Calculation of Radiative and Collisional Atomic Processes

\noindent Relativistic calculations of the atomic structure of Ne\ {\sc ix} have been carried out using the multi-configuration Dirac-Fock (MCDF) method. Accurate X-ray wavelengths and transition probabilities are obtained for transitions involving levels with $n\le10$. We use several techniques to calculate accurate and consistent multipole $A$-coefficients. For example we report a transition rate of $(1.07\pm0.02)\times10^4$\ sec$^{-1}$ for the $z$ line in good agreement with the recent EBIT measurement and with previous calculations using different methods. A subset of the MCDF orbitals for $n\le5$ is used for relativistic {\sl R}-matrix calculations of electron impact excitation and recombination using the Breit-Pauli {\sl R}-matrix and the Dirac {\sl R}-matrix methods. These atomic data, together with other related data from the APED atomic database for Ne\ {\sc ix}, are used to build an extended plasma model for X-ray spectra of the Capella corona observed by the {\sl Chandra} X-ray Observatory. We offer some possible explanations for the problems in the line intensity ratios K$\beta2$/K$\beta1$ and in the inconsistency in the temperature determined by differential emission measurements.