Long-term changes in the ionospheric response to geomagnetic activity

Abstract

Long-term variations in the ionospheric and thermospheric responses to solar extreme ultraviolet (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 response to geomagnetic activity (GA). This study aims to investigate how GA affects the peak electron density of the ionosphere (NmF2) and to assess its stability across different solar cycles (SCs). To account for variability in solar activity, a third-degree polynomial fit model is applied using F30, a proxy for solar extreme ultraviolet radiation. This model is executed separately for each solar cycle, season, and local time (LT) hour. Hourly values of NmF2 from Juliusruh station (54.6° N, 13.4° E) are analyzed from 1964 to 2019. The study examines the seasonal variability in the ionosphere's response to GA using various geomagnetic indices, including Kp, Ap, Am, Kpm, and Hp60. The results indicate higher correlation values (below 0.6) during the local summer months in the morning hours with the Kpm index delayed by approximately 16 hours. Under these conditions, the analysis for Juliusruh reveals no significant changes in the ionosphere's response to GA throughout five solar cycles. Additionally, findings from Juliusruh are consistent with those from five other ionospheric stations (Boulder, Slough/Chilton, Moscow, Port Stanley and Hobart) located at similar geomagnetic latitudes in both hemispheres, which also do not demonstrate any variation over time. The analysis, which examined winter conditions, yielded the same result of no significant variation over time.