P. Heinzel

Prominence fine structures in a magnetic equilibrium: Two-dimensional models with multilevel radiative transfer

In this paper we construct theoretical models for vertical prominence threads which are in magnetohydrostatic (MHS) equilibrium. These models are fully two-dimensional (2D) and take the form of vertically infinite threads hanging in a horizontal magnetic field. A typical example of such a 2D magnetic-dip structure is shown for the case when the central cool parts are surrounded by the prominence-corona transition region (PCTR). We display 2D variations of the pressure, density and temperature. While the pressure variations follow from the MHS equilibrium, the kinetic temperature was specified empirically. As a next step, we have solved the 2D multilevel non-LTE transfer problem in such thread-like structures, in order to predict the spatial variations of the emergent hydrogen spectrum. It is demonstrated that the hydrogen Lyman lines (treated with partial redistribution) show significant spatial variations of the intensity and that an important difference exists between the line profiles emergent along and across the magnetic field lines. We also discuss how these intensity variations compare to recent SOHO/SUMER prominence observations, namely we show the effects of line-profile averaging over the fine-structure threads which are below the instrumental resolution. Finally we make some suggestions for future modelling and observations.

On Lyman-line asymmetries in quiescent prominences

Aims. We study the asymmetries of the synthetic hydrogen Lyman lines and the process responsible for their formation. Methods. To obtain the synthetic Lyman line profiles, we use a multi-thread prominence fine-structure model consisting of identical 2D threads. The 2D thread models are in MHS equilibrium, include an empirical prominence-corona transition region, and solve consistently 2D non-LTE radiative transfer. Each thread of the multi-thread model has a randomly assigned line-of-sight (LOS) velocity. Results. The synthetic Lyman spectrum obtained by multi-thread modelling exhibits substantial asymmetries of the line profiles, even though the LOS velocities of individual threads are only of the order of 10 km s −1 . Moreover, our results indicate that the synthetic Lyman-α profiles may exhibit an opposite asymmetry to that of the higher Lyman lines. Conclusions. The presence and behaviour of the asymmetrical profiles of the synthetic Lyman lines agree with observed profiles acquired by SUMER.

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.

Prominence fine structures in a magnetic equilibrium - II. A grid of two-dimensional models

We construct a grid of 2D vertical-thread models for prominence fine structures which are in magnetohydrostatic (MHS) equilibrium. Such thread models have been described in a previous paper by Heinzel & Anzer (2001), but here we use a modified 2D transfer code with an adaptive MHS grid. Multilevel non-LTE transfer calculations are now performed for a 12-level plus continuum hydrogen model atom, in order to study the behaviour of the Lyman-series lines observed by SOHO/SUMER. Our grid consists of 18 models which cover a range of central column masses, magnetic-field intensities and two parameters characterising the 2D temperature structure of the thread. Since different Lyman lines and their parts (line center, peak, wings) are formed at different places within the thread, the Lyman series may serve as a good diagnostic tool for thermodynamic conditions varying from central cool parts to a prominence-corona transition region. We demonstrate this behaviour for various lines, showing their synthetic profiles as seen from two perpendicular directions along and across the magnetic field lines, respectively, and displaying the respective contribution functions. This study confirms our earlier conclusion that the Lyman line profiles are much more reversed when seen across the field lines, compared to those seen along the lines. The latter can be even unreversed. We also show the geometrical cross-section (shape) of all 18 models. Their thread-like shape with a considerable aspect ratio resembles recent high-resolution H α images. Finally, we discuss the relation of our thread models to the vertical threads studied by Fontenla etxa0al. (1996, ApJ, 466, 496).

On the formation of MgII h and k lines in solar prominences

Aims. With the recent launch of the IRIS mission, it has become urgent to develop the spectral diagnostics using the Mg ii resonance h and k lines. In this paper, we aim to demonstrate the behavior of these lines under various prominence conditions. Our results serve as a basis for analysis of new IRIS data and for more sophisticated prominence modeling. Methods. For this exploratory work, we use a canonical 1D prominence-slab model, which is isobaric and may have three different temperature structures: isothermal, PCTR-like (prominence-corona transition region), and consistent with the radiative equilibrium. The slabs are illuminated by a realistic incident solar radiation obtained from the UV observations. A five-level plus continuum Mg ii model atom is used to solve the full NLTE problem of the radiative transfer. We use the numerical code based on the ALI techniques and apply the partial frequency redistribution for both Mg ii resonance lines. We also use the velocity-dependent boundary conditions to study the effect of Doppler dimming in the case of moving prominences. Finally, the relaxation technique is used to compute a grid of models in radiative equilibrium. Results. We computed the Mg ii h and k line profiles that are emergent from prominence-slab models and show their dependence on kinetic temperature, gas pressure, geometrical extension, and microturbulent velocity. By increasing the line opacity, significant departures from the complete frequency redistribution take place in the line wings. Models with a PCTR temperature structure show that Mg ii becomes ionized to Mg iii in the temperature range between roughly 15 000 and 30 000 K. Doppler dimming is significant for Mg ii resonance lines. At the velocity 300 km s −1 , the line intensity decreases to about 20% of the value for static prominences. Finally, we demonstrate the role of Mg ii h and k radiation losses on the prominence energy balance. Their dominant role is at lower pressures, while the losses due to hydrogen and Ca ii dominate at higher pressures.

Is the magnetic field in quiescent prominences force-free?

Aims. We describe under which conditions the magnetic fields of quiescent prominences are force-free and under which gravity plays the dominant role. Methods. The existing observational determinations of the magnetic field are summarised and the calculation of the plasma β is outlined. We derive the dependence of β on the prominence weight and the field strength. Results. We show that in many cases of well-developed quiescent prominences the field can deviate substantially from the force-free situation and gravity fully determines the structure of the magnetic dips.

On the nature of extended EUV filaments

This paper describes the properties of extended EUV filaments and the theoretical modelling of them. We summarise the general aspects of the depression of EUV-line emission and give an interpretation of recent filament observations in transition-region and coronal lines. The EUV filament was found to be located relatively high in the corona (at least 20u2009000xa0km above the solar surface) and this requires an MHD scenario alternative to the parasitic-polarity model of Aulanier & Schmieder (2002). Here we present a new idea for the support of cool gas in the magnetic arcade of a prominence which is capable of explaining both wide and vertically extended EUV filaments. Our mechanism is based upon the twisting of individual flux tubes, similar to the one which was suggested by Priest etxa0al. (1989). Finally, the consequences of this new model are discussed.

Fast approximate radiative transfer method for visualizing the fine structure of prominences in the hydrogen Hα line

Aims. We present a novel approximate radiative transfer method developed to visualize 3D whole-prominence models with multiple fine structures using the hydrogen Hα spectral line. Methods. This method employs a fast line-of-sight synthesis of the Hα line profiles through the whole 3D prominence volume and realistically reflects the basic properties of the Hα line formation in the cool and low-density prominence medium. The method can be applied both to prominences seen above the limb and filaments seen against the disk. Results. We provide recipes for the use of this method for visualizing the prominence or filament models that have multiple fine structures. We also perform tests of the method that demonstrate its accuracy under prominence conditions. Conclusions. We demonstrate that this fast approximate radiative transfer method provides realistic synthetic Hα intensities useful for a reliable visualization of prominences and filaments. Such synthetic high-resolution images of modeled prominences/filaments can be used for a direct comparison with high-resolution observations.

Radiative equilibrium in solar prominences reconsidered

Aims. We reconsider the question which kinetic temperatures can lead to prominence configurations that are in radiative equilibrium. We compare these temperatures to those from other calculations. Methods. For this purpose we solved the full non-LTE radiative-transfer problem for a gas consisting of hydrogen, helium and calcium. We used simple isobaric 1D slabs and began with isothermal models. Then we solved the radiativerelaxation problem and determined the radiative-equilibrium conditions within the whole slab. Results. By adding the calcium radiative losses, we found that these equilibrium temperatures are considerably lower than those obtained for a pure hydrogen gas. This is because the newly calculated CaII line losses appear to play a significant role in the energy balance, similar to chromospheric conditions. The equilibrium temperatures obtained span the range between 4400 - 9500 K, depending on the gas pressure and slab thickness.

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 kmu2009s -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 kmu2009s -1 and about one-third for flow velocity 150 kmu2009s -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 kmu2009s -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.