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

Goetz, Charlotte; Koenders, C.; Hansen, K. C.; Burch, J. L.; Carr, C.; Eriksson, Anders; Frühauff, Dennis; Güttler, C.; Henri, Pierre; Nilsson, Hans; Richter, Ingo; Rubin, Martin; Sierks, H.; Tsurutani, B. T.; Volwerk, M.; Glaßmeier, Karl‐Heinz

doi:10.1093/mnras/stw3148

Cited by 93 publications

(100 citation statements)

References 35 publications

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“…Figure 5a shows a bar chart of the number of cavity crossings each day during this period (the conspicuous coloring will be explained in connection with Figure 6 below). The distribution is very uneven, with most cavity events occurring in clusters in the end of July, early August, and the end of November 2015, as previously noted by Goetz, Koenders, Hansen, et al (2016) and Henri et al (2017). This is even more clear in Figure 5b, which shows a bar chart of the total time spent inside the cavity during each day.…”

Section: Statistical Survey Of All Cavity Crossingsmentioning

confidence: 60%

“…It should be short enough for secularly varying parameters such as comet outgassing, latitude, and radial distance of the spacecraft to be constant but still long enough to contain sufficient measurements for good statistics. The diamagnetic cavity events came in the form of single events and clusters; the most prominent clusters occurred on 30 July 2015 and 19–21 November 2015 (Goetz, Koenders, Hansen, et al, ). We found that the 50‐hr interval between 08:00 on 19 November 2015 and 10:00 on 21 November 2015 fulfilled the above criteria.…”

Section: Resultsmentioning

confidence: 99%

“…The spacecraft remained inside the cavity for about 25 min during this event. Subsequent analysis has identified a total of 665 cavity crossings in MAG data (Goetz, Koenders, Hansen et al, ), between April 2015 and February 2016 (i.e., some preceding the original detection). They ranged in duration from 8 s up to 40 min, in distance to the nucleus from 40 to 380 km and in heliocentric distance from 1.25 to 2.4 au.…”

Section: Introductionmentioning

confidence: 99%

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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: 60%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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“…is shown in Figure . These waves were detected close to the comet diamagnetic cavity (Goetz, Koenders, Hansen, et al, ; Goetz, Koenders, Richter, et al, ). The individual wave cycles are asymmetric and have steepened edges on the left‐hand side.…”

Section: Resultsmentioning

confidence: 98%

A Review of Alfvénic Turbulence in High‐Speed Solar Wind Streams: Hints From Cometary Plasma Turbulence

et al. 2018

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Solar wind turbulence within high‐speed streams is reviewed from the point of view of embedded single nonlinear Alfvén wave cycles, discontinuities, magnetic decreases (MDs), and shocks. For comparison and guidance, cometary plasma turbulence is also briefly reviewed. It is demonstrated that cometary nonlinear magnetosonic waves phase‐steepen, with a right‐hand circular polarized foreshortened front and an elongated, compressive trailing edge. The former part is a form of “wave breaking” and the latter that of “period doubling.” Interplanetary nonlinear Alfvén waves, which are arc polarized, have a ~180° foreshortened front and with an elongated trailing edge. Alfvén waves have polarizations different from those of cometary magnetosonic waves, indicating that helicity is a durable feature of plasma turbulence. Interplanetary Alfvén waves are noted to be spherical waves, suggesting the possibility of additional local generation. They kinetically dissipate, forming MDs, indicating that the solar wind is partially “compressive” and static. The ~2 MeV protons can nonresonantly interact with MDs leading to rapid cross‐field (~5.5% Bohm) diffusion. The possibility of local (~1 AU) generation of Alfvén waves may make it difficult to forecast High‐Intensity, Long‐Duration AE Activity and relativistic magnetospheric electrons with great accuracy. The future Solar Orbiter and Solar Probe Plus missions should be able to not only test these ideas but to also extend our knowledge of plasma turbulence evolution.

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“…This neutral atmosphere gets ionized by photoionization and electron impact ionization, and the newly born ions get picked up and accelerated by the convective electric field of the solar wind (e.g., Cravens & Gombosi, 2004;Galand et al, 2016;Szegö et al, 2000). From April 2015 to February 2016 Rosetta intermittently encountered a magnetic field free region, known as the diamagnetic cavity (Goetz, Koenders, Hansen, et al, 2016;Goetz, Koenders, Richter, et al, 2016). Both the accelerated population and a relatively cold population with low bulk velocity are observed with RPC-ICA (Nilsson, Stenberg Wieser, Behar, Simon Wedlund, et al, 2015;Nilsson et al, 2017).…”

Section: Introductionmentioning

confidence: 99%

The Influence of Spacecraft Charging on Low‐Energy Ion Measurements Made by RPC‐ICA on Rosetta

Bergman

Wieser

et al. 2020

JGR Space Physics

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Spacecraft charging is problematic for low-energy plasma measurements. The charged particles are attracted to or repelled from the charged spacecraft, affecting both the energy and direction of travel of the particles. The Ion Composition Analyzer (RPC-ICA) on board the Rosetta spacecraft is suffering from this effect. RPC-ICA was measuring positive ions in the vicinity of comet 67P/Churyumov-Gerasimenko, covering an energy range of a few eV/q to 40 keV/q. The low-energy part of the data is, however, heavily distorted by the negatively charged spacecraft. In this study we use the Spacecraft Plasma Interaction Software to model the influence of the spacecraft potential on the ion trajectories and the corresponding distortion of the field of view (FOV) of the instrument. The results show that the measurements are not significantly distorted when the ion energy corresponds to at least twice the spacecraft potential. Below this energy the FOV is often heavily distorted, but the distortion differs between different viewing directions. Generally, ions entering the instrument close to the aperture plane are less affected than those entering with extreme elevation angles.Plain Language Summary The Rosetta spacecraft followed comet 67P/Churyumov-Gerasimenko for 2 years, providing data giving new insights into the nature of comets. The Ion Composition Analyzer (RPC-ICA) on board the spacecraft measures positive ions in the vicinity of the comet. The instrument can measure low-energy ions, which play an important part in the processes taking place in this environment. To fully understand the environment around the comet, we have to understand these low-energy ions. Unfortunately, this part of the RPC-ICA data is distorted by the spacecraft potential. A spacecraft in space interacts with the surrounding environment, which charges the spacecraft surface to a positive or negative potential. Rosetta was commonly charged to a negative potential throughout the mission, which means that the positive ions measured by RPC-ICA were attracted to the spacecraft. Consequently, both the energy and the travel direction of the ions changed before detection. We investigate how the low-energy ions measured by RPC-ICA have been affected by the spacecraft potential. We use the Spacecraft Plasma Interaction Software to model these effects. The results give us a lower energy limit above which we can trust the measurements and show that some parts of the instrument are more heavily affected than others.

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

Cited by 93 publications

References 35 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

A Review of Alfvénic Turbulence in High‐Speed Solar Wind Streams: Hints From Cometary Plasma Turbulence

The Influence of Spacecraft Charging on Low‐Energy Ion Measurements Made by RPC‐ICA on Rosetta

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