Seasonal and long-term variations of PMSE from VHF radar observations at Andenes, Norway

[1] The observation of polar mesosphere summer echoes (PMSE) with VHF radars is an important possibility for the investigation of the polar mesospheric region during summer. This region is characterized by the lowest temperatures in the Earth's atmosphere and the existence of mesospheric ice clouds, visually or by lidars observed as noctilucent clouds (NLC). Using measurements at 53.5 MHz in Andenes, Norway, with the ALOMAR SOUSY radar between 1994-1997 and with the ALWIN radar between 1999-2001 mean seasonal, solar cycle and long-term variations of PMSE have been derived. The seasonal variation of PMSE is characterized by a strong increase during end of May/beginning of June, a rather high level in June and July, and a more gradual decrease during August. The occurrence rate of PMSE is strongly positive correlated with the mesospheric ionization level mainly caused by solar cycle variations of the solar Lyman alpha radiation and also by the flux of precipitating high energetic particles. Long-term trends of PMSE are only very small and not significant due to the limited data series.