Sunspot structure and dynamics

Sammanfattning: Fundamental physical processes occur on small spatial scales in the solar atmosphere and high-spatial-resolution observations are necessary to test theoretical models. This thesis presents high-spatial-resolution observations of sunspots obtained with the Swedish solar telescopes on La Palma: the Swedish Vacuum Solar Telescope (SVST) and its successor the Swedish 1-m Solar Telescope (SST). Spectroscopic observations are used to derive semi-empirical models of the temperature stratification of fine-structure elements in the sunspot penumbra by inversion of the radiation temperatures in the damping wings of the Ca II K line. These models are used to investigate the line formation of spectral line blends in the Ca II K wings. Line-core Dopplershift measurements of a selection of these spectral line blends are used to investigate the height dependence of bulk velocities in the penumbral atmosphere, including the Evershed flow. Examples are found of concentrations of the Evershed flow in the form of channels that tend to coincide with dark filaments. A weak correlation is found between reduced intensity and enhanced velocity but there exist examples of strong flows in bright structures. Numerical tests of flow channels in the penumbral atmosphere suggest that the bulk of the Evershed flow is concentrated in the lower atmosphere and has a velocity slower than 6 km s1. No indication is found of the Evershed flow being elevated above the continuum. A time series of spectrograms with the spectrograph slit positioned at a quasi-fixed position in the penumbra shows that the Evershed flow is variable on a time scale of 8–14 minutes. The time variability does not display any significant height dependence. SST observations of sunspots at unprecedented resolution (better than 0.12 arcseconds) reveal bright penumbral filaments with dark cores. The dark cores have apparent widths of 90 km but are not resolved. Other examples of very narrow dark features in the magnetic-field dominated regions outside sunspots are also found. Observations from the SVST, SST and the Dutch Open Telescope are used to study chromospheric oscillations in the sunspot umbra and penumbra. These observations comprise time series of spectrograms and narrow-band filtergrams covering the line-cores of the Ca II H and K lines. Umbral flashes and running penumbral waves are found to be closely related oscillatory phenomena, combining upward shock propagation with coherent wave spreading over the entire sunspot. Flash brightening is attributed to large redshift by post-shock material higher in the atmosphere.