The Evolution of Ion Charge States in Coronal Mass Ejections

TL;DR

This study models ion charge states in CMEs using in situ and remote observations, incorporating non-Maxwellian electron distributions and heating effects, to better understand the plasma conditions during CME events.

Contribution

It introduces a detailed ionization balance model for CMEs that accounts for non-thermal electron distributions and compares different CME events to observational data.

Findings

Better match to observed charge states with kappa 2-4

Non-thermal electrons significantly influence ionization states

Implications for magnetic reconnection and plasma heating in CMEs

Abstract

We model the observed charge states of the elements C, O, Mg, Si, and Fe in the coronal mass ejections (CMEs) ejecta. We concentrate on "halo" CMEs observed in situ by ACE/SWICS to measure ion charge states, and also remotely by STEREO when in near quadrature with Earth, so that the CME expansion can be accurately specified. Within this observed expansion, we integrate equations for the CME ejecta ionization balance, including electron heating parameterized as a fraction of the kinetic and gravitational energy gain of the CME. We also include the effects of non-Maxwellian electron distributions, characterized as a kappa function. Focusing first on the 2010 April 3 CME, we find a somewhat better match to observed charge states with kappa in the range 2-4, close to the theoretical minimum value of kappa = 3/2, implying a hard spectrum of non-thermal electrons. Similar, but more significant results come from the 2011 February 15 event, although it is quite different in terms of its evolution. We discuss the implications of these values, and of the heating required, in terms of the magnetic reconnection Lundquist number and anomalous resistivity associated with CME evolution close to the Sun.