Modelling the longitudinal asymmetry in sunspot emergence: the role of the Wilson depression

F. Watson, L. Fletcher, S. Dalla, S. Marshall, STFC (Funder), EC (Funder), NASA (Funder), ONR (Funder), NSF (Funder)

Research output: Contribution to journalArticlepeer-review

41 Citations (Scopus)


The distributions of sunspot longitude at first appearance and at disappearance display an east-west asymmetry that results from a reduction in visibility as one moves from disk centre to the limb. To first order, this is explicable in terms of simple geometrical foreshortening. However, the centre-to-limb visibility variation is much larger than that predicted by foreshortening. Sunspot visibility is also known to be affected by the Wilson effect: the apparent 'dish' shape of the sunspot photosphere caused by the temperature-dependent variation of the geometrical position of the tau=1 layer. In this article we investigate the role of the Wilson effect on the sunspot appearance distributions, deducing a mean depth for the umbral tau=1 layer of 500 -aEuro parts per thousand 1500 km. This is based on the comparison of observations of sunspot longitude distribution and Monte Carlo simulations of sunspot appearance using different models for spot growth rate, growth time and depth of Wilson depression.
Original languageEnglish
Pages (from-to)5-19
Number of pages15
JournalSolar Physics
Issue number1
Publication statusPublished - Nov 2009


  • magnetic fields
  • sun
  • photosphere
  • sunspots


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