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.

Plain language summary

The ionosphere, a region of the upper atmosphere that contains charged particles and affects radio communications, is strongly influenced by solar extreme-ultraviolet (EUV) radiation that varies with solar activity. Thus, ionospheric parameters usually show a linear relationship with solar activity level. However, during the last two unusually quiet solar activity minima, around 2008 and 2019, nighttime measurements at Juliusruh, a mid-latitude station, revealed unexpected behavior. The maximum electron density was higher than during the previous, less quiet solar minimum around 1996. This increase occurs at night during a process known as “nighttime enhancement,” which refers to a rise in ionospheric electron density after sunset and is more pronounced during winter. Because this effect appears toward the end of long-term observation records, it can bias estimates of long-term trends in foF2, a key ionospheric parameter that measures the ionospheric maximum electron density. One possible explanation is an increased flow of charged particles from the plasmasphere, a region above the ionosphere that is populated by the up flow of ionospheric plasma during the day and that acts as a nighttime source of ionization. This process can produce higher electron densities than expected from solar EUV radiation alone. Current ionospheric models, which are partly based on observations and empirical relationships, do not reproduce this behavior. Our results indicate that both long-term trend estimates and models should be revised to account for nighttime enhancement effects during periods of low solar activity.