A quarter of a century of polar mesospheric summer echo observations over Andøya: climatology and trends

Abstract. Polar mesosphere summer echoes (PMSE) are strong radar signals received at very high radar frequencies at altitudes between about 80 and 95 km at polar latitudes during summer. PMSE are caused by inhomogeneities in electron density on the radar Bragg scale of 3 m (for a monostatic 53.5 MHz radar), which originate from density fluctuations in negatively charged ice particles. Continuous monitoring of PMSE takes place on the northern Norwegian island of Andøya (69.3° N, 16.0° E) with the ALWIN (1999–2008) and MAARSY radars (since 2011), respectively. Both radars are calibrated which allows the conversion of the received PMSE echo strength to absolute signal power and the determination of the volume radar reflectivity η, which is now available for more than a quarter of a century. The distribution of the mean volume reflectivity covers a range starting at the detection limit of the radars and extending to a maximum value of ∼10-9 m−1 with peak values at around 7.9×10-15 and 1.6×10-15 m−1 for ALWIN and MAARSY, respectively. Seasonal, diurnal, and height variations of PMSE occurrence rates based on the 26-year data set of η are presented. Since PMSE reflectivity is affected by ionization from solar radiation and high-energy particle precipitation, seasonal mean values of PMSE reflectivity have been analyzed with respect to solar and geomagnetic activity. Regarding long term trends, PMSE measurements from the ALOMAR SOUSY radar covering the period 1994–1997 are included which leads to a time series of 32 years, making it the longest measurement series of its kind worldwide.