Regular magnetic fields in coronae of spiral galaxies

Abstract

THE idea that large-scale ordered magnetic fields might reside in the gaseous coronae of spiral galaxies came first from theoretical arguments1 that such fields could be generated in situ by turbulent hydromagnetic dynamos. Here we put this general argument into practice. The partially ionized gas in galactic coronae is in chaotic motion, both turbulent and convective, and owing to galactic rota-tion acquires a non-zero mean helicity. Differential rotation and this helicity generate a magnetic field with predicted magnitude ∼1 νG and with a scale comparable to that of the corona. This field is nearly independent of the disk magnetic field: the two have different symmetry with respect to the galactic equator, the disk field being even and the corona field odd. Consequently, a galactic-scale neutral sheet arises ∼ 1 kpc above one side of the disk. At heights of about 3–5 kpc, the poloidal magnetic field component dominates the corona, a result consistent with observations of polarization in the galaxy NGC4631.