Quasi-biennial and decadal variability obtained from long-term measurements of nighttime radio wave reflection heights over Central Europe Original Research Article

The nighttime reflection height of low-frequency (LF) radio waves at oblique incidence over Central Europe is measured using 1.8 kHz sideband phase comparisons between the sky wave and the ground wave of a commercial 177 kHz LF transmitter. The measurements have been carried out continuously since September 1982, now justifying the analysis of trends and regular variations of the reflection height. In the time series is found a) a long-term negative trend and b) a solar cycle dependence, both confirming earlier measurements and theoretical estimations. Moreover, a significant oscillation of quasi-biennial period is visible in LF reflection heights, indicating a reaction of the midlatitude mesosphere/lower thermosphere region on the equatorial quasi-biennial oscillation.