Polarization of Light: Astrophysical Processes & Mechanisms

[RJ Emery]

What astrophysical processes can result in the polarization of light, and by what mechanisms is that accomplished? (By light, I mean any form of EM waves, from gamma to radio.)

 

[Nereid]

Scattering, synchrotron emission, Kerr rotation, ... no doubt there are more as well.

 

[kocour]

There are many sources of polarization in the universe. Roughly speaking, photons polarization emerges whenever any kind of anisotropy in matter-radiation interaction appears. It is easier to find an answer to the question of what processes do not lead to the light polarization: You need an ideal spherical symmetry of the interaction to avoid light polarization. Even in this case, photons are polarized, but the total polarization is zero due to cancellations of large number of observed photons (distant spherical star free of magnetic fields would produce unpolarized radiation).
Basic processes which lead to polarization include elastic scattering on free electrons (atoms, molecules, ...) (the polarization state of a scattered photon is always changed unless it scatters in the forward/backward direction), inelastic scattering on atoms. In the presence of magnetic or electric fields the polarization state can be modified in a very complicated manner and it can be used for detailed diagnostics of the state of the distant object - Zeeman and Hanle effect. Another phenomenon is so-called impact atomic polarization (polarization state of atoms is changed by anisotropical collisions with charged perturbers hence the emitted radiation is also polarized), bremsstrahlung (in a general sense as a result of charged particle acceleration)

 

[RJ Emery]

Nereid and kocour,

I greatly appreciate the listing of the various processes and their elaboration.