Summer Noontime h m F 2 Long‐Term Trends Inferred From f o F 1 and f o F 2 Ionosonde Observations in Europe

Abstract A new method to retrieve h m F 2 and vertical plasma drift W from f o F 1 and f o F 2 ionosonde observations has been proposed and applied at five European stations for June noontime conditions over the (1958–2017) period. Linear long‐term trends were estimated using the retrieved h m F 2 and W variations. It has been shown that negative and statistically significant h m F 2 trends manifest a pronounced latitudinal dependence: the largest (~2.7 km per decade) is at lower‐latitude station Rome, while the smallest (< 1 km per decade) is at high‐latitude station Sodankylä. It was shown that h m F 2 trends are due to vertical plasma drifts that manifest similar latitudinal dependence: the largest (~1 m/s per decade) is at Rome and the smallest (~0.3 m/s per decade) is at Sodankylä. Vertical plasma drift converted to the meridional northward thermospheric wind manifests an average increase of (11.6 ± 3.0) m/s over the period of ~60 years. This may be related to a decrease in ion drag due to negative trends in the ionospheric F 1 and F 2 regions. An additional factor is a decrease in auroral heating related to long‐term decrease in solar and geomagnetic activity observed for June months over the analyzed period.