Features of additional stratification in ionospheric F2 layer observed for half a solar cycle over Indian low latitudes

[1] The features of the additional stratification in the ionospheric F2 layer often referred to as the F3 layer observed over an Indian low-latitude station Waltair (17.7°N, 83.3°E, magnetic latitude (Mag. Lat.) 8.2°N) during the period 1997–2003 are presented along with the data from two other Indian stations. The observations grossly confirm those reported earlier in the occurrence and seasonal variability of the F3 layer. From an analysis of half a solar cycle ionosonde data (1997–2003), it is observed that the layer appeared more frequently during the summer solstice months of low solar activity period and persisted for longer durations during this season compared with equinox and winter solstice. The best stratification is seen between 10–12 hours IST. The occurrence of the F3 layer does not seem to depend on magnetic activity but the percentage of occurrence decreased with increasing solar activity. The solar activity dependence over Waltair confirms the model predictions of Balan et al. (1998) that the layer becomes less distinct and less frequent as solar activity increases. The ionosonde data for the same period (1997–2003) from an equatorial station, Trivandrum (8.4°N, 76.9°E, Mag. Lat. 0.47°N) and another low-latitude station, SHAR (14°N, 80°E, Mag. Lat. 6.8°N) are also analyzed with a view to examine the effect of the equatorial plasma dynamics on the occurrence of such events.