The comparison between models and observations of optically thin astrophysical plasmas is limited by the diagnostic and simulation techniques today available so much that several long-standing discrepancies, such as those regarding the coronal emission in spectral lines belonging to the Li-like iso-electronic sequence, the elemental abundances in stellar coronae and the use of density diagnostics, have still found no definitive solution. Furthermore, in dynamical plasmas that evolve on timescales less than the ionisation / recombination timescales, metastable levels have an important role in determining spectral line emission and simulations of astrophysical plasmas have generally ignored such effects because of the difficulties in their treatment.
From a diagnostic point of view, a reliable evaluation of the distribution of plasma in electron density would allow to remove ambiguities in the interpretation of spectra related to the magnetic field topology and to the plasma dynamics. It is known that our capability in deriving such distribution is limited mainly by uncertainties in the ionisation and recombination modelling. Recent developments of the Generalised Collisional Radiative (GCR) theory, which include an accurate dependence on electron density via the explicit treatment of metastable and high-lying levels and an accurate treatment of the dielectronic recombination, have been successfully applied to laboratory plasmas, both through numerical simulation and diagnostic techniques. It is expected that the application to astrophysical plasmas will provide the necessary advance for solving long-standing discrepancies and remove much of the ambiguities in the interpretation of spectra from the UV to the soft X-rays.
The project involves:
The calculations of fundamental atomic parameters are tailored to the need for astrophysical applications, in particular to the spectroscopic analysis of coronal plasma (e.g., SOHO/CDS and SOHO/SUMER). These are performed using the UK APAP set of codes.
A a first step we have focused on Mg ions, aiming at obtaining a better accuracy in the measurements of Mg abundance in coronal plasmas.
Preliminary test runs have been carried out using the CINECA CLX linux cluster.
As part of the project, we are carrying out the porting of the R-matrix code for the solution of ion-electron scattering problem to the GRID - PI2S2 infrastructure managed by the COMETA consortium.
First results obtained with the GRID - PI2S2 computers concern the calculation of electron impact cross-sections for Mg VI.
In Fig.1, the cross-section (collision strengths - Omega) as a function of the impacting electron energy (in Ryd) for an allowed transition in Mg VI is shown. Structures below the ionization threshold are due to scattering resonances.
The R-matrix code porting on the GRID-PI2S2 infrastructure is being carried out by Alessandro Lanzafame and Francesco Marziani.
Alessandro Lanzafame. Last edit: 21 April 2008