Solar wind charge exchange in cometary atmospheres

Abstract

Context. 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. Aims. To understand the role of charge-changing reactions in the evolution of the solar wind plasma and to interpret the complex in situ measurements made by Rosetta, numerical or analytical models are necessary.Methods. 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. Results. 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 −1 , 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 −1 solar wind speed are well contained during the Rosetta mission. Conclusions. 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.

Atmospheric Escape Processes and Planetary Atmospheric Evolution

“…The observation of the change in solar wind composition is a proof of charge exchange, for example, at comets (Simon Wedlund, Behar, Kallio, et al. 2019; Simon Wedlund, Bodewits, et al., 2019; Simon Wedlund, Behar, et al., 2019). At Mars, the charge exchange of solar wind protons at the bow shock leads to precipitation of H that can be observed by the effects on the chemistry and by the backscatter (Halekas, 2017), even if the H chemistry at Mars is complex (Chaffin et al., 2017) One more striking example of charge‐exchange processes at Mars is the observation of heavier ions, such as O + , that later lead to sputtering (Leblanc et al., 2015, 2018).…”

“…The latter charge‐exchanged ions, originating from solar wind He 2+ ions (composing about 4% of the bulk of the undisturbed solar wind), were present throughout the mission from a heliocentric distance ranging from 3.4 to 2 AU (Simon Wedlund et al., 2016; Simon Wedlund, Behar, Kallio, et al. 2019; Simon Wedlund, Bodewits, et al., 2019; Simon Wedlund, Behar, et al., 2019). The net effect of the charge transfer of He 2+ solar wind ions with the neutral atmosphere of the comet (composed of molecules M) is the production of ENAs following the typical sequence of electron capture reactions (double charge transfer, and stripping reactions are ignored here for simplicity): …”

“…This total conversion depends on many parameters: outgassing rate, heliocentric distance, solar wind density, and speed (Simon Wedlund, Behar, Kallio, et al. 2019; Simon Wedlund, Bodewits, et al., 2019; Simon Wedlund, Behar, et al., 2019). The effect of minor solar wind species (multiply‐charged heavy ions) can be seen in the production of X‐rays through charge exchange emission with the cometary atmosphere (Cravens, 1997).…”

Solar wind charge exchange in cometary atmospheres

“…Burch et al (2015) detected H − negative ions in the early phase of the mission for the first time. They were confirmed to be arising from two consecutive singleelectron captures from the initial solar wind protons (Burch et al 2015;Simon Wedlund et al 2019b).…”

“…As solar wind H + and He 2+ ions impinge upon an atmosphere, charge-changing reactions start fractionating the initial charge state distribution into a mixture of H + , H 0 , and H − on the one hand, and of He 2+ , He + , and He 0 species on the other (Simon Wedlund et al 2019b). This effectively results in the production of fast energetic neutral atoms (ENAs), which are not bound to the plasma, but may interact further with the atmosphere downstream of the SWCX collision (see Fig.…”