We analyze multi-frequency radio observations obtained on the Very Long Baseline Array (VLBA) of selected AGN, that seem to have a B-field structure with a central "spine" of B-field orthogonal to the jet and a longitudinal B-field near one or both edges of the jet. Two explanations for this structure have been discussed in the literature: shocks making the central orthogonal field combined with a jet-medium interaction making a "sheath" of longitudinal field, or both components produced by a helical magnetic field. We will discuss evidence from our observations supporting the latter of these two pictures.
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