Stokes Profile Analysis and Vector Magnetic Fields. VI. Fine Scale Structure of a Sunspot

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Abstract

The vector magnetic field structure of a small, symmetric sunspot observed very close to disk center has been explored using data from the High Altitude Observatory/National Solar Observatory Advanced Stokes Polarimeter (ASP). This instrument provides, for the first time, quantitative information on sunspot photospheric vector magnetic fields with high angular resolution, as derived from full Stokes profiles of the Zeeman-sensitive Fe I line pair at 630 nm. We find the following. (1) The penumbra contains narrow, radial "spines" of more intense magnetic field more vertically oriented than their surroundings; (2) the magnetic field diverges azimuthally away from the spine axes, as it should for a more intense intrusion of field embedded in a gravitational stratified atmosphere; (3) there are dark protrusions out from the umbra which traverse much of the penumbra and which are characterized by more vertical magnetic field; (4) the observed structure of the magnetic field in both the umbra and penumbra is stable over the 30 minute duration of our observing sequence; (5) we do not find a tight correlation of field inclination with intensity in the penumbra; (6) at the outer edge of the penumbra, the field penetrates the photosphere at an average angle of about 70° from the vertical; (7) outside of the penumbra the sunspot field forms a canopy which is directly observed by our technique; (8) outside of, but close to the sunspot, there are small magnetic features of mixed polarity that are rather highly inclined to the vertical, presumably as influenced by the sunspot canopy; (9) away from the sunspot, the isolated magnetic elements have nearly vertical orientation. These observations suggest that magnetic buoyancy, in addition to thermal buoyancy, plays an important role in maintaining the observed structure in sunspot penumbra.

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