Bibcode
Orozco Suárez, D.; Quintero Noda, C.; Ruiz Cobo, B.; Collados Vera, M.; Felipe, T.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 607, id.A102, 12 pp.
Advertised on:
11
2017
Revista
Número de citas
5
Número de citas referidas
5
Descripción
Context. Determining empirical atmospheric models for the solar
chromosphere is difficult since it requires the observation and analysis
of spectral lines that are affected by non-local thermodynamic
equilibrium (NLTE) effects. This task is especially difficult in sunspot
umbrae because of lower continuum intensity values in these regions with
respect to the surrounding brighter granulation. Umbral data is
therefore more strongly affected by the noise and by the so-called
scattered light, among other effects. Aims: The purpose of this
study is to analyze spectropolarimetric sunspot umbra observations taken
in the near-infrared Si I 1082.7 nm line taking NLTE effects into
account. Interestingly, we detected emission features at the line core
of the Si I 1082.7 nm line in the sunspot umbra. Here we analyze the
data in detail and offer a possible explanation for the Si I 1082.7 nm
line emission. Methods: Full Stokes measurements of a sunspot
near disk center in the near-infrared spectral range were obtained with
the GRIS instrument installed at the German GREGOR telescope. A point
spread function (PSF) including the effects of the telescope, the
Earth's atmospheric seeing, and the scattered light was constructed
using prior Mercury observations with GRIS and the information provided
by the adaptive optics system of the GREGOR telescope during the
observations. The data were then deconvolved from the PSF using a
principal component analysis deconvolution method and were analyzed via
the NICOLE inversion code, which accounts for NLTE effects in the Si I
1082.7 nm line. The information of the vector magnetic field was
included in the inversion process. Results: The Si I 1082.7 nm
line seems to be in emission in the umbra of the observed sunspot after
the effects of scattered light (stray light coming from wide angles) are
removed. We show how the spectral line shape of umbral profiles changes
dramatically with the amount of scattered light. Indeed, the continuum
levels range, on average, from 44% of the quiet Sun continuum intensity
to about 20%. Although very low, the inferred levels are in line with
current model predictions and empirical umbral models. The Si I 1082.7
nm line is in emission after adding more that 30% of scattered light so
that it is very sensitive to a proper determination of the PSF.
Additionally, we have thoroughly investigated whether the emission is a
byproduct of the particular deconvolution technique but have not found
any evidence to the contrary. Only the circular polarization signals
seem to be more sensitive to the deconvolution strategy because of the
larger amount of noise in the umbra. Interestingly, current umbral
empirical models are not able to reproduce the emission in the
deconvolved umbral Stokes profiles. The results of the NLTE inversions
suggests that to obtain the emission in the Si I 1082.7 nm line, the
temperature stratification should first have a hump located at about log
τ = -2 and start rising at lower heights when moving into the
transition region. Conclusions: This is, to our knowledge, the
first time the Si I 1082.7 nm line is seen in emission in sunspot
umbrae. The results show that the temperature stratification of current
umbral models may be more complex than expected with the transition
region located at lower heights above sunspot umbrae. Our finding might
provide insights into understanding why the sunspot umbra emission in
the millimeter spectral range is less than that predicted by current
empirical umbral models.
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