Four‐fluid MHD simulations of the plasma and neutral gas environment of comet 67P/Churyumov‐Gerasimenko near perihelion

Huang, Zhenguang; Tóth, G.; Gombosi, T. I.; Jia, Xianzhe; Rubin, Martin; Fougere, N.; Tenishev, V.; Combi, M. R.; Bieler, André; Hansen, K. C.; Shou, Y.; Altwegg, Kathrin

doi:10.1002/2015ja022333

Cited by 44 publications

(46 citation statements)

References 50 publications

(74 reference statements)

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“…We point out here that R * primarily follows the (reciprocal of the) neutral density, which in turn largely varies in response to changes in the spacecraft latitude, shown in Figure 5e (black line), as a consequence of the seasonal variations in outgassing over the comet nucleus. We also note that the conspicuous lack of cavity events at small R * around the turn of the months August-September and October-November can possibly be explained by the low solar zenith angle (also known as phase angle), shown in Figure 5e (red line) at these times; the extent of the cavity is expected to be smaller closer to the Sun-comet line (Huang et al, 2016;Koenders et al, 2015). Figure 5f shows the total H 2 O production rate model by Hansen et al (2016, black line) and the local production rate calculated as 4 n n r 2 ⋅ u, for a neutral outflow velocity u of 1 km/s (gray dots).…”

Section: Statistical Survey Of All Cavity Crossingsmentioning

confidence: 80%

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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Section: Statistical Survey Of All Cavity Crossingsmentioning

confidence: 80%

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

et al. 2018

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“…They find that the distance to the nucleus is about 50 100 km depending on the phase angle, the angle between Sun-comet line and the spacecraft comet line. Four-fluid simulations presented in Huang et al (2016) show that with an asymmetric outgassing profile the cavity extends to 100 km. Goetz et al (2016) reported that the extension of the cavity was much larger than theoretically expected, based on the analysis of magnetic field data during one cavity detection on July 26, 2015.…”

Section: Introductionmentioning

confidence: 98%

Structure and evolution of the diamagnetic cavity at comet 67P/Churyumov–Gerasimenko

Goetz

Koenders

Hansen

et al. 2016

Mon. Not. R. Astron. Soc.

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The long duration of the Rosetta mission allows us to study the evolution of the diamagnetic cavity at comet 67P/Churyumov-Gerasimenko in detail. From April 2015 to February 2016 665 intervals could be identified where Rosetta was located in a zeromagnetic-field region. We study the temporal and spatial distribution of this cavity and its boundary and conclude that the cavity properties depend on the long-term trend of the outgassing rate, but do not respond to transient events at the spacecraft location, such as outbursts or high neutral densities. Using an empirical model of the outgassing rate, we find a functional relationship between the outgassing rate and the distance of the cavity to the nucleus. There is also no indication that this unexpectedly large distance is related to unusual solar wind conditions. Because the deduced shape of the cavity boundary is roughly elliptical on small scales and the distances of the boundary from the nucleus are much larger than expected we conclude that the events observed by Rosetta are due to an instability of the cavity boundary itself. We support this conjecture with calculations regarding the non-equal pressures on both sides of the boundary crossings.

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“…Similar effects are expected at comet 67P. Recently, Huang et al (2016) used a four-fluid MHD model to investigate the effect of a more realistic nucleus illumination model on comet 67P's plasma environment at perihelion. They find a bow shock standoff distance increasing from 6000 km to 10 000 km when including an asymmetric gas outflow, suggesting that an equivalent higher outgassing rate in a symmetric Haser neutral model should be used to account for the increase in neutral density on the subsolar side.…”

Section: Limitationsmentioning

confidence: 69%

“…This would substantially increase the neutral atmosphere and plasma interaction region, likely expanding the cometopause in the direction of the subsolar point, with cometary ions becoming the major species at larger cometocentric distances and possibly at the maximum distance of Rosetta's excursion. The effect of an illumination-driven model of the cometary neutral outgassing should also increase the bow shock distance substantially, as shown by Huang et al (2016) with their multi-fluid MHD model.…”

Section: Discussion In the Context Of Rosettamentioning

confidence: 87%

“…They find a bow shock standoff distance increasing from 6000 km to 10 000 km when including an asymmetric gas outflow, suggesting that an equivalent higher outgassing rate in a symmetric Haser neutral model should be used to account for the increase in neutral density on the subsolar side. In both cases, rotation and anisotropic outgassing source, the coarse mesh resolution in our current cometary hybrid simulations prevents such a detailed analysis; the results of Huang et al (2016) should in particular be kept in mind when interpreting the absolute bow shock standoff distances in our own simulations.…”

Section: Limitationsmentioning

confidence: 99%

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Hybrid modelling of cometary plasma environments

Wedlund

Alho

Gronoff

et al. 2017

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Context. The ESA/Rosetta mission made it possible to monitor the plasma environment of a comet, from near aphelion to perihelion conditions. To understand the complex dynamics and plasma structures found at the comet, a modelling effort must be carried out in parallel. Aims. Firstly, we present a 3D hybrid model of the cometary plasma environment including photoionisation, solar wind charge exchange, and electron ionisation reactions; this model is used in stationary and dynamic conditions (mimicking the solar wind variations), and is thus especially adapted to a weakly outgassing comet such as 67P/Churyumov-Gerasimenko, the target of the ESA/Rosetta mission. Secondly, we use the model to study the respective effects of ionisation processes on the formation of the dayside macroscopic magnetic and density boundaries upstream of comet 67P in perihelion conditions at 1.3 AU. Thirdly, we explore and discuss the effects of these processes on the magnetic field line draping, ionisation rates, and composition in the context of the Rosetta mission. Methods. We used a new quasi-neutral hybrid model, originally designed for weakly magnetised planetary bodies, such as Venus, Mars, and Titan, and adapted here to comets. Ionisation processes were monitored individually and together following a probabilistic interaction scheme. Three-dimensional paraboloid fits of the bow shock surface, identified for a magnetosonic Mach number equal to 2, and of the cometopause surface, were performed for a more quantitative analysis. Results. We show that charge exchange and electron ionisation play a major role in the formation of a bow shock-like structure far upstream, while photoionisation is the main driver at and below the cometopause boundary, within 1000 km cometocentric distance. Charge exchange contributes to 42% of the total production rate in the simulation box, whereas production rates from electron ionisation and photoionisation reach 33% and 25%, respectively. We also discuss implications for Rosetta's observations, regarding the detection of the bow shock and the cometopause.

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Four‐fluid MHD simulations of the plasma and neutral gas environment of comet 67P/Churyumov‐Gerasimenko near perihelion

Cited by 44 publications

References 50 publications

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

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

Structure and evolution of the diamagnetic cavity at comet 67P/Churyumov–Gerasimenko

Hybrid modelling of cometary plasma environments

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