Nicolas Labrosse

Physics of Solar Prominences: I—Spectral Diagnostics and Non-LTE Modelling

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 (i.e. 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.

Hinode, TRACE, SOHO, and Ground-based Observations of a Quiescent Prominence

A quiescent prominence was observed by several instruments on 2007 April 25. The temporal evolution was recorded in Hα by the Hinode SOT, in X-rays by the Hinode XRT, and in the 195 A channel by TRACE. Moreover, ground-based observatories (GBOs) provided calibrated Hα intensities. Simultaneous extreme-UV (EUV) data were also taken by the Hinode EIS and SOHO SUMER and CDS instruments. Here we have selected the SOT Hα image taken at 13:19 UT, which nicely shows the prominence fine structure. We compare this image with cotemporaneous ones taken by the XRT and TRACE and show the intensity variations along several cuts parallel to the solar limb. EIS spectra were obtained about half an hour later. Dark prominence structure clearly seen in the TRACE and EIS 195 A images is due to the prominence absorption in H I, He I, and He II resonance continua plus the coronal emissivity blocking due to the prominence void (cavity). The void clearly visible in the XRT images is entirely due to X-ray emissivity blocking. We use TRACE, EIS, and XRT data to estimate the amount of absorption and blocking. The Hα integrated intensities independently provide us with an estimate of the Hα opacity, which is related to the opacity of resonance continua as follows from the non-LTE radiative-transfer modeling. However, spatial averaging of the Hα and EUV data have quite different natures, which must be taken into account when evaluating the true opacities. We demonstrate this important effect here for the first time. Finally, based on this multiwavelength analysis, we discuss the determination of the column densities and the ionization degree of hydrogen in the prominence.

Formation of helium spectrum in solar quiescent prominences

We present new non-LTE modelling of the helium spectrum emitted by quiescent solar prominences. The calculations are made in the frame of a one-dimensional plane-parallel slab. The physical parameters of our models are the electron temperature, the gas pressure, the slab width, the microturbulent velocity and the height above the solar surface. In this paper, we present isothermal isobaric models for a large range of temperature and pressure values. This work brings considerable improvements over the calculations of Heasley and co-workers (Heasley et al. 1974; Heasley & Milkey 1976, 1978, 1983) with the inclusion in our calculations of partial redistribution eects in the formation of the H i Ly ,L y ,H ei 584 Aa nd Heii 304 A lines. In addition we consider detailed incident proles for the principal transitions. The statistical equilibrium equations are solved for a 33 bound levels (He i and He ii) plus continuum atom, and the radiative transfer equations are solved by the Feautrier method with variable Eddington factors. In this way we obtain the helium level populations and the emergent line proles. We discuss the influence of the physical parameters on the helium level populations and on the main helium spectral lines. The eect of helium abundance in the prominence plasma is also studied. Some relations between singlet and triplet lines are given, as well as between optically thin or thick lines, He i and He ii lines, and between the He i 5876 Aa nd Hi 4863 A lines. In a future work this numerical code will be used for the diagnostic of the prominence plasma by comparing the results with SUMER observations.

Solar Science with the Atacama Large Millimeter/Submillimeter Array—A New View of Our Sun

The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere—a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA’s scientific potential for studying the Sun for a large range of science cases.

Non-LTE Radiative Transfer in Model Prominences. I. Integrated Intensities of He I Triplet Lines

In this work we use new results of radiative transfer calculations out of local thermodynamical equilibrium to study the triplet lines emitted by neutral helium in solar quiescent prominences. We compare two types of prominence atmospheres: isothermal and isobaric models versus nonisothermal and nonisobaric ones. We can thus investigate the effect of the presence of a prominence-to-corona transition region (PCTR) on the emergent intensities in detail. It is found that the presence of the PCTR affects the emitted intensities of the triplet lines, even though they are formed in the central parts of the prominence. We show that the inclusion of a transition region reduces the impact of collisional excitation at high temperatures in comparison with the isothermal and isobaric case. A simple study of helium energy level populations shows how statistical equilibrium is changed when a transition region is present. This points to the necessity of including an interface between the prominence body and the corona to predict all emergent intensities, whatever the region of formation of the radiation. We have found a correlation between most of the He I triplet line ratios and the altitude of the model prominence. Comparisons of our predicted intensity ratios with observations yield generally good agreement. Remaining discrepancies may be resolved by extrapolating our predicted results to higher altitudes.

Radiative transfer in cylindrical threads with incident radiation - VI. A hydrogen plus helium system

Context: Spectral lines of helium are commonly observed on the Sun. These observations contain important information about physical conditions and He/H abundance variations within solar outer structures. Aims: 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. Methods: 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 hydrogen and helium atoms, using cylindrical coordinates and prescribed solar incident radiation. Electron density is determined by the ionization equilibria of both atoms. Two-dimensional effects are included. Results: 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. Conclusions: 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.

EUV lines observed with EIS/Hinode in a solar prominence

We report on observations of a solar prominence obtained on 26 April 2007 using the Extreme Ultraviolet Imaging Spectrometer on Hinode. Several regions within the prominence are identified for further analysis. Selected profiles for lines with formation temperatures between log(T)=4.7-6.3, as well as their integrated intensities, are given. The line profiles are discussed. We pay special attention to the He II line which is blended with coronal lines. Our analysis confirms that depression in EUV lines can be interpreted by two mechanisms: absorption of coronal radiation by the hydrogen and neutral helium resonance continua, and emissivity blocking. We present estimates of the He II line integrated intensity in different parts of the prominence according to different scenarios for the relative contribution of absorption and emissivity blocking on the coronal lines blended with the He II line. We estimate the contribution of the He II 256.32 line in the He II raster image to vary between ~44% and 70% of the rasters total intensity in the prominence according to the different models used to take into account the blending coronal lines. The inferred integrated intensities of the He II line are consistent with theoretical intensities obtained with previous 1D non-LTE radiative transfer calculations, yielding a preliminary estimate for the central temperature of 8700 K, central pressure of 0.33 dyn/cm^2, and column mass of 2.5 10^{-4} g/cm^2. The corresponding theoretical hydrogen column density (10^{20} cm^{-2}) is about two orders of magnitude higher than those inferred from the opacity estimates 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 region by resonant scattering of the incident radiation.

Effect of motions in prominences on the helium resonance lines in the extreme ultraviolet

Context. 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. Aims. 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. Methods. 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. Results. The motion of the prominence plasma induces a Doppler dimming effect on the resonance lines of He i and He ii .T he velocity effects are particularly important for the He ii λ 304 A line as it is mostly formed by resonant diffusion of incident radiation under prominence conditions. The He i resonance lines at 584 and 537 A 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. Conclusions. 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.

Plasma diagnostic in eruptive prominences from SDO/AIA observations at 304 Å

SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, Scotlande-mail:Nicolas.Labrosse@glasgow.ac.ukPreprint online version: November 22, 2011ABSTRACTContext.Theoretical calculations have shown that when solar prominences move away from the surface of the Sun, their radiativeoutput is affected via the Doppler dimming or brightening effects.Aims.In this paper we ask whether observational signatures of the changes in the radiative output of eruptive prominences can befound in EUV (extreme ultraviolet) observations of the first resonance line of ionised helium at 304 A. We also investigate whetherthese observations can be used to perform a diagnostic of the plasma of the eruptive prominence.Methods.We first look for suitable events in the SDO /AIA database. The variation of intensity of arbitrarily selected features in the304 channel is studied as a function of velocity in the plane of the sky. These results are then compared with new non-LTE radiativetransfer calculations of the intensity of the Heii304 resonance line.Results.We find that observations of intensities in various parts of t he four eruptive prominences studied here are sometimes consis-tent with the Doppler dimming effect on the Heii304 A line. However, in some cases, one observes an increase in intensity in the 304channel with velocity, in contradiction to what is expected from the Doppler dimming effect alone. The use of the non-LTE modelsallows us to explain the different behaviour of the intensity by changes in the plasma parameters inside the prominence, in particularthe column mass of the plasma and its temperature.Conclusions.The non-LTE models used here are more realistic than what wasused in previous calculations. They are able to repro-duce qualitatively the range of observations from SDO/AIA analysed in this study. Thanks to non-LTE modelling, we can infer theplasma parameters in eruptive prominences from SDO/AIA observations at 304 A.Key words. Sun: filaments, prominences – Line: formation – Radiative tr ansfer – Sun: corona

A global 2.5‐dimensional three fluid solar wind model with alpha particles

[1] A global 2.5-dimensional three fluid solar wind model is presented. Two ion species, namely protons and alpha particles, are heated by an empirical energy flux while electrons are heated by the classical heat flux and Coulomb coupling with ions. It is found that for a reasonable relative speed between alpha particles and protons at 1 AU to be achieved, the alphas need to be preferentially heated in the inner corona. No external heating is applied in the streamer base, the closed magnetic field region. A hot coronal boundary, the electron heat flux, and Coulomb coupling keep plasma species in equilibrium inside the streamer, and a nonisothermal streamer is found. The abundance of alpha particles varies within the streamer base. It is small in the streamer core compared with streamer legs, and alphas continuously drain out of the streamer core along magnetic field due to gravitational settling. The settling operates over a timescale of several days. Alpha particles in the slow wind have a smaller abundance than in the fast wind at 1 AU, in agreement with observations. This is mainly determined in the near-Sun region. For the coronal alpha abundances in the range 0.015-0.15, it is found that alpha particles play a negligible role in determining the magnetic field. In this sense, treating alphas as test particles is justified. However, alphas have an important impact on solar wind parameters. Coulomb collisions and heating drag alphas into the solar wind. The Coulomb friction with protons by itself is, however, unable to drive into the slow solar wind a flux of alphas flowing at roughly the same speed of protons as observed by in situ measurements at 1 AU.