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One of the tools that can provide evidence about the existence of helicalmagnetic fields in AGN is the observation of rotation measure gradients acrossthe jet. Such observations have been previously made successfully, proving thatsuch gradients are far from being rare, but common and typically persistentover several years, although some of them may show a reversal in the directionalong the jet. Further studies of rotation measure gradients can help us in ourunderstanding of the magnetic field properties and structure in the base of thejets. We studied Very Long Baseline Array (VLBA) polarimetric observations of 8sources consistent of some quasars and BL Lacs at 12, 15, 22, 24 and 43 GHz andwe find that all but two sources show indications of rotation measuregradients, either parallel or perpendicular to the jet. We interpret gradientsperpendicular to the jet as indications of the change of the line of sight ofthe magnetic field due to its helicity, and gradients parallel to the jet asthe decrease of magnetic field strength and/or electron density as we movealong the jet. When comparing our results with the literature, we findtentative evidence of a rotation measure gradient flip, which can be explainedas a change of the pitch angle or jet bending.

Probing Helical Magnetic Fields in AGN by Rotation Measure Gradients Studies