We present for the first time detailed comparison of heights of MU radar field-aligned echoes and sporadic E (Es) layer by applying an FM-CW sounding technique for observing Es virtual height with high spatial (750 m) and time (one minute) resolutions. The production of typical, discrete quasi-periodic (QP) radar echoes appearing after sunset is closely related to the oscillation of Es height, supporting a theory of the QP echo generation. No Es height oscillations, however, were observed for the QP echoes appearing around midnight. Continuous radar echoes appearing after sunrise are located at 90--100 km heights that are lower than the Es heights (≥100 km), suggesting that a new mechanism other than the gradient-drift plasma instability is necessary for producing these morning continuous echoes. ¿ 1998 American Geophysical Union