Ionization Fractions of Slow Ions in a Plasma with Kappa Distributions for the Electron Velocity

Details Ionization Fractions of Slow Ions in a Plasma with Kappa Distributions for the Electron Velocity Ionization Fractions of Slow Ions in a Plasma with Kappa Distributions for the Electron Velocity

Jun 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003apjs..146..443w&link_type=abstract

The Astrophysical Journal Supplement Series, Volume 146, Issue 2, pp. 443-457.

Astronomy and Astrophysics

Astronomy

10

Atomic Data, Atomic Processes, Plasmas

Scientific paper

The interpretation of a wide variety of astrophysical observations requires an understanding of how ionization fractions depend on plasma parameters. Observations have indicated that electron velocity distributions in space plasmas generally have enhanced high-energy tails. Instead of a Maxwellian distribution, they are better described by a kappa distribution, characterized by the kinetic temperature, T, and a parameter, κ, that quantifies the deviation from a Maxwellian. We calculate and tabulate the equilibrium ionization fractions of N, O, Ne, Mg, S, Si, Ar, Ca, Fe, and Ni, based on a balance of ionization and recombination processes, for 104K<=T<=108K (or up to 109 K for Fe and Ni) and for various Maxwellian and kappa distributions. For a Maxwellian distribution of electrons, the mean charge as a function of temperature is characterized by plateaux corresponding to closed-shell charge states, with transitions over narrow ranges of logT. However, for kappa distributions, which are more realistic models of the observed electron distributions in coronal or space plasmas, those transitions are substantially broader. We find that a lower κ value (more suprathermal electrons) frequently leads to a higher mean charge, especially for low temperatures, but can also lead to a lower mean charge in certain temperature ranges; these effects are associated with the sharp energy thresholds and resonances of ionization and dielectronic recombination cross sections, respectively. The results provide information for various applications in which observed ionization fractions are used as diagnostics of astrophysical plasmas.

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