Abstract

In a standard theoretical picture of the formation and launching of astrophysical jets, the jets should acquire helical magnetic fields, due essentially to the combination of the rotation of the central black hole and accretion disk and the jet outflow. One way such helical fields may be manifest is through their tendency to give rise to gradients in the observed local Faraday rotation across the jet, due to the systematic variation of the line-of-sight component of the helical magnetic field across the jet. Indeed, transverse Faraday rotation gradients have been observed across a number of AGN jets on both parsec and kiloparsec scales, providing direct evidence that they carry toroidal or helical magnetic fields. The directions of the observed transverse Faraday rotation gradients provide statistical evidence for the operation of a cosmic “battery” in the accretion disks of AGN , which couples the direction of rotation and the direction of the poloidal (axial) field of the jet. If this “battery” is indeed operating, we have detected the presence of ordered currents and magnetic fields on a grand scale.