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 (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.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a better resolution in the published version. New version reflects minor changes brought after proof editin

DIAGNOSTIC LINE RATIOS FOR HIGHLY IONIZED IONS OF THE BERYLLIUM ISOELECTRONIC SEQUENCE AND A COMPARISON WITH SOLAR OBSERVATIONAL DATA

Observations of the relative line strengths of Beryllium like ions in a high temperature plasma can be used to derive the electron temperature and density of the emitting region through diagnostic line ratios [1, 2]. However, to calculate such ratios reliably, accurate atomic data must be employed, especially for the electron impact excitation rates of the relevant transitions [3]. In this paper we compare the theoretical S XIII and Ar XV emission line ratios R1 = I (2s2p 1P - 2s2 1S) / I (2s2p 3P1 - 2s2 1S), and R2 = I (2s2p 1P - 2s2 1S) / I (2p2 3P2 - 2s2p 3P2) with observational data for several solar flares, obtained using the SO82A instrument on board Skylab. Good agreement is found between theory and observation for R1 for both ions, which provides support for the electron impact excitation rates adopted in the calculations. However, in the case of R2, all the observed values for S XIII and Ar XV are much smaller than the theoretical estimates, which is probably due to blending in the 2p2 3P2 - 2s2p 3P2) lines

Electron densities in the coronae of the Sun and Procyon from extreme-ultraviolet emission line ratios in Fe XI

New R-matrix calculations of electron impact excitation rates for Fe XI are used to determine theoretical emission line ratios applicable to solar and stellar coronal observations. These are subsequently compared to solar spectra of the quiet Sun and an active region made by the Solar EUV Rocket Telescope and Spectrograph (SERTS-95), as well as Skylab observations of two flares. Line blending is identified, and electron densities of 10(9.3), 10(9.7), greater than or equal to 10(10.8), and greater than or equal to 10(11.3) cm(-3) are found for the quiet Sun, active region, and the two flares, respectively. Observations of the F5 IV-V star Procyon, made with the Extreme Ultraviolet Explorer (EUVE) satellite, are compared and contrasted with the solar observations. It is confirmed that Procyon's average coronal conditions are very similar to those seen in the quiet Sun, with N-e = 10(9.4) cm(-3). In addition, although the quiet Sun is the closest solar analog to Procyon, we conclude that Procyon's coronal temperatures are slightly hotter than solar. A filling factor of 25(-12)(+38)% was derived for the corona of Procyon