CHARACTERISTICS OF POLAR MESOSPHERE SUMMER ECHOES (PMSE) OBSERVED WITH THE EISCAT-224 MHZ RADAR AND POSSIBLE EXPLANATIONS OF THEIR ORIGIN

We describe experiments carried out with the EISCAT VHF radar during the MAC/SINE campaign. These experiments included observations of the polar mesosphere summer echoes (PMSE), which were studied with a high spectral resolution program. The fine structure of the spectra imply that very thin and non-random transient structures of reflectivity occur frequently in the mesopause region. We find no clear relation between the echo power and the coherence time which could support the hypothesis of scatter from turbulence or partial reflection. In addition, the estimates of radar reflectivity let us discard incoherent scatter and pure turbulence scatter as the cause of the PMSE. We also discuss the relation of the PMSE and cluster ions, electric fields, charge accumulation and atmospheric gravity waves.