Abstract
We analyze spectropolarimetric sunspot umbra observations taken in the near-infrared Si i 1082.7 nm line taking NLTE effects into account. The data were obtained with the GRIS instrument installed at the German GREGOR telescope. A point spread function (PSF) was constructed using prior Mercury observations with GRIS and the information provided by the adaptive optics system of the GREGOR telescope. The data were then deconvolved from the PSF using a principal component analysis deconvolution method and were analyzed via the NICOLE inversion code. 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 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%. The inferred levels are in line with current model predictions and empirical umbral models. 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 tau -2 and start rising at lower heights when moving into the transition region. 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.
Highlights
In the literature, there are several proposed empirical models specific to sunspot umbrae (Avrett 1981; Maltby et al 1986; Obridko & Staude 1988; Collados et al 1994; Severino et al 1994; Socas-Navarro 2007; Fontenla et al 2009)
The data were deconvolved from the point spread function (PSF) using a principal component analysis deconvolution method and were analyzed via the NICOLE inversion code, which accounts for non-local thermodynamic equilibrium (NLTE) effects in the Si i 1082.7 nm line
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
Summary
There are several proposed empirical models specific to sunspot umbrae (Avrett 1981; Maltby et al 1986; Obridko & Staude 1988; Collados et al 1994; Severino et al 1994; Socas-Navarro 2007; Fontenla et al 2009). Modeling the temperature and vector magnetic magnetic field stratification in the photospheric part of sunspot umbrae is relatively easy, this is not the case for the chromosphere and layers above In these regions, most observed spectral lines suffer from non-local thermodynamic equilibrium effects (NTLE), which makes it difficult to determine the height variation of several thermodynamic parameters accurately. Other umbral models do not take into account the chromosphere (Collados et al 1994)
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