Solar chromospheric emission and magnetic structures from plages to intranetwork: Contribution of the very quiet Sun

Abstract

We need to establish a correspondence between the magnetic structures generated by models and usual stellar activity indexes to simulate radial velocity time series for stars less active than the Sun. This is necessary to compare the outputs of such models with observed radial velocity jitters and is critical to better understand the impact of stellar activity on exoplanet detectability. We propose a coherent picture to describe the relationship between magnetic activity, including the quiet Sun regions, and the chromospheric emission using the Sun as a test-bench and a reference. We analyzed a time series of MDI magnetograms jointly with chromospheric emission time series obtained at Sacramento Peak and Kitt Peak observatories. This has allowed us to study the variability in the quiet Sun over the solar cycle, and then, based on available relations between magnetic fields in active structures and chromospheric emission, to propose an empirical reconstruction of the solar chromospheric emission based on all contributions. We show that the magnetic flux covering the solar surface, including in the quieted regions, varies in phase with the solar cycle, suggesting a long-term relationship between the global dynamo and the contribution of all components of solar activity. We have been able to propose a reconstruction of the solar S-index, including a relationship between the weak field component and its chomospheric emission, which is in good agreement with the literature. This allows us to explain that stars with a low average chromospheric emission level exhibit a low variability. We conclude that weak flux regions significantly contribute to the chromospheric emission; these regions should be critical in explaining the lower variability associated with the lower average activity level in other stars as compared to the Sun and estimated from their chromospheric emission.

Accepted in Astronomy and Astrophysics 23/03/2018 13 pages, 9 figures

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• Sun-as-a-Star Spectrum Variations 1974-2006
• The variability of Sun-like stars: reproducing observed photometric trends
• Relation between photospheric magnetic field and chromospheric emission
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• Activity time series of old stars from late F to early K. II. Radial velocity jitter and exoplanet detectability
• Long-term stellar activity variations and their effect on radial-velocity measurements
• Relationship between Ca and H$\alpha$ chromospheric emission in F-G-K stars: indication of stellar filaments?
• On the contribution of quiet Sun magnetism to solar irradiance variations: Constraints on quiet Sun variability and grand minimum scenarios
• Testing the Spectroscopic Extraction of Suppression of Convective Blueshift
• Unsigned magnetic flux proxy from solar optical intensity spectra
• Pollux: A weak dynamo-driven dipolar magnetic field and implications for its probable planet
• Solar-Cycle Variation of quiet-Sun Magnetism and Surface Gravity Oscillation Mode