The Use of Faraday Rotation Sign Maps as a Diagnostic for Helical Jet
  Magnetic Fields
  We present maps of the sign of the Faraday Rotation measure [sign(RM)]
obtained from multi-frequency radio observations on the Very Long Baseline
Array (VLBA). Many of the Active Galactic Nuclei (AGN) considered have B-field
structures with a central "spine" of B-field orthogonal to the jet and/or a
longitudinal B-field near one or both edges of the jet. This structure can
plausibly be interpreted as being caused by a helical/toroidal jet magnetic
field. Faraday Rotation is a rotation of the plane of polarization that occurs
when the polarized radiation passes through a magnetized plasma. The sign of
the RM is determined by the direction of the line-of-sight B-Field in the
region causing the Faraday Rotation, and an ordered toroidal or helical
magnetic field associated with an AGN jet will thus produce a distinctive
bilateral distribution of positive and negative RMs across the jet. We present
and discuss sign(RM) maps and their possible interpretation regarding the
magnetic field geometries for several sources.
