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, mass-loading the solar wind through an effective conversion of fast light solar wind ions into slow heavy cometary ions. Aims. To understand these processes and place them in the context of a solar wind plasma interacting with a neutral atmosphere, numerical or analytical models are necessary. Inputs of these models, such as collision cross sections and chemistry, are crucial. Methods. Book-keeping and fitting of experimentally measured charge-changing and ionization cross sections of hydrogen and helium particles in a water gas are discussed, with emphasis on the low-energy/low-velocity range that is characteristic of solar wind bulk speeds (<20 keV u−1/2000 km s−1). Results. We provide polynomial fits for cross sections of charge-changing and ionization reactions, and list the experimental needs for future studies. To take into account the energy distribution of the solar wind, we calculated Maxwellian-averaged cross sections and fitted them with bivariate polynomials for solar wind temperatures ranging from 105 to 106 K (12–130 eV). Conclusions. Single- and double-electron captures by He2+ dominate at typical solar wind speeds. Correspondingly, single-electron capture by H+ and single-electron loss by H− dominate at these speeds, resulting in the production of energetic neutral atoms (ENAs). Ionization cross sections all peak at energies above 20 keV and are expected to play a moderate role in the total ion production. However, the effect of solar wind Maxwellian temperatures is found to be maximum for cross sections peaking at higher energies, suggesting that local heating at shock structures in cometary and planetary environments may favor processes previously thought to be negligible. This study is the first part in a series of three on charge exchange and ionization processes at comets, with a specific application to comet 67P/Churyumov-Gerasimenko and the Rosetta mission.

Atmospheric Escape Processes and Planetary Atmospheric Evolution

“…Charge transfer has recently been evidenced by the ESA/Rosetta ion spectrometers at comet 67P/Churyumov-Gerasimenko (67P), with the observation of H − ions (Burch et al, 2015), and He + fast ions (Nilsson et al, 2015). 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, Bodewits, 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 set of reactions is equivalent to coupled differential flux continuity equations, which can be solved analytically for the simplified case or numerically (Simon Wedlund et al, 2016;Simon Wedlund, Bodewits, et al, 2019).…”

“…These processes lead to the almost total conversion of the solar wind into ENAs, potentially escaping or sputtering the nucleus, by the time the solar wind impinges within a few tens of kilometers from the comet's surface, in the case of a highly outgassing nucleus (perihelion conditions). This total conversion depends on many parameters: outgassing rate, heliocentric distance, solar wind density, and speed Simon Wedlund, Bodewits, et al, 2019;. The effect of minor solar wind species (multiplycharged heavy ions) can be seen in the production of X-rays through charge exchange emission with the cometary atmosphere (Cravens, 1997).…”

“…The observation of the change in solar wind composition is a proof of charge exchange, for example, at comets Simon Wedlund, Bodewits, 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(Leblanc et al, , 2018.…”

Solar wind charge exchange in cometary atmospheres

“…Because the neutral atmosphere of a comet is in expansion and may extend to millions of kilometers in space (Combi et al 2004), SWCX reactions may strongly affect the solar wind massloading upstream of the nucleus, and thus the formation and dynamics of bow shock and cometopause structures (Gombosi 1987;Simon Wedlund et al 2017), and around the diamagnetic cavity (Ip 1990). Moreover, the projectile solar wind ion is usually much faster (∼400 km s −1 ) than the neutral species it hits (∼1 km s −1 ), with the result that the charge-exchanged species mostly keep their initial kinetic energy (Simon Wedlund et al 2019a). Momentum transfer due to charge exchange between ion projectiles and neutral targets may also take place, and dominates at low impact energies the total momentum transfer cross sections (Banks & Kockarts 1973;Ip 1990).…”