Periodicity Variation in Solar UV Radiation (115 – 300 nm) During 2004 – 2019

Abstract Solar ultraviolet (UV) irradiance data between 115 and 300 nm as obtained by the SORCE (Solar Radiation and Climate Experiment) satellite during the years 2004 till end 2019 have been analysed by spectral analysis techniques to study the variability of particularly the Rieger (∼ 155 days) as well as the ∼ 27-day synodic rotation period. Results obtained using Lomb-Scargle periodogram as well as Morlet wavelet spectral analysis revealed both a strong wavelength and time-dependant behaviour for both periodicities. A spectral analysis investigation of the solar UV irradiances between 2004 and 2019 provides the first evidence that the variation of the power of the Rieger periodicity fluctuates substantially with time as well as wavelength between 115 – 300 nm in comparison to the 27-day period, particularly during the abnormally low minima of Solar Cycles 23 and 24. A possible explanation for this behaviour is the anomalous behaviour of the solar dynamo, accompanied by the development of Rossby waves during this time interval.