| dc.description.abstract | The temporal behaviour of various periodicities of hydrogen Lyman-alpha (Ly-α) between 1950 and 2020 have been analysed using Lomb-Scargle and Morlet wavelets spectral analysis techniques. Daily mean values of Ly-α 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 (≈ 27 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 ≈ 27-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-α can probably be attributed to the anomalous asymmetrical structure of the solar magnetic field during Cycles 23 and
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