Backscatter spectra with extremely narrow spectral width of only 2 to 7 m/s and small mean Doppler velocity of 10 to 30 m/s associated with plasma irregularity patches in nighttime sporadic E layers have been obtained with the Chung-Li VHF radar. The radar interferometer shows these echoes to be highly aspect sensitive with striated structures in the horizontal and azimuthal planes. By considering the spatial distributions of the Es echoes and the geometry of the effective radar beam, we propose a schematic model in which multiple thin and tilted Es layers with steep electron density gradients are drifting across the illuminated region. The results show that the layers appear to be tilted toward east at angles of 3 degrees to 14 degrees, and their horizontal drift velocities (obtained by tracking the patches with the interferometer phase) imply primarily eastward motions at speeds of 7.5 to 56.2 m/s.
