Polar mesosphere summer echoes (PMSE) are strong enhancements of received
signal power at very high radar frequencies occurring at altitudes between
about 80 and 95 km at polar latitudes during summer. PMSE are caused by
inhomogeneities in the electron density of the radar Bragg scale within the
plasma of the cold summer mesopause region in the presence of negatively
charged ice particles. Thus the occurrence of PMSE contains information about
mesospheric temperature and water vapour content but also depends on the
ionisation due to solar wave radiation and precipitating high energetic
particles. Continuous and homogeneous observations of PMSE have been done on
the North-Norwegian island Andøya (69.3° N, 16.0° E) from
1999 until 2008 using the ALWIN VHF radar at 53.5 MHz. In 2009 the
Leibniz-Institute of Atmospheric Physics in Kühlungsborn, Germany (IAP)
started the installation of the Middle Atmosphere Alomar Radar System
(MAARSY) at the same location. The observation of mesospheric echoes could be
continued in spring 2010 starting with an initial stage of expansion of
MAARSY and is carried out with the completed installation of the radar since
May 2011. Since both the ALWIN radar and MAARSY are calibrated, the received
echo strength of PMSE from 14 yr of mesospheric observations could be
converted to absolute signal power. Occurrence frequencies based on different
common thresholds of PMSE echo strength were used for investigations of the
solar and geomagnetic control of the PMSE as well as of possible long-term
changes. The PMSE are positively correlated with the solar Lyman α
radiation and the geomagnetic activity. The occurrence frequencies of the
PMSE show slightly positive trends but with marginal significance levels
