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.
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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.
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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