摘要：Context.The vertical component of the magnetic field was found to reach a constant value at the boundary between penumbra and umbra of stable sunspots in a recent statistical study of Hinode/SP data. This finding has profound implications as it can serve as a criterion to distinguish between fundamentally different magneto-convective modes operating in the sun.Aims.The objective of this work is to verify the existence of a constant value for the vertical component of the magnetic field (B⊥) at the boundary between umbra and penumbra from ground-based data in the near-infrared wavelengths and to determine its value for the GREGOR Infrared Spectrograph (GRIS@GREGOR) data. This is the first statistical study on the Jurčák criterion with ground-based data, and we compare it with the results from space-based data (Hinode/SP and SDO/HMI).Methods.Eleven spectropolarimetric data sets from the GRIS@GREGOR slit-spectograph containing fully-fledged stable sunspots were selected from the GRIS archive. SIR inversions including a polarimetric straylight correction are used to produce maps of the magnetic field vector using the Fe I 15648 Å and 15662 Å lines. Averages ofB⊥along the contours between penumbra and umbra are analyzed for the 11 data sets. In addition, contours at the resultingB⊥constare drawn onto maps and compared to intensity contours. The geometric difference between these contours, ΔP, is calculated for each data set.Results.Averaged over the 11 sunspots, we find a value ofB⊥const= (1787 ± 100) gauss. The difference from the values previously derived from Hinode/SP and SDO/HMI data is explained by instrumental differences and by the formation characteristics of the respective lines that were used. Contours atB⊥=B⊥constand contours calculated in intensity maps match from a visual inspection and the geometric distance ΔPwas 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 forB⊥than forB∥by a factor of 2.4.Conclusions.Our results provide further support to the Jurčák criterion with the existence of an invariable valueB⊥constat the umbra–penumbra boundary. This fundamental property of sunspots can act as a constraining parameter in the calibration of analysis techniques that calculate magnetic fields. It also serves as a requirement for numerical simulations to be realistic. Furthermore, it is found that the geometric difference, ΔP, between intensity contours and contours atB⊥=B⊥constacts as an index of stability for sunspots.
关键词：ensunspotsSun: fundamental parametersSun: infraredSun: photosphereSun: magnetic fieldsSun: evolution
