Measurement of Interstellar Magnetization by Synchrotron Polarization Variance

TL;DR

This paper introduces a new method using polarization intensity variance to measure the magnetization of the Galactic interstellar medium, confirming its effectiveness and applying it to survey data to reveal turbulence properties.

Contribution

The study proposes a novel polarization variance technique for ISM magnetization measurement and establishes a power-law relation with the Alfvénic Mach number, validated through comparison and application to survey data.

Findings

ISM turbulence is predominantly sub-Alfvénic with Mach numbers between 0.5 and 1.0.

The polarization intensity variance correlates with the Alfvénic Mach number as a power law.

The method reliably estimates magnetic turbulence properties from polarization data.

Abstract

Since synchrotron polarization fluctuations are related to the fundamental properties of the magnetic field, we propose the polarization intensity variance to measure the Galactic interstellar medium (ISM) magnetization. We confirm the method's applicability by comparing it with the polarization angle dispersion and its reliability by measuring the underlying Alfv\'enic Mach number of MHD turbulence. With the finding of the power-law relation of $\mathcal{A} \propto M_{\rm A}^{2}$ between polarization intensity variance $\mathcal{A}$ and Alfv\'enic Mach number $M_{\rm A}$, we apply the new technique to the Canadian Galactic Plane Survey (CGPS) data, achieving Alfv\'enic Mach number of the Galactic ISM. Our results show that the low-latitude Galactic ISM is dominated by sub-Alf\'enic turbulence, with $M_{\rm A}$ approximately between 0.5 and 1.0.