The Role of Magnetic Forces in Sunspot Equilibrium

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Abstract
Detailed measurements have been made to determine the configuration of the magnetic field over a large sunspot, using the Oxford 35-m solar telescope and the 11-m spectroscope. The heliographic direction of the magnetic vector and the field strength have been derived, for more than 200 points specified by solar latitude and longitude, from measurements of Zeeman split lines in the λ 5250 Å region. Treanor's method has been used to obtain the magnetic field directions with an estimated precision, generally, of a few degrees. The sight-line observations are fully corrected for the effects of telescope polarization before being used to obtain heliographic directions. The field strength observations show that the isogauss lines, in this somewhat irregular spot, run for the most part parallel to the umbra–penumbra boundary. The magnetic field lines in stable regions of the spot are found to be untwisted and lie in planes perpendicular to the isogauss lines. The horizontal forces in stable regions of sunspots are briefly discussed using the ‘magnetic line curvature ’approach developed by Jakimiec. The results obtained from our precise measurements imply that the magnetic field, at least in the penumbra, cannot be force-free, and some estimate is made of the magnetic field contribution to the horizontal forces between the umbra and the photosphere.

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