Influence of collisions on ion dynamics in the inner comae of four comets

Abstract

Context. Collisions between cometary neutrals in the inner coma of a comet and cometary ions that have been picked up into the solar wind flow and return to the coma lead to the formation of a broad inner boundary known as a collisionopause. This boundary is produced by a combination of charge transfer and chemical reactions, both of which are important at the location of the collisionopause boundary. Four spacecraft measured ion densities and velocities in the inner region of comets, exploring the part of the coma where an ion-neutral collisionopause boundary is expected to form. Aims. The aims are to determine the dominant physics behind the formation of the ion-neutral collisionopause and to evaluate where this boundary has been observed by spacecraft. Methods. We evaluated observations from three spacecraft at four different comets to determine if a collisionopause boundary was observed based on the reported ion velocities. We compared the measured location of the ion-neutral collisionopause with measurements of the collision cross sections to evaluate whether chemistry or charge exchange are more important at the location where the collisionopause is observed. Results. Based on measurements of the cross sections for charge transfer and for chemical reactions, the boundary observed by Rosetta appears to be the location where chemistry becomes the more probable result of a collision between H2O and H2O+ than charge exchange. Comparisons with ion observations made by Deep Space 1 at 19P/Borrelly and Giotto at 1P/Halley and 26P/Grigg-Skjellerup show that similar boundaries were observed at 19P/Borrelly and 1P/Halley. The ion composition measurements made by Giotto at Halley confirm that chemistry becomes more important inside of this boundary and that electron-ion dissociative recombination is a driver for the reported ion pileup boundary.

Steepening of magnetosonic waves in the inner coma of comet 67P/Churyumov–Gerasimenko

“…Hence, for the following analysis, k B T i,0 ≈ 1 eV is assumed. For a solar wind primarily mass-loaded with H 2 O + the ion-neutral momentum transfer cross section was estimated to be σ in = 8 × 10 −19 m 2 (Mendis et al, 1986;Buti and Eviatar, 1989;Hajra et al, 2018b;Mandt et al, 2019). However, as stated by Mendis et al (1986) and Gunell et al (2017) the uncertainty of the cross section is significant, as no reliable laboratory measurements are available.…”

ROSINA ion zoo at Comet 67P

“…Overall, the H 2 O + peak intensity exhibits this trend, increasing with decreasing heliocentric distance (and hence increasing ν, at least for the photoionisation, Heritier et al 2018), though its variability over a day or during a month, in particular near perihelion is still puzzling (Beth et al 2016). Possible reasons, not investigated in this paper, include the variability of the generally-negative spacecraft potential (Odelstad et al 2018), the interaction between corotating interaction regions and coronal mass ejections (Hajra et al 2018) with the 67P's ionosphere, and to the proximity of the diamagnetic cavity (Goetz et al 2016a) or plasma boundaries (Mandt et al 2019). NH 3, but for 18-19 u•q −1 .…”

ROSINA ion zoo at Comet 67P

“…Overall, the H 2 O + peak intensity exhibits this trend, increasing with decreasing heliocentric distance (and hence increasing ν, at least for the photoionisation, Heritier et al 2018), though its variability over a day or during a month, in particular near perihelion is still puzzling (Beth et al 2016). Possible reasons, not investigated in this paper, include the variability of the generally-negative spacecraft potential (Odelstad et al 2018), the interaction between corotating interaction regions and coronal mass ejections (Hajra et al 2018) with the 67P's ionosphere, and to the proximity of the diamagnetic cavity (Goetz et al 2016a) or plasma boundaries (Mandt et al 2019). NH + 4 is produced by proton transfer between NH 3 and protonated molecules, mainly H 2 O + and H 3 O + (Geiss et al 1991;Beth et al 2016).…”

“…TOF spectrometers have a much higher time resolution acquiring several u q −1 all at once, at the expense of a lower sensitivity. Beside fostering the ion-neutral chemistry, very active comets exhibit several boundaries (Mandt et al 2019) and regions including the diamagnetic cavity (Cravens 1989). Ion composition may differ inside and outside such a cavity (Balsiger et al 1986b).…”