Long-term observations of VHF scintillation and total electron content near the crest of the equatorial anomaly in the Indian longitude zone

Ionospheric VHF scintillation (SI greater than or equal to 3 dB and saturated level).,and total electron content (TEC) data obtained at Calcutta (subionospheric 21 degrees N, 92.7 degrees E geographic, 27 degrees N dip) and ionosonde data at Kodaikanal (10.2 degrees N, 77.5 degrees E geographic, 3.5 degrees N dip) for the period 1977-1990 have been analysed to show the importance of electrodynamic drift near the magnetic equator in controlling nighttime ionospheric F region ionization and irregularities in the equatorial region. Such long-term observations extending over a period of more than 13 years are possibly being reported for the first time from a location situated near the equatorial anomaly crest. Frequent and intense VHF scintillations near the equatorial anomaly crest during equinoctial and December solstice months around solar maximum years, have been identified with the equatorial F region irregularities. Simultaneous measurement of TEC at the same location shows that during solar maximum years the high F region ambient ionization is sustained for several hours in the postsunset period, often showing secondary enhancements during equinoctial months. Under solar minimum epoch, when scintillation is sparse, TEC in the above period shows a rapid decrease. At Kodaikanal, situated near the magnetic equator, during the equinoctial and December solstice months of solar maximum years, h'F values rise by more than 100 km in about an hour around sunset. These features are seldom observed during solar minimum epoch. A causative connection among h'F variation near the magnetic equator and the maintenance of high ambient ionization and occurrence of scintillation near the anomaly crest is established. Further, scintillation occurrence during the May-July months shows a remarkable hysteresis effect with solar activity level.