  We observed 17 nearby low luminosity FR I radio galaxies using the NRAO Very
Long Baseline Array (VLBA) at 1.67 GHz, as part of a multi-wavelength study of
a complete sample of 21 sources selected by radio flux density from the Uppsala
General Catalogue of Galaxies. We detected radio emission from all 17 galaxies.
At a FWHM resolution of ~ 10x4 mas, five galaxies show only an unresolved radio
core, ten galaxies show core-jet structures, and two galaxies show twin-jet
structures. Comparing these VLBA images with images previously obtained with
the NRAO VLA, we find that all detected VLBA jets are well-aligned on parsec
scales with the VLA jets on kilo parsec scales, and that the jet to counter-jet
surface brightness ratios, or the sidedness, decreases systematically with
increasing distance along the jet. We attribute the sidedness to the Doppler
boosting effect and its decline to the deceleration of the jets. We show that a
distribution of Lorentz factor centered near Gamma = 5 can reproduce our VLBA
detection statistics for core, core-jet and twin-jet sources. We also note that
the luminosity per unit length, L_j, of the VLBA jets drops quickly with
distance, r, along the jet, approximately as L_j being proportional to
r^(-2.0). We discuss three different mechanisms to explain this jet fading: (1)
the decrease of Doppler boosting due to jet deceleration, (2) synchrotron
losses, and (3) expansion losses in constant velocity but adiabatically
spreading jets. Mechanisms (1) and (2) are inconsistent with the observations,
while mechanism (3) is consistent with the observations provided the magnetic
field lines in the jets are aligned perpendicular to the jet axis. This implies
that the deceleration of the jets required by the Unified Scheme does not occur
on the tens of parsec scales, but must occur on larger scales.
