Wilson depression in sunspots

Recent advances in the understanding of the Wilson depression in Sunspots

November 13, 2018

By Björn Löptien

In sunspots, the geometric height of the visible solar surface is depressed compared to the quiet Sun. This so-called Wilson depression is caused by the Lorentz force of the strong magnetic field within the spot. Hence, the Wilson depression is strongly connected to the geometry of the magnetic field of the sunspot. A good knowledge of the Wilson depression is therefore important for understanding the structure of sunspots. So far, the Wilson depression could not be determined with a high accuracy since existing methods for measuring it suffer from strong systematic errors. Recently, we derived a novel, robust method for measuring the Wilson depression that is based on minimizing the divergence of the magnetic field vector. Tests performed on synthetic data derived from MHD simulations of sunspots indicate that our method exhibits an accuracy of about 100 km, significantly better than what previous methods could achieve. Applying this approach to sunspots observed with Hinode shows that the Wilson depression lies in the range between 500-700 km. These results are published in A&A (ADS)

Maps of the continuum intensity (left) and of the inferred Wilson depression (right) of AR 10923, observed on 14 November 2006 with Hinode. In both panels, the white and black contours show the inner and outer boundary of the penumbra.
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