Zeeman-Doppler imaging (ZDI or tomographic magnetic imaging) is the use of a spectropolarimeter to image the magnetic field of a star, making use of the Zeeman effect. It is essentially Doppler imaging (extracting details of the star's rotation from the effect of Doppler shifts on spectral lines) extended to make use of the observed Zeeman effect, which is the division of what would be a single emission line into multiple lines from atoms within a magnetic field. Or it can be the complimentary effect on absorption lines. The magnetic field interacts with orbiting electrons, slightly affecting the atom's energy levels. The polarization of the resulting lines are also affected, so by measuring both the polarity and the wavelength of lines from a stellar atmosphere, the star's magnetic field can be reconstructed through tomography. Instruments currently used for this:
The Bcool project uses the technique.