Comparative Analysis of the Solar Wind: Modeling Charge State Distributions in the Heliosphere

TL;DR

This paper models the non-equilibrium ionization in the solar wind using MHD simulations and compares the results with in situ measurements to understand charge state distributions and freeze-in distances.

Contribution

It introduces a NEI modeling approach combined with MHD simulations to analyze solar wind charge states and compares them with observational data for the first time.

Findings

Charge state densities differ between slow and fast solar wind.

Freeze-in distances correlate with electron density and velocity.

Model results match some observed charge state ratios.

Abstract

Non-equilibrium ionization (NEI) is a key process often times neglected when modeling astrophysical plasmas with thermodynamical timescales much shorter than the timescales for ionization and recombination. In this paper, we perform NEI modeling on a magnetohydrodynamic (MHD) simulation of the solar wind during the Whole Sun Month (Carrington Rotation 1913 from 1996 August 22 to September 18), and compare the resulting charge state distributions with in situ measurements made with the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses. We trace the wind trajectory back to 20$R_\odot$ using the velocity measured by Ulysses at about 4 AU, and obtain the plasma flow trajectory within 20$R_\odot$ using the MHD simulation data performed by Predictive Science Inc. We assume that the wind started from the solar surface and was initially in ionization equilibrium, and analyze the time-dependent ionization state of solar wind along each of the trajectories. In our analysis we: (1) obtain charge state densities and ratios for slow and fast winds based on the NEI model, and compare them with in situ observations; and (2) measure the "freeze-in" distance for several ions observed by SWICS to determine a possible correlation between when the ionization states become fixed and the electron density and outward velocity of the plasma at the freeze-in height. This study provides a stringent test on comparing charge state distributions from the outer corona predicted from simulations with in situ measurements made in the far heliosphere. This showcases the challenges in matching plasma conditions near the corona to those observed in interplanetary space.