Climatology of intermediate descending layers (or 150 km echoes) over the equatorial and low-latitude regions of Brazil during the deep solar minimum of 2009

Abstract

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.

Some Differences in the Dynamics of the Intermediate Descending Layers Observed During Periods of Maximum and Minimum Solar Flux

“…While the radar registered ILs occurring simultaneously at different heights in the time intervals of ~13:00–14:00 LT, and ~16:30–17:00 LT, the Digisonde showed only one single layer in the time intervals of 15:35 LT to 16:35 LT and 17:20 LT to 18:10 LT. We can observe that the time intervals in which the ILs were observed in the radar were not coincident with the register from the Digisonde. As mentioned by Dos Santos et al (2019), simultaneous ILs were a common feature observed over SL, but during this specific day, they were not observed by the Digisonde as by the radar. Another point is that both the ILs from the Digisonde were formed from the detachment of the F1 layer.…”

“…Journal of Geophysical Research: Space Physics observe that the time intervals in which the ILs were observed in the radar were not coincident with the register from the Digisonde. As mentioned by Dos Santos et al (2019), simultaneous ILs were a common feature observed over SL, but during this specific day, they were not observed by the Digisonde as by the radar. Another point is that both the ILs from the Digisonde were formed from the detachment of the F1 layer.…”

“…As mentioned by Dos Santos et al (2019), as the intermediate layers descend in height they may evolve into a normal Es layer, in the form of the c-type Es layer, for example. We do not count specifically the number of cases in which the ILs transformed into a normal Es layer attaining altitudes lower than~130 km, but the analysis of the ILs total duration revealed that over the equatorial region the duration of ILs in 2003 was considerably shorter than in 2009.…”

“…For more details on the Digisonde, see Reinisch (1986). The criteria used for processing the h'IL and ftIL data from the ionograms using the SAO-explorer software were the same as those described by Dos Santos et al (2019).…”

“…The day‐to‐day variability observed in the ILs parameters such as the height, frequency of occurrence, and descent velocity may result from variations of tides, the electric fields, the metallic ion, or due to the influences of gravity waves (Nygrén et al, 1990; Wilkinson et al, 1992). In addition, Dos Santos et al (2019) also mentioned that the IL formation from a detachment of the F1 layer base was one of the most commonly observed characteristics.…”