Occurrence of polar mesosphere summer echoes at very high latitudes

Observations of polar mesosphere summer echoes (PMSE) have been carried out during the summer periodes 1999-2001 and 2003-2004 at the very high latitude of 78 N using the SOUSY Svalbard Radar (53.5 MHz) at Longyear- byen. Although the measurements could not be done con- tinuously in these seasons, PMSE have been detected over more than 6600 h of 9300 h of observation time overall. Using this data base, particular PMSE occurrence charac- teristics have been determined. PMSE at Svalbard appear from the middle of May to the end of August with an al- most permanent total occurrence in June and July. Diur- nal variations are observable in the height-depend occur- rence rates and in PMSE thickness, they show a maximum around 09:00-10:00 UTC and a minimum around 21:00- 22:00 UTC. PMSE occur nearly exclusively between a height of 80 km and 92 km with a maximum near 85 km. How- ever, PMSE appear not simultaneously over the entire height range, the mean vertical PMSE extension is around 4-6 km in June and July. Furthermore, typically PMSE are separated into several layers, and only 30% of all PMSE are single lay- ers. The probability of multiple layers is greater in June and July than at the beginning and the end of the PMSE season and shows a marked 5-day-variation. The same variation is noticeable in the seasonal dependence of the PMSE occur- rence and the PMSE thickness. We finally discuss potential geophysical processes to explain our observational results.