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We investigate the magnetic field in an intermediate-velocity filament forwhich the H$\alpha$ intensity in the WHAM survey correlates with excess Faradayrotation 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 relationbetween H-alpha emission measure and dispersion measure from Berkhuijsen et al.(2006). In view of the uncertainties in the derived magnetic field strength, wepropose an alternative use of the available data, rotation measure and emissionmeasure, to derive a lower limit to the Alfven speed, weighted by electrondensity n_e^1.5. We find lower limits to the Alfven speed that are comparableto, or larger than the sound speed in a 10^4 K plasma, and conclude that themagnetic field is dynamically important. We discuss the role ofintermediate-velocity gas as a locus of Faraday rotation in the interstellarmedium, and propose this lower limit to the Alfven speed may also be applicableto Faraday rotation by galaxy clusters.

Magnetic field strength in an intermediate-velocity ionized filament in the First Galactic Quadrant