Possible solar cycle influence on midlatitude mesosphere/lower thermosphere horizontal winds over Germany

At Collm (51.3° N, 13.0° E) mesosphere/lower thermosphere (MLT) zonal and meridional winds have been measured for more than four decades using low-frequency spaced receiver measurements on 177, 225, and 270 kHz, and very high frequency meteor radar observations on 36.2 MHz. We analyse the combined time series of monthly mean horizontal winds at 90 km height from 1979 to 2024 with respect to long-term changes, and in particular to the effect of the 11-year solar cycle (SC). In the 1980s and 1990s there is a negative correlation with solar extreme ultraviolet proxies in spring and early summer, with a weaker jet in the lower thermosphere during solar maximum. This correspondence weakens after the year 2000. In winter, there is a weak positive effect towards stronger westerly winds during solar maximum. Together, the results suggest that solar forcing remains an important, season-dependent driver of mid-latitude MLT dynamics, but its expression is modulated by the recently weakened SCs 24–25, or modified by changing trends of forcing from below. These results are largely in agreement with partial reflection radar observations of mesospheric winds over Juliusruh (54.6° N, 13.4° E) at 3.18 MHz. We also compare them with decadal variations of gravity wave proxies, but these results are partly inconclusive.