Magnetic field strength in an intermediate-velocity ionized filament in
  the First Galactic Quadrant
  We investigate the magnetic field in an intermediate-velocity filament for
which the H$\alpha$ intensity in the WHAM survey correlates with excess Faraday
rotation of extragalactic radio sources over the length of the filament from b
~ 20 degr. to b ~ 55 degr. The density-weighted mean magnetic field is 2.8 +/-
0.8 microgauss, derived from rotation measures and an empirical relation
between H-alpha emission measure and dispersion measure from Berkhuijsen et al.
(2006). In view of the uncertainties in the derived magnetic field strength, we
propose an alternative use of the available data, rotation measure and emission
measure, to derive a lower limit to the Alfven speed, weighted by electron
density n_e^1.5. We find lower limits to the Alfven speed that are comparable
to, or larger than the sound speed in a 10^4 K plasma, and conclude that the
magnetic field is dynamically important. We discuss the role of
intermediate-velocity gas as a locus of Faraday rotation in the interstellar
medium, and propose this lower limit to the Alfven speed may also be applicable
to Faraday rotation by galaxy clusters.
