EDP Sciences
Free access
Volume 488, Number 2, September III 2008
Page(s) 781 - 793
Section Numerical methods and codes
DOI http://dx.doi.org/10.1051/0004-6361:200809981
Published online 01 July 2008

A&A 488, 781-793 (2008)
DOI: 10.1051/0004-6361:200809981

A fast method for Stokes profile synthesis

Radiative transfer modeling for ZDI and Stokes profile inversion
T. A. Carroll, M. Kopf, and K. G. Strassmeier

Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
    e-mail: tcarroll@aip.de

Received 15 April 2008 / Accepted 12 June 2008

Context. The major challenges for a fully polarized radiative transfer driven approach to Zeeman-Doppler imaging are still the enormous computational requirements. In every cycle of the iterative interplay between the forward process (spectral synthesis) and the inverse process (derivative based optimization) the Stokes profile synthesis requires several thousand evaluations of the polarized radiative transfer equation for a given stellar surface model.
Aims. To cope with these computational demands and to allow for the incorporation of a full Stokes profile synthesis into Doppler- and Zeeman-Doppler imaging applications as well as into large scale solar Stokes profile inversions, we present a novel fast and accurate synthesis method for calculating local Stokes profiles.
Methods. Our approach is based on artificial neural network models, which we use to approximate the complex non-linear mapping between the most important atmospheric parameters and the corresponding Stokes profiles. A number of specialized artificial neural networks, are used to model the functional relation between the model atmosphere, magnetic field strength, field inclination, and field azimuth, on one hand and the individual components (I,Q,U,V) of the Stokes profiles, on the other hand.
Results. We performed an extensive statistical evaluation and show that our new approach yields accurate local as well as disk-integrated Stokes profiles over a wide range of atmospheric conditions. The mean rms errors for the Stokes I and V profiles are well below 0.2% compared to the exact numerical solution. Errors for Stokes Q and U are in the range of 1%. Our approach does not only offer an accurate approximation to the LTE polarized radiative transfer it, moreover, accelerates the synthesis by a factor of more than 1000.

Key words: line: formation -- line: profiles -- polarization -- radiative transfer -- stars: magnetic fields -- Sun: magnetic fields

© ESO 2008