Gunár et al. 2012, St Andrews, UK

Comparison of gravity-induced and force-free magnetic dips
in quiescent prominences

S. Gunár, D.H. Mackay, P. Heinzel & U. Anzer

From the HINODE-6 meeting, Aug 14 – 17, 2012, St Andrews, UK

Abstract:

High-resolution observations of quiescent-prominence fine structures such as those obtained by Hinode/SOT show great variety of blob and thread-like small-scale features. Moreover, these fine structures exhibit a dynamical behaviour that represents a challenge for the understanding of quiescent prominences.

Despite a long-lasting modelling effort and a large number of superb observations of quiescent prominences, the nature of the magnetic field supporting prominences remains an open question. Today, two distinct modelling techniques are employed to describe the prominence fine structures. On one hand, large-scale prominence force-free magnetic field simulations and extrapolations can realistically represent their global magnetic structure and also positions of fine structures corresponding to the locations of dipped field lines. On the other hand, local multi-dimensional models of gravity-induced magnetic dips, usually of the Kippenhahn-Schlutter (KS) type, with realistic atmospheric models of prominence fine structures are able to produce the synthetic spectra in good agreement with observations and explain some dynamical aspects of quiescent prominences.

We present here the first comparison of local gravity-induced 2D KS-type magnetic dip models of prominence fine structures with fine structures located in the force-free dips produced by global 3D MHD simulations. In both cases we employ realistic models of the atmospheric structure including the prominence-corona transition region. We solve the 2D non-LTE radiative transfer problem for multi-level hydrogen atom and compare the hydrogen synthetic spectra of the gravity-induced and force-free models. We also discuss the validity of the force-free approximation for various values of the model parameters.

A combination of the gravity-induced and force-free prominence modelling holds a significant potential for a deeper understanding of quiescent prominences.

Poster

Gunár et al. 2012, A&A 543, A93

Dynamics of quiescent prominence fine structures analyzed
by 2D non-LTE modelling of the Hα line

S. Gunár, P.Mein, B. Schmieder, P. Heinzel & N.Mein

Aims: We analyze the dynamics of the prominence fine structures of a quiescent prominence observed on April 26, 2007 during a coordinated campaign of several spaceborne and ground-based instruments. We use Lyman spectra observed by SOHO/SUMER and the Hα line spectra obtained by MSDP spectrograph working at the Meudon Solar Tower.

Methods: We employ the 2D multi-thread prominence fine-structure modelling that includes randomly distributed line-of-sight (LOS) velocities of individual threads to derive models producing synthetic Lyman lines in good agreement with the SOHO/SUMER observations. We then use these models to produce synthetic Hα line spectra that we compare with the observed spectra using three statistical parameters: the line integrated intensity, the line full-width at half-maximum (FWHM), and the Doppler velocity derived
from shifts of the line profiles.

Results: We demonstrate that the 2D multi-thread models that produce synthetic Lyman spectra in agreement with observations also generate synthetic Hα spectra in good agreement with the observed ones. The statistical analysis of the FWHMand Doppler velocities of the synthetic Hα line profiles show that the overall LOS velocities in the April 26, 2007 prominence at the time of the observations were below 15 km/s and in the prominence core were close to 10 km/s. In combination with the analysis of the Lyman spectra, we determine several physical parameters of the observed prominence fine-structures that show that the April 26, 2007 prominence had a relatively low-mass weakly magnetized structure.We are also able to impose some constraints on the prominence core temperature, which may be relatively low, with values below 6000 K.

Conclusions: The combination of 2D non-LTE prominence fine-structure modelling with the statistical analysis of the observed and synthetic Lyman and Hα spectra allows us to analyze the influence of the model input parameters and the velocity fields on the synthetic Hα line profiles, thus determine the overall dynamics of the observed prominence as well as the physical parameters of its plasma.

Gunár et al. 2012, Mysore, India

Prominence fine structure dynamics as inferred from 2D non-LTE models

S. Gunár, P. Mein, B. Schmieder, P. Heinzel & N. Mein

From the 39th COSPAR Scientific Assembly, July 14 – 22, Mysore, India

Abstract:

2D multi-thread prominence fine structure models are able to produce synthetic Lyman spectra in very good agreement with spectral observations by SOHO/SUMER including the spectral line asymmetries. The synthetic differential emission measure curves derived from these models are also in a good agreement with observations. Now we show that these models are also able to produce synthetic H-alpha line profiles in very good agreement with observations which allows us to analyze not only the physical parameters of the prominence fine-structure plasma but also some aspects of its dynamical behaviour.

We compare the synthetic H-alpha spectra with the observed spectra of the April 26, 2007 prominence using three statistical parameters: the line integrated intensity, the line full-width at the half-maximum (FWHM), and the Doppler velocity derived from shifts of the line profiles. This statistical analysis allows us to conclude that the overall statistical distribution of the LOS velocities in the April 26, 2007 prominence at the time of the observations was below +/-15 km/s and in the prominence core was close to +/-10 km/s.

In combination with the analysis of the Lyman spectra we determine several physical parameters of the observed prominence fine structures which show that the April 26, 2007 prominence was relatively less massive. We are also able to put some constrains on the prominence core temperature that might be relatively low, reaching values below 6000 K.

Presentation: in pdf

Gunár et al. 2011, Pasadena, CA, USA

2D prominence fine structure models: recent progress and results

From the 2011 PROM Workshop, Dec 13- 15, 2011, Pasadena, CA, USA

Presentation: in pdf

Gunár et al. 2011, Kiel, Germany

Synthetic DEM curves of prominence fine structures

S. Gunár, S. Parenti, U. Anzer, P. Heinzel & J.-C. Vial

From THE SUN 360 – STEREO-4/SDO-2/SOHO-25 Workshop, July 24- 30, 2011, Kiel, Germany

Abstract: We combine for the first time the hydrogen Lyman-line observations and modelling with the UV and EUV lines observations and the differential emission measure (DEM) determination for a prominence observed on June 8, 2004. We use a trial-and-error method to derive the 2D multi-thread prominence fine-structure models producing synthetic Lyman spectra in good agreement with the observations. We then employ a newly developed numerical method for the forward determination of the DEM from 2D multi-thread models and we compare the synthetic DEM curves with those derived from observations using inversion techniques. The available observations of the June 8, 2004 prominence allow us to determine the range of input parameters of the models producing synthetic Lyman spectra in good agreement with the observations. We select three models, which well represent this parametric-space area and compute the synthetic DEM curves for multi-thread realizations of these models. The so obtained DEM curves of the selected models very well agree with the DEM curves derived from the observations. We show that the evaluation of the prominence fine-structure DEM is complementary to the analysis of the prominence hydrogen Lyman spectra and that its combination with the detailed radiative-transfer modelling of prominence fine structure provides a useful tool for investigating the prominence temperature structure from the cool core to the prominence-corona transition region.

Poster

Gunár et al. 2011b, A&A 535, A122

Synthetic differential emission measure curves of prominence
fine structures II. The SoHO/SUMER prominence of 8 June 2004

S. Gunár, S. Parenti, U. Anzer, P. Heinzel & J.-C. Vial

Aims: This study is the first attempt to combine the prominence observations in Lyman, UV, and EUV lines with the determination of the prominence differential emission measure derived using two different techniques, one based on the inversion of the observed UV and EUV lines and the other employing 2D non-LTE prominence fine-structure modeling of the Lyman spectra.

Methods: We use a trial-and-error method to derive the 2D multi-thread prominence fine-structure model producing synthetic Lyman spectra in good agreement with the observations. We then employ a numerical method to perform the forward determination of the DEM from 2D multi-thread models and compare the synthetic DEM curves with those derived from observations using inversion techniques.

Results: A set of available observations of the June 8, 2004 prominence allows us to determine the range of input parameters, which contains models producing synthetic Lyman spectra in good agreement with the observations.We select three models, which represent this parametric-space area well and compute the synthetic DEM curves for multi-thread realizations of these models. The synthetic DEM curves of selected models are in good agreement with the DEM curves derived from the observations.

Conclusions: We show that the evaluation of the prominence fine-structure DEM complements the analysis of the prominence hydrogen Lyman spectra and that its combination with the detailed radiative-transfer modeling of prominence fine structures provides a useful tool for investigating the prominence temperature structure from the cool core to the prominence-corona transition region.

Gunár et al. 2011a, A&A 528, A47

Synthetic differential emission measure curves
of prominence fine structures

S. Gunár, P. Heinzel & U. Anzer

Aims: We use 2D single and multi-thread prominence fine-structure models to obtain the synthetic DEM curves. These are then compared with the DEM curves derived from observations.

Methods: We use the temperature and electron density structure resulting from the 2D models and numerically compute the average synthetic DEM curves for different orientations of the threads with respect to the line of sight.

Results: We show that the synthetic DEM curves obtained by 2D modelling are similar to the DEM curves derived from observations of quiescent prominences.

Conclusions: The DEM curves derived from observations, which are most reliable above temperatures of 20 000 K, can be extended towards cool prominence-core temperatures by supplementing them with synthetic DEM values obtained by modelling hydrogen Lyman spectra originating mainly at temperatures below 20 000 K. On the other hand, the observed DEM can constrain the temperature structure of the prominence fine structures above the formation temperatures of the Lyman spectrum.

Gunár et al. 2010, A&A 514, A43

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

S. Gunár, P. Schwartz, B. Schmieder, P. Heinzel, & U. Anzer

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.

Gunár et al. 2009, Boulder, CO

Atmospheric Structures

2D Radiative Transfer in Heterogeneous Atmospheric Structures

S. Gunár, P. Heinzel & U. Anzer

From the RECENT DIRECTIONS IN ASTROPHYSICAL QUANTITATIVE SPECTROSCOPY AND RADIATION HYDRODYNAMICS conference, March 30 – April 3, 2009, Boulder, CO.

Abstract: In stellar atmospheres we observe highly heterogeneous structures, such as the solar chromosphere, solar prominences, stellar winds, etc. We have developed a 2D radiative transfer code which solves a multi-level non-LTE problem for hydrogen within an externally irradiated heterogeneous medium. Mutual radiative interaction between individual fine structures is consistently taken into account for the case of solar prominences.

Poster:

Gunár et al. 2008, A&A 490, 307

On Lyman-line asymmetries in quiescent prominences

S. Gunár, P. Heinzel, U. Anzer & B. Schmieder

Aims: We study the asymmetries of the synthetic hydrogen Lyman lines and the process of their formation.

Methods: To obtain the synthetic Lyman line profiles, we use a multi-thread prominence finestructure model consisting of identical 2D threads. The 2D thread models are in MHS equilibrium, include an empirical PCTR and consistently solve the 2D non-LTE radiative transfer. Each thread of the multi-thread model has a randomly assigned LOS velocity.

Results: The synthetic Lyman spectrum obtained by multi-thread modelling exhibits substantial asymmetries of the line profiles even thought the LOS velocities of individual threads are only of the order of 10 km/s. Moreover, our results show that the synthetic Lyman- alpha profiles may exhibit an opposite asymmetry as compared to higher Lyman lines.

Conclusions: The presence and the behaviour of the asymmetrical profiles of the synthetic Lyman spectrum is in agreement with observed profiles taken by SOHO/SUMER.

of identical 2D threads. The 2D thread models are in MHS equilibrium, include an empirical prominence-corona transition region, and
s$^{-1}$. Moreover, our results indicate that the synthetic Lyman-$\alpha$ profiles may exhibit an opposite asymmetry