1. Introduction

The study of electron density and temperature diagnostics of solar plasmas using emission lines was first extensively reviewed by Gabriel & Jordan (1972), with more recent work including that of Feldman et al. (1992), Dwivedi (1994) and Mason & Monsignori Fossi (1994). A common procedure involves searching for intensity ratios which are sensitive to electron density () and/or electron temperature (). In the present paper we consider the case of the MgVI ion which has five fine-structure levels in the ground configuration, each of which is sensitive to electron density values relevant to the solar transition region. Therefore, although lines of MgVI are relatively weak in the solar spectrum, some of the line ratios are potentially very useful for electron density/temperature diagnostics in different solar phenomena such as the quiet-Sun, active regions and solar flares (Bhatia & Mason 1980; Raju & Gupta 1993).

   

Orbital Orbital

1s 75.47140 1 11.69120 44.75401 1 3.14288

4.96191 1 19.81840 -440.48331 2 3.19387

0.10120 2 5.30819 1104.09003 3 3.19394

13.81971 2 9.99860 -1038.47746 4 3.60095

0.00039 2 4.07241

2p 25.22256 2 4.44405

2s -21.53204 1 11.69120 21.80006 2 7.10836

-0.71817 1 19.81840 10.49715 2 3.64321

19.64494 2 5.30819 4.90313 2 14.97880

-51.70492 2 9.99860

33.51006 2 4.07241 49.25869 2 2.52065

-117.35998 3 3.18186

3s 9.86858 1 9.50846

-23.25848 2 3.67081 74.19637 3 4.38082

10.98560 3 2.34889

At present there is a paucity of atomic data available for the electron-impact excitation of the MgVI ion. An early calculation by Saraph et al. (1969) produced LS collision strengths for transitions among the 2s²2pS, ²D and ²P levels using the exact resonance distorted wave method. Only a limited number of incident electron energies were considered in this work which makes the Maxwellian averaging necessary to produce the astrophysically important effective collision strengths impossible. Davis et al. (1976) presented results for electron-impact excitation rate coefficients for a few selected transitions (2s²2pS - 2s2pP, 2s²2pP - 2s2pP) in MgVI. The calculations were carried out in the distorted wave approximation with exchange when required. Many of the rate coefficients produced by this work were expressed in terms of a simple two parameter fit. The most elaborate calculation to date was performed by Bhatia & Mason (1980), again utilizing the distorted wave method. They evaluated fine-structure collision strengths for transitions among the 2s²2p³ and 2s2p⁴ levels in MgVI. The data, however, were presented for only three incident electron energies (10, 15, 20 Ryd.) and the effective collision strengths required for many astrophysical and plasma applications were not evaluated.

Clearly there is a need for a large and sophisticated close-coupling calculation to evaluate for the first time accurate effective collision strengths for the electron-impact excitation of MgVI. The purpose of the present work is to perform the most elaborate evaluation to date which (a) includes a large number of target eigenstates in the wavefunction representation of the MgVI ion; (b) incorporates channel coupling and configuration-interaction effects by the inclusion of higher-lying levels; (c) produces collision strengths at a very fine mesh of incident electron energies to properly resolve the autoionizing resonances in the collision cross sections, which are known to significantly enhance the resulting effective collision strengths, and finally (d) to produce the astrophysically important fine-structure effective collision strengths over a wide range of electron temperatures.