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

Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
e-mail: nadege.meunier@univ-grenoble-alpes.fr

Received: 18 March 2017
Accepted: 23 March 2018

Abstract

Context. 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.

Aims. We propose a coherent picture to describe the relationship between magnetic activity, including the so-called quiet Sun regions, and the chromospheric emission using the Sun as a test-bench and a reference.

Methods. We analyzed a long time series of Michelson Doppler imaging (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.

Results. 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.

Conclusions. 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.