2022ApJ...927...49C

Understanding how the Galactic magnetic field threads the multiphase interstellar medium (ISM) remains a considerable challenge, as different magnetic field tracers probe dissimilar phases and field components. We search for evidence of a common magnetic field shared between the ionized and neutral ISM by comparing 1.4 GHz radio continuum polarization and H I line emission from the Galactic Arecibo L-Band Feed Array Continuum Transit Survey (GALFACTS) and Galactic Arecibo L-Band Feed Array H I (GALFA-H I) survey, respectively. We compute the polarization gradient of the continuum emission and search for associations with diffuse/translucent H I structures. The polarization gradient is sensitive to changes in the integrated product of the thermal electron density and line-of-sight field strength (B _||) in warm ionized gas, while narrow H I structures highlight the plane-of-sky field orientation in cold neutral gas. We identified one region in the high Galactic latitude Arecibo sky, G216+26 centered on (l, b) ∼ (216°, +26°), containing filaments in the polarization gradient that are aligned with narrow H I structures roughly parallel to the Galactic plane. We present a comparison of multiphase observations and magnetic field tracers of this region, demonstrating that the warm ionized and cold neutral media are connected likely via a common magnetic field. We quantify the physical properties of a polarization gradient filament associated with Hα emission, measuring a line-of-sight field strength B _|| = 6 ± 4 µG and a plasma beta β={2.1}_{-2.1}^{+3.1}. We discuss the lack of widespread multiphase magnetic field alignments and consider whether this region is associated with a short-timescale or physically rare phenomenon. This work highlights the utility of multitracer analyses for understanding the magnetized ISM.