Relationships Between Pre-sunset Electrojet Strength, Pre-reversal Enhancement and Equatorial Spread-F Onset

Equatorial spread-F (ESF) is electron density irregularity occurring in the post-sunset equatorial F-region ionosphere that causes interruption and degradation to various satellite communication and navigation systems. To date, it has been clarified that various physical parameters and processes control ESF occurrence and development. In addition to such controlling factors, recent observations suggest a relationship between ESF occurrence and equatorial electrojet (EEJ) strength. To investigate this relationship in more detail, we analyzed the virtual height of the bottom side F-region (h’F) and ESF onsets at Chumphon, Thailand (10.7 °N, 99.4 °E; 3.3 °N magnetic latitude) with the EEJ ground strength at Phuket, Thailand (8.1 °N, 98.3 °E; 0.1 °N magnetic latitude) during the period from November 2007 to October 2008. We found that increase in the F-layer height and ESF onsets during the evening hours were well connected with the EEJ ground strength before sunset, namely, both the height increase and ESF onsets were suppressed when the integrated EEJ ground strength for the period from 1 to 2 h prior to sunset was negative. The finding suggests observationally that the pre-sunset E-region dynamo current and/or electric field are related to the F-region dynamics and ESF onsets around sunset.