THE MAGNETOHYDRODYNAMICS OF UMBRAL FLUX TUBES .2. SPECTROSCOPIC PROPERTIES

We apply a recently developed magnetohydrodynamic model for flux tube systems within sunspot umbrae for a range of model parameters appropri ate to umbral dots. The results are used to generate synthetic observa tions of umbral dots, which we then compare to recent observational da ta taken from the literature. Below the umbral surface, our umbral flu x tube models are characterized by (1) reduced (but nonzero) field str ength relative to the surrounding umbra, (2) a relative temperature en hancement, and (3) an assumed upflow which advects heat toward the sur face, thus making the dot brighter than its surroundings. In this pape r 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 w ithin the umbral flux tubes, relative to the surrounding umbra, provid es specific quantitative constraints for selection among parameters sp ecified at the lower boundary of our models. Available observations do not clearly delimit line strengths; however, empirical models based o n recent observations by Sobotka et al. suggest that our models with s trong upflow (15 m s-1) and modest temperature enhancement relative to the surrounding umbra at 100 km below the visible surface best charac terize umbral dots. We show that, even though the flow velocity is lar ge well above the photospheric surface of our model umbral flux tubes, the line-of-sight component of velocity as inferred from the zero cro ssing 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 fr om our models are correspondingly quite weak (< 3%).