Characterization of the umbra-penumbra boundary by the vertical component of the magnetic field -- Analysis of ground-based data from the GREGOR Infrared Spectrograph
Abstract
The vertical component of the magnetic field (B) was found to reach a constant value at the umbra-penumbra boundary of stable sunspots in a recent statistical study of Hinode/SP data. The objective of this work is to verify the existence of a constant value for B at the umbra-penumbra boundary from ground-based data in the near-infrared wavelengths and to determine its value for GREGOR Infrared Spectrograph (GRIS) data. This is the first statistical study on the Jurcak criterion with ground-based data, and we compare it with the results from Hinode and HMI data. Eleven spectropolarimetric data sets from the GRIS slit-spectograph containing stable sunspots were selected from the GRIS archive (sdc.leibniz-kis.de). SIR inversions including a polarimetric straylight correction are used to produce maps of the magnetic field vector using the Fe 1564.8 and 1566.2 nm lines. Averages of B along umbra-penumbra boundaries are analyzed for the 11 data sets. Geometric differences between contours at the resulting B const value and contours in intensity, P, are calculated. Averaged over the 11 sunspots, we find a value of B=1787100 gauss. Contours at B=B const and contours calculated in intensity maps match from a visual inspection and the geometric distance P was found to be on the order of 2 pixels. Furthermore, the standard deviation between different data sets of averages along umbra--penumbra contours is smaller for B than for B by a factor of 2.4. Our results provide further support to the Jurcak criterion with the existence of an invariable value B const at the umbra--penumbra boundary. We also found that the geometric difference, P, between intensity contours and contours at B=B const acts as an index of stability for sunspots.