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Abstract

For more than 60 years, field strength measurements of the broadcasting station, Allouis (Central France), have been used 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 Europe. The advantages of this method are the low costs and the simplicity of operation. Results of the extended reanalyzed fifth release of standard-phase heights (SPH) are presented. The SPH-series are anti-correlated to the solar cycle as known because stronger photo-ionization is linked with a higher number of electrons, which decreases the height of a constant level of electrons, namely the reflection height of long radio waves. Furthermore, the statistical analysis of the SPH-series shows a significant overall trend in the order of hundred meters per decade induced by a shrinking stratosphere due to global warming. Observational evidence for a quasi-bidecadal oscillation in the summer mesopause region over Western Europe is shown. The strong intra-decadal variability is related to QBO and ENSO kind of oscillations. The derived thickness temperature of the mesosphere decreased statistically significantly over the period 1959-2019 after pre-whitening with summer means of solar sunspot numbers. The trend value is in the order of about -1 K/ decade if the stratopause trend is excluded. The amount of linear regression is weaker, about -0.8 K/ decade from 1964 to 1986 (2 SCs), but stronger, about -1.6 K/ decade during 1996-2019 (last 2 SCs) in agreement with a stronger increase of near-surface temperature.