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.
Volume 94, 2020
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