Dynamic unmagnetized plasma in the diamagnetic cavity around comet 67P/Churyumov–Gerasimenko

Rajkumar Hajra 1 Pierre Henri 1 Xavier Vallières 1 Jérome Moré 1 Nicolas Gilet 1 Gaëtan Wattieaux 2 Charlotte Goetz 3 Ingo Richter 3 Bruce Tsurutani 4 Herbert Gunell 5 Hans Nilsson 6 Anders Eriksson 7 Zoltán Németh 8 James Burch 9 Martin Rubin 10
1 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
2 LAPLACE - LAboratoire PLasma et Conversion d'Energie
3 Technische Universität Braunschweig [Braunschweig]
4 JPL - Jet Propulsion Laboratory
5 BIRA-IASB - Belgian Institute for Space Aeronomy / Institut d'Aéronomie Spatiale de Belgique
6 IRF - Swedish Institute of Space Physics [Kiruna]
7 IRF - Swedish Institute of Space Physics [Uppsala]
8 Wigner Research Centre for Physics
9 SwRI - Southwest Research Institute [Boulder]
10 Physikalisches Institut [Bern]
Abstract : The Rosetta orbiter witnessed several hundred diamagnetic cavity crossings (unmagnetized regions) around comet 67P/Churyumov–Gerasimenko during its two year survey of the comet. The characteristics of the plasma environment inside these diamagnetic regions are studied using in situ measurements by the Rosetta Plasma Consortium instruments. Although the unmagnetized plasma density has been observed to exhibit little dynamics compared to the very dynamical magnetized cometary plasma, we detected several localized dynamic plasma structures inside those diamagnetic regions. These plasma structures are not related to the direct ionization of local cometary neutrals. The structures are found to be steepened, asymmetric plasma enhancements with typical rising-to-descending slope ratio of ∼2.8 (±1.9), skewness ∼0.43 (±0.36), mean duration of ∼2.7 (±0.9) min and relative density variation ΔN/N of ∼0.5 (±0.2), observed close to the electron exobase. Similar steepened plasma density enhancements were detected at the magnetized boundaries of the diamagnetic cavity as well as outside the diamagnetic region. The plausible scalelength and propagation direction of the structures are estimated from simple plasma dynamics considerations. It is suggested that they are large-scale unmagnetized plasma enhancements, transmitted from the very dynamical outer magnetized region to the inner magnetic field-free cavity region.
Keywords : comets: individual: 67P/Churyumov–Gerasimenko comets: general methods: observational methods: data analysis
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Rajkumar Hajra, Pierre Henri, Xavier Vallières, Jérome Moré, Nicolas Gilet, et al.. Dynamic unmagnetized plasma in the diamagnetic cavity around comet 67P/Churyumov–Gerasimenko. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2018, 475 (3), pp.4140-4147. ⟨10.1093/mnras/sty094⟩. ⟨insu-01825213⟩

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