Abstract
Abstract. This study presents statistical quantification of the correlation between the equatorial electrojet (EEJ) and the occurrence of the equatorial ionisation anomaly (EIA) over the East African sector. The data used were for quiet geomagnetic conditions (Kp ≤ 3) during the period 2011–2013. The horizontal components, H, of geomagnetic fields measured by magnetometers located at Addis Ababa, Ethiopia (dip lat. ∼1∘ N), and Adigrat, Ethiopia (dip lat. ∼6∘ N), were used to determine the EEJ using differential techniques. The total electron content (TEC) derived from Global Navigation Satellite System (GNSS) signals using 19 receivers located along the 30–40∘ longitude sector was used to determine the EIA strengths over the region. This was done by determining the ratio of TEC over the crest to that over the trough, denoted as the CT : TEC ratio. This technique necessitated characterisation of the morphology of the EIA over the region. We found that the trough lies slightly south of the magnetic equator (0–4∘ S). This slight southward shift of the EIA trough might be due to the fact that over the East African region, the general centre of the EEJ is also shifted slightly south of the magnetic equator. For the first time over the East African sector, we determined a threshold daytime EEJ strength of ∼ 40 nT that is mostly associated with prominent EIA occurrence during a high solar activity period. The study also revealed that there is a positive correlation between daytime EEJ and EIA strengths, with a strong positive correlation occurring during the period 13:00–15:00 LT. Keywords. Ionosphere (equatorial ionosphere)
Highlights
One of the factors that determines the distribution of ambient ion and electron density in the low-latitude F region ionosphere is the vertical E × B drift, which is roughly directly proportional to the equatorial electrojet (EEJ) during daytime (Anderson et al, 2002)
Otherwise, based on the cases observed over Africa and India, we suggest that the location of the equatorial ionisation anomaly (EIA) trough over a particular longitude depends on the alignment of the centre of the EEJ with respect to the magnetic equator
We have established the statistics of the correlation between daytime EEJ strength and the occurrence of the EIA over the East African sector
Summary
One of the factors that determines the distribution of ambient ion and electron density in the low-latitude F region ionosphere is the vertical E × B drift, which is roughly directly proportional to the equatorial electrojet (EEJ) during daytime (Anderson et al, 2002). The large enhancements in electron densities on either side of the magnetic equator significantly affect radio frequency signals passing through the ionosphere and groundto-ground high-frequency communication systems (Anderson et al, 2004) Due to such problems, several studies have been undertaken to understand the influence of the EEJ on the development of the EIA. Yue et al (2015) derived peak electron density in the F2 layer (NmF2) from the Global Position System (GPS) Radio Occultation (RO) observations made by the Constellation Observation System for Meteorology, Ionosphere and Climate (COSMIC) mission to study the morphology of the EIA statistically during 2006–2014 They found that NmF2 increases more significantly with solar activity in the crest region than that of the trough region.
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