Jet evolution in nearby FRI radio galaxies

We have observed 17 sources in a complete sample of FRIs in nearby UGC galaxies with the VLBA at 1.67 GHz. We detect core-jets in 60% and twin-jets in 10%. The data (alignment, sidedness ratios, detection statistics) are consistent with relativistic Doppler boosting effects. The luminosity per unit length of the jets fades with distance l from the core as L-j proportional to l(-2). The fading is not consistent with either deceleration of the jets (with subsequent reduction of Doppler boosting) or synchrotron losses. However, the fading can be explained by adiabatic losses in a constant velocity jet with constant opening angle and magnetic field perpendicular to the jet axis. If confirmed by VLBA polarimetry, this would imply that the jets must decelerate on scales larger than tens of parsec