Interstellar Polarization Survey. IV. Characterizing the Magnetic Field Strength and Turbulent Dispersion Using Optical Starlight Polarization in the Diffuse Interstellar Medium

Angular dispersion functions are typically used to estimate the fluctuationsin polarization angle around the mean magnetic field orientation in denseregions, such as molecular clouds. The technique provides accurate turbulent toregular magnetic field ratios, ⟨ B_t^2⟩^1/2/B_pos, which areoften underestimated by the classic Davis-Chandrasekhar-Fermi method. We assessthe technique's suitability to characterize the turbulent and regularplane-of-sky magnetic field in low-density structures of the nearbyinterstellar medium (ISM), particularly when the turbulence outer scale,δ, is smaller than the smallest scale observed, ℓ_min. We useoptical polarization maps of three intermediate-latitude fields (|b| ≳7.^∘5) with dimensions of 0.^∘3 × 0.^∘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 associatedwith the structure coupled to the magnetic field. We produced maps of theplane-of-sky magnetic field strength (B_pos), mass density (ρ), andturbulent velocity dispersion (σ_v,turb). In the regions withwell-defined structures at d<400 pc, the average B_pos ranges from∼3 μG to ∼9 μG, depending on the method and physicalproperties. In the region where structures extend up to 1000 pc, B_posvaries from ∼1 μG to ∼3 μG. The results agree with previousestimations in the local, diffuse ISM. Finally, optical starlight polarizationand thermal dust polarization at 353 GHz consistently reveal a highly regularplane-of-sky magnetic field orientation unfazed by diffuse dust structuresobserved at 12 μm.