Faraday Rotation in Pulsar Magnetosphere

TL;DR

This paper investigates the intrinsic Faraday rotation in pulsar magnetospheres, revealing it is negligible in symmetric plasmas but significant in asymmetric electron-ion plasmas, with a unique wavelength dependence and implications for pulsar emission observations.

Contribution

It provides a detailed numerical analysis of Faraday rotation effects in various plasma conditions within pulsar magnetospheres, highlighting the significance of plasma asymmetry.

Findings

Faraday rotation is negligible in symmetric electron-positron plasmas.

Significant Faraday rotation occurs in asymmetric electron-ion plasmas.

Rotation measure increases with frequency and differs for opposite magnetic poles.

Abstract

The magnetosphere of a pulsar is composed of relativistic plasmas streaming along the magnetic field lines and corotating with the pulsar. We study the intrinsic Faraday rotation in the pulsar magnetosphere by critically examining the wave modes and the variations of polarization properties for the circularly polarized natural modes under various assumptions about the magnetosphere plasma properties. Since it is difficult to describe analytically the Faraday rotation effect in such a plasma, we use numerical integrations to study the wave propagation effects in the corotating magnetosphere. Faraday rotation effect is identified among other propagation effects, such as wave mode coupling and the cyclotron absorption. In a highly symmetrical electron-positron pair plasma, the Faraday rotation effect is found to be negligible. Only for asymmetrical plasmas, such as the electron-ion streaming plasma, can the Faraday rotation effect become significant, and the Faraday rotation angle is found to be approximately proportional to $\lambda^{0.5}$ instead of the usual $\lambda^2$-law. For such electrons-ion plasma of pulsar magnetosphere, the induced rotation measure becomes larger at higher frequencies, and should have opposite signs for the emissions from opposite magnetic poles.