  The neutral hydrogen structure of high-velocity cloud A0 (at about -180 km/s)
has been mapped with a 9.1 arcmin resolution. Gaussian decomposition of the
profiles is used to separately map families of components defined by
similarities in center velocities and line widths. About 70% of the HI gas is
in the form of a narrow, twisted filament whose typical line widths are of
order 24 km/s. Many bright features with narrow line widths of order 6 km/s,
clouds, are located in and near the filament. A third category with properties
between those of the filament and clouds appears in the data. The clouds are
not always co-located with the broader line width filament emission as seen
projected on the sky. Under the assumption that magnetic fields underlie the
presence of the filament, a theorem is developed for its stability in terms of
a toroidal magnetic field generated by the flow of gas along field lines. It is
suggested that the axial magnetic field strength may be derived from the excess
line width of the HI emission over and above that due to kinetic temperature by
invoking the role of Alfven waves that create what is in essence a form of
magnetic turbulence. At a distance of 200 pc the axial and the derived toroidal
magnetic field strengths in the filament are then about 6 microGauss while for
the clouds they are about 4 microGauss. The dependence of the derived field
strength on distance is discussed.
