  We investigate the magnetic field configuration of the M87 jet at arc-second
scales by using archival polarimetric VLA data at 8, 15, 22 and 43 GHz. By
stacking images over three years in order to enhance the sensitivity, we
reveal, for the first time, systematic transverse gradients of the Faraday
rotation measure in several knots along the jet. Combining this result with
polarization properties and the dynamics of the jet, we suggest the magnetic
structure in several knots at kiloparsec scales consists of a systematically
wrapped, tightly wound helical configuration. Our analysis brings us a new
paradigm where the M87 jet is a fundamentally current carrying system produced
in the vicinity of the supermassive black hole, transferring a huge amount of
the electromagnetic energy over the host galaxy scale.
