Photometric Properties of Network and Faculae Derived from HMI Data Compensated for Scattered-Light

TL;DR

This study uses full-disk, scattered-light corrected images from HMI to analyze the photometric properties of solar faculae and network, revealing differences based on magnetic flux and their impact on solar irradiance estimates.

Contribution

First to demonstrate that full-disk observations reproduce small-scale photometric properties of faculae and network, highlighting differences based on magnetic flux and proximity to active regions.

Findings

Network is brighter than faculae for magnetic flux > 300 G, especially near the limb.

Network exhibits higher center-to-limb variation than previously reported.

The contribution of faculae and network to solar irradiance variability is overestimated by at least 11%.

Abstract

We report on the photometric properties of faculae and network as observed in full-disk, scattered-light corrected images from the Helioseismic Magnetic Imager. We use a Lucy-Richardson deconvolution routine that corrects an image in less than one second. Faculae are distinguished from network through proximity to active regions. This is the first report that full-disk observations, including center-to-limb variations, reproduce the photometric properties of faculae and network observed previously only in sub-arcsecond resolution, small field-of-view studies, i.e. that network, as defined by distance from active regions, exhibit higher photometric contrasts. Specifically, for magnetic flux values larger than approximately 300 G, the network is brighter than faculae and the contrast differences increases toward the limb, where the network contrast is about twice the facular one. For lower magnetic flux values, network appear darker than faculae. Contrary to reports from previous full-disk observations, we also found that network exhibits a higher center-to-limb variation. Our results are in agreement with reports from simulations that indicate magnetic flux alone is a poor proxy of the photometric properties of magnetic features. We estimate that the contribution of faculae and network to Total Solar Irradiance variability of the current Cycle 24 is overestimated by at least 11\% due to the photometric properties of network and faculae not being recognized as different. This estimate is specific to the method employed in this study to reconstruct irradiance variations, so caution should be paid when extending it to other techniques.