On the amplification of large-scale magnetic fields in spiral galaxies by galactic winds

Abstract

A new mechanism is presented for the amplification of nonaxisymmetric large-scale magnetic fields in the disks of spiral galaxies. Using a local theory, we show how a galactic wind with components axial and radial to the disk can drive magnetic field growth. The wind model postulates a large-scale plasma flow in which the axial component varies with radial position across the disk, and the radial component varies with axial position above the disk. The radial dependence of the axial wind component induces axial magnetic field from the radial field. The axial dependence of the radial wind component induces radial magnetic field from the axial field. These two inductive processes permit regeneration by coupling the evolution of the field components. Azimuthal field is induced from radial field by differential rotation, according to the usual mechanism. The shear in the large-scale wind leads to indefinite field amplification in situ (regenerative) without introducing a mean helicity of small-scale plasma motions (the α-effect), as is done in most other theories of galactic magnetic field generation. The model is nevertheless formulated within the framework of turbulent magnetohydrodynamics since turbulent diffusion of magnetic field is an important feature. Results from a kinematic, local (one-dimensional) numerical model are presented which illustrate the time evolution and axial spatial structure of large-scale magnetic field for various wind motions. The physical essence of wind-driven magnetic field amplification is demonstrated by a "Lotka-Volterra" spatially integrated model that highlights the means by which coupling between field components mediates their time evolution. Show more