Asymmetrical appearance of dark-cored filaments in sunspot penumbrae

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Abstract
Recent sunspot observations at unprecedented resolution have led to the discovery of dark cores in the bright filaments that form the penumbra ([CITE]). The discovery paper considered spots at disk center only, so the properties of the dark-cored filaments remain largely unknown. Here we analyze a speckle-reconstructed time series of G-band and blue continuum images of a sunspot acquired with the Dutch Open Telescope. The target was located at an heliocentric angle of 27 deg. We confirm the existence of dark-cored penumbral filaments also in spots outside the disk center, and report on distinct differences between the center and limb-side penumbra. In the inner center-side penumbra, filaments are detected as two narrow bright streaks separated by a central obscuration. These structures move together as a single entity. On the limb side, dark cores are hardly seen. The time series is used to determine the sizes (~200-250 km), proper motions (~280 m s-1), and lifetimes (45 min) of typical dark-cored filaments.

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