Wavefronts of the magnetic field in galaxies: asymptotic and numerical approaches

E. A. Mikhailov

Faculty of Physics, Lomonosov Moscow State University, bldg. 1, str. 2, Leninskie Gory, GSP-1, 119991 Moscow, Russia

Abstract
There are regular magnetic fields of several microgauss in the inner parts of spiral galaxies. Their evolution is described by the dynamo mechanism based on differential rotation and alpha-effect (which describes the turbulent motions of the interstellar medium). Such conditions take place in the outer parts, such as the periphery of the disk or outer rings. So a large-scale magnetic field can exist there. It is assumed that the galaxy disk is very thin, so the model for the field can be reduced to a system of two simple non-linear equations in partial derivatives. These equations describe the magnetic wavefront which is asosiated with the Kolmogorov-Petrovsky-Piskunov effect that is usual for parabolic differential equations in mathematical physics. The wavefront connects different stationary solutions of the system of equations. It has been shown that this front can pass with velocities proportional to the first or second power of the small parameter which characterizes turbulent diffusion. Results were obtained both by numerical modelling and by the asymptotic contrast structure theory. Figs 4, Refs 14.