A study carried out using ionospheric spread-F and VHF scintillation data of 1998 and 1999 I-STEP campaigns from a chain of five stations covering the equatorial and subtropical latitudes (geog. lat. 8° to 23°N) in the Indian sector indicates that the spread-F and scintillation activity can be classified into two broad categories (plasma bubble and BSS) depending on time duration of events and their onset at different latitudes. Recurrent short duration equatorial spread-F (ESF) and scintillation activity thought to be due to plasma depletions/bubbles are associated with a sharp F-region height rise at equatorial and subtropical latitudes. Such events are found to be more probable during equinox and winter solstice months, while the long duration events associated with the bottom side sinusoidal (BSS) irregularities are found to be less probable.