7.4 Faraday depolarization
Radiation of wavelength λ which starts off linearly polarized in a particular direction and travels through a plasma has its direction of polarization rotated by an angle
where ne is the electron density, B| | is the component of the magnetic field which is parallel to the direction in which the wave is travelling, and x is the distance along the line of propagation. This is known as Faraday rotation, or the Faraday effect. The integral is known as the rotation measure.
Radiation from an extended synchrotron source travelling through an ionized medium will have a rotation measure which depends on the electron density and magnetic field along the propagation path. The rotation measure will, therefore, vary for radiation emitted from different locations within an extended source. Consequently, even if the radiation were initially very highly linearly polarized, the different values of rotation measure for various emission locations, will reduce the observed polarization fraction. This effect is known as Faraday depolarization.