Long-term studies of the summer wind in the mesosphere and lower thermosphere at middle and high latitudes over Europe

Continuous wind measurements using partial-reflection radars and specular meteor radars have been carried out for nearly 2 decades (2004-2022) at middle and high latitudes over Germany ( similar to 54 circle N) and northern Norway ( similar to 69 circle N), respectively. They provide crucial data for understanding the long-term behavior of winds in the mesosphere and lower thermosphere. Our investigation focuses on the summer season, characterized by the low energy contribution from tides and relatively stable stratospheric conditions. This work presents the long-term behavior, variability, and trends of the maximum velocity of the summer eastward, westward, and southward winds. In addition, the geomagnetic influence on the summer zonal and meridional wind is explored at middle and high latitudes. The results show a mesospheric westward summer maximum located around 75 km with velocities of 35-54 m s - 1 , while the lower-thermospheric eastward wind maximum is observed at similar to 97 km with wind speeds of 25-40 m s - 1 . A weaker southward wind peak is found around 86 km, ranging from 9 to 16 m s - 1 . The findings indicate significant trends at middle latitudes in the westward summer maxima with increasing winds over the past decades, while the southward winds show a decreasing trend. On the other hand, only the eastward wind in July has a decreasing trend at high latitudes. Evidence of oscillations around 2-3, 4, and 6 years modulate the maximum velocity of the summer winds. In particular, a periodicity between 10.2 and 11.3 years found in the westward component is more significant at middle latitudes than at high latitudes, possibly due to solar radiation. Furthermore, stronger geomagnetic activity at high latitudes causes an increase in eastward wind velocity, whereas the opposite effect is observed in zonal jets at middle latitudes. The meridional component appears to be disturbed during high geomagnetic activity, with a notable decrease in the northward wind strength below approximately 80 km at both latitudes.