Longitudinal Differences in the Spread F Characteristics in the American Zone and Implications on the F-Region Dynamo

摘要

A comparative study is carried out of the spread F occurrence rates at Fortaleza (4 deg S, 38 deg W) and Huancayo (12 deg S, 75 deg W), two stations located along the magnetic equator in the American zone. While there are similarities in the gross features of the range type spread F occurrences, profound differences, on a seasonal basis, are present in the case of the frequency type spread F at the two stations. Profound dissimilarities are observed in the evening enhancement in the F region vertical drift velocity (V) measured over Jicamarca (and the F region height changes observed over Huancayo), with respect to that observed over Fortaleza, in that the seasonal trends in the local times of prereversal V peak occurrences are in exactly opposite sense at the two stations. An attempt is made to interpret these results in terms of the differences in the magnetic field declinations at the two stations that cause significant differences in the conjugate E region sunset directions and, hence, in differing rates of F region polarization electric field developments at these stations.