We present semiempirical model atmospheres for the darkest parts of large sunspot umbrae, regions we call umbral cores. Our approach is based on general-purpose computational procedures that are applicable to different types of stellar atmospheres. We show that recent umbral intensity measurements of the spectral energy distribution may be accounted for by an umbral core atmospheric model that varies with time during the solar cycle; the observed center-limb variation can be accounted for by the properties of the model. Three umbral core models are presented, corresponding to the early, middle, and late phases of the solar cycle. These three models also may be regarded as having the properties of dark, average, and bright umbral cores respectively. The effects of atomic, opacity, and abundance data uncertainties on the model calculations are briefly discussed. We also give for comparison a new reference model for the average quiet solar photosphere.

No abstract

No abstract

No abstract

High-resolution filtergrams of a sunspot region observed at seven wavelengths in Hc~ with a resolution of ~-A have been used to investigate the three-dimensional structure of the chromospheric Evershed flow and its time variation.The flow channels have the form of loops whose cross-section alters with height and distance from the spot. The loops probably reach a height of 5000 km. The presence of a compression shock is suggested by the observations. Many velocity channels appear to alter their direction with position in the line, giving the impression of a screw-like motion resembling a Beltrami flow.The flow channel alters with a characteristic time of 8• rain. It is suggested that this relatively short time constant represents the characteristic time of perturbations of the flow channels and the superpenumbra fibrils. The lifetime of the flow channel istelf is estimated to be 70 rain.The observations are discussed in terms of material moving inwards towards the spot along curved flux loops.