Abstract. In this work, we have performed a study for the first time on the
climatology of the intermediate descending layers (ILs) over Brazilian
equatorial and low-latitude regions during the extreme solar minimum period
of 2009. The result of this study shows that the occurrence frequency of the
ILs is very high, being > 60 % over São Luís
(2∘ S, 44∘ W; inclination: −3.8∘) and > 90 % in Cachoeira Paulista (22.42∘ S, 45∘ W; inclination:
−33.5∘). In most cases the ILs occur during the day at altitudes
varying from 130 to 180 km and they may descend to lower altitudes
(∼100 km) in a time interval of a few minutes to hours. The
main driving force for the ILs at the low-latitude region, may be considered
to be the diurnal tide (24 h) followed in smaller dominance by the
semidiurnal (12 h), terdiurnal (8 h) and quarter-diurnal (6 h) components. In
the magnetic equatorial sector, similar behavior was seen, with the exception of
the semidiurnal tide, which in general does not appear to have influenced the
IL's dynamics (except in summer). Additionally, the IL mean descent
velocity over São Luís and Cachoeira Paulista shows a day-to-day
variability that may be associated with a wave-like perturbation with a
periodicity of some days. Some peculiarities in the IL dynamics were noted,
such as the presence of the ILs during the night hours. Ascending and
descending ILs appeared to have been formed from some connection with the
ionospheric F layer. Quite often, these characteristics are observed in the
presence of strong signatures of the gravity wave propagation as suggested
by the F layer traces in the ionogram. The descending intermediate layer
over Brazil appears to have been formed through a process of F1 layer base
detachment. An interesting case study showed that an ascending ILs,
initially detected at ∼130 km, reached the base of the F2
layer, due probably to the gravity wave propagation and/or the effect of a
prompt penetration electric field.