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The idea that systematic Faraday Rotation gradients across the parsec-scalejets of AGNs can reveal the presence of helical magnetic (B) fields has beenaround since the early 1990s. These gradients are taken to be due to thesystematic variation of the line of sight-B-field across the jet. We presenthere the parsec-scale Faraday Rotation distributions for the BL Lac objects0716+714 and 1749+701, based on polarization data obtained with the Very LongBaseline Array (VLBA) at two wavelengths near each of the 2cm, 4cm and 6cmbands (0716+714) and at four wavelengths in the range 18-22 cm (1749+701). TheRotation Measure (RM) maps for both these sources indicate systematic gradientsacross their jets, as expected if these jets have helical B fields. Thesignificance of these transverse RM gradients is > 3 sigma in all cases. Wepresent the results of Monte Carlo simulations directly demonstrating thepossibility of observing such transverse RM gradients even if the intrinsic jetstructure is much narrower than the observing beam. We observe an intriguingnew feature in these sources, a reversal in the direction of the gradient inthe jet as compared to the gradient in the core region. This provides newevidence to support models in which field lines emerging from the centralregion of the accretion disk and closing in the outer region of the accretiondisk are both "wound up" by the differential rotation of the disk. The netobserved RM gradient will essentially be the sum effect of two regions ofhelical field, one nested inside the other. The direction of the net RMgradient will be determined by whether the inner or outer helix dominates theRM integrated through the jet, and RM gradient reversals will be observed ifthe inner and outer helical fields dominate in different regions of the jet.This potentially provides new insights about the overall configuration of thejet B fields.

Connecting Magnetic Towers with Faraday Rotation Gradients in Active Galactic Nuclei Jets