  We developed theory of particle acceleration inside the relativistic rotating
electron-positron force-free jet with spiral magnetic field. We considered
perturbation of stationary magnetic field structure and found that acceleration
takes place in the regions where the Alfven resonant condition with the
eigenmodes in the jet is fulfilled. Acceleration process and synchrotron losses
combined together form power law energy spectrum of ultrarelativistic electrons
and positrons with index between 2 and 3 depending upon initial energy of
injected particles. Synchrotron emission of these electrons and positrons in
spiral magnetic field of rotating force-free jet has been calculated.
Polarization properties of the radiation has been obtained and compared with
existing VLBI polarization measurements of parsec-scale jets in BL Lac sources
and quasars. Our results give explanation of observed bimodality in alignment
between electric field vector and jet axis. Degree of polarization and velocity
of observed proper motion of bright knots depend upon angular rotational
velocity of the jet. Thus, comparing them to each other, we can estimate
angular rotational velocity in jets. We obtain that the fact, that generally in
BL Lac objects electric field vector is oriented parallel to the jet axis while
in quasars perpendicular to the jet axis, may be due to intrinsically larger
angular rotational velocity and large winding of magnetic field in BL Lac jets
than in quasar jets.
