Optical observations of plasma bubble westward drifts over Brazilian tropical region

Previous studies based on experimental observations of plasma bubbles occurrences over the Brazilian region revealed that they often propagate eastward in the postsunset sector. Optical observations of the OI630 nm airglow made possible to carry out the present study based on the observed signatures on the OI630 nm airglow. From September 2000 to April 2007 at São João do Cariri (7.4∘S7.4∘S; 36.5∘W36.5∘W) 16 events were found to propagate westward presenting a defined seasonality, six of them occurring during June–July–August, seven of them during September–October–November, the others three occurring during March–April–May, and none during December–January–February. Ten observed cases occurred under quiet or weak magnetic periods showing that the intense magnetic activity is not the only cause of the westward direction propagation of the plasma bubbles.