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A solar tornado observed by EIS: Plasma diagnostics

The term `solar tornadoes' has been used to describe apparently rotating magnetic structures above the solar limb, as seen in high resolution images and movies from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO). These often form part of the larger magnetic structure of a prominence, however the links between them remain unclear. Here we present plasma diagnostics on a tornado-like structure and its surroundings, seen above the limb by the Extreme-ultraviolet Imaging Spectrometer (EIS) aboard the Hinode satellite. We aim to extend our view of the velocity patterns seen in tornado-like structures with EIS to a wider range of temperatures and to provide insight into the physical characteristics of the plasma. Using Gaussian fitting to fit and de-blend the spectral lines seen by EIS, we calculated line-of-sight velocities and non-thermal line widths. Along with information from the CHIANTI database, we used line intensity ratios to calculate electron densities at each pixel. Using a regularised inversion code we also calculated the differential emission measure (DEM) at different locations in the prominence. The split Doppler-shift pattern is found to be visible down to a temperature of around log(T) = 6.0. At temperatures lower than this, the pattern is unclear in this data set. We obtain an electron density of log(n_e) = 8.5 when looking towards the centre of the tornado structure at a plasma temperature of log(T) = 6.2, as compared to the surroundings of the tornado structure where we find log(n_e) to be nearer 9. Non-thermal line widths show broader profiles at the tornado location when compared to the surrounding corona. We discuss the differential emission measure in both the tornado and the prominence body, which suggests that there is more contribution in the tornado at temperatures below log(T) = 6.0 than in the prominence.