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 simultaneously (during September and October of 1989) from two satellites at Sikandarabad (dip 42.0 degrees N) and Chenglepet (dip 10.5 degrees N), along a common magnetic meridian (149.0 degrees E), are analyzed. Scintillation occurrences on a daily basis at these two locations are examined in relation to presunset hour variations in h'F and their rate of rise, 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 scintillation at 21 degrees N magnetic latitude. The characteristics and occurrence pattern of scintillations at these two stations suggest that they are equatorial plasma bubble induced events. The scintillations and spread F at the equatorial location occur whenever h'F rises to levels of more than 400 km during evening hours. However, at 21 degrees N magnetic latitude, scintillations were observed only on those nights when 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 inhibited during magnetic disturbances at both the locations except during postmidnight hours, where it is found to increase. With increasing magnetic 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. Scintillation intensity at low latitudes is found to be positively correlated with ionospheric electron content values observed during evening hours (2000 LT), as well as with their diurnal maximum values. In addition, scintillation intensity is also found to be dependent on the geometries of the ray paths to the respective locations and their intersections with the magnetic field lines.