Photometric Trends in the Visible Solar Continuum and Their Sensitivity to the Center-to-Limb Profile

TL;DR

This study analyzes how the definition of the center-to-limb intensity profile affects measurements of solar continuum contrast and their correlation with the solar cycle, highlighting the importance of magnetic structures and imaging techniques.

Contribution

It demonstrates that variations in the center-to-limb profile shape significantly influence solar cycle contrast trends, emphasizing the need for radiometric imaging for accurate assessments.

Findings

Contrast trends depend on the magnetic flux density used in quiet-sun definition.

Changes in the profile shape over the solar cycle cause contradictory contrast results.

Low contrast magnetic structures are highly sensitive to profile definition.

Abstract

Solar irradiance variations over solar rotational time-scales are largely determined by the passage of magnetic structures across the visible solar disk. Variations on solar cycle time scales are thought to be similarly due to changes in surface magnetism with activity. Understanding the contribution of magnetic structures to total solar irradiance and solar spectral irradiance requires assessing their contributions as a function of disk position. Since only relative photometry is possible from the ground, the contrasts of image pixels are measured with respect to a center-to-limb intensity profile. Using nine years of full-disk red and blue continuum images from the Precision Solar Photometric Telescope at the Mauna Loa Solar Observatory (PSPT/MLSO), we examine the sensitivity of continuum contrast measurements to the center-to-limb profile definition. Profiles which differ only by the amount of magnetic activity allowed in the pixels used to determine them yield oppositely signed solar cycle length continuum contrast trends; either agreeing with the result of Preminger et al. (2011) showing negative correlation with solar cycle or disagreeing and showing positive correlation with solar cycle. Changes in the center-to-limb profile shape over the solar cycle are responsible for the contradictory contrast results, and we demonstrate that the lowest contrast structures, internetwork and network, are most sensitive to these. Thus the strengths of the full-disk, internetwork, and network photometric trends depend critically on the magnetic flux density used in the quiet-sun definition. We conclude that the contributions of low contrast magnetic structures to variations in the solar continuum output, particularly to long-term variations, are difficult, if not impossible, to determine without the use of radiometric imaging.