Hydrogen Lyman-Alpha Periodicity Behaviour During Various Solar Cycles

The hydrogen Lyman‐alpha (Ly-α) line at 121.567 nm is the strongest solar vacuum ultraviolet emission line and the main excitation source for atomic hydrogen resonant scattering in cool material in the solar system. During quiescent solar conditions, the wings of the Ly-α line are formed in mid‐chromosphere, whereas the core is formed higher up at the base of the transition region from where it is radiated into the upper chromosphere, playing an important role in the radiative energy transport of a cool star like the Sun. Energy losses through the Ly-α emission are the most important radiative losses in the lower transition region, where the approximate temperature ranges from 8000 K to 30 000 K. The spectral irradiance behaviour of Ly-α is therefore important to characterise dynamic processes occurring in the solar atmosphere, particularly solar flares.

The temporal behaviour of various periodicities of 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 (152-156 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) conditions, 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 24.