The relationship between equatorial ionization anomaly and nighttime equatorial spread F in East Africa

Abstract

We study the relationship of equatorial ionization anomaly (EIA) to the occurrence of nighttime equatorial spread F (ESF) using data from ground-based GPS stations and a horizontal meridional neutral wind model for the East African sector. Anomaly strength, symmetry of anomaly crests, horizontal meridional wind magnitude and direction, and magnitude of rate of TEC index (ROTI) are computed and analyzed. We find mean crest to trough ratio (CTR) greater than or equal to 1.4 and mean north crest to south crest ratio (CCR) within 0.9 and 1.1 in the time between 16:00 UT (19:00 LT) and 18:00 UT (21:00 LT) as good conditions for the occurrence of ESF. EIA strength and symmetry of anomaly crests are better related with ESF occurrence between conjugate stations (≈ ± 11.5o magnetic latitudes) than between other north-south stations. ROTI is found to be relatively large when CCR of EIA are symmetrical (CCR between 0.9 and 1.1) and CTR is greater than or equal to 1.4, but decreases as the CCR of EIA shows asymmetry and/or CTR of EIA is less than 1.4. EIA strength and symmetry of anomaly crests between 18:00 UT to 20:45 UT are not well related with spread F occurrences. Post-sunset spread F occurrence close to anomaly crest locations are found to be greater than spread F occurrence at magnetic equatorial station. Shifting of EIA crest locations on either side of the magnetic equator is found to depend on season and meridional wind magnitude and direction. The relative difference in spread F occurrence among the four seasons is also discussed.