Effects of the terdiurnal tide on the sporadic E (E-s) layer development at low latitudes over the Brazilian sector

Sporadic E (E-s) layers are patches of high ionization observed at around 100-140 km height in the E region. Their formation at low latitudes is primarily associated with the diurnal and semidiurnal components of the tidal winds via the ion convergence driven by the wind shear mechanism. However, recent studies have shown the influence of other tidal modes, such as the terdiurnal tide. Therefore, this work investigates the effect of terdiurnal tide-like oscillations on the occurrence and formation of the E-s layers observed over Palmas (10.17 degrees S, 48.33 degrees W; dip lat. -7.31 degrees), a low-latitude station in Brazil. The analysis was conducted from December 2008 to November 2009 by using data collected from CADI (Canadian Advanced Digital Ionosonde). Additionally, the E Region Ionospheric Model (MIRE) was used to simulate the terdiurnal tidal component in the E-s layer development. The results show modulations of 8 h periods on the occurrence rates of the E-s layers during all seasonal periods. In general, we see three well-defined peaks in a superimposed summation of the E-s layer types per hour in summer and autumn. We also observed that the amplitude modulation of the terdiurnal tide on the E-s occurrence rates minimizes in December in comparison to the other months of the summer season. Other relevant aspects of the observations, with complementary statistical and periodogram analysis, are highlighted and discussed.