# Solar wind charge exchange in cometary atmospheres. II. Analytical model

## TL;DR

This paper develops an analytical model for solar wind charge exchange processes at comets, providing insights into ion distributions, neutral atom production, and solar wind interactions, validated with data from the Rosetta mission to comet 67P.

## Contribution

It introduces a comprehensive analytical formalism for charge-changing reactions at comets, including effects of double charge exchange and solar wind parameters, enhancing understanding of cometary atmospheres.

## Key findings

- Double charge exchange significantly affects helium neutral atom production below 200 km/s solar wind speed.
- Model predicts a sharp decrease in solar wind ion fluxes near perihelion, forming a solar wind ion cavity.
- Retrieval methods for outgassing rates and upstream solar wind fluxes from in situ measurements are discussed.

## Abstract

Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet because they mass-load the solar wind through an effective conversion of fast, light solar wind ions into slow, heavy cometary ions. The ESA/Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P) provided a unique opportunity to study charge-changing processes in situ. An extended analytical formalism describing solar wind charge-changing processes at comets along solar wind streamlines is presented. It is based on a thorough book-keeping of available charge-changing cross sections of hydrogen and helium particles in a water gas. After presenting a general 1D solution of charge exchange at comets, we study the theoretical dependence of charge-state distributions of (He$^{2+}$, He$^+$, He$^0$) and (H$^+$, H$^0$, H$^-$) on solar wind parameters at comet 67P. We show that double charge exchange for the He$^{2+}$-H$_2$O system plays an important role below a solar wind bulk speed of 200 km/s , resulting in the production of He energetic neutral atoms, whereas stripping reactions can in general be neglected. Retrievals of outgassing rates and solar wind upstream fluxes from local Rosetta measurements deep in the coma are discussed. Solar wind ion temperature effects at 400 km/s solar wind speed are well contained during the Rosetta mission. As the comet approaches perihelion, the model predicts a sharp decrease of solar wind ion fluxes by almost one order of magnitude at the location of Rosetta, forming in effect a solar wind ion cavity. This study is the second part of a series of three on solar wind charge-exchange and ionization processes at comets, with a specific application to comet 67P and the Rosetta mission.