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 ($B_{\perp }$) 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 $B_{\perp }$ 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 $B_{\perp }$ along umbra-penumbra boundaries are analyzed for the 11 data sets. Geometric differences between contours at the resulting $B_{\perp }^{\mathrm{const}}$ value and contours in intensity, $\Delta P$, are calculated. Averaged over the 11 sunspots, we find a value of $B_{\perp }=1787\pm 100$ gauss. Contours at $B_{\perp }=B_{\perp }^{\mathrm{const}}$ and contours calculated in intensity maps match from a visual inspection and the geometric distance $\Delta P$ 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 $B_{\perp }$ than for $B_{\parallel }$ by a factor of 2.4. Our results provide further support to the Jurcak criterion with the existence of an invariable value $B_{\perp }^{\mathrm{const}}$ at the umbra--penumbra boundary. We also found that the geometric difference, $\Delta P$, between intensity contours and contours at $B_{\perp }=B_{\perp }^{\mathrm{const}}$ acts as an index of stability for sunspots.

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Cited by in corpus (7)

• Magnetic properties on the boundary of an evolving pore
• No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots
• Long-Term Evolution of Three Light Bridges Developed on the Same Sunspot
• Solar and stellar flares: frequency, active regions and stellar dynamo
• The MODEST catalog of depth-dependent spatially coupled inversions of sunspots observed by Hinode/SOT-SP
• Clarifying Physical Properties of Magnetic Fields in Sunspots
• High-resolution observations of two pores with the integral field unit (IFU) of the GREGOR Infrared Spectrograph (GRIS)