VHF radar measurements with high temporal and vertical resolution of 4 s and 75 m, respectively, have been carried out in northern Norway (69¿N) in the summer of 1987 to study the structure of polar mesospheric summer echoes as well as dynamical processes involved. The results, which have been analysed using bispectral methods, reveal nonlinear interactions to exist in the gravity wave period range. Iso-lines of conserved quantities deduced from the vertical velocity field show strong resemblance with simultaneously observed echo power variations. The results indicate a correlation between signal-to-noise ratio and the vertical gradient of the temperature. ¿ 1997 American Geophysical Union