Understanding Sun-as-a-Star Variability of Solar Balmer Lines

Abstract

Precise, high-cadence, long-term records of stellar spectral variability at different temporal scales lead to better understanding of a wide variety of phenomena including stellar atmospheres and dynamos, convective motions, and rotational periods. Here, we investigate the variability of solar Balmer lines (Hα, β, γ, δ) observed by space-borne radiometers (OSIRIS, SCIAMACHY, OMI, and GOME-2), combining these precise, long-term observations with high-resolution data from the ground-based NSO/ISS spectrograph. We relate the detected variability to the appearance of magnetic features on the solar disk. We find that on solar-rotational timescales (about 1 month), the Balmer line activity indices (defined as line-core to line-wing ratios) closely follow variations in the total solar irradiance (which is predominantly photospheric), thus frequently (specifically, during passages of sunspot groups) deviating from behavior of activity indices that track chromospheric activity levels. On longer timescales, the correlation with chromospheric indices increases, with periods of low correlation or even anticorrelation found at intermediate timescales. Comparison of these observations with estimates from semiempirical irradiance reconstructions helps quantify the contributions of different magnetic and quiet features. We conclude that both the lower sensitivity to network and in part the higher sensitivity to filaments and prominences, may result in complex, time-dependent relationships between Balmer and other chromospheric indices observed for the Sun and solar-like stars. The fact that core and wings contribute in a similar manner to the variability, and current knowledge of Balmer-lines formation in stellar atmospheres, supports the notion that Balmer line core-to-wing ratio indices behave more like photospheric rather than chromospheric indices.

Sun-as-a-star variability of Hα and Ca II 854.2 nm lines

“…Recent works by Marchenko et al (2021); Criscuoli et al (2023b) showed that the variability on the rotational scale of core-towing ratio indices derived from Balmer lines is strongly modulated by the passage of sunspots, and almost insensitive to the presence of network and plage, thus following patterns more similar to the ones observed for photospheric, rather than chromospheric indices. Analysis of sun-as-a-star observations acquired with the Integrated Sunlight Spectrometer at SOLIS (Criscuoli et al, 2023b) and with the Fourier Transform Spectrometer at McMath-Pierce Solar telescope (Livingston et al, 2007;Meunier and Delfosse, 2009;Livingston et al, 2010) indicate that on the solar cycle scale, the Hα core-to-wing ratio index is highly correlated (r ≥ 0.7) with indices derived from the Ca II K line. Meunier and Delfosse (2009); Criscuoli et al (2023b) also found that the correlation between the two indices is not constant over the magnetic cycle, but increases during periods of increasing activity and decreases during periods of low activity.…”

“…Meunier and Delfosse (2009); Criscuoli et al (2023b) also found that the correlation between the two indices is not constant over the magnetic cycle, but increases during periods of increasing activity and decreases during periods of low activity. Criscuoli et al (2023b) suggested that the observed complex relation between the Hα and Ca II K indices should be ascribed to the temperature response function of the Hα core picking in the lower layers of the photosphere. In this work we expand the analysis presented in Criscuoli et al (2023b) by studying the variability of both the width and core of Hα and comparing them to the variability of the same indices derived from the Ca II 854.2 nm line.…”

“…However, several observational studies conducted on samples of F-G-K stars reported that the variability of Balmer (especially Hα) and Ca II K indices can exhibit correlations, anti-correlations, or no discernible pattern (e.g., Strassmeier et al, 1990;Cincunegui et al, 2007;Gomes da Silva et al, 2014;Flores et al, 2018;Meunier et al, 2022). Recent works by Marchenko et al (2021); Criscuoli et al (2023b) showed that the variability on the rotational scale of core-towing ratio indices derived from Balmer lines is strongly modulated by the passage of sunspots, and almost insensitive to the presence of network and plage, thus following patterns more similar to the ones observed for photospheric, rather than chromospheric indices. Analysis of sun-as-a-star observations acquired with the Integrated Sunlight Spectrometer at SOLIS (Criscuoli et al, 2023b) and with the Fourier Transform Spectrometer at McMath-Pierce Solar telescope (Livingston et al, 2007;Meunier and Delfosse, 2009;Livingston et al, 2010) indicate that on the solar cycle scale, the Hα core-to-wing ratio index is highly correlated (r ≥ 0.7) with indices derived from the Ca II K line.…”

“…Analysis of sun-as-a-star observations acquired with the Integrated Sunlight Spectrometer at SOLIS (Criscuoli et al, 2023b) and with the Fourier Transform Spectrometer at McMath-Pierce Solar telescope (Livingston et al, 2007;Meunier and Delfosse, 2009;Livingston et al, 2010) indicate that on the solar cycle scale, the Hα core-to-wing ratio index is highly correlated (r ≥ 0.7) with indices derived from the Ca II K line. Meunier and Delfosse (2009); Criscuoli et al (2023b) also found that the correlation between the two indices is not constant over the magnetic cycle, but increases during periods of increasing activity and decreases during periods of low activity. Criscuoli et al (2023b) suggested that the observed complex relation between the Hα and Ca II K indices should be ascribed to the temperature response function of the Hα core picking in the lower layers of the photosphere.…”

Advancements in solar spectral irradiance measurements by the TSIS-1 spectral irradiance monitor and its role for long-term data continuity

“…NRLSSI2) using the Mg II chromospheric emissions have been developed to quantify the spectral dependence of the variability since photospheric faculae underlie the bright chromospheric plage of these active regions (Lean, 2000). Alternatively, the more physically based model approach to spectral irradiance reconstruction utilizes either full-disc magnetograms and intensity images (e.g., Yeo et al, 2015;Krivova et al, 2011) or Ca II K and red continuum images (e.g., Fontenla et al, 1999;Ermolli et al, 2011;Criscuoli et al, 2023) to define the fraction of the solar surface covered by the sunspots and faculae in conjunction with spectral intensities from semi-empirical model atmospheres (e.g., Fontenla et al, 1999;Unruh et al, 1999).…”