Comparisons of measured and calculated potential magnetic fields

Photospheric line-of-sight and transverse magnetic field data obtained with the Marshall Space Flight Center vector magnetograph system for an isolated sunspot are described. A study of the linear polarization patterns and of the calculated transverse field lines indicates that the magnetic field of the region is very nearly potential. The Hα fibril structures of this region as seen in high resolution photographs corroborate this conclusion. Consequently, a potential field calculation is described using the measured line-of-sight fields together with assumed Neumann boundary conditions; both as necessary and sufficient for a unique solution. The computed transverse fields are then compared with the measured transverse fields to verify the potential field model and assumed boundary values. The implications of these comparisons on the validity of magnetic field extrapolations using potential theory are discussed. (Auth.)