  In this paper we study the feasibility of inferring the magnetic field from
polarized multi-line spectra using two methods: The pseudo line approach and
The PCA-ZDI approach. We use multi-line techniques, meaning that all the lines
of a stellar spectrum contribute to obtain a polarization signature. The use of
multiple lines dramatically increases the signal to noise ratio of these
polarizations signatures. Using one technique, the pseudo-line approach, we
construct the pseudo-line as the mean profile of all the individual lines. The
other technique, the PCA-ZDI approach proposed recently by Semel et al. (2006)
for the detection of polarized signals, combines Principle Components Analysis
(PCA) and the Zeeman Do ppler Imaging technique (ZDI). This new method has a
main advantage: the polarized signature is extracted using cross correlations
between the stellar spectra nd functions containing the polarization properties
of each line. These functions are the principal components of a database of
synthetic spectra. The synthesis of the spectra of the database are obtained
using the radiative transfer equations in LTE. The profiles built with the
PCA-ZDI technique are denominated Multi-Zeeman-Signatures. The construction of
the pseudo line as well as the Multi-Zeeman-Signatures is a powerful tool in
the study of stellar and solar magnetic fields. The information of the physical
parameters that governs the line formation is contained in the final polarized
profiles. In particular, using inversion codes, we have shown that the magnetic
field vector can be properly inferred with both approaches despite the magnetic
field regime.
