The Magnetohydrodynamics of Umbral Flux Tubes. II. Spectroscopic Properties

Astronomy and Astrophysics – Astronomy

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Magnetohydrodynamics: Mhd, Sun: Sunspots

Scientific paper

We apply a recently developed magnetohydrodynamic model for flux tube systems within sunspot umbrae for a range of model parameters appropriate to umbral dots. The results are used to generate synthetic observations of umbral dots, which we then compare to recent observational data taken from the literature.
Below the umbral surface, our umbral flux tube models are characterized by (1) reduced (but nonzero) field strength relative to the surrounding umbra, (2) a relative temperature enhancement, and (3) an assumed upflow which advects heat toward the surface, thus making the dot brighter than its surroundings. In this paper we vary the free parameters which characterize our model to explore the range of physical conditions within umbral dots as admitted by our model. We find that the equivalent width of lines of Fe I and Fe ii within the umbral flux tubes, relative to the surrounding umbra, provides specific quantitative constraints for selection among parameters specified at the lower boundary of our models. Available observations do not clearly delimit line strengths; however, empirical models based on recent observations by Sobotka et al. suggest that our models with strong upflow (15 m s-1) and modest temperature enhancement relative to the surrounding umbra at 100 km below the visible surface best characterize umbral dots.
We show that, even though the flow velocity is large well above the photospheric surface of our model umbral flux tubes, the line-of-sight component of velocity as inferred from the zero crossing of the Stokes V profile is (in general) smaller than 0.5 km s-1. Likewise, the amplitude asymmetry of the Stokes V profiles arising from our models are correspondingly quite weak (<3%).

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