Magnetic Fields in the Milky Way and Other Spiral Galaxies

The average strength of the total magnetic field in the Milky Way, derived from radio synchrotron data under the energy equipartition assumption, is 6μG locally and ≃ 10μG at 3 kpc Galactic radius. Optical and synchrotron polarization data yield a strength of the local regular field of ≃ 4μG (an upper limit if anisotropic fields are present), while pulsar rotation measures give ≃ 1.5μG (a lower limit if small-scale fluctuations in regular field strength and in thermal electron density are anticorrelated). In spiral arms of external galaxies, the total [regular] field strength is up to ≃ 35μG [≃ 15μG]. In nuclear starburst regions the total field reaches ≃ 50μG. – – Little is known about the global field structure in the Milky Way. The local regular field may be part of a “magnetic arm” between the optical arms, a feature that is known from other spiral galaxies. Unlike external galaxies, rotation measure data indicate several global field reversals in the Milky Way, but some of these could be due to field distortions. The Galaxy is surrounded by a thick radio disk of similar extent as around many edge-on spiral galaxies. While the regular field of the local disk is of even symmetry with respect to the plane (quadrupole), the regular field in the inner Galaxy or in the halo may be of dipole type. The Galactic center region hosts highly regular fields of up to milligauss strength which are oriented perpendicular to the plane.