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 \approx 20 \arcdeg to b \approx 55 \arcdeg .
The density-weighted mean magnetic field is 2.8 \pm 0.8 \mu G , 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 Alfvén speed , weighted by electron density n _ { e } ^ { 3 / 2 } .
We find lower limits to the Alfvén 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 Alfvén speed may also be applicable to Faraday rotation by galaxy clusters .