  We investigate the properties of a `solar tornado' observed on 15 July 2014,
and aim to link the behaviour of the plasma to the internal magnetic field
structure of the associated prominence. We made multi-wavelength observations
with high spatial resolution and high cadence using SDO/AIA, the IRIS
spectrograph and the Hinode/SOT instrument. Along with spectropolarimetry
provided by the THEMIS telescope we have coverage of both optically thick
emission lines and magnetic field information. AIA reveals that the two legs of
the prominence are strongly absorbing structures which look like they are
rotating, or oscillating in the plane of the sky. The two prominence legs,
which are both very bright in Ca II (SOT), are not visible in the IRIS Mg II
slit-jaw images. This is explained by the large optical thickness of the
structures in Mg II which leads to reversed profiles, and hence to lower
integrated intensities at these locations than in the surroundings. Using lines
formed at temperatures lower than 1 MK, we measure relatively low Doppler
shifts on the order of +/- 10 km/s in the tornado-like structure. Between the
two legs we see loops in Mg II, with material flowing from one leg to the
other, as well as counterstreaming. It is difficult to interpret our data as
showing two rotating, vertical structures which are unrelated to the loops.
This kind of `tornado' scenario does not fit with our observations. The
magnetic field in the two legs of the prominence is found to be preferentially
horizontal.
