Special runs of the Arecibo incoherent scatter radar were made to provide high spatial resolution (8.7 km) and high temporal resolution (about 1 min) electron density data at discrete heights in the altitude range 200--500 km. The observed electron density fluctuations were spectrally analyzed and interpreted in terms of gravity wave induced traveling ionospheric disturbances. In many records, two kinds of persistent spectral dips for all heights were obtained. The first dip, occurring in the frequency range 0.025--0.055 min-1 (18- to 40-min period), is shown to be caused by the absence of ionospheric response when the gravity wave normal is perpendicular to the earth's magnetic field and therefore is of geomagnetic origin. The second dip, occurring in the frequency range 0.09--0.12 min-1 (8- to 11-min period), is shown to be related to high attenuation of those waves whose frequencies are slightly above the Brunt-V¿is¿l¿ frequency.