A New Constraint on the Relative Disorder of Magnetic Fields between Neutral Interstellar Medium Phases

Abstract Utilizing Planck polarized dust emission maps at 353 GHz and large-area maps of the neutral hydrogen (H i ) cold neutral medium (CNM) fraction ( f CNM ), we investigate the relationship between dust polarization fraction ( p 353 ) and f CNM in the diffuse high latitude ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close="∣" open="∣"> <mml:mi>b</mml:mi> </mml:mfenced> <mml:mo>&gt;</mml:mo> <mml:mn>30</mml:mn> <mml:mo>°</mml:mo> </mml:math> ) sky. We find that the correlation between p 353 and f CNM is qualitatively distinct from the p 353 –H i column density ( N H i ) relationship. At low column densities ( N H i &lt; 4 × 10 20 cm −2 ) where p 353 and N H i are uncorrelated, there is a strong positive p 353 – f CNM correlation. We fit the p 353 – f CNM correlation with data-driven models to constrain the degree of magnetic field disorder between phases along the line of sight. We argue that an increased magnetic field disorder in the warm neutral medium (WNM) relative to the CNM best explains the positive p 353 – f CNM correlation in diffuse regions. Modeling the CNM-associated dust column as being maximally polarized, with a polarization fraction p CNM ∼ 0.2, we find that the best-fit mean polarization fraction in the WNM-associated dust column is 0.22 p CNM . The model further suggests that a significant f CNM -correlated fraction of the non-CNM column (an additional 18.4% of the H i mass on average) is also more magnetically ordered, and we speculate that the additional column is associated with the unstable medium. Our results constitute a new large-area constraint on the average relative disorder of magnetic fields between the neutral phases of the interstellar medium, and are consistent with the physical picture of a more magnetically aligned CNM column forming out of a disordered WNM.