Effect of Anisotropic Collisions on Solar Scattering Polarization

TL;DR

This paper explores how anisotropic collisions and magnetic fields can generate circular scattering polarization in the Sun's atmosphere, offering new diagnostic tools for solar and astrophysical plasmas.

Contribution

It demonstrates the mechanisms by which anisotropic collisions and magnetic fields produce circular scattering polarization, expanding the understanding of polarization phenomena in astrophysical environments.

Findings

Circular SP can be generated by anisotropic collisions in the presence of magnetic fields.

Conditions exist where linear SP is accompanied by circular SP.

Theoretical calculations of density matrix elements relate to circular SP presence.

Abstract

Scattering of anisotropic radiation by atoms, ions or molecules is sufficient to generate linear polarization observable in stars and planets atmospheres, circumstellar environments, and in particular in the Sun's atmosphere. This kind of polarization is called scattering polarization (SP) or second solar spectrum (SSS) if it is formed near the limb of the solar photosphere. Generation of linear SP can typically be reached more easily than circular SP. Interestingly, the later is often absent in observations and theories. Intrigued by this, we propose to demonstrate how circular SP can be created by anisotropic collisions if a magnetic field is present. We also demonstrate how anisotropic collisions can result in the creation of circular SP if the radiation field is anisotropic. We show that under certain conditions, linear SP creation is accompanied by the emergence of circular SP which can be useful for diagnostics of solar and astrophysical plasmas. We treat an example and calculate the density matrix elements of tensorial order $k\!=\!1$ which are directly associated with the presence of circular SP. This work should encourage theoretical and observational research to be increasingly oriented towards circular SP profiles in addition to linear SP in order to improve our analysis tools of astrophysical and solar observations.