A QUANTITATIVE COMPARISON OF VECTOR MAGNETIC-FIELD MEASUREMENT AND ANALYSIS TECHNIQUES

We make a quantitative comparison between spectral vs filter measureme nt and analysis techniques for extraction of solar vector magnetic fie lds from polarimetric data using as a basis the accurately calibrated, high angular resolution Stokes profile data from the Advanced Stokes Polarimeter. It is shown that filter-based measurements deliver qualit ative images of the field alignment for sunspots that are visually sim ilar to images derived from the more detailed analysis of the Stokes p rofiles. However, quantitative comparison with least-squares fits to t he full Stokes profiles show that both the strength of the field predi cted by the filter-based analysis and its orientation contain substant ial errors. These errors are largest for plage regions outside of suns pots, where the field strengths are inferred to be only a fraction of their true values, and errors in the orientation of 40-50 degrees are common. Within sunspots, errors of 20 degrees are commonplace. The gre atest source of these errors is the inability of the filter-based meas urements to account for the small fill fraction of magnetic fields or, equivalently, scattered light in the instrument, which reduce the deg ree of polarization. The uncertainties of the full profile fitting met hods are also discussed, along with the errors introduced by coarser w avelength sampling of the observed Stokes profiles. The least-squares fitting procedure operates best when the profiles are sampled at least as frequently as one Doppler width of the line.