Hydrogen Lyman-Alpha Periodicity Behaviour During Various Solar Cycles

Astrophysics and Space Science (2023) 368:6 https://doi.org/10.1007/s10509-023-04163-9 RESEARCH Hydrogen Lyman-Alpha Periodicity Behaviour During Various Solar Cycles Pieter Kotzé1,2 Received: 6 June 2022 / Accepted: 13 January 2023 © The Author(s) 2023 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 24. Keywords Solar cycle · Lyman-Alpha · 27-day periodicity · Rieger periodicity · Asymmetric solar magnetic field 1 Introduction 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 im-  P. Kotzé 1 Centre for Space Research, North-West University, Potchefstroom, South Africa 2 Department of Physics, Stellenbosch University, Stellenbosch, South Africa portant to characterise dynamic processes occurring in the solar atmosphere, particularly solar flares (Fontenla et al. 1988; Milligan and Chamberlin 2016; Milligan et al. 2020). It is important to note that Ly-α radiation can vary quite substantially during a solar cycle. A study by Woods et al. (2000) reported that the mean variability of Ly-α due to the 27-day solar rotation across Solar Cycles 18–22 was 5% at solar minimum which increased to 11% at solar maximum. Studies focussing on the identification and behaviour of periodicities in solar indices and observational data (Pap et al. 1990; Zou and Li 2014) has been of high interest for several years to understand solar variability and space weather. A long-term analysis of solar activity led e.g. to the identification of the well-known 11-year sunspot and 22-year magnetic cycles (Hale 1924). Solar activity is to a large extent modulated by the ≈ 27-day Carrington rotation together with its different harmonics (e.g. 13.5-day 2nd harmonic). During a solar cycle lasting ≈ 11 years, solar behaviour is predominantly driven by changes in the solar magnetic dynamo (Solanki et al. 2006). The Rieger periodicity (Rieger et al. 1984), first detected in solar flares at ≈ 155 days, has since been observed in several solar activ- 6 Page 2 of 6 P. Kotzé Fig. 1 A combined plot of Ly-α and smoothed monthly mean sunspot values (SSN) between 1950 and 2020. The respective solar cycle numbers are also shown ity indices, e.g., Mg II (Kotzé 2020). A wavelet investigation of sunspot data by Krivova and Solanki (2002) revealed that the power at the 1.3 year and the Rieger periodicity at 156 days fluctuates approximately in phase, correlating with the strength of a solar cycle as determined by the number of sunspots. This finding led to the conclusion that the 1.3year solar dynamo period (Richardson et al. 1994; Mursula and Zieger 2000), as well as the Rieger period have a strong underlying common magnetic origin, most probably at the bottom of the solar convection zone. Several studies have shown that periodicities can vary in strength and amplitude during various phases of a solar cycle and that the ratio of powers can also indicate the dominance of a certain period during e.g., solar maximum or solar minimum. Solar cycles 23 and 24 were characterised by extremely low solar activity levels. During 2008 the number of sunspot-less days were 268 (73%), while during 2019 it was 281 (77%) days (https://www.spaceweather.com/). In addition, an anomalously weak and asymmetric solar dynamo (Love et al. 2012) resulted in unusual behaviour of several periodicities and harmonics of the synodic solar rotation period as observed across a wide range of geomagnetic and solar parameters (Chowdhury et al. 2015). Mursula and Zieger (1996) made a detailed analysis of the 13.5-day periodicity of the solar chromosphere, the near-Earth solar wind, interplanetary magnetic field and geomagnetic activity during solar cycles 20, 21 and 22 and concluded that the 13.5-day periodicity is a real quasi-periodicity whose amplitude varies substantially with time, sometimes reaching values larger than the amplitude of the 27-day synodic rotation periodicity. In this article we report how the Rieger, 27-day as well as the 13.5-day periodicities in solar Ly-α vary with time during various solar cycles since 1950. In the past the Rieger period has been identified in the range 150–180 days, while in this study it is detected between 152–155 days. In particular, the focus will be placed on the behaviour of these periodicities during solar cycles 23 and 24 when the solar dynamo showed anomalous (fewer sunspots during 2008 than in any year since 1913) and asymmetric characteristics. These results for the solar hydrogen Ly-α spectral line have not been reported before in the literature. 2 Data analysis The Ly-α line is produced in the solar transition region and radiated into the upper chromosphere (Vernazza et al. 1981). It has been observed that this spectral line not only varies quite substantially over a 27-day period, but its variability over a solar cycle is about a factor of 2 (Woods et al. 2000). Since the first composite compilation of a Ly-α data set (Woods and Rottman 1997; Woods et al. 2000), several upgrades with improvements have been (...truncated)