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Characterization of the umbra-penumbra boundary by the vertical component of the magnetic field -- Analysis of ground-based data from the GREGOR Infrared Spectrograph | | P. Lindner
; R. Schlichenmaier
; N. Bello González
; | | Date: |
21 Apr 2020 | | 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
$B_perp^{
m const}$ value and contours in intensity, $Delta P$, are
calculated. Averaged over the 11 sunspots, we find a value of
$B_perp=1787pm100$ gauss. Contours at $B_perp=B_perp^{
m const}$ 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 $B_perp^{
m const}$ at the
umbra--penumbra boundary. We also found that the geometric difference, $Delta
P$, between intensity contours and contours at $B_perp=B_perp^{
m const}$
acts as an index of stability for sunspots. | | Source: | arXiv, 2004.9956 | | Services: | Forum | Review | PDF | Favorites | |
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