NIGHTTIME ENHANCEMENTS IN IONOSPHERIC ELECTRON-CONTENT - SEASONAL ANDSOLAR-CYCLE VARIATION

Various characteristics of anomalous night-time enhancement in ionosph eric electron content (IEC) at Lunping (14.08 degrees N geomagnetic), a station near the crest of the equatorial anomaly, have been presente d by considering the IEC data for the 21st solar cycle. Out of a total of 1053 enhancements, 354 occur in pre-midnight and 699 occur in post -midnight events at Lunping. The occurrence is more frequent during su mmer, less during the equinox and least during winter months. All the characteristics of the enhancements have seasonal dependencies and the y reach their maximum values during summer months. The occurrence of t he pre-midnight events show positive and post-midnight events show neg ative correlation with solar activity. The results have been discussed and compared with those at low-latitude stations in India and Hawaii and at the mid-latitude station, Tokyo.