The umbra/penumbra boundary in mature spots
The umbra and penumbra of sunspots, the most prominent and renowned manifestations of solar activity, are separated by a boundary that has always been defined by an intensity threshold. Using data from the Hinode satellite, Jurcak and collaborators found that the umbra-penumbra boundary in stable sunspots is characterised by an invariant vertical component of the magnetic field. This law is known as the Jurcak criterion.
The figure shows a sample of sunspots scanned with the Gregor Infrared Spectrograph attached to the GREGOR telescope at the Observatorio del Teide (Tenerife, Spain). The white contours outline the umbral boundary as seen in intensity and the independently defined red contours outline the vertical component of the magnetic field at 1843 Gauss. This value is consistent with the results achieved with the Hinode data and validates the Jurcak criterion in the infrared part of the spectrum.
Image credit: P. Lindner (KIS), Jan Jurcak (AI, Czech Academy of Sciences), Nazaret Bello González (KIS), Rolf Schlichenmaier (KIS)