No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots

@article{Loptien2020NoUC,
  title={No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots},
  author={B. Loptien and A. Lagg and M. Noort and S. Solanki},
  journal={arXiv: Solar and Stellar Astrophysics},
  year={2020}
}
Context. It has been reported that the boundary between the umbra and the penumbra of sunspots occurs at a canonical value of the strength of the vertical magnetic field, independently of the size of the spot. This critical field strength is interpreted as to be the threshold for the onset of magnetoconvection. Aims. Here we investigate the reasons why this criterion, also called the Jurcak criterion in the literature, does not always identify the boundary between umbra and penumbra. Methods… Expand

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