Latitudinal gradient in the plasma bubble zonal velocities as observed by scanning 630‐nm airglow measurements

Equatorial ionospheric plasma bubble zonal velocities were measured using two 630‐nm scanning photometers. Simultaneous scanning measurements were made in two east‐west planes that were tilted 30° northward and southward of the vertical. The results of measurements carried out during 5 days covering the summer equinoctial period of 1988, presented here, show evidence of marked latitudinal gradient in the bubble zonal velocities, the velocities decreasing with increasing latitude over our low‐latitude observing site, Cachoeira Paulista (−28°dip). We suggest that this velocity gradient is consistent with expectations at solar minimum, when our measurements may be associated with the region above the maximum in the eastward plasma velocity height profile.