Spectropolarimetry of sunspot penumbrae

Sunspots, the most prominent manifestation of the solar magnetism, are fascinating research subjects. Especially sunspot penumbrae, with their high structuring, have attracted much attention from which ample observational and theoretical scientific work has originated. Recent observations with improved solar instrumentation have revealed that sunspot penumbrae possess much finer structures than was thought a few decades ago. Thus, we are still at the beginning of understanding penumbrae with their still enigmatic intensity pattern, their magnetic field structures, and the basic processes which cause the structuring and the Evershed flow (Evershed 1909).

This Thesis work is based on two-dimensional high-spatial-resolution spectropolarimetric observations of sunspot penumbrae. They were obtained in April 2002 and May 2003 with the “Göttingen” Fabry-Pérot spectrometer at the Vacuum Tower Telescope at the Observatorio del Teide (Tenerife). Speckle methods were applied for image reconstruction which resulted in a spatial resolution of 0.5 arcsec in the magnetograms of the penumbrae. Stokes I and V profiles of the Fe II 614.9 nm line, which exhibits no instrumental Stokes Q/U → V crosstalk, and of the Fe I line pair at 630.2 nm, have been analysed. From the analysis of intensity maps corresponding to different atmospheric layers it has been found that, on scales larger than 0.5 arcsec, the intensity pattern of penumbrae stays the same in the continuum and core images of the 630.15n m line, which stem from 0 km and 300 km (above τ= 1), respectively. Yet at scales of 0.5 arcsec and smaller the pattern in the two spectral features it is found to be different.

From the comparison of velocity and intensity maps, one can see that in the limb side of sunspots the Evershed flow is carried by dark filaments and on the centre side by bright features and their somewhat weakened tails. This can be explain in terms of the picture in which the velocity of hot rising gas is best seen on the centre side, while on the limb side the horizontal outward and possibly downward flows are seen when the gas has cooled down.

Strong line-of-sight components of the magnetic fields are not found in bright filaments but in dark structures, somewhat displaced from the darkest parts. Their positions do not coincide with those of the strongest velocity fields. In general, these results are compatible with the picture of low lying flow channels coincident with the horizontal magnetic field, or possibly emerging and diving down into sub-photospheric layers, like a “sea serpent”.

To complete the analysis on the penumbral fine-structure, forward modelling has been performed. The aim has been to synthesise Stokes profiles which account for the asymmetries of the observed profiles. For that purpose, a numerical code was developed from existing ones to solve the one-dimensional radiative transfer problem. The assumed penumbral model contains two atmospheric components coexisting in the same resolution element. It has been possible to rule out “cool upflows” as an appropriate scenario of the penumbral structure. Moreover, synthetic profiles generated from the assumption of hot upflows have lead to the simulation of observed profiles from penumbral grains. This would mean that the grains do indeed carry hot material to the photosphere in up-flows. However, strongly asymmetric profiles are not easily reproducible with the considered model.

Some further dynamic phenomena are discussed along these pages, which demonstrate the richness of processes in penumbrae, and reveal unexpected properties.