VLBA and Chandra Observations of Jets in FRI radio galaxies: Constraints
  on Jet Evolution
  (ABRIDGED) We present here the results from new Very Long Baseline Array
observations at 1.6 and 5 GHz of 19 galaxies of a complete sample of 21 UGC FRI
radio galaxies. New Chandra data of two sources, viz., UGC00408 and UGC08433,
are combined with the Chandra archival data of 13 sources. The 5 GHz
observations of ten "core-jet" sources are polarization-sensitive, while the
1.6 GHz observations constitute second epoch total intensity observations of
nine "core-only" sources. Polarized emission is detected in the jets of seven
sources at 5 GHz, but the cores are essentially unpolarized, except in M87.
Polarization is detected at the jet edges in several sources, and the inferred
magnetic field is primarily aligned with the jet direction. This could be
indicative of magnetic field "shearing" due to jet-medium interaction, or the
presence of helical magnetic fields. The jet peak intensity $I_\nu$ falls with
distance $d$ from the core, following the relation, $I_\nu\propto d^a$, where
$a$ is typically -1.5. Assuming that adiabatic expansion losses are primarily
responsible for the jet intensity "dimming", two limiting cases are considered:
[1] the jet has a constant speed on parsec-scales and is expanding gradually
such that the jet radius $r\propto d^0.4$; this expansion is however
unobservable in the laterally unresolved jets at 5 GHz, and [2] the jet is
cylindrical and is accelerating on parsec-scales. Accelerating parsec-scale
jets are consistent with the phenomenon of "magnetic driving" in Poynting flux
dominated jets. Chandra observations of 15 UGC FRIs detect X-ray jets in nine
of them. The high frequency of occurrence of X-ray jets in this complete sample
suggests that they are a signature of a ubiquitous process in FRI jets.
