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

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.

8 pages, 7 figures. Accepted for publication in Astronomy & Astrophysics

References in corpus (5)

• Spatially resolved vertical vorticity in solar supergranulation using helioseismology and local correlation tracking
• Tracking granules on the Sun's surface and reconstructing horizontal velocity fields: I. the CST algorithm
• Comparison of large-scale flows on the Sun measured by time-distance helioseismology and local correlation tracking technique
• Helioseismology with Solar Orbiter
• Data compression for local correlation tracking of solar granulation

Cited by in corpus (12)

• The Polarimetric and Helioseismic Imager on Solar Orbiter
• Global-scale equatorial Rossby waves as an essential component of solar internal dynamics
• Time-distance helioseismology of solar Rossby waves
• Exploring the latitude and depth dependence of solar Rossby waves using ring-diagram analysis
• Measuring solar active region inflows with local correlation tracking of granulation
• Large-scale photospheric motions determined from granule tracking and helioseismology from SDO/HMI data
• Improved Measurements of the Sun's Meridional Flow and Torsional Oscillation from Correlation tracking on MDI \& HMI magnetograms
• Removal Of Active Region Inflows Reveals a Weak Solar Cycle Scale Trend In Near-surface Meridional Flow
• Sectoral r modes and periodic RV variations of Sun-like stars
• Contribution of flows around active regions to the north-south helioseismic travel-time measurements
• Multi-height probing of horizontal flows in the solar photosphere
• Evolution of solar surface inflows around emerging active regions