Abstract
The unique geometry of the geomagnetic field lines over the equatorial ionosphere coupled with the E-W electric field causes the equatorial ionization anomaly (EIA) and equatorial spread-F (ESF). Ionosonde data obtained at a chain of four stations covering equator to anomaly crest region (0.3 to 33 °N dip) in the Indian sector are used to study the role of EIA and the associated processes on the occurrence of ESF. The study period pertains to the equinoctial months (March, April, September and October) of 1991. The ratios of critical frequency of F-layer (Æ0F2) and electron densities at an altitude of 270 km between Ahmedabad (33 °N dip) and Waltair (20 °N dip) are found to shoot up in the afternoon hours on spread-F days showing strengthening of the EIA in the afternoon hours. The study confirms the earlier conclusions made by Raghava Rao et al. and Alex et al. that a well-developed EIA is one of the conditions conducive for the generation of ESF. This study also shows that the location of the crest is also important in addition to the strength of the anomaly.
Highlights
The sunset in the equatorial and low latitude ionosphere is characterised by a rapid rise in the height of the Flayer due to the enhancement of the eastward electric field before it reverts to westward, which forms a platform for the generation of various plasma irregularities
In the low-latitude regions, it is known that the critical frequency of the F-layer and the electron density vary significantly as a function of local time, influenced by the phenomenon of equatorial ionization anomaly (EIA)
It is known that the occurrence of equatorial plasma instabilities quite frequent during the evening hours, do not occur every day, as one would expect if it were due to the gravitational drift alone
Summary
The sunset in the equatorial and low latitude ionosphere is characterised by a rapid rise in the height of the Flayer due to the enhancement of the eastward electric field before it reverts to westward, which forms a platform for the generation of various plasma irregularities.
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