Magnetic flux accumulation at the Galactic center and its implications forthe strength of the pregalactic magnetic field

We study the inflow of disk gas toward the Galactic center during the lifet ime of the Galaxy and its effect on magnetic field lines frozen-in to the i nterstellar plasma. While compression leads to a large amplification of the "vertical" magnetic field (pointing perpendicular to the disk), ambipolar diffusion efficiently removes from the disk magnetic flux components orient ed parallel to the Galactic plane. Turbulent interchange motions of nearly parallel vertical field lines at the Galactic center enhance the efficiency of magnetic reconnection of neighboring regions of oppositely directed ver tical held. This suggests that the sign of the present-day vertical field a t the Galactic center is uniform. If the Galactic-center held originates in the entrainment of a pregalactic field B-0 in radially inflowing interstel lar plasma, then observations of the vertical flux through the central 200 pc of our Galaxy yield a measure of the pregalactic field that depends on t he total mass accreted into the central 200 pc during the Galaxy's lifetime . If this mass is 3 x 10(9) M. and if the surface density of disk gas is ro ughly constant over the lifetime of the Galaxy, then B-0 greater than or si milar to 2 x 10(-7) G, regardless of the angle of the pregalactic field wit h respect to the Galactic plane. The abundance of mechanisms for radial acc retion of disk gas suggests that strong magnetic fields should be a generic feature of the centers of spiral galaxies. We also note that cosmic-ray co nfinement in the strong vertical held at the Galactic center is expected to be poor.