Atomic data and spectral line intensities for Ni XV

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Abstract

Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for Ni XV. We include in the calculations the 9 lowest configurations, corresponding to 126 fine structure levels: 3s2 3p2, 3s3p3, 3s2 3p3d, 3p4, 3s3p2 3d, and 3s2 3p4l with l=,s,p,d,f. Collision strengths are calculated at five incident energies for all transitions: 7.8, 18.5, 33.5, 53.5, and 80.2 Ry above the threshold of each transition. An additional energy, very close to the transition threshold, has been added, whose value is between 0.004 and 0.28 Ry depending on the levels involved. Calculations have been carried out using the Flexible Atomic Code and the distorted-wave approximation. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates calculated in the present work, statistical equilibrium equations for level populations are solved at electron densities covering the 108–1014 cm−3 range and at an electron temperature of logTe(K)=6.4, corresponding to the maximum abundance of Ni XV. Spectral line intensities are calculated, and their diagnostic relevance is discussed. This dataset will be made available in the next version of the CHIANTI database.

Introduction

In recent years, we have started a program to calculate atomic data and transition rates for atomic transitions that emit lines that have been observed in astrophysical spectra, and yet have been neglected in the literature. These include ions for which a large number of calculations are available for low-energy configurations, but whose high-energy configurations have not been considered (i.e., C-like, N-like, and O-like systems), or ions that have never or only seldom been studied. The present work focuses on one of the latter ions: Si-like Ni, or Ni XV. This ion emits a number of bright allowed transitions in the EUV range, many of which have been observed over the years in solar spectra from active plasmas [1], [2], [3], [4], [5], [6], [7], [8], [9]. Observations of Ni XV in the visible were made during eclipses [10], [11] and were used to determine the electron density in the corona [12]. Many of these lines can be used to form strongly density sensitive intensity ratios that can be applied to the measurement of the electron density of active solar and stellar plasmas. Also, configurations with principal quantum number n=4 emit a number of observable lines in the soft X-rays between 45 and 80 Å that can be used for plasma diagnostics.

While several authors calculated Ni XV A values, the available datasets of electron impact excitation rate coefficients for Ni XV are surprisingly scarce. The CHIANTI database [13], [14] includes the same unpublished distorted-wave calculations made by one of the CHIANTI team members (Mason) since its first release in 1996. To our knowledge comprehensive datasets of electron impact collision excitation rate coefficients even for the lowest configurations have never been published. The aim of the present work is to provide a complete dataset of energy levels, A values and electron collisional excitation rates for Ni XV that allows prediction of the spectrum of this ion and use for plasma diagnostic purposes. We also discuss here their diagnostic relevance.

Section snippets

Atomic data

To calculate a complete set of atomic data and transition rates and to assess its accuracy, we have used two suites of codes: the Flexible Atomic Code (FAC) by Gu [15] and the University College London (UCL) suite of codes [16], [17].

FAC is a relativistic configuration interaction program. The radial wavefunctions for single-electron orbitals are obtained with a self-consistent field method based on the Dirac formulation. The Dirac–Coulomb Hamiltonian is diagonalized to obtain energy levels and

Assessment of results

We have compared the energy levels and A values obtained in the present work with results from previous calculations and–where available–from observations. We have also compared the results we obtained with both FAC and SST adopting the three different atomic models.

FAC and SST energies calculated with the same model tend to agree to within 1.5%, the differences being slightly larger in the ground configuration. The results obtained using Model 3 are rather different from those of the two

Level populations and relative line intensities

In the absence of absorption of solar blackbody radiation and proton excitation, the level populations are obtained by solving the equations dNidt=NeNi(j>iCije+j<iCijd)+j>iNjAjiNij<iAij+Ne(j>iNjCjid+j<iNjCjie)dNidt=0for steady state where Nj is the number density of level j, Ne is the electron density, and Aji (s−1) is the spontaneous radiative transition rate from level j to level i. The equations have been solved at electron densities of logNe(cm3)=8, 9, 10, 11, 12, 13, and 14. Level

Comparison with observations

Ni XV spectral lines have been observed in the solar EUV spectrum on a number of occasions, emitted from active region or subflare plasmas. Visible lines have been observed in quiescence from coronographic observations made by Fisher and Pope [12].

Ni XV lines provide several strongly density sensitive intensity ratios that can be used for density diagnostics in active region conditions. Their density sensitivity is due to the levels of the ground configuration reaching equilibrium populations

Conclusions

In the present work we have calculated a complete set of level energies, oscillator strengths, A values, and collision strengths for 9 configurations of Ni XV, corresponding to 126 fine structure levels. We used three different sets of configurations to describe the target ion, and two different suites of programs (FAC and UCL), to assess the accuracy of our results. We have also compared the radiative rates and level lifetimes we obtained with results from other calculations for a few key

Acknowledgments

The work of Enrico Landi is supported by NASA grants NNX10AM17G and NNX11AC20G. Calculations were carried out using Discover computer of the NASA Center for Computation Science. The authors thank the referee for his/her valuable comments that helped them to improve the original manuscript.

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