Characteristics of hydrogen Lyman lines during solar cycles 23–24 and solar flares

Abstract Solar Extreme Ultraviolet (EUV; 10–124 nm) emissions impact the Earth's ionosphere. The Lyman-alpha line (Lyα; 121.6 nm) ionizes NO molecules in the D-region (60–100 km), while the Lyman-beta line (Lyβ; 102.6 nm) ionizes O₂ in the E-region (100–150 km). Despite Lyβ's weaker irradiance, the higher O₂ density in the E-region suggests Lyβ could significantly influence the ionosphere. Limited Lyβ observations require estimation and substitution from Lyα. Lemaire et al. (2012) analyzed the relationship between Lyα and Lyβ during solar cycle 23 through fluctuations in the Lyα/β ratio and found that the Lyα/β ratio was small during solar maximum and large during solar minimum. Based on these results, we used data from the Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED)/Solar Extreme Ultraviolet Experiment (SEE), the Solar Dynamics Observatory (SDO)/Extreme Ultraviolet Variability Experiment (EVE), and the Geostationary Operational Environmental Satellite (GOES)/Extreme Ultraviolet Sensor (EUVS)-E to derive the Lyα/β ratio during solar cycle 24 and for M-class or larger solar flares occurring between 2010 and 2014. We derived an updated third-order polynomial fit incorporating the revised instrumental calibrations reported by Lemaire et al. (2012). The long-term variation in the Lyα/β ratio during solar cycle 24 did not differ significantly from that observed during solar cycle 23. However, during flares, the Lyα/β ratio fluctuated much more rapidly than during the solar cycle. This was due to Lyβ increase exceeding that of Lyα, reflecting complex nonthermal behaviors in the flaring solar atmosphere. Particularly, the strong non-LTE radiative transfer effects caused by the difference in optical thickness between the Lyα and Lyβ may contribute this Lyα/β ratio decrease.