Characteristics of slant sporadic-E layers observed at low-latitudes

This study investigates the occurrence, characteristics, and formation mechanisms of slant sporadic-E layers (Es_s). The Es_s-type layers observed at the Brazilian low-latitude stations of Jataí (17.9°S, 51.7°W) and São José dos Campos (23.2°S, 45.8°W), are analyzed using ionosonde data recorded for four months (April, June, September, and December) of 2016. Parameters such as top frequency (ftEs), blanketing frequency (fbEs), and virtual height (h’Es) were scaled from ionograms to characterize the slant (Es_s) traces. The results show that Es_s-type layers predominantly occur at night, forming between 95 and 120 km altitudes, with monthly and local variations. Model simulations using meteor radar-derived winds revealed that strong and stable zonal wind shear are associated with increased Es_s-type layer activity. In addition, wavelet spectral analyses of ftEs and fbEs showed that tidal periodicities (diurnal, semidiurnal, terdiurnal, and quarterdiurnal) and their interactions with gravity waves seem to play fundamental roles in the formation of Es_s-type layers. A comparison of ΔF (ftEs-fbEs) during Es_s-type events confirmed the presence of strong plasma density gradients, supporting the hypothesis that the slanted traces in ionograms result mostly from oblique reflections in inhomogeneous Es layer structures. However, the appearance of slant Es traces may in some cases be related to an actual tilt of the layer. Other relevant aspects of the observations associated with the possible physical mechanisms behind the formation of Es_s-type layers at low latitudes are highlighted and discussed