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
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 of B⊥ along the contours between penumbra and umbra are analyzed for the 11 data sets. In addition, contours at the resulting B⊥const are 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 of B⊥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 at B⊥ = B⊥const and contours calculated in intensity maps match from a visual inspection and the geometric distance ΔP 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.
Conclusions: Our results provide further support to the Jurčák criterion with the existence of an invariable value B⊥const at 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 at B⊥ = B⊥const acts as an index of stability for sunspots.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- June 2020
- DOI:
- 10.1051/0004-6361/202037716
- arXiv:
- arXiv:2004.09956
- Bibcode:
- 2020A&A...638A..25L
- Keywords:
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- sunspots;
- Sun: fundamental parameters;
- Sun: infrared;
- Sun: photosphere;
- Sun: magnetic fields;
- Sun: evolution;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- A&