Range spread F (RSF) data for a 12-year period (1978–1990) over the Brazilian low-latitude station, Cachoeira Paulista, are analyzed to determine the fine structure of the seasonal pattern of the irregularity occurrence, which appears consistent throughout an entire solar cycle. The RSF occurrence for one of these years is compared also with that over the equatorial station, Fortaleza, to show that the seasonal pattern discussed here corresponds to that of plasma bubble irregularity developments. A striking result that has come out of the present analysis concerns two secondary minima, during the epoch of broad annual maximum, in the RSF that occur in association with the two nodal points at the magnetically conjugate E layer sunset local times, that is, at the perfect alignment of the sunset terminator with the magnetic meridian of the station. The occurrence of these minima, seen in individual solar minimum and solar maximum years (as also in the overall mean behavior), is interpreted on the basis of a simplified F region dynamo development model that considers also asymmetric conjugate E layer decay conditions. Competing roles of a prereversal zonal electric field enhancement and of its height gradient seem to determine the occurrences of these secondary minima in RSF.
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