A STATISTICAL STUDY OF POLAR MESOSPHERE SUMMER ECHOES OBSERVED BY EISCAT

A comprehensive survey of data on 'Polar Mesosphere Summer Echoes', ob served by the EISCAT VHF radar during 1988-1993, confirms that (1) the se echoes are a summer phenomenon, with a season lasting from June to August; (2) PMSE occur most frequently around noon and midnight, and t hus follow a semidiurnal pattern; (3) PMSE occur at a mean height of 8 5 +/- 2 km; (4) there is often a good correlation between the vertical Doppler velocity and the rate of change of echo height, which suggest s that the echoing structures move bodily, perhaps in response to grav ity waves. Previous results on the lack of correlation between the occ urrence of PMSE and noctilucent clouds are reinforced.