Comparing UV/EUV line parameters and magnetic field in a quiescent prominence with tornadoes

Context. Understanding the close relationship between the plasma and the magnetic field is important to describe and explain the observed complex dynamics of solar prominences. Aims. We determine if a close relationship between plasma and magnetic field parameters measured in a well-observed solar prominence with high spatial resolution can be found. Methods. We select a prominence observed on 15 July 2014 from space (IRIS, Hinode, SDO) and from the ground (THEMIS). We perform a robust co-alignment of the data sets using a 2D cross-correlation technique. We derive the magnetic field parameters from spectropolarimetric measurements of the He I D3 line taken by THEMIS. Line ratios and line-of-sight velocities from the Mg II h and k lines observed by IRIS are compared with magnetic field strength, inclination and azimuth. Electron densities are calculated using Hinode/EIS Fe XII line ratios and also compared with THEMIS and IRIS data. Results. We find Mg II k/h ratios of around 1.4 everywhere, similar to values found previously in prominences. We also find that the magnetic field is strongest (around 30 G) and predominantly horizontal in the tornado-like legs of the prominence. The k3 Doppler shift is found to be between +/- 10 km/s everywhere. Electron densities at a temperature of 1.5e6 K are found to be around 1e9 /cm3. No significant correlations are found between the magnetic field parameters, and any of the other plasma parameters inferred from EUV spectroscopy, which may be explained by the large differences in the temperatures of the lines used in this study. Conclusions. This is the first time that a detailed statistical study of plasma and magnetic field parameters has been carried out at high spatial resolution in a prominence. Our results provide important constraints on future models of the plasma and magnetic field in these structures.