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

B. Löptien, A. C. Birch, T. L. Duvall, L. Gizon, J. Schou

Research output: Contribution to journalArticlepeer-review


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 toward disk center, which is also known as the shrinking-Sun effect. Aims. We aim to study the nature of the shrinking-Sun effect for continuum intensity data and to derive a simple model that can explain its origin. Methods. We derived LCT flow maps by running the LCT code Fourier Local Correlation Tracking (FLCT) on tracked and remapped continuum intensity maps provided by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). 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 the HMI data and 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 toward disk center.

Original languageEnglish (US)
Article numberA130
JournalAstronomy and Astrophysics
StatePublished - 2016


  • Methods: data analysis
  • Radiative transfer
  • Sun: granulation
  • Sun: photosphere

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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