The role of the Fraunhofer lines in solar brightness variability

TL;DR

This study investigates how Fraunhofer lines, molecular, and atomic lines contribute to solar brightness variability across different timescales, highlighting the dominant role of Fraunhofer lines on longer timescales and molecular lines on the 11-year cycle.

Contribution

It provides a detailed analysis of the spectral contributions to solar brightness variability, emphasizing the importance of Fraunhofer lines and molecular lines over various timescales.

Findings

Fraunhofer lines determine the amplitude of solar brightness variability on timescales >1 day.

Molecular lines significantly contribute to the 11-year solar activity cycle.

Approximately 25% of the 11-year solar irradiance variability originates from molecular lines.

Abstract

The solar brightness varies on timescales from minutes to decades. A clear identification of the physical processes behind such variations is needed for developing and improving physics-based models of solar brightness variability and reconstructing solar brightness in the past. This is, in turn, important for better understanding the solar-terrestrial and solar-stellar connections. We estimate the relative contributions of the continuum, molecular, and atomic lines to the solar brightness variations on different timescales. Our approach is based on the assumption that variability of the solar brightness on timescales greater than a day is driven by the evolution of the solar surface magnetic field. We calculated the solar brightness variations employing the solar disc area coverage of magnetic features deduced from the MDI/SOHO observations. The brightness contrasts of magnetic features relative to the quiet Sun were calculated with a non-LTE radiative transfer code as functions of disc position and wavelength. By consecutive elimination of molecular and atomic lines from the radiative transfer calculations, we assessed the role of these lines in producing solar brightness variability. We show that the variations in Fraunhofer lines define the amplitude of the solar brightness variability on timescales greater than a day and even the phase of the total solar irradiance variability over the 11-year cycle. We also demonstrate that molecular lines make substantial contribution to solar brightness variability on the 11-year activity cycle and centennial timescales. In particular, our model indicates that roughly a quarter of the total solar irradiance variability over the 11-year cycle originates in molecular lines. The maximum of the absolute spectral brightness variability on timescales greater than a day is associated with the CN violet system between 380 and 390 nm.