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.

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.

A quiescent prominence was observed by several instruments on 2007 April 25. The temporal evolution was recorded in Hα by Hinode/SOT, in X-rays by Hinode/XRT and in the 195Å channel by TRACE. Moreover, ground-based observatories (GBO) provided calibrated Hα intensities. Simultaneous EUV data were also taken by the Hinode/EIS and SOHO/SUMER-CDS spectrometers. 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 co-temporal ones taken by 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 195Å images is due to the prominence absorption in HI, HeI and HeII resonance continua plus the coronal emissivity blocking due to the prominence void (cavity). The void clearly visible in XRT images is entirely due to X-ray emissivity blocking, since no prominence structure is seen in the XRT images because of negligible absorption at X-ray wavelengths. We use TRACE, EIS and XRT data to estimate the amount of absorption and blocking. Independently, the Hα integrated intensities provide us with an estimate of the Hα opacity and this is related to the opacity of resonance continua as follows from the non-LTE radiative-transfer modeling. Therefore, we have an independent check of the results obtained from TRACE/XRT and EIS/XRT. 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 multi-wavelength analysis, we discuss the determination of the column densities and the ionization degree of hydrogen in the prominence.

IRIS observation of the initiation of a solar flare suggests that turbulence helps to heat the Sun’s lower atmosphere.

Abstract. 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 effects in the formation of the H i Lyα, Lyβ, He i λ 584Å and He ii λ 304Å lines. In addition we consider detailed incident profiles 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 profiles. We discuss the influence of the physical parameters on the helium level populations and on the main helium spectral lines. The effect 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 λ 5876Å and H i λ 4863Å 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.