Measurement of a Magnetic Field in a Leading Arm High Velocity Cloud
  Using a recent catalogue of extragalactic Faraday rotation derived from the
NRAO VLA Sky Survey we have found an agreement between Faraday rotation
structure and the HI emission structure of a High Velocity Cloud (HVC)
associated with the Leading Arm of the Magellanic System. We suggest that this
morphological agreement is indicative of Faraday rotation through the HVC.
Under this assumption we have used 48 rotation measures through the HVC,
together with estimates of the electron column density from H-\alpha\
measurements and QSO absorption lines to estimate a strength for the
line-of-sight component of the coherent magnetic field in the HVC of <B_{||}> >
6 {\rm \mu G}$. A coherent magnetic field of this strength is more than
sufficient to dynamically stabilize the cloud against ram pressure stripping by
the Milky Way halo and may also provide thermal insulation for the cold cloud.
We estimate an upper limit to the ratio of random to coherent magnetic field of
$B_{r}/B_{||} < 0.8$, which suggests that the random field does not dominate
over the coherent field as it does in the Magellanic Clouds from which this HVC
likely originates.
