Rotation Measure Variations across the Lobes of Extragalactic Radio Sources

It is shown through a high-resolution, three-dimensional smoothed particle hydrodynamics simulation of a transonic shear layer that large-scale nonlinear surface waves can form on the lobes of extragalactic radio sources through the successive merging of smaller waves generated by the Kelvin-Helmholtz instability. A theory is proposed relating these surface waves to substantial variations in rotation measure. The magnetic field is advected into the surrounding medium by an associated low level of turbulence, and changes direction on a scale approximately equal to the half-wavelength of the Kelvin-Helmholtz waves. The mixing of magnetic field from the lobe with the surrounding thermal plasma causes large positive and negative rotation measure excursions. The peak-to-peak variation in rotation measure is estimated and favorably compared with values in five regions of Cygnus A and in the northern lobe of a newly studied radio source PKS 2104 - 25N.