Impact of nighttime foF2 enhancement at Juliusruh on ionospheric trend estimation and model accuracy

Abstract

The last two solar minima, around 2008 and 2019, were exceptionally low, with ionospheric electron densities even lower than expected. An irregular variation among the last three minima is observed in data from Juliusruh ionospheric station. Annual mean data indicates that the F2-layer critical frequency (foF2) during nighttime was slightly higher during the deep minima of 2008/2019 compared to the 1996 minimum, despite the lower solar activity levels. This implies a deviation from the expected direct association between foF2 and solar EUV proxies. This effect is more noticeable in winter during the post-midnight enhancement; however, the irregular behavior is sufficient to affect the annual mean, possibly affecting long-term trend estimations using the linear regression method. The enhanced plasmaspheric flux expected to be more noticeable, relative to the background ionization, during lower solar activity may lead to stronger nighttime ionization than that expected from the direct effect of solar activity. These findings suggest that long-term ionospheric trend estimations and models require revision to incorporate nighttime enhancement effects and dependence on solar activity levels during minima, improving accuracy in regions influenced by such phenomena.