Faraday rotation

Faraday rotation is the rotation of a wave’s linear polarization as it travels through a magnetized medium. In plasma, the rotation depends on the magnetic field along the path and the wave’s wavelength.

Circular birefringence: A difference in propagation speed or refractive index between left- and right-circularly polarized waves. Its unequal phase accumulation turns an initially linear polarization into a rotated one.
Rotation measure: The coefficient relating polarization-angle rotation to the square of wavelength in a Faraday-thin source. It summarizes the wavelength-dependent rotation measured in many radio observations.
Linear polarization: An electromagnetic wave polarization state in which the electric field oscillates along a fixed direction. Faraday rotation changes this direction while ideally preserving the wave’s linear state.
Cotton–Mouton effect: Magnetic-field-induced linear birefringence in which the polarization eigenmodes are aligned with and across the field. It can change polarization differently from Faraday rotation, especially when the field is transverse.
Plasma frequency: The natural oscillation frequency of electrons displaced from equilibrium in a plasma. It helps determine how a plasma’s refractive response varies with radio frequency.
Rotation measure synthesis: A method that reconstructs polarized emission as a function of Faraday depth from broadband measurements. It separates complex Faraday-rotating structures that a single rotation measure cannot describe.
Plasma: An ionized gas containing mobile charged particles and collective electromagnetic behavior. Its free electrons and ions provide the medium through which radio waves acquire rotation.
Plasma dispersion: The frequency-dependent propagation delay of electromagnetic waves in a plasma. It delays pulses rather than rotating their polarization angle.
Electron cyclotron frequency: The angular frequency at which an electron gyrates around a magnetic field line. It sets the magnetic scale behind the plasma’s unequal response to circular polarizations.
Pulsar: A rotating neutron star whose beams of radiation can be observed as periodic pulses. Pulsar polarization and dispersion measurements constrain magnetic fields in intervening plasma.

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