Uncertainties on atomic data. A case study: N iv

Abstract

We consider three recent large-scale calculations for the radiative and electron-impact excitation data of N iv, carried out with different methods and codes. The scattering calculations employed the relativistic Dirac R-matrix (DARC) method, the intermediate coupling frame transformation (ICFT) R-matrix method, and the B-spline R-matrix (BSR) method. These are all large-scale scattering calculations with well-tested and sophisticated codes, which use the same set of target states. One concern raised in previous literature is related to the increasingly large discrepancies in the effective collision strengths between the three sets of calculations for increasingly weak and/or high-lying transitions. We have built three model ions and calculated the intensities of all the main spectral lines in this ion. We have found that, despite such large differences, excellent agreement (to within ± 20%) exists between all the spectroscopically-relevant line intensities. This provides confidence in the reliability of the calculations for plasma diagnostics. We have used the differences in the radiative and excitation rates amongst the three sets of calculations to obtain a measure of the uncertainty in each rate. Using a Monte Carlo approach, we have shown how these uncertainties affect the main theoretical ratios which are used to measure electron densities and temperatures.

R-matrix electron-impact excitation data for the C-like iso-electronic sequence

“…As shown later in Section 4, the atomic structures obtained in the present work show relatively large differences with respect to experiment values, especially for the first few ions in the isoelectronic sequence, which require R-matrix calculations with pseudo-states. In general, this inaccuracy does not significantly affect plasma diagnostics using spectroscopically and astrophysically important lines (Del Zanna et al 2019).…”

“…Consequently, the ICFT method is inherently significantly faster than the classic Breit-Pauli Rmatrix (BPRM) method (Berrington et al 1995), B-spline Rmatrix (BSR) code (Zatsarinny 2006), and Dirac atomic Rmatrix code (DARC 1 ). We refer readers to Fernández-Menchero et al (2016), Aggarwal (2017), andDel Zanna et al (2019) for recent comparisons among different R-matrix methods and the impact on plasma diagnostics.…”

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

“…Uncertainties in the radiative rates (oscillator strengths and A-values) can be assessed by comparing the values in the length and velocity forms [45], or more generally comparing the results of different calculations, as done e.g., for Fe XIII [74] and for N IV [75]. Other useful information is provided by laboratory measurements of radiative lifetimes.…”

Atomic data and uncertainties with PyAtomDB

“…They remain hard to calculate with meaningful accuracy, and even harder to implement in an analysis framework. Often, approaches compare in detail existing discrepancies between calculations, for example, Del Zanna et al (). However, for tackling the larger set of all uncertainties, a more general approach is required.…”