Sk. Chakraborty et al., Long-term observations of VHF scintillation and total electron content near the crest of the equatorial anomaly in the Indian longitude zone, RADIO SCI, 34(1), 1999, pp. 241-255
Ionospheric VHF scintillation (SI greater than or equal to 3 dB and saturat
ed level).,and total electron content (TEC) data obtained at Calcutta (subi
onospheric 21 degrees N, 92.7 degrees E geographic, 27 degrees N dip) and i
onosonde 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 imp
ortance of electrodynamic drift near the magnetic equator in controlling ni
ghttime ionospheric F region ionization and irregularities in the equatoria
l 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 sit
uated near the equatorial anomaly crest. Frequent and intense VHF scintilla
tions near the equatorial anomaly crest during equinoctial and December sol
stice months around solar maximum years, have been identified with the equa
torial F region irregularities. Simultaneous measurement of TEC at the same
location shows that during solar maximum years the high F region ambient i
onization is sustained for several hours in the postsunset period, often sh
owing 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 eq
uinoctial and December solstice months of solar maximum years, h'F values r
ise 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 i
onization and occurrence of scintillation near the anomaly crest is establi
shed. Further, scintillation occurrence during the May-July months shows a
remarkable hysteresis effect with solar activity level.