The density-magnetic field relation in the atomic ISM

Abstract

We present numerical experiments aimed to study the correlation between the magnetic field strength, B, and the density, n, in the cold atomic interstellar medium (CNM). We analyse 24 magnetohydrodynamic models with different initial magnetic field intensities (B 0 = 0.4, 2.1, 4.2, and 8.3 μG) and/or mean densities (2, 3, and 4 cm -3 ), in the presence of driven and decaying turbulence, with and without self-gravity, in a cubic computational domain with 100 pc by side. Our main findings are as follows: (i) For forced simulations that reproduce the main observed physical conditions of the CNM in the solar neighbourhood, a positive correlation between B and n develops for all the B 0 values. (ii) The density at which this correlation becomes significant (≲30 cm -3 ) depends onB 0 but is not sensitive to the presence of self-gravity. (iii) The effect of self-gravity, when noticeable, consists of producing a shallower correlation at high densities, suggesting that, in the studied regime, self-gravity induces motions along the field lines. (iv) Self-gravitating decaying modelswhere theCNMis subsonic and sub- Alfvénic with β ≲ 1 develop a high-density positive correlation whose slopes are consistent with a constant β(n). (v) Decaying models where the low-density CNM is subsonic and sub- Alfvénic with β > 1 show a negative correlation at intermediate densities, followed by a high-density positive correlation.