Astronomy & Astrophysics, Volume 597, id.A60, 4 pp.
Jurčák, J.; Bello González, N.; Schlichenmaier, R.; Rezaei, R.
Context. We recently presented evidence that stable umbra-penumbra boundaries are characterised by a distinct canonical value of the vertical component of the magnetic field, Bstablever. In order to trigger the formation of a penumbra, large inclinations in the magnetic field are necessary. In sunspots, the penumbra develops and establishes by colonising both umbral areas and granulation, that is, penumbral magneto-convection takes over in umbral regions with Bver