  We study the magnetic field evolution of an isolated spiral galaxy, using
isolated Milky Way-mass galaxy formation simulations and a novel prescription
for magnetohydrodynamic (MHD) supernova feedback. Our main result is that a
galactic dynamo can be seeded and driven by supernova explosions, resulting in
magnetic fields whose strength and morphology is consistent with observations.
In our model, supernovae supply thermal energy, and a low level magnetic field
along with their ejecta. The thermal expansion drives turbulence, which serves
a dual role by efficiently mixing the magnetic field into the interstellar
medium, and amplifying it by means of turbulent dynamo. The computational
prescription for MHD supernova feedback has been implemented within the
publicly available ENZO code, and is fully described in this paper. This
improves upon ENZO's existing modules for hydrodynamic feedback from stars and
active galaxies. We find that the field attains $\mu G$-levels over Gyr-time
scales throughout the disk. The field also develops large-scale structure,
which appears to be correlated with the disk's spiral arm density structure. We
find that seeding of the galactic dynamo by supernova ejecta predicts a
persistent correlation between gas metallicity and magnetic field strength. We
also generate all-sky maps of the Faraday rotation measure from the
simulation-predicted magnetic field, and present a direct comparison with
observations.
