Magnetic fields in our Galaxy: How much do we know? (II) Halo fields and the global field structure

I review the large scale global magnetic field structure of our Galaxy, using all information available for disk fields, halo fields and magnetic fields near the Galactic center (GC). Most of the knowledge was obtained from the rotation measures (RMs) of pulsars and extragalactic radio sources. In the local disk of our Galaxy, RM and dispersion measure (DM) data of nearby pulsars yield the strength of regular field as 1.8μG, with a pitch angle of about 8, and a bisymmetric spiral structure. There are at least four, maybe five, field reversals from the Norma arm to the outskirts of our Galaxy. The regular fields are probably stronger in the interarm regions. The directions of regular magnetic fields are coherent along spiral arms over more than 10kpc. The regular fields get stronger with decreasing radius from the GC. In the thick disk or Galactic halo, large scale toroidal magnetic fields, with opposite field directions in the Southern and Northern Galaxy, have been revealed by the antisymmetric RM sky towards the inner Galaxy. This signature of the A0 dynamo-mode field structure is strengthened by the indication of a poloidal field of dipole form, that is the transition of the RM signs probably shifted from l ∼ 0 to l ∼ +10. The local vertical field is probably a part of this dipole field. The field structure of the A0 dynamo-mode strikingly continues towards the region near the GC. We predict that the direction of the dipole fields near the GC should point towards the Southern pole. In short, the magnetic fields in the Galactic disk have a bisymmetric spiral structure of primordial nature, while in the halo and near the GC the A0 dynamo seems to dominate, so that the fields consist of toroidal fields with opposite directions below and above the Galactic plane and poloidal fields of dipole form.