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 hmF2 and vertical plasma drift W from foF1 and foF2 ionosonde observations has been proposed and applied of five European stations for June noontime conditions over the (1958-2017) period. Linear long-term trends were estimated using the retrieved hmF2 and W variations. It has been shown that negative and statistically significant hmF2 trends manifest a pronounced latitudinal dependence: the largest (~2.7 km per decade) is at lower-latitude station Rome, while the smallest (< 1km per decade) is at high-latitude station Sodankyla. It was shown that hmF2 trends are due to vertical plasma drifts that manifest similar latitudinal dependence: the largest (~ m/s per decade) is at Rome and the smallest (~0.3 m/s per decade) is at Sodankyla. 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 F1 and F2 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.