More than 60 years of measurements of Standard-Phase-Heights over Western Europe – Trends and long-term oscillations of the mesosphere

For more than 60 years, field strength measurements of the broadcasting station, Allouis (Central France), have been received at Kühlungsborn (54° N, 12° E), Mecklenburg, Northern Germany. Beginning with the year 1959 these so-called indirect phase-height measurements of low frequency radio waves (with a frequency of about 162 kHz) are used to examine trends and the long-term oscillations over Western Europe. The advantages of this method are the low costs and the simplicity of operation. Results of the updated fifth release (R5, 1959-2019) of standard-phase heights (SPH) are presented.

The statistical analysis of the SPH series shows a significant overall trend with a decrease of 116 m per decade indicating a subsidence of the long-radio wave reflection height of about 700 m during R5. As expected the daily time series of SPH shows in its spectrum dominant modes which are typical for the solar cycle, ENSO and for QBO bands, indicating solar and lower atmospheric influences. Solar cycle and ENSO (-QBO)-like band-pass show a growing increase of SPH up to 1987, followed by a decrease afterwards.

For summer months during solar minimum years, without solar influences and without stratopause altitude trend, a thickness temperature trend of the mesosphere is significant with a trend value of -0.47 ± 0.43 K/ decade. The overall cooling of the intrinsic mesospheric temperature during 60 years of observation is in the order of 3 K.