Nocturnal and Seasonal Variation of Na and K Layers Simultaneously Observed in the MLT Region at 23°S

Abstract

The present work shows for the first time the study of morphology of the mesopause sodium (Na) and potassium (K) layers simultaneously observed by a dual-beam LIDAR in the Southern Hemisphere. We analyze these two alkali metal layers from November 2016 to February 2019 measured at São José dos Campos (23.1°S, 45.9°W) and present their nocturnal and seasonal behavior. The night of 25 April 2017 was investigated as a representative of the vertical descending structure frequently observed in the metal density data at this location. We discuss here the chemical and dynamical contribution in the formation of these distinct layers, using simultaneous meteor radar wind and ionosonde data. These downward structures are seen in 66% of Na/K LIDAR data and seem to occur preferentially around April. The good agreement with diurnal tide and the presence of Es layer, suggest a combining mechanism in the formation of these structures. Moreover, semiannual variations are observed in both layers; however, they present different maxima location. The Na density presents its maxima around May and September; whereas, K density shows a strong maximum around July and a weaker one around December. Likewise, semiannual variation with maxima at the equinoxes is observed in the centroid height for both layers. However, the column abundances of these two metals show distinct seasonal variation: Annual variation peaking from one equinox to the other is observed in Na and semiannual with maxima in the solstices in K. The same behavior of centroid height for both layers indicates the same mechanism acting in the seasonal variation, which is not yet completely understood.