3D-NUMERICAL SIMULATIONS OF THE MAGNETIC-FIELD EVOLUTION IN A TURBULENT INTERSTELLAR GAS

A fully three-dimensional kinematic model is applied to simulate the e volution of the magnetic field in a small (120 pc in size) volume of t he turbulent intersteller medium in the presence of the field diffusio n. Turbulent motions are approximated by a sequence of non-overlapping in time vortices having the form of rotating gas columns much longer than the parcel size. A combined action of vertical vortices and those inclined at the angle of 30-degrees to the galactic plane is consider ed. Due to the Coriolis force and density stratification their motions have a non-zero helicity. The magnetic field is found to evolve towar ds the structure resembling a fragment of a twisted flux tube, well-or dered over the volume analyzed. Its sense of twisting is determined by the vortex helicity. Possible observational consequences of the prese nce of twisted magnetic tubes in galactic disks are studied, as well. Helical fields are found to cause a specific asymmetry of the polarize d emission, similar to that observed in polarization maps of galaxies with large inclination angles (e.g. M31).