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Journal ArticleDOI

Influence of resonances on spectral formation of x-ray lines in Fe XVII.

17 Jun 2002-Physical Review Letters (American Physical Society)-Vol. 89, Iss: 1, pp 013202
TL;DR: Large-scale relativistic close coupling calculations reveal the precise effect of resonances in collisional excitation of x-ray lines of Ne-like Fe XVII and significant resonance enhancement of the collision strengths of forbidden and intercombination transitions is shown.
Abstract: New theoretical results from large-scale relativistic close coupling calculations reveal the precise effect of resonances in collisional excitation of x-ray lines of Ne-like Fe XVII. By employing the Breit-Pauli R-matrix method and an 89-level eigenfunction expansion, including up to n = 4 levels, significant resonance enhancement of the collision strengths of forbidden and intercombination transitions is shown. The present results should help resolve long-standing discrepancies; in particular, the present line ratios of three benchmark diagnostic lines 3C, 3D, and 3E at 15.014, 15.265, and 15.456 A, respectively, are in excellent agreement with two independent measurements on electron-beam ion traps. Strong energy dependence in cross sections due to resonances is demonstrated.
Citations
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Journal ArticleDOI
TL;DR: In this paper, the role of indirect processes in the soft X-ray line formation of iron L-shell ions, using a newly developed, relativistic, multiconfigurational atomic package, was investigated.
Abstract: We present a systematic study of the role of indirect processes in the soft X-ray line formation of iron L-shell ions, using a newly developed, relativistic, multiconfigurational atomic package. These indirect processes involve the neighboring charge states of the target ion, namely, radiative recombination, dielectronic recombination, and resonance excitation. For Fe XVII, the inner-shell collisional ionization (CI) is also relevant. Lines originating from 3s and 3p upper levels of Fe XVII-XX are found to be significantly affected by these processes, with some lines being enhanced by nearly a factor of 2 at the temperature of maximum fractional abundance in CI equilibrium. Such enhancement, although not enough to completely explain the observed line ratios from various astrophysical sources, is a vast improvement over the previous models neglecting these processes. Rate coefficients for individual processes are tabulated, which can be conveniently included in spectral models to correctly account for these effects. As a by-product of this investigation, the total recombination and ionization rates of all Fe L-shell ions are tabulated as a function of temperature.

431 citations

Journal ArticleDOI
TL;DR: The available atomic data used for interpreting and modeling x-ray observations can be divided into several levels of detail, ranging from compilations which can be used with direct inspection of raw data, such as line finding lists, to synthetic spectra which attempt to fit to an entire observed dataset simultaneously as mentioned in this paper.
Abstract: The available atomic data used for interpreting and modeling x-ray observations are reviewed The applications for these data can be divided into several levels of detail, ranging from compilations which can be used with direct inspection of raw data, such as line finding lists, to synthetic spectra which attempt to fit to an entire observed dataset simultaneously This review covers cosmic sources driven by both electron ionization and photoionization and touches briefly on planetary surfaces and atmospheres The applications to x-ray astronomy, the available data, and recommendations for astronomical users are all reviewed, and an attempt to point out the applications where the shortcomings are greatest is also given

101 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present complete collisional-radiative modelling results for the soft x-ray emission lines of Fe16+ in the 15 A-17 A range.
Abstract: We present complete collisional-radiative modelling results for the soft x-ray emission lines of Fe16+ in the 15 A–17 A range. These lines have been the subject of much controversy in the astrophysical and laboratory plasma community. Radiative transition rates are generated from fully relativistic atomic structure calculations. Electron-impact excitation cross sections are determined using a fully relativistic R-matrix method employing 139 coupled atomic levels through n = 5. We find that, in all cases, using a simple ratio of the collisional rate coefficient times a radiative branching factor is not sufficient to model the widely used diagnostic line ratios. One has to include the effects of collisional-radiative cascades in a population model to achieve accurate line ratios. Our line ratio results agree well with several previous calculations and reasonably well with tokamak experimental measurements, assuming a Maxwellian electron-energy distribution. Our modelling results for four EBIT line ratios, assuming a narrow Gaussian electron-energy distribution, are in generally poor agreement with all four NIST measurements but are in better agreement with the two LLNL measurements. These results suggest the need for an investigation of the theoretical polarization calculations that are required to interpret the EBIT line ratio measurements.

68 citations

Journal ArticleDOI
TL;DR: In this article, the authors have re-examined the original solar spectra, identifying the Fe XVI λ15.21 line and measuring its flux to account for the contribution of Fe XVI to the λ 15.26 flux.
Abstract: Resonance scattering has often been invoked to explain the disagreement between the observed and predicted line ratios of Fe XVII λ15.01 to Fe XVII λ15.26 (the "3C/3D" ratio). In this process photons of λ15.01, with its much higher oscillator strength, are preferentially scattered out of the line of sight, thus reducing the observed line ratio. Recent laboratory measurements, however, have found significant inner-shell Fe XVI lines at 15.21 and 15.26 A, suggesting that the observed 3C/3D ratio results from blending. Given our new understanding of the fundamental spectroscopy, we have reexamined the original solar spectra, identifying the Fe XVI λ15.21 line and measuring its flux to account for the contribution of Fe XVI to the λ15.26 flux. Deblending brings the 3C/3D ratio into good agreement with the experimental ratio; hence, we find no need to invoke resonance scattering. Low opacity in Fe XVII λ15.01 also implies low opacity for Fe XV λ284.2, ruling out resonance scattering as the cause of the fuzziness of TRACE and SOHO-EIT 284 A images. The images must, instead, be unresolved due to the large number of structures at this temperature. Insignificant resonance scattering implies that future instruments with higher spatial resolution could resolve the active region plasma into its component loop structures.

51 citations


Cites background from "Influence of resonances on spectral..."

  • ...Furthermore, new theoretical models are converging toward a ratio closer to three (e.g. Doron & Behar 2002; Chen & Pradhan 2002; Gu 2003), though all published models continue to exceed the measurements by at least 10%....

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Journal ArticleDOI
TL;DR: In this paper, the authors used an electron beam ion trap and an X-ray microcalorimeter for the n = 3 to n = 2 Fe XVII emission lines in the 15-17 A range, along with new theoretical predictions for electron energy distributions.
Abstract: New laboratory measurements using an electron beam ion trap and an X-ray microcalorimeter are presented for the n = 3 to n = 2 Fe XVII emission lines in the 15-17 A range, along with new theoretical predictions for a variety of electron energy distributions. This work improves upon our earlier work on these lines by providing measurements at more electron-impact energies (seven values from 846 to 1185 eV), performing an in situ determination of the X-ray window transmission, taking steps to minimize the ion impurity concentrations, correcting the electron energies for space charge shifts, and estimating the residual electron energy uncertainties. The results for the 3C/3D and 3s/3C line ratios are generally in agreement with the closest theory to within 10%, and in agreement with previous measurements from an independent group to within 20%. Better consistency between the two experimental groups is obtained at the lowest electron energies by using theory to interpolate, taking into account the significantly different electron energy distributions. Evidence for resonance collision effects in the spectra is discussed. Renormalized values for the absolute cross sections of the 3C and 3D lines are obtained by combining previously published results and shown to be in agreement with the predictions of converged R-matrix theory. This work establishes consistency between results from independent laboratories and improves the reliability of these lines for astrophysical diagnostics. Factors that should be taken into account for accurate diagnostics are discussed, including electron energy distribution, polarization, absorption/scattering, and line blends.

49 citations