Intensity contrast from MHD simulations and HINODE observations

Abstract

Context. Changes in the solar surface area, which is covered by small-scale magnetic elements, are thought to cause long-term changes in the solar spectral irradiance, which are important for determining the impact on Earth's climate.
Aims: To study the effect of small-scale magnetic elements on the total and spectral irradiance, we derive their contrasts from 3-D MHD simulations of the solar atmosphere. These calculations are necessary because measurements of small-scale flux tube contrasts are confined to a few wavelengths and affected by scattered light and instrument defocus, even for space observations.
Methods: To test the contrast calculations, we compare rms contrasts from simulations with those obtained with the broad-band filter imager mounted on the Solar Optical Telescope (SOT) onboard the Hinode satellite and also analyse centre-to-limb variations (CLV). The 3-D MHD simulations include the interaction between convection and magnetic flux tubes. They are performed by assuming non-grey radiative transfer and using the MURaM code. The simulations have an average vertical magnetic field of 0 G, 50 G, and 200 G. Emergent intensities are calculated with the spectral synthesis code ATLAS9 and are convolved with a theoretical point-spread function to account for the properties of the observations' optical system.
Results: We find reasonable agreement between simulated and observed intensity distributions in the visible continuum bands. Agreement is poorer for the CN and G-bands. The analysis of the simulations uncovers a potentially more realistic centre-to-limb behaviour than calculations based on 1-D model atmospheres.
Conclusions: We conclude that starting from 3-D MHD simulations represents a powerful approach to obtaining intensity contrasts for a wide wavelength coverage and different positions across on the solar disk. This also paves the way for future calculations of facular and network contrast as a function of magnetic fluxes.