A New Probe of Line-of-sight Magnetic Field Tangling

The Galactic neutral hydrogen (HI) sky at high Galactic latitudes is suffused with linear structure. Particularly prominent in narrow spectral intervals, these linear HI features are well aligned with the plane-of-sky magnetic field orientation as measured with optical starlight polarization and polarized thermal dust emission. We analyze the coherence of the orientation of these features with respect to line-of-sight velocity, and propose a new metric to quantify this HI coherence. We show that HI coherence is linearly correlated with the polarization fraction of 353 GHz dust emission. HI coherence constitutes a novel method for measuring the degree of magnetic field tangling along the line of sight in the diffuse interstellar medium. We propose applications of this property for HI-based models of the polarized dust emission in diffuse regions, and for studies of frequency decorrelation in the polarized dust foreground to the cosmic microwave background.