On magnetic field "reconstruction" (Research Note)

Context. Solanki and colleagues have presented intriguing 3D "reconstructions" of magnetic fields from the vector polarimetry of the He I 1083 nm multiplet. Aims. In this Research Note I re-examine the reconstruction technique used. Methods. Using a simple dipole field, I examine the reconstruction technique as applied to the theoretical fields. I assume that the He line forms in two locations, (1) along the magnetic loops and (2) in a horizontal plane. Results. The planar interpretation can account for all aspects of the data, but the loop interpretation has geometrical and physical problems. Conclusions. The data by themselves are not sufficient to determine which picture is more applicable. Nevertheless I argue that the planar interpretation makes more physical sense and that the early reconstructions lead to spurious results. I suggest additional tests that might help constrain the problem further.