The magnetic field at the boundary between umbra and penumbra

The boundary between the umbra and the penumbra of sunspots has untill recently been defined by an intensity value only. Using Hinode satellite data, Jurcak et. al 2018 found an empirical law of the magnetic nature of the umbra-penumbra boundary in stable sunspots: an invariant vertical component of the magnetic field.

The validity of this empirical law and the value of this new constant is now being examined for different wavelengths and data sources. The figure shows intensity images of sunspots observed with the GRIS@GREGOR spectograph. Data of this instrument was used to infer maps of the vertical magnetic component of the magnetic field, Bver.  Iso-contours from intensity maps are shown in white and the independently calculated iso-contours from Bver maps with Bver=1843 Gauss are shown in red. For most of the datasets, the two contours coincide well. The value of Bver=1843 Gauss is consistent with the results from Jurcak et. al 2018 with Hinode data. 

References:

  • Philip Lindner, Masterthesis
  • Jurčák, J.; Rezaei, R.; González, N. Bello; Schlichenmaier, R.; Vomlel, J. 2018, A&A 611 L4