2025ApJ...981..164G

The possibility of generating a magnetic field by a dynamo effect with anisotropic electrical conductivity rather than turbulent flow has been demonstrated theoretically and experimentally. If the electrical conductivity is anisotropic, the electrical currents will flow preferentially in certain directions rather than others, and a simple differential rotation will suffice to generate a magnetic field. In a galaxy with spiral arms, it is reasonable to assume that the electrical conductivity will be twice as large along the arms than in the perpendicular direction, suggesting the possibility of an anisotropic dynamo. However, a further geometrical criterion must be satisfied to obtain a dynamo. It is given by ${{\rm{\Omega }}}^{{\prime} }\cdot \sin p\gt 0$, where p is the pitch angle of the spiral arms, with $p\in [-\frac{\pi }{2},\frac{\pi }{2}]$, and ${{\rm{\Omega }}}^{{\prime} }$ is the radial shear of the angular velocity. We find that the usual spiral-arm galaxies, which satisfy $| {{\rm{\Omega }}}^{{\prime} }| \lt 0$, do not satisfy this dynamo condition because they have trailing arms instead of leading arms. Even galaxy NGC 4622, which has both trailing and leading arms, does not satisfy this dynamo condition either. This is confirmed by numerical simulations of the induction equation. Thus, for all spiral-arm galaxies known to date, the anisotropy of the spiral arms cannot explain the existence of galactic magnetic fields until further notice.