The shrinking Sun: a systematic error in local correlation tracking of solar granulation

TL;DR

This paper investigates the systematic shrinking-Sun effect in local correlation tracking of solar granulation, attributing it to radiative transfer-induced asymmetries and granule expansion, and provides a simple explanatory model.

Contribution

It identifies the physical origin of the shrinking-Sun effect and demonstrates its presence in both observational and synthetic data, offering a new understanding of this systematic error.

Findings

The shrinking-Sun effect is present in both HMI observations and synthetic simulations.

The effect arises from radiative transfer effects causing apparent asymmetry in granulation.

A simple model explains the effect as a result of viewing angle and granule expansion.

Abstract

Context. Local correlation tracking of granulation (LCT) is an important method for measuring horizontal flows in the photosphere. This method exhibits a systematic error that looks like a flow converging towards disk center, also known as the shrinking-Sun effect. Aims. We aim at studying the nature of the shrinking-Sun effect for continuum intensity data and at deriving a simple model that can explain its origin. Methods. We derived LCT flow maps by running the local correlation tracking code FLCT on tracked and remapped continuum intensity maps provided by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory. We also computed flow maps from synthetic continuum images generated from STAGGER code simulations of solar surface convection. We investigated the origin of the shrinking-Sun effect by generating an average granule from synthetic data from the simulations. Results. The LCT flow maps derived from HMI and from the simulations exhibit a shrinking-Sun effect of comparable magnitude. The origin of this effect is related to the apparent asymmetry of granulation originating from radiative transfer effects when observing with a viewing angle inclined from vertical. This causes, in combination with the expansion of the granules, an apparent motion towards disk center.