Faraday rotation variations along radio jets: the magnetic field in   galaxy and group halos
  Our modelling of FR I radio jets as decelerating, relativistic flows allows us to derive their orientations accurately. We present images of Faraday rotation for two of these these objects (3C 31 and NGC 315) and show that the fluctuations of rotation measure (RM) are larger in the fainter (receding) jets, as expected if the rotation occurs in the hot galaxy/group halos. The gas density is much lower in NGC 315 and the RM fluctuations are only just detectable. 
R. A. Laing (1), J. R. Canvin (2), W. D. Cotton (3), A. H. Bridle (3),   P. Parma (4) ((1) ESO, (2) University of Sydney, (3...