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The Dusty Plasma Disk Around the Janus/Epimetheus Ring
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0001-9958-0241
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0002-2107-5859
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0002-7933-0322
Univ Iowa, Dept Phys & Astron, Iowa City, IA 52242 USA.
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2018 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, no 6, p. 4668-4678Article in journal (Refereed) Published
Abstract [en]

We report on the electron, ion, and dust number densities and the electron temperatures obtained by the Radio and Plasma Wave Science instruments onboard Cassini during the Ring-Grazing orbits. The numerous ring passage observations show a consistent picture as follows: (1) Beyond 0.1 R-S above and below the equator the electron and ion densities are quasi-neutral with a distribution similar to the one obtained in the plasma disk. (2) A sharp ion density enhancement occurs at vertical bar Z vertical bar < 0.1 R-S, to more than 200 cm(-3) at the equator, while the electron density remains low only to values of 50cm(-3). The electron/ion density ratio is <= 0.1 at the equator. (3) Micrometer-sized dust has also been observed at the equator. However, the region of intense dust signals is significantly narrower (vertical bar Z vertical bar<0.02 R-S) than the enhanced ion density regions. (4) The electron temperature (T-e) generally decreases with decreasing Z with small T-e enhancements near the equator. We show that the dust size characteristics are different depending on the distance from the equator, and the large micrometer-sized grains are more perceptible in a narrow region near the equator where the power law slope of the dust size distribution becomes less steep. As a result, different scale heights are obtained for nanometer and micrometer grains. Throughout the ring, the dominant part of the negative charges is carried by the small nanometer-sized grains. The electron/ion density ratio is variable from orbit to orbit, suggesting changes in the dust charging over time scales of weeks. Plain Language Summary The Radio and Plasma Wave Science instrument onboard Cassini observed a dusty plasma during the Ring-Grazing orbits. Dusty plasma is composed of, in addition to the electrons and ions, charged dust grains, and those grains play an important role in the plasma dynamics. The observed electron, ion, and dust number densities and the electron temperatures showed the layered structure of the faint Janus/Epimetheus rings. The core of the dusty ring composed of micron-sized dust is surrounded by a dusty plasma consisting of the ions and the negatively charged nanometer grains and further surrounded by the pristine plasma. The electron/ion density ratio of the dusty plasma varies from orbit to orbit, implying that the dust charging characteristics of the dusty ring change over time scales of weeks.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION , 2018. Vol. 123, no 6, p. 4668-4678
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:uu:diva-362176DOI: 10.1002/2017JA024917ISI: 000439803100014OAI: oai:DiVA.org:uu-362176DiVA, id: diva2:1253347
Funder
Swedish National Space Board, Dnr 174/15Available from: 2018-10-04 Created: 2018-10-04 Last updated: 2018-10-04Bibliographically approved

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Morooka, Michiko W.Wahlund, Jan-ErikAndrews, David J.

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