Plasma bubble zonal drift characteristics observed by airglow images over Brazilian tropical region

using airglow images observed at s？o jo？o do cariri (7.4os, 36.5ow; dip angle: -11os) from september 2000 to april 2007, plasma bubblezonal drifts for ninety-seven geomagnetically quiet nights (dst index higher than -30 nt) were calculated. the plasma bubble eastward averaged drifts presented an increase from 18:00 to 22:00 local time (lt), and followed by a deceleration. the plasma bubbles observed during high solar activity were faster after 22:00 lt. eastward drifts observed during the summer, after 21:00 lt, were higher than drifts observed during the equinox months. few nights had plasma bubble eastward drifts higher than 100 m/s and almost all bubbles disappeared after 02:00 lt. averaged zonal drifts observed between -9.5 and -5.5os were almost constant, primarily,around 23:00 lt.