasting. Several of such models have been developed; the most com- monly used is the International Reference Ionosphere (IRI) (Bilitza, 2001). On the other hand, analyses of real time observations of ionospheric parameters at different loca- tions on the globe are the significant indices for improving the overall effectiveness of these models. Previous studies by several authors have presented reports on the variability of ionospheric parameters. For instance, Jayachandran et al. (1995), Forbes et al. (2000), Rishbeth and Mendillo (2001), Kouris and Fotiadis (2002), Bilitza et al. (2004), Fotiadis et al. (2004), Bradley et al. (2004), Zhang et al. (2007), Chou and Lee (2008) and Atac et al. (2009) investigated the variability of foF2, hmF2, and MUF at different latitudes, while Zhang and Holt (2008) characterized the variability in electron density and plasma temperatures at mid-latitudes. A large propor- tion of these efforts have been focussed on the high- and mid-latitudes, thereby leaving the equatorial latitudes, especially the African equatorial latitude, underrepre- sented. This is directly correlated with the sparse distribu- tion of ionospheric stations in Africa. The present study tends to bridge this gap by analyzing observed foF2 data from three equatorial stations in Africa with a view to investigating the diurnal, seasonal, solar, and latitudinal variability of foF2. The overall aim is to provide African inputs that will be of assistance at improv- ing existing forecasting models. 0273-1177/$36.00 Ó 2010 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2010.01.003 * Corresponding author. Tel.: +234 8055419769. E-mail address: akalaovie2004@yahoo.com (A.O. Akala). www.elsevier.com/locate/asr Available online at www.sciencedirect.com Advances in Space Research 45 (2010) 1311–1314