ALBERT

Description: In this work, we report for the first time the climatology of intermediate descending layers over Brazilian equatorial and low latitudes regions during the extreme solar minimum period of 2009. The occurrence frequency of this layer is very high, being 〉 60 % over São Luís (2° S; 44° W, I: −5.7°) and 〉 90 % in Cachoeira Paulista (22.42° S; 45° W, I: −34.4°). Our results reveal that in most of the cases the intermediate layers (IL’s) appear during the day at altitudes varying from 130 to 180 km and present a descent movement that reaches the lower altitudes (~ 100 km) in a time interval of a few minutes to hours. Differently from other longitudinal sectors, the diurnal tide (24 h) can be considered as the main cause of IL’s for the low latitude region, followed by a smaller dominance of semidiurnal (12 h), terdiurnal (8 h) and quarter-diurnal (6 h) tide components. In the equatorial sector, similar behavior was found, with the exception of the semidiurnal tide, which in general does not appear to influence the IL’s dynamics (except in summer). The IL’s mean descent velocities over São Luís and Cachoeira Paulista show a day-to-day variability that may be associated with gravity waves propagation. Some peculiarities in the IL’s dynamics have been noted, such as the presence of the IL’s during the night hours, ascending IL’s, simultaneous IL’s, and descending IL’s been formed from some connection with the ionospheric F layer. Quite often, these characteristics are observed in the presence of strong signatures in the ionogram F-layer trace similar to those caused by the gravity wave propagation. We will show further that the descending intermediate layer over Brazil can be formed through a process of F1 layer base detachment. Besides that, we will present an interesting case study in which an ascending IL’s, initially detected at ~ 140 km, reached the base of F2 layer, probably due to the gravity wave propagation and/or due to the effect of the prompt penetration electric field.