Recent data collected near the magnetic equator depict one kind of ionospheric perturbation in the nightime hours variously as bubbles or plumes. Theoretical studies show that the underside of the ionosphere is subjected to Rayleigh-Taylor instabilities which, when they are triggered, will cause a region of depleted ionization to rise as bubbles. When such regions are traversed by a probing radio wave, the associated Faraday effect is expected toshow depletions of the electron content. This paper describes some experimental results obtained at Natal, Brazil (35.23¿W, 5.85¿S, dip--9.6¿), by monitoring radio signals transmitted by the geostationary satellites Marisat 1 and SMS 1. Using ionization depletions as indications of bubbles, statistical studies of occurrence, size, and magnitude of perturbations are carried out. The most probable depletions for the propagation path under study have values in the range 1--4¿1016 el/m2, but depletions as large as 1.2¿1017 el/m2 have also been observed. The average durations for each observed bubble may vary from less than 2 to over 30 min with an average of 8 min. The experimental data further show that the scintillation rate may increase suddenly when these bibbles either form along or drift across the propagation path.