Open questions on prominences from coordinated observations by IRIS, Hinode, SDO/AIA, THEMIS, and the Meudon/MSDP

Abstract

ContextResults. The magnetic field is found to be globally horizontal with a relatively weak field strength (8-15 Gauss). On the other hand, the Ca II movie reveals turbulent-like motion that is not organized in specific parts of the prominence. We tested the addition of a turbulent magnetic component. This model is compatible with the polarimetric observations at those places where the plasma turbulence peaks. On the other hand, the Mg II line profiles show multiple peaks well separated in wavelength. This is interpreted by the existence of small threads along the line of sight with a large dispersion of discrete values of Doppler shifts, from 5 km s −1 (a quasi-steady component) to 60-80 km s −1 . Each peak corresponds to a Gaussian profile, and not to a reversed profile as was expected by the present non-LTE radiative transfer modeling. This is a very surprising behavior for the Mg II line observed in prominences. Conclusions. Turbulent fields on top of the macroscopic horizontal component of the magnetic field supporting the prominence give rise to the complex dynamics of the plasma. The plasma with the high velocities (70 km s −1 to 100 km s −1 if we take into account the transverse velocities) may correspond to condensation of plasma along more or less horizontal threads of the arch-shape structure visible in 304 Å. The steady flows (5 km s −1 ) would correspond to a more quiescent plasma (cool and prominence-corona transition region) of the prominence packed into dips in horizontal magnetic field lines. The very weak secondary peaks in the Mg II profiles may reflect the turbulent nature of parts of the prominence.

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

“…One particular THEMIS spectrum at a given slit position is taken with the exposure time of 2 s, and overall, the observing procedure is the same as in Schmieder et al (2013Schmieder et al ( , 2014 and Levens et al (2016a,b). We emphasize that in our case a modulation cycle of six images, spanning the three polarization states with either positive or negative signs, is repeated ten times to increase the signal-to-noise ratio.…”

“…The primary reduction is the same as applied in Schmieder et al (2013Schmieder et al ( , 2014 and Levens et al (2016a,b), and it involves data from all ten modulation cycles. The raw THEMIS spectra are reduced using the IDL package DeepStokes, whose main characteristics are outlined in López .…”

“…The data reduction includes geometry corrections of inclination and curvature of spectral lines, dark current and bias subtraction, flat-field correction, and a careful handling of the polarization signals. Since the reduction yields Stokes Q, U, V profiles with very small amplitudes, typically 0.2 -0.4% of the Stokes I peak intensity, we repeat the reduction taking data only from five modulation cycles as in Schmieder et al (2013Schmieder et al ( , 2014 and Levens et al (2016a,b), but without improvement. Therefore, we aim the analysis only at the Stokes I profiles.…”

“…Schmieder et al (2013) also found mostly horizontal magnetic fields by He i D3 in a prominence pillar showing signatures of magnetosonic waves propagating transverse to the magnetic field with a velocity of about 10 km s −1 , a period of about 300 s, and a wavelength of around 2 000 km. The low signal-to-noise ratio of the He i D3 polarization signatures was interpreted in Schmieder et al (2014) as a possible manifestation of a turbulent field superimposed on the macroscopic horizontal component of the prominence magnetic field. The results of these recent studies made use of He i D3 spectropolarimetry obtained by the Téléscope Heliographique pour l'Étude du Magnetisme et des Instabilités Solaires (THEMIS) solar telescope.…”

Prospects of Solar Magnetometry—From Ground and in Space

“…Due to its low instrumental polarization, THEMIS is well-suited for high-precision polarimetric studies. Studies have focused on magnetic fields in prominences (e.g., López Ariste et al 2006;Schmieder et al 2014) and the second solar spectrum (Faurobert & Arnaud 2003;Faurobert et al 2009). López Ariste et al (2012) investigated the scattering polarization of Na in the exosphere of Mercury, which could be used in the future to study its magnetic field.…”