Multiwavelength spectroscopic observations of a quiescent prominence

TL;DR

This study combines coordinated space- and ground-based spectroscopy (IRIS, SUMER, HSFA2, and SDO/AIA) to reconstruct the three-dimensional velocity field and thermodynamic state of a quiescent solar prominence. By matching multi-line observations (H$\alpha$, Ca II H, Mg II h&k, Ly$\alpha$) with a grid of NLTE radiative-transfer models (PRODOP), the authors derive electron and hydrogen densities, temperatures, and ionization degrees across 23 co-spatial regions, and demonstrate that a macro-velocity broadening of about $20\,\mathrm{km\,s^{-1}}$ is needed to fit observed profiles. They find POS speeds up to $\sim20\,\mathrm{km\,s^{-1}}$ and LOS speeds exceeding $90\,\mathrm{km\,s^{-1}}$, with densities in the $10^{9}-10^{10}\ \mathrm{cm^{-3}}$ range and temperatures from $7\times10^{3}$ to $1.4\times10^{4}$ K, while hydrogen ionization degrees span $0.40-0.91$. The results support a dynamic, multi-thread prominence plasma not confined to magnetic dips and highlight the importance of NLTE, multi-line inversions for constraining prominence thermodynamics and dynamics.

Abstract

In this paper we focus on the analysis of the multiwavelength spectroscopic observations of a quiescent prominence. The spectral and geometrical parameters in the prominence were derived and used to constrain the NLTE radiative transfer models. Applying this method with multiwavelength observations provides a good opportunity to reduce the large range of thermodynamic parameters in solar prominences. We used time-slice and optical flow methods in order to derive the plane-of-sky (POS) velocities, and used gravity center and peak position methods on Mg II h&k and H I Ly-alpha profiles to compute the line-of-sight (LOS) velocities. We used the integrated intensities and FWHM values of the H-alpha, Ca II H, and Mg II h&k lines to compare with the NLTE radiative transfer computations. Ionization degree and thickness of the prominence plasma could be further derived. Opposite flows are observed along two strands between prominence barbs. The POS velocity can reach 20 km/s and the largest LOS velocity is > 90 km/s. The derived electron densities range from 6.5e9 cm-3 to 2.7e10 cm-3, and the derived total hydrogen densities range from 7.4e9 cm-3 to 6.6e10 cm-3. The temperature ranges from 7 000 to 14 000 K. The ionization degree of hydrogen is in the range of 0.40 to 0.91. The comparison between averaged and modeled profiles of Mg II and Ly-alpha lines shows that macro-velocities of 15 km/s and 20 km/s are required, respectively. The bulk motions among prominence barbs indicate that the prominence plasma is not confined within magnetic dips but exhibits a large-scale behavior. The presence of high-speed cool plasma flows, along with a wide range of plasma densities and temperatures, suggests that the prominence plasma is far from thermodynamic equilibrium and is inherently dynamic in nature.

Figures (18)

• Figure 1: Example of IRIS observations. Left: IRIS SJI in $2796\,\mathrm{\AA}$. The dark line in the center corresponds to the position of the spectrograph slit, and the dotted cyan lines mark the two extreme positions of the slit. Right: Mg ii h&k spectra taken along the slit in (a). Both the two images are shown in logarithmic scale.
• Figure 2: Coalignment between the IRIS and SUMER observations. The dash-dotted line in the left panel in white represents the position of the SUMER slit. The right panel shows how the normalized integrated intensity changes along the slit. The blue line corresponds to the intensity of the pixels of the IRIS SJI along the white line from the bottom to the top. The black line corresponds to the Ly$\alpha$ integrated intensity at 09:13:33 UT along the SUMER slit.
• Figure 3: Example of HSFA2 observations. Left: HSFA2 SJI in H$\alpha$. The vertical line in white corresponds to the position of the spectrograph slit, the two vertical cyan lines mark the two extreme positions of the slit, and two horizontal lines are used for the coalignment of the SJI and spectra. Right: H$\alpha$ and Ca ii H spectra taken along the vertical slit in the SJI with scattered light removed. SJI and spectra are coaligned in the vertical direction.
• Figure 4: Example of the scatter light subtraction process for H$\alpha$ and Ca ii H spectra taken with the HSFA2 spectrograph. Black line: Raw prominence spectrum. Brown line: Scattered light spectrum. Red line: Normalized scatter light spectrum. Green line: Prominence spectrum after scattered light subtraction. Blue line: Fit Gaussian profile. Dashed line: Wavelength limits taken to scattered light normalization.
• Figure 5: Images of AIA $304\,\mathrm{\AA}$ and IRIS $2796\,\mathrm{\AA}$ at 09:13 UT. The inner and outer prominence boundaries and the limb position in H$\alpha$ observed by HSFA2 at 09:22 UT are marked in blue.