Effects of density on the oxygen ionization equilibrium in collisional plasmas

Abstract

<jats:title>Abstract</jats:title> <jats:p>The ion populations most frequently adopted for diagnostics in collisional plasmas are derived from the density independent, coronal approximation. In higher density, lower temperature conditions, ionisation rates are enhanced once metastable levels become populated, and recombination rates are suppressed if ions recombine into Rydberg levels. As a result, the formation temperatures of ions shift, altering the diagnostics of the plasma. To accurately model the effect of ionisation from metastable levels, new electron impact, ionisation cross sections have been calculated for oxygen, both for direct ionisation and excitation–auto-ionisation of the ground and metastable levels. The results have been incorporated into collisional radiative modelling to show how the ionisation equilibrium of oxygen changes once metastable levels become populated. Suppression of dielectronic recombination has been estimated and also included in the modelling, demonstrating the shifts with density in comparison to the coronal approximation. The final results for the ionisation equilibrium are used in differential emission measure modelling to predict line intensities for many lines emitted by O ii-O vi in the solar transition region. The predictions show improved agreement by 15-40% for O ii, O vi and the inter-combination lines of O iii-O v, when compared to results from coronal approximation modelling. While there are still discrepancies with observations of these lines, this could, to a large part, be explained by variability in the observations.</jats:p>

Atomic Data for Plasma Spectroscopy: The CHIANTI Database, Improvements and Challenges

“…As it turns out that CI from (populated) excited states is very important for the calculation of the charge state distribution, an effort is ongoing to calculate DI with FAC, and EA with the available R-matrix CE rates. Rates for the carbon and oxygen ions have been produced [55,56], and further work is ongoing.…”

“…One way to estimate the DR suppression is therefore to use the recent DR project rates and apply the variation with density found by [67]. This approximation has been adopted by [55,56] using directly the effective rates of [67], and by [68] using the empirical formulae of [69] which aim to reproduce the same effective rates. The [69] formulae will be introduced in CHIANTI version 10 to improve the modeling of high density plasma (on a side note, earlier empirical formulae by [70] were affected by a few problems).…”

“…The DR suppression was however only included approximately, as mentioned above. A similar model was built for oxygen [56]; an example of the resulting ion charge states is given in Figure 2.…”

Photoionization Models for High-density Gas

“…Density effects in coronal models leading to the suppression of dielectronic recombination (DR) have been addressed by Summers (1972). Dufresne et al (2020) explored the effects of DR suppression and metastable populations on ionization balance and line diagnostics in the solar transition region.…”

Modelling low charge ions in the solar atmosphere

“…The present work is part of a series of papers with the aim of building on those pioneering works, in order to answer the questions raised above, and to provide modelling suitable for the transition region. Dufresne & Del Zanna (2019) and Dufresne et al (2020) develop the coronal approximation of carbon and oxygen by adding the effects modelled by Burgess & Summers (1969) and Nussbaumer & Storey (1975). It is shown that, by including the effects of density on the electron collisional processes, predicted intensities are in better agreement with observations for the lines emitted below 100 000 K, when compared to coronal approximation modelling.…”