Hydrogen in the strong magnetic field of the white dwarf PG 1031+234

According to the classical Zeeman effect, the π-components of Zeeman split lines are linearly polarized but not circularly polarized at all. However, in the magnetic white dwarf PG 1031+234, the absorption features formed by stationary (dλ/dB=0) π-transitions of Zeeman split hydrogen lines are, with respect to the ambient continuum, strongly circularly polarized. This is explained in terms of an opacity effect in a magnetic field geometry which differs clearly from a centered dipole and in connection with a continuum polarization where the ratio of the linear to circular polarization depends not only on the angle between the line of sight and the field vector, but on the field strength, too