  While the magnetic field in quiescent prominences has been widely
investigated, less is known about the field in activated prominences. We
introduce observational results on the magnetic field structure of an activated
filament in a flaring active region. We study, in particular, its magnetic
structure and line-of-sight flows during its early activated phase, shortly
before it displays signs of rotation. We invert the Stokes profiles of the
chromospheric He I 10830 A triplet and the photospheric Si I 10827 A line
observed in this filament by the VTT on Tenerife. Using these inversion results
we present and interpret the first maps of velocity and magnetic field obtained
in an activated filament, both in the photosphere and the chromosphere. Up to 5
different magnetic components are found in the chromospheric layers of the
filament, while outside the filament a single component is sufficient to
reproduce the observations. Magnetic components displaying an upflow are
preferentially located towards the centre of the filament, while the downflows
are concentrated along its periphery. Also, the upflowing gas is associated
with an opposite-polarity magnetic configuration with respect to the
photosphere, while the downflowing gas is associated with a same-polarity
configuration. The activated filament has a rather complex structure.
Nonetheless, it is compatible with a flux rope, although with a distorted one,
in the normal configuration. The observations are best explained by a rising
flux rope in which a part of the filament material is still stably stored
(upflowing material, rising with the field), while a part is no longer stably
stored and flows down along the field lines.
