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Radiative transfer in cylindrical threads with incident radiation VII. Multi-thread models  

Nicolas Labrosse   Submitted: 2016-01-12 02:10

We solved the radiative transfer and statistical equilibrium equations in a two-dimensional cross-section of a cylindrical structure oriented horizontally and lying above the solar surface. The cylinder is filled with a mixture of hydrogen and helium and is illuminated at a given altitude from the solar disc. We constructed simple models made from a single thread or from an ensemble of several threads along the line of sight. This first use of two-dimensional, multi-thread fine structure modelling combining hydrogen and helium radiative transfer allowed us to compute synthetic emergent spectra from cylindrical structures and to study the effect of line-of-sight integration of an ensemble of threads under a range of physical conditions. We analysed the effects of variations in temperature distribution and in gas pressure. We considered the effect of multi-thread structures within a given field of view and the effect of peculiar velocities between the structures in a multi-thread model. We compared these new models to the single thread model and tested them with varying parameters. These new computations show, for the first time, the effect of integrating the radiation emitted in H and He lines by several cylindrical threads that are static or moving along the line of sight. They can be used to interpret high-spatial and spectral resolutions of cylindrical structures found in the solar atmosphere, such as cool coronal loops or prominence threads.

Authors: N. Labrosse and A.S. Rodger
Projects: None

Publication Status: A&A (in press)
Last Modified: 2016-01-13 12:40
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Plasma diagnostic in eruptive prominences from SDO/AIA observations at 304  

Nicolas Labrosse   Submitted: 2011-11-21 05:17

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Labrosse N. & McGlinchey K.
Projects: SDO-AIA

Publication Status: Accepted, A&A
Last Modified: 2011-11-22 03:20
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EUV lines observed with EIS/Hinode in a solar prominence  

Nicolas Labrosse   Submitted: 2011-05-11 15:27

We report on observations of a solar prominence obtained on 26 April 2007using the Extreme Ultraviolet Imaging Spectrometer on Hinode. Several regionswithin the prominence are identified for further analysis. Selected profilesfor lines with formation temperatures between log(T)=4.7-6.3, as well as theirintegrated intensities, are given. The line profiles are discussed. We payspecial attention to the He II line which is blended with coronal lines. Ouranalysis confirms that depression in EUV lines can be interpreted by twomechanisms: absorption of coronal radiation by the hydrogen and neutral heliumresonance continua, and emissivity blocking. We present estimates of the He IIline integrated intensity in different parts of the prominence according todifferent scenarios for the relative contribution of absorption and emissivityblocking on the coronal lines blended with the He II line. We estimate thecontribution of the He II 256.32 line in the He II raster image to vary between~44% and 70% of the raster's total intensity in the prominence according to thedifferent models used to take into account the blending coronal lines. Theinferred integrated intensities of the He II line are consistent withtheoretical intensities obtained with previous 1D non-LTE radiative transfercalculations, yielding a preliminary estimate for the central temperature of8700 K, central pressure of 0.33 dyn/cm^2, and column mass of 2.5 10-4g/cm^2. The corresponding theoretical hydrogen column density (1020 cm-2)is about two orders of magnitude higher than those inferred from the opacityestimates at 195 {AA}. The non-LTE calculations indicate that the He II 256.32{AA} line is essentially formed in the prominence-to-corona transition regionby resonant scattering of the incident radiation.

Authors: Nicolas Labrosse, Brigitte Schmieder, Petr Heinzel, Tetsuya Watanabe
Projects: Hinode/EIS,Hinode/SOT,Hinode/XRT

Publication Status: Accepted, A&A
Last Modified: 2011-05-12 15:06
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Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling  

Nicolas Labrosse   Submitted: 2010-01-11 18:58

This review paper outlines background information and covers recent advances made via the analysis of spectra and images of prominence plasma and the increased sophistication of non-LTE (ie when there is a departure from Local Thermodynamic Equilibrium) radiative transfer models. We first describe the spectral inversion techniques that have been used to infer the plasma parameters important for the general properties of the prominence plasma in both its cool core and the hotter prominence-corona transition region. We also review studies devoted to the observation of bulk motions of the prominence plasma and to the determination of prominence mass. However, a simple inversion of spectroscopic data usually fails when the lines become optically thick at certain wavelengths. Therefore, complex non-LTE models become necessary. We thus present the basics of non-LTE radiative transfer theory and the associated multi-level radiative transfer problems. The main results of one- and two-dimensional models of the prominences and their fine-structures are presented. We then discuss the energy balance in various prominence models. Finally, we outline the outstanding observational and theoretical questions, and the directions for future progress in our understanding of solar prominences.

Authors: N. Labrosse, P. Heinzel, J.-C. Vial, T. Kucera, S. Parenti, S. Gunar, B. Schmieder, G. Kilper
Projects: None

Publication Status: Accepted, Space Science Reviews
Last Modified: 2010-01-12 08:51
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Automatic Detection of Limb Prominences in 304 Å EUV Images  

Nicolas Labrosse   Submitted: 2009-12-08 03:14

A new algorithm for automatic detection of prominences on the solar limb in 304 Å EUV images is presented, and results of its application to SOHO/EIT data discussed. The detection is based on the method of moments combined with a classifier analysis aimed at discriminating between limb prominences, active regions, and the quiet corona. This classifier analysis is based on a Support Vector Machine (SVM). Using a set of 12 moments of the radial intensity profiles, the algorithm performs well in discriminating between the above three categories of limb structures, with a misclassification rate of 7%. Pixels detected as belonging to a prominence are then used as starting point to reconstruct the whole prominence by morphological image processing techniques. It is planned that a catalogue of limb prominences identified in SOHO and STEREO data using this method will be made publicly available to the scientific community.

Authors: Nicolas Labrosse, Silvia Dalla, Steve Marshall
Projects: SoHO-EIT

Publication Status: Solar Physics, accepted
Last Modified: 2009-12-08 09:52
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Radiative transfer in cylindrical threads with incident radiation VI. A hydrogen plus helium system  

Nicolas Labrosse   Submitted: 2009-05-22 03:42

Spectral lines of helium are commonly observed on the Sun. These observations contain important informations about physical conditions and He/H abundance variations within solar outer structures. The modeling of chromospheric and coronal loop-like structures visible in hydrogen and helium lines requires the use of appropriate diagnostic tools based on NLTE radiative tranfer in cylindrical geometry. We use iterative numerical methods to solve the equations of NLTE radiative transfer and statistical equilibrium of atomic level populations. These equations are solved alternatively for the hydrogen and helium atoms, using cylindrical coordinates and prescribed solar incident radiation. Electron density is determined by the ionization equilibria of both atoms. Two-dimension effects are included. The mechanisms of formation of the principal helium lines are analyzed and the sources of emission inside the cylinder are located. The variations of spectral line intensities with temperature, pressure, and helium abundance, are studied. The simultaneous computation of hydrogen and helium lines, performed by the new numerical code, allows the construction of loop models including an extended range of temperatures.

Authors: Pierre Gouttebroze and Nicolas Labrosse
Projects: None

Publication Status: Accepted for publication in Astronomy and Astrophysics
Last Modified: 2009-05-22 09:23
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Subject will be restored when possible  

Nicolas Labrosse   Submitted: 2008-04-29 08:18

In this study we show how hydrogen and helium lines modelling can be used to make a diagnostic of active and eruptive prominences. One motivation for this work is to identify the physical conditions during prominence activation and eruption. Hydrogen and helium lines are key in probing different parts of the prominence structure and inferring the plasma parameters. However, the interpretation of observations, being either spectroscopic or obtained with imaging, is not straightforward. Their resonance lines are optically thick, and the prominence plasma is out of local thermodynamic equilibrium due to the strong incident radiation coming from the solar disk. In view of the shift of the incident radiation occurring when the prominence plasma flows radially, it is essential to take into account velocity fields in the prominence diagnostic. Therefore we need to investigate the effects of the radial motion of the prominence plasma on hydrogen and helium lines. The method that we use is the resolution of the radiative transfer problem in the hydrogen and helium lines out of local thermodynamic equilibrium. We study the variation of the computed integrated intensities in H and He lines with the radial velocity of the prominence plasma. We can confirm that there exist suitable lines which can be used to make a diagnostic of the plasma in active and eruptive prominences in the presence of velocity fields.

Authors: Nicolas Labrosse, Jean-Claude Vial, Pierre Gouttebroze
Projects: None

Publication Status: to appear in Annales Geophysicae
Last Modified: 2008-09-23 21:07
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Effect of motions in prominences on the helium resonance lines in the extreme ultraviolet  

Nicolas Labrosse   Submitted: 2007-02-21 02:34

Extreme ultraviolet resonance lines of neutral and ionised helium observed in prominences are difficult to interpret as the prominence plasma is optically thick at these wavelengths. If mass motions are taking place, as is the case in active and eruptive prominences, the diagnostic is even more complex. We aim at studying the effect of radial motions on the spectrum emitted by moving prominences in the helium resonance lines and at facilitating the interpretation of observations, in order to improve our understanding of these dynamic structures. We develop our non-local thermodynamic equilibrium radiative transfer code formerly used for the study of quiescent prominences. The new numerical code is now able to solve the statistical equilibrium and radiative transfer equations in the non-static case by using velocity-dependent boundary conditions for the solution of the radiative transfer problem. This first study investigates the effects of different physical conditions (temperature, pressure, geometrical thickness) on the emergent helium radiation. The motion of the prominence plasma induces a Doppler dimming effect on the resonance lines of He I and He II. The velocity effects are particularly important for the He II λ 304 Å line as it is mostly formed by resonant diffusion of incident radiation under prominence conditions. The He I resonance lines at 584 and 537 ? also show some sensitivity to the motion of the plasma, all the more when thermal emission is not too important in these lines. We also show that it is necessary to consider partial redistribution in frequency for the scattering of the incident radiation. This set of helium lines offers strong diagnostic possibilities that can be exploited with the SOHO spectrometers and with the EIS spectrometer on board the Hinode satellite. The addition of other helium lines and of lines from other elements (in particular hydrogen) in the diagnostics will further enhance the strength of the method.

Authors: Labrosse, N.; Gouttebroze, P.; Vial, J.-C.
Projects: None

Publication Status: A&A 463, 1171-1179 (2007)
Last Modified: 2007-02-21 11:06
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Abstracts by Author
Radiative transfer in cylindrical threads with incident radiation VII. Multi-thread models
Plasma diagnostic in eruptive prominences from SDO/AIA observations at 304
EUV lines observed with EIS/Hinode in a solar prominence
Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling
Automatic Detection of Limb Prominences in 304 A EUV Images
Radiative transfer in cylindrical threads with incident radiation VI. A hydrogen plus helium system
Subject will be restored when possible
Effect of motions in prominences on the helium resonance lines in the extreme ultraviolet

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