The times of reversal of east-west electrostatic field in the ionosphere near the equator in the Indian zone have been estimated from the measurements of ionospheric drift at Thumba. The reversal of electric field in the morning from westward to eastward is delayed with respect to the sunrise at 100 km by 1.5 hr during winter and by about 3 hr during summer months. The reversal in the evening from eastward to westward occurs around 2100 hrs,i.e., well after sunset during winter months and around 16–17 hrs,i.e., well before sunset during summer months. The electric field in the American zone is known to reverse 1–2 hr after the sunrise and sunset at 100 km; the duration of daytime eastward electric field varies with season between 12 and 16 hr. In the Indian zone, duration of the eastward field during the J months is only 8 hr. These longitudinal differences in the reversal times of electrostatic field are suggested to be the cause of longitudinal differences in the equatorial ionosphere,viz., high incidence of blanketing sporadicE layer in the Indian zone and the longitudinal differences in the occurrence of spreadF.

The recordings of the amplitudes of radio beacon signals on 40, 140 and 360 MHz from ATS-6 (at 34° E longitude) recorded at Ootacamund, India (11.43° N, 76.70°E, dip 4°N, elevation angle 41°) have revealed largest occurrence of scintillations for about 60% of cases around 2200 hr during the nighttime, and two secondary peaks (25% of cases) around 0900 hr and 1400 hr during the daytime.

During the daytime, the scintillation decreases approximately as the inverse of the frequency for higher frequencies while for lower frequencies the law is valid till scintillation index at 40 MHz does not exceed 0.9. The temporal variation of daytime scintillation shows impulsive character, the duration of activity lasts for 1–2 hours at a time.

During the nighttime, the scintillation decreases inversely with frequency for weak and moderate scintillation activity. The scintillation index at 360 MHz becomes independent of that at 140 MHz when the index at 140 MHz exceeds 0.85. For the set of frequencies 40–140 MHz, on some occasions scintillation index at 40 MHz is seen to be less than that at 140 MHz. The nighttime scintillations are in general stronger and remain so for extended length of time.

The daytime scintillations are suggested to be due to blanketing or some other non-q type of sporadicE layer. The nighttime scintillations are most probably due to spreadF condition and the abnormal frequency variation of the scintillations may be due to multiple scattering layer during periods of intense spreadF.