Has the relationship between ionospheric behavior and geomagnetic activity changed over the past five solar cycles?

Abstract

Long-term variations in the ionospheric and thermospheric responses to solar EUV radiation have been documented in numerous studies. Model simulations indicate that these long-term changes in the thermosphere may influence the intensity of the ionospheric relationship with geomagnetic activity (GA). This study investigates the relationship between GA and the peak electron density of the ionosphere (NmF2) and examines its consistency across different solar cycles to identify any observable long-term changes. Hourly values of NmF2 from six midlatitude ionospheric stations are analyzed from 1964 to 2019. The residuals from a dedicated background model are analyzed for correlation with several GA indices to detect the ionospheric relationship with GA. Kpm was identified as the most appropriate index, yielding the highest absolute correlation with the residuals of NmF2. The observed patterns align with the well-established storm-time characteristics at midlatitudes, showing a predominant negative response during summer morning hours and a positive response during winter afternoons. During summer conditions, a predominant negative association with GA, which exhibits sufficiently high correlations for statistical analysis. However, no significant long-term changes in the relationship between NmF2 and GA were identified within the Kpm range of 0–5. During winter conditions, the results show the well-known positive relationship with GA during daytime, but the variability is so strong that no significant correlation to GA indices can be found, and detection of long-term changes is not possible. Consequently, the long-term changes in the relationship between the ionosphere and GA reported in model-based studies cannot yet be confirmed by observations.