Variability of the lunar semidiurnal tidal amplitudes in the ionosphere over Brazil
Abstract. The variability in the amplitudes of the lunar semidiurnal tide was investigated using maps of Total Electron Content over Brazil from January 2011 to December 2014. Long period variability showed that the annual variation is always present in all investigated magnetic latitudes and it represents the main component of the temporal variability. Semiannual and intra-seasonal (~ 120 days) oscillations were the second and third components, respectively, but they presented significant temporal and spatial variation without a well-defined pattern. Among the short period oscillations in the amplitude of the lunar tide, the most pronounced ones were concentrated between 7–11 days. These oscillations were stronger around the equinoxes, in special between September and November in almost all latitudes. In some years, as in 2013 and 2014, for instance, they appeared with large power spectral density in the winter hemisphere. There was also observed evidence of antisymetry in the amplitudes maxima and minima of the 7–11 days oscillation with respect to the magnetic equator. These characteristics are compatible with normal mode westward propagating quasi 10 days planetary wave with horizontal wavenumber equal to 1. Besides, using data from a meteor radar located at low latitudes in Brazil for November 2013, when the amplitude of the 7–11 days oscillation was strong, it was possible to identify the presence of quasi 10 days oscillation in the both zonal and meridional component of the horizontal winds. These results suggest a possible coupling process by modulation of the lunar semidiurnal tidal amplitudes that allows the propagation of the 7–11 days waves into the thermosphere-ionosphere system.