Permanent changes in sporadic E layers over Fortaleza, Brazil

Abstract

Data available on sporadic E layers in ionograms, from 1975 to 1995 over Fortaleza (38°W, 4°S) in Brazil are analysed as an extension of a previous study to examine permanent changes in the occurrence characteristics of the different types of E s layers. The so called q type E s produced by plasma instability (type II) process of the equatorial electrojet and the other types of E s produced by wind/wind shear mechanism are studied separately. The results show that while the occurrence of the former variety steadily decreased during the period of analysis, the latter type showed marked increases. These characteristics are shown to be associated mainly with the steady drift of the magnetic equator to northward of Fortaleza at a rate of ∼ 0.2 degree per year. The long term trends are, however, modulated by solar activity cycle in F 10.7 flux. Also there is a suggestion of a possible effect from the June 1991 Pinatubo volcanic eruption. The competing roles of the neutral winds and electric fields in the formation of the low latitude sporadic E layer is demonstrated.