Anomalous persistence of bisymmetric magnetic structures in spiral galaxies

We discuss non-linear evolution of a self-excited bisymmetric magnetic field in a galactic disc with spiral arms. We assume that the level at which the dynamo saturates is modulated by the spiral pattern. Using asymptotic estimates and two-dimensional numerical simulations, we show that a bisymmetric magnetic field can have a lifetime in excess of 10-15 Gyr in a region around the corotation radius whose radial extent is a few kiloparsecs. The field is trapped by the spiral arms, i.e. it rotates rigidly together with the spiral pattern. Our numerical model roughly resembles the galaxy M51, and our results show a promising degree of agreement with the magnetic pattern observed in this galaxy.