Astronomy& Astrophysicsmanuscript no. aa ' ESO 2024 May 15, 2024 The physical mechanism behind magnetic eld alignment in interstellar clouds Guido Granda-Muæoz 1; 2 ,Enrique VÆzquez-Semadeni 1 and Gilberto C. Gmez 1 1 Instituto de Radioastronoma y Astrofsica, Universidad Nacional Autnoma de MØxico, Apdo. Postal 3-72, Morelia, MichoacÆn 58089, MØxico e-mail:e.vazquez@irya.unam.mx,g.gomez@irya.unam.mx 2 Departamento de Ciencias, Facultad de Artes Liberales, Universidad Adolfo IbÆæez, Av. Padre Hurtado 750, Viæa del Mar, Chile e-mail:guido.granda@edu.uai.cl Received XXX; accepted YYY ABSTRACT Context. A tight correlation between interstellar clouds contours and their local magnetic eld orientation has been widely observed. However, the physical mechanisms responsible for this correlation remain unclear. Aims. We investigate the alignment mechanism between the magnetic eld and interstellar clouds. Methods. We perform three- and two-dimensional MHD simulations of warm gas streams in the thermally-bistable atomic interstellar medium (ISM) colliding with velocities of the order of the velocity dispersion in the ISM. In these simulations, we follow the evolution of magnetic eld lines, identify and elucidate the physical processes causing their evolution. Results. The collision produces a fast MHD shock, and a condensation front roughly one cooling length behind it, on each side of the collision front. A cold dense layer forms behind the condensation front, onto which the gas settles, decelerating smoothly. We nd that the magnetic eld lines, initially oriented parallel to the ow direction, are perturbed by the fast MHD shock, across which the magnetic eld uctuations parallel to the shock front are amplied. The downstream perturbations of the magnetic eld lines are further amplied by thecompressivedownstream velocity gradient between the shock and the condensation front caused by the settlement of the gas onto the dense layer. This mechanism causes the magnetic eld to become increasingly parallel to the dense layer, and the development of a shear ow around the latter. Furthermore, the bending-mode perturbations on the dense layer are amplied by the non-linear thin-shell instability (NTSI), stretching the density structures formed by the thermal instability, and rendering them parallel to the bent eld lines. By extension, we suggest that a tidal stretchingvelocity gradient such as that produced in gas infalling into a self-gravitating structure must straighten the eld lines along the accretion ow, orienting them perpendicular to the density structures. We also nd that the upstream superalfvØnic regime transitions to a transalfvØnic regime between the shock and the condensation front, and then to a subalfvØnic regime inside the condensations. Finally, in two-dimensional simulations with a curved collision front, the presence of the magnetic eld inhibits the generation of turbulence by the shear around the dense layer. Conclusions. Our results provide a feasible physical mechanism for the observed transition from parallel to perpendicular relative orientation of the magnetic eld and the density structures as the density structures become increasingly dominated by self-gravity. Key words. Magnetic elds ISM: clouds ISM: magnetic elds ISM: kinematics and dynamics 1. Introduction Studying the role of magnetic elds in the formation and evo- lution of atomic and molecular clouds (MCs) has been an im- portant research topic for both observational and theoretical as- tronomy. Magnetic elds are thought to be an important ingre- dient in the dynamics of the ISM, providing a possible support mechanism against gravitational collapse, and guiding the gas ow in the surroundings of lamentary structures, among many other e ects. In addition, a tight correlation between the ori- entation of the magnetic eld and cold atomic clouds (CACs) has been identied in the last decade. For example, Clark et al. (2015) found that the plane of the sky magnetic eld, measured using polarized thermal dust emission, is aligned with atomic hydrogen structures detected with HI emission and Clark et al. (2014) also observed a similar alignment in HI bers , which are thin long dense structures identied in HI emission using the Rolling Hough transform. In addition, Planck Collabora- tion et al. (2016a) found that the plane of the sky magnetic eld, detected using polarized thermal dust emission, is aligned with molecular structures traced by dust, and Planck Collabo- ration et al. (2016b) found that the relative orientation of the projected magnetic eld and dust laments changes from par- allel to perpendicular when sampling higher density regions in nearby MCs. Skalidis et al. (2022) studied the role of the mag- netic eld in the transition between HI H 2 using multiple trac- ers to investigate the gas properties of Ursa Minor. They found that turbulence is transalfvØnic and that the gas probably accu- mulates along magnetic eld lines and generates overdensities where molecular gas can form. However, the origin of these alignments remains unclear, as most of the observational evidence refers to spatial and orien- tation correlations without a clear understanding of the causal- ity involved. Therefore, it is crucial to understand the interplay between the precursors of molecular clouds (MCs) and mag- netic elds. Since CACs are thought to constitute the primordial place for the early stages of the evolution of molecular clouds and, eventually, star-forming regions (e.g., Koyama & Inutsuka 2002; Audit & Hennebelle 2005; VÆzquez-Semadeni et al. 2006; Heitsch et al. 2006; Heiner et al. 2015; Seifried et al. 2017), un- Article number, page 1 of 11 arXiv:2405.08702v1 [astro-ph.GA] 14 May 2024