  We present high spatial resolution spectropolarimetric observations of a
quiescent hedgerow prominence taken in the He I 1083.0 nm triplet. The
observation consisted of a time series in sit-and-stare mode of 36 minutes of
duration. The spectrograph's slit crossed the prominence body and we recorded
the time evolution of individual vertical threads. Eventually, we observed the
development of a dark Rayleigh-Taylor plume that propagated upward with a
velocity, projected onto the plane of the sky, of 17 km/s. Interestingly, the
plume apex collided with the prominence threads pushing them aside. We inferred
Doppler shifts, Doppler widths, and magnetic field strength variations by
interpreting the He I Stokes profiles with the HAZEL code. The Doppler shifts
show that clusters of threads move coherently while individual threads have
oscillatory patterns. Regarding the plume we found strong redshifts (9-12 km/s)
and large Doppler widths (10 km/s) at the plume apex when it passed through the
prominence body and before it disintegrated. We associate the redshifts with
perspective effects while the Doppler widths are more likely due to an increase
in the local temperature. No local variations of the magnetic field strength
associated with the passage of the plume were found; this leads us to conclude
that the plumes are no more magnetized than the surroundings. Finally, we found
that some of the threads oscillations are locally damped, what allowed us to
apply prominence seismology techniques to infer additional prominence physical
parameters.
