OBSERVATIONS OF POLAR MESOSPHERE SUMMER ECHOES AT EISCAT DURING SUMMER 1991

Polar mesosphere summer echoes (PMSE) have been observed with the Euro pean incoherent scatter (EISCAT) VHF radar operating on 224 MHz during summer 1991. The mean height of PMSE was about 85 km during daytime a nd about 87 km during nighttime, with a markedly enhanced frequency of occurrence during daytime. The occurrence frequency of PMSE is higher during geomagnetically quiet periods than during disturbed periods. T here are indications of a PMSE modulation by vertical velocities, whic h may be caused by atmospheric gravity waves. Comparisons of PMSE stru ctures detected with the EISCAT VHF radar on 224 MHz and the Cornell U niversity portable radio interferometer, which was operated at the sam e time on 46.9 MHz, demonstrate some common features when operated col located but marked differences when operated at a distance of some 126 km.