The Magnetic Field of the Milky Way

There is strong evidence that the Milky Way contains an ordered, large-scale magnetic field. The field configuration has been explored mostly by modeling pulsar (Faraday) rotation and dispersion measures. The ratio of a rotation to dispersion measure gives the integral of the magnetic field along the line of sight to the pulsar, weighted by electron density. Different researchers have reached the conclusion that, in the plane of the Galaxy, the magnetic field is predominantly azimuthal and it experiences reversals at different galacto-centric radii. However, even though astronomers work with essentially the same catalog of pulsar, they do not agree on whether the configuration is best described by a spiral or circular pattern.

The "face-on" view of the Milky Way shown at the left is only a schematic model of a possible configuration of the magnetic field. The locations of the Sun and the Galactic Center (GC) are indicated on the model. The arrows indicate the direction of the magnetic field, assumed to lie along spiral arms. Models like this one need to be compared to real measurements in order to evaluate how faithfully they conform to the actual magnetic field.

A new method of modeling the magnetic field of the Milky Way uses Zeeman splitting of hydroxyl (OH) masers in regions of star formation. Zeeman splitting of atomic or molecular lines indicates the direction and magnitude of the line of sight component of the magnetic field in situ in a cloud of gas. Thus, Zeeman splitting and rotation measure data provide complementary information. Recently, a large survey of OH masers was completed with the Very Large Array, and many magnetic field directions have been determined. These data will help constrain models of the magnetic field of the Milky Way.

(References: Han and Qiao 1994, Astron. Astroph., 288, 759; Reid and Silverstein 1990, ApJ, 361, 483; Reid 2000, to appear in Joint Discussion 14 of the IAU 2000)

Further details can be found in a preprint of my recent review talk on

  • "The Magnetic Field of the Milky Way" (postscript: 60 kB)
  • from Joint Discussion 14 at the International Astronomical Union General Assembly held in Manchester, UK in August 2000