P. Schwartz

Properties of prominence fine-structure threads derived from SOHO/SUMER hydrogen Lyman lines

Context. The SOHO/SUMER observations provide us for the first time with the prominence spec tra in the Lyman-� line outside the attenuator together with the higher members of the hydrogen Lyman series. Aims. We derive the prominence fine-structure thread properties by compar ing the SOHO/SUMER hydrogen Lyman series observations with the synthetic Lyman lines. Methods. To obtain the synthetic profiles of the Lyman lines, we use 2D prominence fin e-structure thread models with a PCTR and consistently solved the 2D non-LTE multilevel radiative transfer. The trial-and-erro r method was applied to find the model with the best agreement between the synthetic Lyman line profiles and the observed ones. Results. The properties of the resulting model with the best match of the synthetic and observed line profiles are central (minimum) temperature T0 = 7000 K, maximum column mass in the centre of the thread M0 = 1.1× 10 −4 g cm −2 , horizontal field strength in the middle of the thread Bx(0) = 6 Gauss and the boundary pressure p0 = 0.015 dyn cm −2 . Conclusions. The Lyman line profiles observed by SOHO/SUMER can be better reproduced by using multi-thread models consisting of a set of the 2D prominence fine-structure threads placed perpendicularly to th e line-of-sight, rather than with the single-thread model.

Multi-wavelength study of a high-latitude EUV filament

A large filament was observed during a multi-wavelength coordinated campaign on June 19, 1998 in the Hα line with the Swedish Vacuum Solar Telescope (SVST) at La Palma, in the coronal lines Fe ix/x 171 Å and Fe xi 195 Å with the Transition Region and Coronal Explorer (TRACE) and in EUV lines with the SOHO/CDS spectrometer and the hydrogen Lyman series with the SOHO/SUMER spectrometer. Because of its high-latitude location, it is possible to disentangle the physical properties of the Hα filament and the filament channel seen in EUV lines. TRACE images point out a dark region fitting the Hα fine-structure threads and a dark corridor (filament channel), well extended south of the magnetic inversion line. A similar pattern is observed in the CDS EUV-line images. The opacity of the hydrogen and helium resonance continua at 171 Å is almost two orders of magnitude lower than that at the Hi head (912 Å) and thus similar to the opacity of the Hα line. Since we do not see the filament channel in Hα, this would imply that it should also be invisible in TRACE lines. Thus, the diffuse dark corridor is interpreted as due to the coronal ‘volume blocking’ by a cool plasma which extends to large altitudes. Such extensions were also confirmed by computing the heights from the projection geometry and by simulations of the CDS and TRACE line intensities using the spectroscopic model of EUV filaments (Heinzel, Anzer, and Schmieder, 2003). Finally, our NLTE analysis of selected hydrogen Lyman lines observed by SUMER also leads to a conclusion that the dark filament channel is due to a presence of relatively cool plasma having low densities and being distributed at altitudes reaching the Hα filament.

2D radiative-magnetohydrostatic model of a prominence observed by Hinode, SoHO/SUMER and Meudon/MSDP

Aims. Prominences observed by Hinode show very dynamical and intriguing structures. To understand the mechanisms that are responsible for these moving structures, it is important to know the physical conditions that prevail in fine-structure threads. In the present work we analyse a quiescent prominence with fine structures, which exhibits dynamic behaviour, which was observed in the hydrogen Hα line with Hinode/SOT, Meudon/MSDP and Ondˇ/HSFA2, and simultaneously in hydrogen Lyman lines with SoHO/SUMER during a coordinated campaign. We derive the fine-structure physical parameters of this prominence and also address the questions of the role of the magnetic dips and of the interpretation of the flows. Methods. We calibrate the SoHO/SUMER and Meudon/MSDP data and obtain the line profiles of the hydrogen Lyman series (Lβ to L6), the Ciii (977.03 A) and Svi (933.40 A), and Hα along the slit of SoHO/SUMER that crosses the Hinode/SOT prominence. We employ a complex 2D radiation-magnetohydrostatic (RMHS) modelling technique to properly interpret the observed spectral lines and derive the physical parameters of interest. The model was constrained not only with integrated intensities of the lines, but also with the hydrogen line profiles. Results. The slit of SoHO/SUMER is crossing different prominence structures: threads and dark bubbles. Comparing the observed integrated intensities, the depressions of Hα bubbles are clearly identified in the Lyman, Ciii ,a nd Svi lines. To fit the observations, we propose a new 2D model with the following parameters: T = 8000 K, pcen = 0.035 dyn cm −2 , B = 5 Gauss, ne = 10 10 cm −3 , 40 threads each 1000 km wide, plasma β is 3.5 × 10 −2 . Conclusions. The analysis of Ciii and Svi emission in dark Hα bubbles allows us to conclude that there is no excess of a hotter plasma in these bubbles. The new 2D model allows us to diagnose the orientation of the magnetic field versus the LOS. The 40 threads are integrated along the LOS. We demonstrate that integrated intensities alone are not sufficient to derive the realistic physical parameters of the prominence. The profiles of the Lyman lines and also those of the Hα line are necessary to constrain 2D RMHS models. The magnetic field in threads is horizontal, perpendicular to the LOS, and in the form of shallow dips. With this geometry the dynamics of fine structures in prominences could be interpreted by a shrinkage of the quasi-horizontal magnetic field lines and apparently is not caused by the quasi-vertical bulk flows of the plasma, as Hinode/SOT movies seemingly suggest.

Statistical comparison of the observed and synthetic hydrogen Lyman line profiles in solar prominences

Aims. We analyse a unique set of prominence SOHO/SUMER Lyman spectra by comparing it with synthetic spectra obtained by 2D multi-thread prominence fine-structure models. Methods. We employed a novel statistical approach to the analysis of the observed and synthetic Lyman spectra. We compared the statistical distributions of the line properties of the observed and synthetic Lyman spectra using a set of four statistical criteria. Results. We demonstrate the very good agreement between the observed Lyman spectra and synthetic spectra obtained by modelling. Conclusions. Our set of statistical criteria is well-suited to analyses of the prominence Lyman spectra because of its sensitivity to a number of different parameters governing the conditions in the prominence fine structures.

Statistical analysis of UV spectra of a quiescent prominence observed by IRIS

The paper analyzes the structure and dynamics of a quiescent prominence that occurred on October 22, 2013. We aim to determine the physical characteristics of the observed prominence using MgII k and h, CII (1334 and 1336 A), and SiIV (1394 A) lines observed by IRIS. We employed the 1D non-LTE modeling of MgII lines assuming static isothermal-isobaric slabs. We selected a large grid of models with realistic input parameters and computed synthetic MgII lines. The method of Scargle periodograms was used to detect possible prominence oscillations. We analyzed 2160 points of the observed prominence in five different sections along the slit averaged over ten pixels due to low signal to noise ratio in the CII and SiIV lines. We computed the integrated intensity for all studied lines, while the central intensity and reversal ratio was determined only for both MgII and CII 1334 lines. We plotted several correlations: time evolution of the integrated intensities and central intensities, scatter plots between all combinations of line integrated intensities, and reversal ratio as a function of integrated intensity. We also compared MgII observations with the models. Results show that more than two-thirds of MgII profiles and about one-half of CII 1334 profiles are reversed. Profiles of SiIV are generally unreversed. The MgII and CII lines are optically thick, while the SiIV line is optically thin. The studied prominence shows no global oscillations in the MgII and CII lines. Therefore, the observed time variations are caused by random motions of fine structures with velocities up to 10 km/s. The observed average ratio of MgII k to MgII h line intensities can be used to determine the prominences characteristic temperature. Certain disagreements between observed and synthetic line intensities of MgII lines point to the necessity of using more complex 2D multi-thread modeling in the future.

Oscillatory phenomena in a solar network region

We examine oscillatory phenomena in a solar network region from multi-wavelength, observations obtained by the ground-based Dutch Open Telescope (DOT), and by instruments on the spacecraft Solar and Heliospheric Observatory (SoHO). The observations were obtained during a coordinated observing campaign on October 14, 2005. The temporal variations of the intensities and velocities in two distinct regions of the quiet Sun were investigated: one containing several dark mottles and the other several bright points defining the network boundaries (NB). The aim is to find similarities and/or differences in the oscillatory phenomena observed in these two regions and in different spectral lines formed from the chromosphere to the transition region, as well as propagation characteristics of waves.

Multiwavelength determination of the density and total mass of the EUV filament observed by SoHO/CDS, SoHO/SUMER and MSDP/VTT

It was found that filaments observed in EUV could be much more extended than in Hα. These extended dark structures visible in EUV are named EUV filaments. Their parts seen only in EUV (not observable in Hα because of low opacity at the Hα wavelength) are called EUV-filament extensions (or simply EUV extensions). For the EUV filament observed by SoHO on 15 October 1999 as northern polar crown filament, only a few small dark structures were seen in Hα. This suggests that the mass of the EUV extension is larger than, or at least comparable with, the mass of the parts of the filament observed in Hα. In our previous work we determined the 3D structure of the EUV extension of this EUV filament. In this paper we present the determinations of mass and average density of this EUV extension. For better density estimates we interpret the hydrogen Lyman lines observed by SUMER using non-LTE radiative transfer code. From the best fitting of Lyman lines we obtained a reasonable model of the EUV extension with low gas pressure, temperatures from 2×10 K to 10 K and with extended prominence-corona transition regions.