Interstellar Polarization Survey. IV. Characterizing the magnetic field strength and turbulent dispersion using optical starlight polarization in the diffuse interstellar medium

TL;DR

This study uses optical polarization data to estimate magnetic field strength and turbulence in the diffuse interstellar medium, improving methods to understand magnetic field characteristics in low-density regions.

Contribution

It evaluates the applicability of angular dispersion functions for characterizing magnetic fields in low-density ISM, providing new estimates of magnetic field strength and turbulence.

Findings

Magnetic field strength varies from 1 to 9 microGauss depending on region and method.

The technique yields results consistent with previous local ISM studies.

Optical and thermal dust polarization reveal a highly regular magnetic field orientation.

Abstract

Angular dispersion functions are typically used to estimate the fluctuations in polarization angle around the mean magnetic field orientation in dense regions, such as molecular clouds. The technique provides accurate turbulent to regular magnetic field ratios, $\langle B_t^2\rangle^{1/2}/B_{pos}$, which are often underestimated by the classic Davis-Chandrasekhar-Fermi method. We assess the technique's suitability to characterize the turbulent and regular plane-of-sky magnetic field in low-density structures of the nearby interstellar medium (ISM), particularly when the turbulence outer scale, $\delta$, is smaller than the smallest scale observed, $\ell_{min}$. We use optical polarization maps of three intermediate-latitude fields ($|b| \gtrsim 7.\!\!^{\circ}5$) with dimensions of $0.\!\!^{\circ}3 \times 0.\!\!^{\circ}3$, sourced from the Interstellar Polarization Survey--General ISM (IPS-GI) catalog. We decomposed the HI emission detected by the Galactic All-Sky Survey (GASS) within our fields to estimate the multiphase ISM properties associated with the structure coupled to the magnetic field. We produced maps of the plane-of-sky magnetic field strength ($B_{pos}$), mass density ($\rho$), and turbulent velocity dispersion ($\sigma_{v,turb}$). In the regions with well-defined structures at $d<400$ pc, the average $B_{pos}$ ranges from ${\sim}3 \mu$G to ${\sim}9 \mu$G, depending on the method and physical properties. In the region where structures extend up to $1000$ pc, $B_{pos}$ varies from ${\sim}1 \mu$G to ${\sim}3 \mu$G. The results agree with previous estimations in the local, diffuse ISM. Finally, optical starlight polarization and thermal dust polarization at 353 GHz consistently reveal a highly regular plane-of-sky magnetic field orientation unfazed by diffuse dust structures observed at $12 \mu$m.