Plasma density structures at comet 67P/Churyumov–Gerasimenko

Engelhardt, Ilka; Eriksson, Anders; Wieser, Gabriella Stenberg; Goetz, Charlotte; Rubin, Martin; Henri, Pierre; Nilsson, Hans; Odelstad, Elias; Hajra, Rajkumar; Vallières, Xavier

doi:10.1093/mnras/sty765

Cited by 13 publications

(23 citation statements)

References 35 publications

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“…All of the observed ion-neutral collisionopause crossings are within the predicted region. The ion-pileup boundary crossing observed by Giotto at Halley occurred just outside the area where the electron-neutral collisionopause could be located, providing further support for the proposal that this boundary is formed as a result of cooling of electrons (Ip et al 1988;Gan & Cravens 1990;Häberli et al 1995Häberli et al , 1996Lindgren et al 1997;Eriksson et al 2017;Henri et al 2017;Engelhardt et al 2018). The crossing of the diamagnetic cavity boundary observed by Giotto at Halley was well within the location where the electron-neutral collisionopause could be located, and closest approach was clearly inside of this collisionopause.…”

Section: Observations During Spacecraft Flybys Of Cometssupporting

confidence: 52%

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

Mandt

Eriksson

Beth

et al. 2019

A&A

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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.

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Section: Observations During Spacecraft Flybys Of Cometssupporting

confidence: 52%

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

Mandt

Eriksson

Beth

et al. 2019

A&A

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“…In addition to these warm electrons, clear signatures of cold (

≲

0.1 eV) electrons have also been observed by LAP (Engelhardt et al, ; Eriksson et al, ) and MIP (Gilet et al, ). In LAP, these show up in high‐time‐resolution current measurements at fixed bias voltage in the form pulses of typical duration between a few seconds and a few minutes, and in bias voltage sweeps in the form of very steep slopes in the current‐voltage curve at high positive bias voltages (to be discussed further below).…”

Section: Introductionmentioning

confidence: 86%

Ion Velocity and Electron Temperature Inside and Around the Diamagnetic Cavity of Comet 67P

et al. 2018

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A major point of interest in cometary plasma physics has been the diamagnetic cavity, an unmagnetized region in the innermost part of the coma. Here we combine Langmuir and Mutual Impedance Probe measurements to investigate ion velocities and electron temperatures in the diamagnetic cavity of comet 67P, probed by the Rosetta spacecraft. We find ion velocities generally in the range 2–4 km/s, significantly above the expected neutral velocity ≲1 km/s, showing that the ions are (partially) decoupled from the neutrals, indicating that ion‐neutral drag was not responsible for balancing the outside magnetic pressure. Observations of clear wake effects on one of the Langmuir probes showed that the ion flow was close to radial and supersonic, at least with respect to the perpendicular temperature, inside the cavity and possibly in the surrounding region as well. We observed spacecraft potentials ≲−5 V throughout the cavity, showing that a population of warm (∼5 eV) electrons was present throughout the parts of the cavity reached by Rosetta. Also, a population of cold ( ≲0.11emeV) electrons was consistently observed throughout the cavity, but less consistently in the surrounding region, suggesting that while Rosetta never entered a region of collisionally coupled electrons, such a region was possibly not far away during the cavity crossings.

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“…While properties like amplitude, width and strength of asymmetry can change significantly from event to event, they are still strikingly similar in respect to their general shape. Comparable structures have been observed in the electron density (Engelhardt et al, 2018;Hajra et al, 2018b) and ion energy (Stenberg Wieser et al, 2017). Due to their highly asymmetric shape we will refer to them as steepened wave events in the following.…”

Section: Introductionmentioning

confidence: 76%

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

Ostaszewski¹,

Glaßmeier²,

Goetz³

et al. 2020

Preprint

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Abstract. We present a statistical survey of large amplitude, asymmetric plasma, and magnetic field enhancements at comet 67P/Churyumov-Gerasimenko from December 2014 to June 2016. The aim is to provide a general overview of these structures' properties over the mission duration. At comets, nonlinear wave evolution plays an integral part in the development of turbulence and in particular facilitates the transfer of energy and momentum. As the first mission of its kind, the ESA Rosetta mission was able to study the plasma properties of the inner coma for a prolonged time and during different stages of activity. This enables us to study the temporal evolution of steepened waves and their characteristics. In total, we identified ~70000 events in the magnetic field data by means of machine learning. We observe that the occurrence of wave events is linked to the activity of the comet, where events are primarily observed at high outgassing rates. No clear indications of a relationship between the occurrence rate and solar wind conditions were found. The waves are found to propagate predominantly perpendicular to the background magnetic field, which indicates their compressive nature. Characteristics like amplitude, skewness, and width of the waves were extracted by fitting a skew normal distribution to the magnetic field magnitude of individual events. With increasing massloading the average amplitude of steepened waves decreases while the skewness increases. Using a modified 1D MHD model it was possible to show that such solitary structures can be described by the combination of nonlinear, dispersive, and dissipative effects. By combining the model with observations of amplitude, width, and skewness we obtain an estimate of the effective plasma viscosity in the comet-solar wind interaction region. At 67P/Churyumov-Gerasimenko steepened waves are of particular importance as they dominate the innermost interaction region for intermediate to high activity.

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Plasma density structures at comet 67P/Churyumov–Gerasimenko

Cited by 13 publications

References 35 publications

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

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

Ion Velocity and Electron Temperature Inside and Around the Diamagnetic Cavity of Comet 67P

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

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