S. Jejčič

Hot prominence detected in the core of a coronal mass ejection: Analysis of SOHO/UVCS Lα and SOHO/LASCO visible-light observations

Context. The paper deals with the physics of erupting prominences in the core of coronal mass ejections (CME). Aims. We determine the physical parameters of an erupting prominence embedded in the core of a CME using SOHO/UVCS hydrogen L α and L β lines and SOHO/LASCO visible light observations. In particular we analyze the CME event observed on August 2, 2000. We develop the non-LTE (NLTE; i.e. considering departures from the local thermodynamic equilibrium – LTE) spectral diagnostics based on L α and visible light observations. Methods. Our method is based on 1D NLTE modeling of eruptive prominences and takes into account the effect of large flow velocities, which reach up to 300 km s -1 for the studied event (the so-called Doppler dimming). The NLTE radiative-transfer method can be used for both optically thin and thick prominence structures. We combine spectroscopic UVCS observations of an erupting prominence in the core of a CME with visible light images from LASCO-C2 in order to derive the geometrical parameters like projected thickness and velocity, together with the effective temperature and column density of electrons. These are then used to constrain our NLTE radiative transfer modeling which provides the kinetic temperature, microturbulent velocity, gas pressure, ionization degree, the line opacities, and the prominence effective thickness (geometrical filling factor). Results. Analysis was made for 69 observational points (spatial pixels) inside the whole erupting prominence. Roughly one-half of them show a non-negligible L α optical thickness for flow velocity 300 km s -1 and about one-third for flow velocity 150 km s -1 . All pixels with L α τ 0 ≤ 0.3 have been considered for further analysis, which is presented in the form of statistical distributions (histograms) of various physical quantities such as the kinetic temperature, gas pressure, and electron density for two representative flow velocities (150 and 300 km s -1 ) and non-zero microturbulence. For two pixels co-temporal LASCO visible-light data are also available, which further constrains the diagnostics of the electron density and effective thickness. Detailed NLTE modeling is presented for various sets of input parameters. Conclusions. The studied CME event shows that the erupting prominence expands to large volumes, meaning that it is a low-pressure structure with low electron densities and high temperatures. This analysis provides a basis for future diagnostics using the METIS coronagraph on board the Solar Orbiter mission.

Hot prominence detected in the core of a coronal mass ejection - II. Analysis of the C iii line detected by SOHO/UVCS

Context. We study the physics of erupting prominences in the core of coronal mass ejections (CMEs) and present a continuation of a previous analysis. Aims. We determine the kinetic temperature and microturbulent velocity of an erupting prominence embedded in the core of a CME that occurred on August 2, 2000 using the Ultraviolet Coronagraph and Spectrometer observations (UVCS) on board the Solar and Heliospheric Observatory (SOHO) simultaneously in the hydrogen L α and C iii lines. We develop the non-LTE (departures from the local thermodynamic equilibrium – LTE) spectral diagnostics based on L α and L β measured integrated intensities to derive other physical quantities of the hot erupting prominence. Based on this, we synthesize the C iii line intensity to compare it with observations. Methods. Our method is based on non-LTE modeling of eruptive prominences. We used a general non-LTE radiative-transfer code only for optically thin prominence points because optically thick points do not allow the direct determination of the kinetic temperature and microturbulence from the line profiles. The input parameters of the code were the kinetic temperature and microturbulent velocity derived from the L α and C iii line widths, as well as the integrated intensity of the L α and L β lines. The code runs in three loops to compute the radial flow velocity, electron density, and effective thickness as the best fit to the L α and L β integrated intensities within the accuracy defined by the absolute radiometric calibration of UVCS data. Results. We analyzed 39 observational points along the whole erupting prominence because for these points we found a solution for the kinetic temperature and microturbulent velocity. For these points we ran the non-LTE code to determine best-fit models. All models with τ 0 (L α ) ≤ 0.3 and τ 0 (C iii) ≤ 0.3 were analyzed further, for which we computed the integrated intensity of the C iii line using a two-level atom. The best agreement between computed and observed integrated intensity led to 30 optically thin points along the prominence. The results are presented as histograms of the kinetic temperature, microturbulent velocity, effective thickness, radial flow velocity, electron density, and gas pressure. We also show the relation between the microturbulence and kinetic temperature together with a scatter plot of computed versus observed C iii integrated intensities and the ratio of the computed to observed C iii integrated intensities versus kinetic temperature. Conclusions. The erupting prominence embedded in the CME is relatively hot with a low electron density, a wide range of effective thicknesses, a rather narrow range of radial flow velocities, and a microturbulence of about 25 km s -1 . This analysis shows a disagreement between observed and synthetic intensities of the C iii line, the reason for which most probably is that photoionization is neglected in calculations of the ionization equilibrium. Alternatively, the disagreement might be due to non-equilibrium processes.

PROMINENCE VISIBILITY IN HINODE/XRT IMAGES

In this paper we study the soft X-ray (SXR) signatures of one particular prominence. The X-ray observations used here were made by the Hinode/X-Ray Telescope instrument using two different filters. Both of them have a pronounced peak of the response function around 10 A. One of them has a secondary smaller peak around 170 A, which leads to a contamination of SXR images. The observed darkening in both of these filters has a very large vertical extension. The position and shape of the darkening correspond nicely with the prominence structure seen in SDO/AIA images. First, we have investigated the possibility that the darkening is caused by X-ray absorption. However, detailed calculations of the optical thickness in this spectral range show clearly that this effect is completely negligible. Therefore, the alternative is the presence of an extended region with a large emissivity deficit, which can be caused by the presence of cool prominence plasmas within an otherwise hot corona. To reproduce the observed darkening, one needs a very large extension along the line of sight of the region amounting to around 105 km. We interpret this region as the prominence spine, which is also consistent with SDO/AIA observations in EUV.

Hot prominence detected in the core of a coronal mass ejection: III. Plasma filling factor from UVCS Lyman-α and Lyman-β observations

This work deals with the study of an erupting prominence embedded in the core of a CME and focuses on the derivation of the prominence plasma filling factor. We explore two methods to measure the prominence plasma filling factor that are based on the combination of visible-light and ultraviolet spectroscopic observations. Theoretical relationships for resonant scattering and collisional excitation are used to evaluate the intensity of the H I Lyman-{\alpha} and Lyman-{\beta} lines, in two prominence points where simultaneous and cospatial LASCO-C2 and UVCS data were available. Thermodynamic and geometrical parameters assumed for the calculation are provided by both observations and the results of a detailed 1D non-LTE radiative-transfer model of the prominence, developed in our previous work (Heinzel 2016). The filling factor is derived from the comparison between the calculated and the measured intensities of the two lines. The results are then checked against the non-LTE model in order to verify the reliability of the methods. The resulting filling factors are consistent with the model in both the prominence points when the separation of the radiative and collisional components of the total intensity, required to estimate the filling factor, is performed using both the line intensities. An exploration of the parameter space shows that the results are weakly sensitive to the plasma velocity, but they depends more strongly on the assumed kinetic temperatures. The combination of visible-light and ultraviolet Lyman-{\alpha} and Lyman-{\beta} data can be used to approximately estimate the geometrical filling factor in erupting prominences, but the proposed techniques are reliable only for emission that is optically thin in the lines considered, condition that is not in general representative of prominence plasma.

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.