Small scale plasma and charged aerosol variations and their importance for polar mesosphere summer echoes

Since 1990 a number of campaigns have been conducted in order to study the small scale structure of charged as well as neutral species within Polar Mesosphere Summer Echo (PMSE) layers. Several VHF radars have been used to study this phenomenon from the ground (EISCAT, CUPRI, ALOMAR SOUSY) and sounding rockets have been launched through different types of PMSE layers. The radar observations appear to occur in two classes: (I) One characterized by broad spectral widths and almost no aspect sensitivity, associated with neutral air turbulence; and (II) another by narrow spectral widths and strong aspect sensitivity, which is observed in the absence of neutral air turbulence. Both radar and in situ measurements show a clear dominance of the non-turbulent type II echoes. We will discuss how localised layers of charged aerosols, ions and electrons observed in situ might lead to single and multiple Fresnel type reflections at VHF wavelengths. We will also discuss the possibility that a two-stream type plasma instability may explain the type II echoes. The important parameter in this respect is the electrical field and a horizontal electric field of about 40 mV/m is needed to excite small scale plasma waves. Such a field may have its origin in a differential drift between the heavy aerosols and the much lighter plasma, caused by a varying background wind, windshears and/or waves of different types, that give rise to a polarisation electric field. (C) 1999 COSPAR. Published by Elsevier Science Ltd.