Doppler Imaging of stellar magnetic fields - II. Numerical experiments

We present numerical experiments designed to evaluate the performance of the new Magnetic Doppler Imaging (MDI) code INVERS10. Numerous test runs demonstrate what can be achieved with the MDI method and allow assessment of the systematic errors that can cause distortions of the maps. Our numerical experiments showed that given high-resolution observations in four Stokes parameters the code is capable of reconstructing abundance and magnetic field vector distributions simultaneously and without any prior assumptions about the magnetic field geometry. At the same time we found that in order to achieve reliable reconstruction using only circular polarization data it is necessary to impose additional constraints on the possible structure of the magnetic field. Numerical tests also reveal surprisingly different properties of the MDI maps from conventional scalar maps which we attribute to the complex relation between the orientation of the field and the polarization signal. We conclude that the information about the field is primarily extracted from the changes in magnetic orientation due to rotation rather than from the Doppler shifts critical for scalar Doppler Imaging maps making it possible to apply the MDI to even very slow rotators.