Examining Long‐Term Changes in the Ionospheric Response to Geomagnetic Activity Over the Past Five Solar Cycles

Abstract

Model simulations indicate that long-term changes in the thermosphere may influence the intensity of the ionospheric response to geomagnetic activity (GA). This study investigates the response of the peak electron density of the ionosphere (NmF2) to GA and its consistency across different solar cycles (SCs) to identify any observable long-term variation. Hourly values of NmF2 from six ionospheric stations located at midlatitudes in both hemispheres are examined from 1964 to 2019. Residuals of NmF2 relative to a background model are analyzed for correlations with various GA indices to assess the ionospheric response to GA. Kp modified (Kpm) exhibits the highest absolute correlation with the NmF2 residuals, indicating it as the most suitable index. 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, there is a marked negative response to GA with sufficiently high correlations for statistical analysis. However, no significant long-term changes in the relationship between NmF2 and GA are identified within the Kpm range of 1–5. This is mainly attributed to the combination of small changes in CO2 concentration (compared to simulation results from models) and increasing solar wind forcing during solar cycle 23 and 24 than previous solar cycles. During winter, very low correlation between NmF2 and GA prehibits the analysis of any long-term changes in their relationship.