Electromagnetic Mechanism for the Origin of Knots in Parsec Scale Jets

We consider the propagation and stability of the helical electromagnetic perturbations in a relativistic force-free Pointing flux dominated jet (magnetic helix). Perturbations are found to dissipate energy in the vicinity of the resonant surfaces inside the jet, where the resonance of the perturbation with the Alfven waves occurs. The energy of the perturbations is converted into the energy of accelerated particles, which emit synchrotron radiation. The intensity and polarization of this radiation is modulated with the perturbation. Perturbations with vanishing group velocity are the most important and could be associated with the bright knots in parsec scale jets.