Friday, 22 October 2010

The Galactic Center Magnetosphere

Mark Morris
Department of Physics & Astronomy, University of California, Los Angeles, CA 90095-1547,

Abstract. The magnetic field within a few hundred parsecs of the center of the Galaxy is
an essential component of any description of that region. The field has several pronounced
observational manifestations: 1) morphological structures such as nonthermal radio filaments
(NTFs) – magnetic flux tubes illuminated by synchrotron emission from relativistic electrons
– and a remarkable, large-scale, helically wound structure, 2) relatively strong polarization of
thermal dust emission from molecular clouds, presumably resulting from magnetic alignment of
the rotating dust grains, and 3) synchrotron emission from cosmic rays. Because most of the
NTFs are roughly perpendicular to the Galactic plane, the implied large-scale geometry of the
magnetic field is dipolar. Estimates of the mean field strength vary from tens of microgauss to
∼ a milligauss. The merits and weaknesses of the various estimations are discussed here. If the
field strength is comparable to a milligauss, then the magnetic field is able to exert a strong
influence on the dynamics of molecular clouds, on the collimation of a Galactic wind, and on
the lifetimes and bulk motions of relativistic particles. Related to the question of field strength
is the question of whether the field is pervasive throughout the central zone of the Galaxy, or
whether its manifestations are predominantly localized phenomena. Current evidence favors the
pervasive model.