Long term reduction of F2 layer peak electron density at mid-latitudes

Abstract

The talk focuses on studying long-term trends of the F2 layer peak electron density (NmF2) derived from vertical sounding measurements at 10 ionosonde stations situated in mid-latitudes at European, Asian and American longitude sectors. The ionosonde data cover a time period of 71 years thus fulfilling an important condition for long-term trend (LTT) studies. To derive long term changes of the behavior of NmF2 measurements around noontime, its relationship with the solar 30 cm radio flux index F30 has been investigated. Confirming earlier results, the long-term behavior of 11 years sliding averages of NmF2 data shows a substantial decrease from reference values. The reference level, specific for each ionosonde station, is defined by a linear model of noontime NmF2 averages as a function of the F30 solar index. Whereas the 11 years running average values of NmF2 are proportional to corresponding F30 values within a small variability range of less than ±1% until 1985, NmF2 decouples substantially from this linear relationship with F30 at ionosonde stations afterwards. The reduction of NmF2 may reach up to 1.6·1011m-3 or about 20% referred to the linear model in 2022. The reduction is different at Northern and Southern hemisphere. The derived noticeable long-term trends of ionospheric peak electron density are assumed to be correlated with climate changes at the Earth surface due to greenhouse gases.