Intensity contrast of solar network and faculae

TL;DR

This paper investigates the intensity contrast of solar network and faculae using HMI data, revealing differences in their behavior across the solar disc and emphasizing the importance of spectral line contributions to solar irradiance variations.

Contribution

It provides observational constraints on the intensity contrast of network and faculae, highlighting differences in their center-to-limb variation and the significance of spectral lines in irradiance models.

Findings

Network features are brighter per unit magnetic flux than faculae.

Continuum and line core contrasts exhibit opposite center-to-limb variations.

Spectral line changes significantly influence solar irradiance variations.

Abstract

This study aims at setting observational constraints on the continuum and line core intensity contrast of network and faculae, specifically, their relationship with magnetic field and disc position. Full-disc magnetograms and intensity images by the HMI instrument onboard SDO were employed. Bright magnetic features, representing network and faculae, were identified and the relationship between their intensity contrast at continuum and line core with magnetogram signal and heliocentric angle examined. Care was taken to minimize the inclusion of the magnetic canopy and straylight from sunspots and pores as network and faculae. In line with earlier studies, network features, on a per unit magnetic flux basis, appeared brighter than facular features. Intensity contrasts in the continuum and line core differ considerably, most notably, they exhibit opposite centre-to-limb variations. We found this difference in behaviour to likely be due to the different mechanisms of the formation of the two spectral components. From a simple model based on bivariate polynomial fits to the measured contrasts we confirmed spectral line changes to be a significant driver of facular contribution to variation in solar irradiance. The discrepancy between the continuum contrast reported here and in the literature was shown to arise mainly from differences in spatial resolution and treatment of magnetic signals adjacent to sunspots and pores. HMI is a source of accurate contrasts and low-noise magnetograms covering the full solar disc. For irradiance studies it is important to consider not just the contribution from the continuum but also from the spectral lines. In order not to underestimate long-term variations in solar irradiance, irradiance models should take the greater contrast per unit magnetic flux associated with magnetic features with low magnetic flux into account.