Spectral Characteristics of the He I D3 Line in a Quiescent Prominence Observed by THEMIS

We analyze the observations of a quiescent prominence acquired by the Telescope Heliographique pour l'Etude du Magnetisme et des Instabilites Solaires (THEMIS) in the He I 5876 A (He I D3) multiplet aiming to measure the spectral characteristics of the He I D3 profiles and to find for them an adequate fitting model. The component characteristics of the He I D3 Stokes I profiles are measured by the fitting system approximating them with a double Gaussian. This model yields an He I D3 component peak intensity ratio of $5.5\pm0.4$, which differs from the value of 8 expected in the optically thin limit. Most of the measured Doppler velocities lie in the interval $\pm5$ km/s, with a standard deviation of $\pm1.7$ km/s around the peak value of 0.4 km/s. The wide distribution of the full-width at half maximum has two maxima at 0.25 A and 0.30 A for the He I D3 blue component and two maxima at 0.22 A and 0.31 A for the red component. The width ratio of the components is $1.04\pm0.18$. We show that the double-Gaussian model systematically underestimates the blue wing intensities. To solve this problem, we invoke a two-temperature multi-Gaussian model, consisting of two double-Gaussians, which provides a better representation of He I D3 that is free of the wing intensity deficit. This model suggests temperatures of 11.5 kK and 91 kK, respectively, for the cool and the hot component of the target prominence. The cool and hot components of a typical He I D3 profile have component peak intensity ratios of 6.6 and 8, implying a prominence geometrical width of 17 Mm and an optical thickness of 0.3 for the cool component, while the optical thickness of the hot component is negligible. These prominence parameters seem to be realistic, suggesting the physical adequacy of the multi-Gaussian model with important implications for interpreting He I D3 spectropolarimetry by current inversion codes.Comment: 25 pages,1 movie, 10 figures, 2 tables, 2 equations. The final publication is available at Springer via http://dx.doi.org/10.1007/s11207-017-1118-z The supplementary movie is available for viewing and download at https://www.dropbox.com/s/7tskvnc593tlbyv/Prominence_HeID3_GONG_AIA.mpg?dl=

Structure of prominence legs: Plasma and magnetic field

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.Comment: 13 pages, 14 figures, one tabl