Periodicities in Solar Microwave Emission During Solar Cycles 20-24

Abstract We present a detailed time-series analysis of solar radio flux at five discrete frequencies 1.0, 2.0, 3.75, 9.4, and 17 GHz and daily sunspot numbers (SSNs) spanning Solar Cycles 20 to 24, using the Fast Fourier Transform, Lomb–Scargle periodogram, and Wavelet techniques. These complementary methods identify persistent and transient periodicities by integrating global spectral estimation, reliable detection in unevenly sampled data, and time–frequency localization, enabling a comprehensive characterization of solar activity across different atmospheric layers. Our results reveal that radio fluxes at 2.0 GHz and 2.8 GHz exhibit the strongest correlation with SSN, indicating their close association with active-region dynamics and magnetic field concentrations. In contrast, emissions at 17 GHz show minimal correlation with SSN, suggesting an origin in distinct, possibly non-thermal, coronal processes. Prominent rotational periodicities in the range of 26–31 days are consistently detected in the SSN and lower-frequency fluxes, but are largely absent in the 17 GHz band. Cycle-wise analysis highlights Solar Cycle 21 as the most active, with the highest number of statistically significant periodic components. These findings underscore the utility of multi-frequency radio observations combined with advanced spectral techniques in revealing the complex, cycle-dependent nature of solar magnetic activity.