Calculation of mean charges of slow ions in a plasma with a kappa distribution for the electron velocity

Abstract

The mean equilibrium charges of ions are important as diagnostics of high temperature plasmas and in understanding the charge state abundance of solar energetic particles. The equilibrium charge states are derived from a balance of ionization and recombination processes. For slow ions, only the collisions with surrounding electrons are important; therefore,the electron velocity distribution is important for our calculation. According to space observations, the enhanced number of high-energy electrons is well modeled by a kappa distribution, a distribution function with a parameter, K. We consider the formation of equilibrium charge states of N, O, Ne, Mg, S, Si, Ar, Ca, Fe, and Ni as well as the ionization fractions as a function of temperature and the K value. We find that the electron velocity distributions affect the mean equilibrium charges, which can be higher at lower K values (for a greater number of high-energy electrons). At lower temperature, the kappa distribution has a strong influence on the ionic charge states of all elements. In addition, in the range of coronal temperatures (~10x10x10x10x10x10 - 10x10x10x10x10x10x10 K), our results show a significant effect of different K values on the mean equilibrium charges of N, O, Ne, Ar, and Ca