Solar Magnetic Field Studies Using the 12 Micron Emission Lines. III. Simultaneous Measurements at 12 and 1.6 Microns

We report total magnetic field strength measurements made simultaneously at two heights in the solar atmosphere using two infrared lines: the upper photospheric 12.32 μm line of Mg I and the lower photospheric 1.56 μm line of Fe I. We used the dual outputs of the McMath-Pierce Fourier transform spectrometer to obtain high spectral resolution data for both lines using a single input aperture, eliminating spatial registration errors. The results show a decrease of field strength with height in sunspots and plages, demonstrating field divergence between the lower and upper photosphere. Sunspot penumbral field strengths decrease by 12% between the two heights, while plages show decreases as large as a factor of 3. The plage results are consistent with thin-flux-tube models, provided that merging of flux tubes occurs. However, the sunspot results imply height gradients in total field strength that are large (2 G km -1 ) at the inner edge of the penumbra and decrease to smaller values (0.5 G km -1 ) at the outer penumbral edge. The magnitude of these gradients and their radial variation imply that sunspots do not conform to the "similarity assumption" commonly invoked in classic analytic models of sunspot magnetostatic structure.