The role of the Fraunhofer lines in solar brightness variability

Abstract

Context. 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. Aims. We estimate the relative contributions of the continuum, molecular, and atomic lines to the solar brightness variations on different timescales. Methods. 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. Results. 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.

Are solar brightness variations faculae- or spot-dominated?

“…One can expect that the facular contrast in Sun-like stars depends on the stellar brightness temperature (cf. Beeck et al 2015) and on the stellar metallicity (since the facular contrast is strongly affected by the weak atomic and molecular lines, see Shapiro et al 2015). A small change of the facular contrast might break a delicate balance between facular and spot contributions to Strömgren b and y photometry and significantly increase the stellar brightness variability.…”

“…Figure 3 demonstrates that the spectral dependences of the spot and facular components are remarkably different. The facular component has a very complex profile due to the strong influence of Fraunhofer lines (Unruh et al 1999(Unruh et al , 2008Shapiro et al 2015). The effect of Fraunhofer lines on the spot component is much more subtle and exposes itself as barely visible spikes superposed on otherwise undulating dependence (see discussion in Unruh et al 2008;Shapiro et al 2015).…”

“…(4)). As a result, the transition from the faculae-to the spot-dominated regime of rotational variability happens already around 400 nm, immediately longward of the CN violet system (which amplifies the facular contrast and thus ensures the faculae-dominated regime of variability, see discussion in Shapiro et al 2015).…”

The Influence of Metallicity on Stellar Differential Rotation and Magnetic Activity

“…9). (Shapiro et al 2015). Together with the inclination of the rotation axis, metallicity is also known to have an effect on the visibility of activity-related phenomena like spots and faculae (Shapiro et al 2014).…”

Variation of the Mn I 539.4 nm line with the solar cycle

“…Furthermore, the fact that SATIRE-S reproduces the correct level of the variation in the Mn line provides strong support to the overall spectral profile of the irradiance variability computed with SATIRE-S. On the one hand, Yeo et al (2015) have recently demonstrated that the magnitude of the changes in the UV returned by SATIRE-S agrees well with the most stable satellite measurements. On the other hand, since spectral lines determine the amplitude and even the A33, page 7 of 8 phase of the visible irradiance variations (Shapiro et al 2015), our finding here indicates that the magnitude of the variability in the visible is also adequate. Finally, the model also reproduces the changes in the TSI that are dominated by changes in the UV and the visible and are measured far more reliably than changes in the spectral irradiance (Yeo et al 2014a).…”