Vacuum birefringence in a rotating magnetic field

We calculate the vacuum polarization-induced ellipticity acquired by a linearly polarized laser beam of angular frequency omega on traversing a region containing a transverse magnetic field rotating with a small angular velocity Omega around the beam axis. The transmitted beam contains the fundamental frequency omega and weak sidebands of frequency omega +/- 2 Omega, but no other sidebands. To first order in small quantities, the ellipticity acquired by the transmitted beam is independent of Omega, and is the same as would be calculated in the approximation of regarding the magnetic field as fixed at its instantaneous angular orientation, using the standard vacuum birefringence formulae for a static magnetic field. Also to first order, there is no rotation of the polarization plane of the transmitted beam. Analogous statements hold when the magnetic field strength is slowly varying in time.