Anisotropic Structure of Synchrotron Polarization

Abstract

Abstract Since polarized synchrotron intensity arising from magnetized turbulence is anisotropic along the direction of mean magnetic field, it can be used to trace the direction of the mean magnetic field. In this paper, we provide a statistical description of anisotropy of polarized synchrotron intensity. We use a second-order structure function and a new statistics, quadrupole moment (QM), at different wavelengths. The second-order structure function visualizes anisotropy of polarized intensity (PI) along the direction of mean magnetic field. Using QM, we quantify the degree of anisotropy displayed in the structure function. Since Faraday rotation, which depends on wavelength, can change the structure of PI and result in depolarization, we study how the anisotropic structure changes with wavelength. First, we consider polarized synchrotron emission arising from one spatial region, in which synchrotron emission and Faraday rotation occur simultaneously. Second, we also consider polarized synchrotron emission from two spatially separated regions. When the wavelength is very small, the observed polarization exhibits the averaged structures of both foreground and background regions. As the wavelength increases and Faraday rotation becomes important, depolarization wipes out large-scale structures, while small-scale anisotropy begins to reflect that of the foreground region, where the depolarization effect has relatively weak influence.