DAY-TO-DAY VARIABILITY IN THE OCCURRENCE OF EQUATORIAL AND LOW-LATITUDE SCINTILLATIONS IN THE INDIAN ZONE

Nighttime ionospheric scintillation data at 4 GHz, recorded simultaneo usly (during September and October of 1989) from two satellites at Sik andarabad (dip 42.0 degrees N) and Chenglepet (dip 10.5 degrees N), al ong a common magnetic meridian (149.0 degrees E), are analyzed. Scinti llation occurrences on a daily basis at these two locations are examin ed in relation to presunset hour variations in h'F and their rate of r ise, as well as with solar and magnetic activity conditions. The main objective of the study is to identify the ionospheric conditions over the magnetic equator necessary for observing intense gigahertz scintil lation at 21 degrees N magnetic latitude. The characteristics and occu rrence pattern of scintillations at these two stations suggest that th ey are equatorial plasma bubble induced events. The scintillations and spread F at the equatorial location occur whenever h'F rises to level s of more than 400 km during evening hours. However, at 21 degrees N m agnetic latitude, scintillations were observed only on those nights wh en h'F rises to more than 500 km with dh'F/dt of 30 m/s or more. Also, whenever h'F was less than 400 km, no scintillations were observed at any of the locations. Scintillation activity was found to be inhibite d during magnetic disturbances at both the locations except during pos tmidnight hours, where it is found to increase. With increasing magnet ic activity, h'F values during evening hours decrease. The day-to-day variability in the occurrence of scintillations seems to be controlled mainly by the electric field, neutral winds, and magnetic activity. S cintillation intensity at low latitudes is found to be positively corr elated with ionospheric electron content values observed during evenin g hours (2000 LT), as well as with their diurnal maximum values. In ad dition, scintillation intensity is also found to be dependent on the g eometries of the ray paths to the respective locations and their inter sections with the magnetic field lines.