Analysis of the polarized emission from radio jets from Active Galactic Nuclei (AGN) is a powerful tool for investigating their magnetic (B) field structure. The radio emission from these jets is synchrotron radiation originating from relativistic charged particles accelerated by B-fields. Synchrotron radiation is intrinsically polarized and has a theoretical upper limit to its polarized fraction of ∼70% for a uniform B-field and a random pitch-angle distribution for the radiating electrons [
1]. The polarization angle,
, can be used to estimate the direction of the B-field projected on the plane of the sky. The line-of-sight B-field however can also be investigated using Faraday rotation, the change in the angle of linear polarization as the electromagnetic wave passes through a region with free electrons and magnetic field. The ‘rotation’ is caused by a delay between the left and right circularly polarized components of the electromagnetic wave. The rotation measure (RM) is defined as follows:
where
denotes the polarization angle,
is the intrinsic polarization angle,
is the magnetic field,
is a path element along the line of sight,
is the electron density, and
is the observing wavelength; by fitting
versus
the fitted slope and intercept on the
axis give the RM and intrinsic polarization angle,
. A gradient in the RM transverse to the jet indicates a similar trend in the line-of-sight magnetic field. Such a gradient is a strong indicator for a toroidal magnetic field component, which may be confining the jet. The direction of this gradient also gives information regarding the direction of the toroidal B-field component, which, in turn implies a direction for the associated electrical current in the jet using the right hand rule from basic physics. This publication focuses on the larger scale RM gradients using observations made with the VLA. Reliable transverse RM gradients on kiloparsec scale have previously only been detected in 2 other extragalactic radio sources [
2,
3]; our detection of significant RM gradients in Coma A and 3C 465 adds substantially to these results. The method by which the gradients considered here were analysed is identical to the method used in [
2,
3,
4,
5]. The significance of an RM gradient was calculated using its end point values and associated errors. This is a conservative estimate as the error in the RM increases at the edges of the jet as there is less polarized intensity. The significance was calculated by dividing the total change in the RM by the errors at the end points of the gradients added in quadrature.