Temporal and Periodic Variations of the Solar Flare Index During the Last Four Solar Cycles and Their Association with Selected Geomagnetic-Activity Parameters

We studied the temporal and periodic variations of the monthly solar flare index (FI) and selected geomagnetic-activity parameters (Ap, Dst, Scalar B, and aa) measured during Solar Cycles 21 – 24 (from January 1, 1975 to December 31, 2020) and report the following findings: 1) all data sets except the FI peak values gradually decreased after 1992, while the FI peak values began their gradual decrease in 1982; 2) all data sets show double or multiple peaks during the maximum phase of solar cycles; 3) the FI shows meaningful correlations with the investigated geomagnetic-activity parameters; 4) the 11-year sunspot-cycle periodicity and periodicities lower than 3.9 months were observed in all data sets without exception; 5) the FI time series exhibits a unique period of 4.8 – 5.2 months that is not present in all the other indices, while geomagnetic aa, Ap, and Dst indices show a unique 6 – 6.1 months periodicity that does not appear in the scalar B and FI; 6) crosswavelet transform (XWT) spectra between FI and other parameters generally show phase mixing in the short (2 – 8 months) period range, while all parameters used in this study were found to be inphase and highly correlated with the 11-year solar-activity period. All these results show that the FI variations are one of the main drivers of the geomagnetic activity.