Numerical models of the galactic dynamo driven by supernovae and superbubbles

Abstract

We calculate the temporal evolution and spatial structure of the large-scale magnetic field in our Galaxy, in the framework of an axisymmetric SN-driven dynamo model. We consider various parameter regimes, allowing for anisotropies in the dynamo parameters, the existence of an effective vertical escape of the field (analogous to a Galactic wind carrying field lines away from the midplane), vertical variations in the Galactic rotation curve... In the linear regime, axisymmetric (m = 0) modes are always easier to excite than bisymmetric (m = 1) modes. Amongst the former, the even (SO) mode often has the larger growth rate, while the odd (AO) mode generally oscillates more readily. Under typical conditions, the SO and AO modes have very similar properties; both grow monotonically with time at an exponential rate similar or equal to 0.45 Gyr(-1), which suggests that the Galactic magnetic field has presently reached a state close to saturation. In the absence of vertical escape, the magnetic field oscillates and only its AO component is amplified. Oscillatory behaviors are also found when the azimuthal alpha-parameter is enhanced by at least a factor of 3 or when the magnetic diffusivities are reduced by a factor > 1.7 with respect to their reference values; in both cases, the switch from monotonous to oscillatory behavior is accompanied by an increase in the growth rate. A height-dependence in the Galactic rotation velocity profoundly modifies the magnetic field morphology and is conducive to oscillatory decay. The nonlinear solutions obtained when the dynamo parameters are forced to decrease with increasing magnetic field strength are generally more spread out in space. For the growing modes, the field amplification saturates when its intensity in the peak region reaches similar to 20 mu G, corresponding to a magnetic pressure of roughly four times the local gas pressure. The time to saturation, which depends on the seed field strength adopted, is typically of the order of a few 10 Gyr. Nonlinear mode interactions may produce long-term changes both in the even vs. odd parity and in the monotonous vs. oscillatory temporal behavior of the large-scale magnetic field.