2016ApJ...826..202S

We investigate the magnetic field in an intermediate-velocity filament for which the Hα intensity in the WHAM survey correlates with excess Faraday rotation of extragalactic radio sources over the length of the filament from b ≃ 20° to b ≃ 55°. The density-weighted mean magnetic field is 2.8 ±0.8 µG , derived from rotation measures and an empirical relation between Hα emission measure and dispersion measure from Berkhuijsen et al. 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}^{3/2} . 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 that this lower limit to the Alfven speed may also be applicable to Faraday rotation by galaxy clusters.