Polar Mesosphere Summer Echoes (PMSE) have been studied at Andenes (69¿N, 16¿E), Norway, using VHF radar observations from 1994--1997 and since 1999. One remarkable feature of all PMSE is the fact that the radar echoes often occur in the form of two or more distinct layers that can persist for periods of up to several hours. Until now, the layering mechanism leading to these multiple structures is not well understood. Here we are studying the occurrence and the mean altitude distribution of these multiple layer events. To discuss the processes that are potentially responsible for the creation of these multiple structures, we apply a microphysical model of the generation and growth of mesospheric ice particles under the influence of temperature and wind variations caused by a long period gravity wave. Finally, this model approach is tested with gravity wave characteristics derived from MF and VHF radar wind measurements during the PMSE observations.