Hydrogen Lyman-Alpha Periodicity Behaviour During Various Solar Cycles

Abstract

The temporal behaviour of various periodicities of hydrogen Lyman-alpha (Ly-alpha ) between 1950 and 2020 have been analysed using Lomb-Scargle and Morlet wavelets spectral analysis techniques. Daily mean values of Ly-alpha radiance (https://lasp.colorado.edu/lisird/data/composite_lyman_alpha/) for each individual year were used in this investigation to obtain the temporal behaviour of particularly the Rieger periodicity (150–180 days), the synodic solar rotation periodicity (approx27 days) as well as the elusive 13.5-day periodicity. Results obtained showed that the Rieger periodicity dominates at solar maximum during strong solar cycle conditions (Cycles 19, 21, 22, and 23), while the approx27-day periodicity is in most cases dominant during solar minima. On the other hand, the 13.5-day periodicity only appears above the 95% statistical confidence level during solar maxima of Cycles 19, 21, 22 and 24. Contrary to all previous Cycles since 1950, the 13.5-day periodicity appears exceptionally strong and above the 95% confidence level during the downward and minimum phases of Cycles 23 (2006) and 24 (2016) when its power exceeds that of the 27-day periodicity. This peculiar behaviour of the 13.5-day periodicity in Ly-alpha can probably be attributed to the anomalous asymmetrical structure of the solar magnetic field during Cycles 23 and 24.