Magnetic double refraction
From Glossary of Meteorology
magnetic double refraction
(Or magnetic birefringence; also Voigt effect, Cotton–Mouton effect.) Double refraction (or linear birefringence) induced in a medium as a consequence of a magnetic field applied to it; discovered in 1902 by Voigt, who showed that light propagating in a vapor to which a strong, perpendicular magnetic field is applied exhibits double refraction.
The Cotton–Mouton effect (discovered in 1907) is the Voigt effect in a liquid. All double refraction results from anisotropy. Magnetic fields applied to otherwise isotropic media provide the anisotropy necessary for magnetic double refraction. Radio waves propagating in the ionosphere may exhibit magnetic double refraction as a consequence of the geomagnetic field.