The fine scale dynamics of a sunspot penumbra

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Sun: Photosphere - Sunspots

Scientific paper

We have used two-dimensional spectroscopy to study the dynamics of a sunspot penumbra at different heliocentric angles. The study utilises spatially highly resolved digital spectra, obtained with the Swedish Vacuum Solar Telescope on La Palma, and provides images of parameters measured in these spectra. The presented measurements were made in the line Fe I 557.6 nm, which has no magnetic sensitivity.
Images of line shift show the Evershed outflow in the mid- outer penumbra. Its structure at 16 degrees heliocentric angle suggests an inclination of the flow in the order of 5 degrees. Related to the flow are strong variations in line width and equivalent width, which are correlated at small scales. The spatial variations in general show weak or non-existent correlation to intensity structures, although a tendency for bright structures to correspond to local blue shift is seen on both sides of the spot.
Enhanced line width is present mainly in the flow regions and it increases with heliocentric angle in those areas. A comparison to line shift at 50% line depth suggests the origin to be velocity gradients with depth, but a proper evaluation of this hypothesis requires more detailed analysis of the data as well as modelling and radiative transfer calculations.

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