A Steady-State Picture of Solar Wind Acceleration and Charge State Composition Derived from a Global Wave-Driven MHD Model

TL;DR

This study uses a global MHD model to investigate steady-state solar wind ionization, revealing a potential subclass of highly ionized slow wind originating from coronal hole boundaries, with implications for understanding solar wind composition.

Contribution

It presents the first comprehensive charge state modeling across all latitudes in a realistic magnetic field, incorporating ionization calculations along open field lines in a steady-state solar atmosphere.

Findings

Slow wind shows higher charge states, consistent with observations.

Modeled ion fractions are under-predicted, indicating ionization rates may be underestimated.

Suprathermal electrons improve model agreement with observations.

Abstract

The higher charge states found in slow ($<$400km s$^{-1}$) solar wind streams compared to fast streams have supported the hypothesis that the slow wind originates in closed coronal loops, and released intermittently through reconnection. Here we examine whether a highly ionized slow wind can also form along steady and open magnetic field lines. We model the steady-state solar atmosphere using AWSoM, a global magnetohydrodynamic model driven by Alfv{\'e}n waves, and apply an ionization code to calculate the charge state evolution along modeled open field lines. This constitutes the first charge states calculation covering all latitudes in a realistic magnetic field. The ratios $O^{+7}/O^{+6}$ and $C^{+6}/C^{+5}$ are compared to in-situ Ulysses observations, and are found to be higher in the slow wind, as observed; however, they are under-predicted in both wind types. The modeled ion fractions of S, Si, and Fe are used to calculate line-of-sight intensities, which are compared to EIS observations above a coronal hole. The agreement is partial, and suggests that all ionization rates are under-predicted. Assuming the presence of suprathermal electrons improved the agreement with both EIS and Ulysses observations; importantly, the trend of higher ionization in the slow wind was maintained. The results suggest there can be a sub-class of slow wind that is steady and highly ionized. Further analysis shows it originates from coronal hole boundaries (CHB), where the modeled electron density and temperature are higher than inside the hole, leading to faster ionization. This property of CHBs is global, and observationally supported by EUV tomography.