Fast inversion of Zeeman line profiles using central moments - II. Stokes V moments and determination of vector magnetic fields

摘要

Context. In the case of unresolved solar structures or stray light contamination, inversion techniques using four Stokes parameters of Zeeman profiles cannot disentangle the combined contributions of magnetic and nonmagnetic areas to the observed Stokes I . Aims. In the framework of a two-component model atmosphere with filling factor f , we propose an inversion method restricting input data to Q , U, and V profiles, thus overcoming ambiguities from stray light and spatial mixing. Methods. The V -moments inversion (VMI) method uses shifts S _( V ) derived from moments of V -profiles and integrals of Q ~(2) , U ~(2) , and V ~(2) to determine the strength B and inclination ψ of a magnetic field vector through least-squares polynomial fits and with very few iterations. Moment calculations are optimized to reduce data noise effects. To specify the model atmosphere of the magnetic component, an additional parameter δ , deduced from the shape of V -profiles, is used to interpolate between expansions corresponding to two basic models. Results. We perform inversions of HINODE SOT/SP data for inclination ranges 0 & ψ & 60° and 120 & ψ & 180° for the 630.2 nm Fe? i line. A damping coefficient is fitted to take instrumental line broadening into account. We estimate errors from data noise. Magnetic field strengths and inclinations deduced from VMI inversion are compared with results from the inversion codes UNNOFIT and MERLIN. Conclusions. The VMI inversion method is insensitive to the dependence of Stokes I profiles on the thermodynamic structure in nonmagnetic areas. In the range of Bf products larger than 200 G, mean field strengths exceed 1000 G and there is not a very significant departure from the UNNOFIT results because of differences between magnetic and nonmagnetic model atmospheres. Further improvements might include additional parameters deduced from the shape of Stokes V profiles and from large sets of 3D-MHD simulations, especially for unresolved magnetic flux tubes.