On the production of He$^+$ of solar origin in the solar wind

TL;DR

This paper investigates the unexpected presence of solar-origin He$^+$ ions in the solar wind, proposing charge exchange with dust neutrals as a key formation mechanism, which current models underestimate.

Contribution

It introduces an empirical neutral density profile and suggests charge exchange with dust neutrals as a significant source of He$^+$ in the solar wind, challenging existing ionization models.

Findings

He$^+$$ ions are present in the solar wind with properties indicating solar origin.

Current ionization models underestimate He$^+$ populations by several orders of magnitude.

Charge exchange with circumsolar dust neutrals can produce observed He$^+$ levels.

Abstract

Solar wind measurements in the heliosphere are predominantly comprised of protons, alphas, and minor elements in a highly ionized state. The majority of low charge states, such as He$^{+}$, measured in situ are often attributed to pick up ions of non-solar origin. However, through inspection of the velocity distribution functions of near Earth measurements, we find a small but significant population of He$^+$ ions in the normal solar wind whose properties indicate that it originated from the Sun and has evolved as part of the normal solar wind. Current ionization models, largely governed by electron impact and radiative ionization and recombination processes, underestimate this population by several orders of magnitude. Therefore, to reconcile the singly ionized He observed, we investigate recombination of solar He$^{2+}$ through charge exchange with neutrals from circumsolar dust as a possible formation mechanism of solar He$^{+}$. We present an empirical profile of neutrals necessary for charge exchange to become an effective vehicle to recombine He$^{2+}$ to He$^{+}$ such that it meets observational He$^{+}$ values. We find the formation of He$^{+}$ is not only sensitive to the density of neutrals but also to the inner boundary of the neutral distribution encountered along the solar wind path. However, further observational constraints are necessary to confirm that the interaction between solar $\alpha$ particles and dust neutrals is the primary source of the He$^{+}$ observations.