TITLE: Signatures of hot plasma in a quiescent prominence observed in the
He I D3 line

AUTHORS: Július Koza, Ján Rybák, Peter Gömöry, Matúš Kozák

ABSTRACT:

We present results of analysis of the He I 5876 A (He I D3) spectral line
profiles observed in a quiescent prominence by the THEMIS solar telescope.
Spectral characteristics of components of He I D3 Stokes I profiles are
measured by the fitting system approximating them with a double Gaussian.
This model yields the He I D3 component peak intensity ratio of 5.5+/-0.4,
which differs from the value of eight expected in the optically-thin
limit. We show that the double-Gaussian model underestimates
systematically the He I D3 blue wing intensities. We also test the
double-Lorentzian and the double-Voigt model showing that the former
produces unsatis-factory fits and the latter is physically invalid. To
solve this problem, we invoke a two-temperature multi-Gaussian model,
consisting of two double-Gaussians, which provides a satis-fying
representation of He I D3 free of the wing intensity deficit. This model
suggests tempera-tures of 11.5 kK and 91 kK respectively for the cool and
the hot component of the target prom-inence. Remarkably, these
temperatures agree very well with the central and boundary tempera-tures
of 10 kK and 100 kK for the 2D model of prominence fine structure applied
earlier in ra-diative transfer modeling of hydrogen Lyman lines by, e.g.,
Gunar et al. 2007 (Astronomy & Astrophysics 472, 929) and Schwartz et al.
2015 (Astronomy & Astrophysics 577, A92). This may be evidence of the
physical adequacy of the multi-Gaussian model and even another evi-dence
of hot plasma existing in the target prominence. The cool and hot
components of the typ-ical 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 one 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, which still assume
one-component isothermal model of a prominence slab. Therefore the
multi-Gaussian model should be considered for advanced diagnostics of this
line in solar prominences.