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Plasma and Dust at Saturn's Icy Moon Enceladus and Comet 67P/Churyumov-Gerasimenko
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. (RPF)ORCID iD: 0000-0002-0957-3847
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Saturn’s moon Enceladus and comet 67P/Churyumov-Gerasimenko both are examples of icy solar system objects from which gas and dust flow into space. At both bodies, the gas becomes partly ionized and the dust grains get charged. Both bodies have been visited by spacecraft carrying similar Langmuir probe instruments for observing the plasma and the charged dust. The conditions at Enceladus and the comet turn out to be different, so we emphasize different aspects of their plasma environments. At Enceladus, we concentrate on the characteristic plasma regions and charged dust. At the comet, we investigate cold electrons.

At Enceladus, internal frictional heating leads to gas escaping from cracks in the ice in the south pole region. This causes a plume of gas, which becomes partially ionized, and dust, becoming charged. We have investigated the plasma and charged nanodust in this region by the use of the Langmuir Probe (LP) of the Radio and Plasma Wave Science (RPWS) instrument on Cassini. The dust charge density can be calculated from the quasineutrality condition, the difference between ion and electron density measurements from LP. We found support for this method by comparing to measurements of larger dust grains by the RPWS electric antennas. We use the LP method to find that the plasma and dust environment of Enceladus can be divided into at least three regions. In addition to the well known plume, these are the plume edge and the trail region.

At the comet, heat from the Sun sublimates ice to gas dragging dust along as it flows out into space. When gas molecules are hit by ionizing radiation we get a plasma. Models predict that the electron temperature just after ionization is around 10 eV, but that this collisions with the neutral gas should cool the electrons to below 0.1 eV. The Langmuir Probe instrument LAP has previously been used to show that the warm component exists at the comet. We present the first measurements of the cold component, co-existing with the warm component. We find that that the cold plasma often is observed as brief pulses in the LAP data, which we interpret as filamentation of the cold plasma.

Place, publisher, year, edition, pages
Uppsala University, 2016. , p. 67
National Category
Fusion, Plasma and Space Physics
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
URN: urn:nbn:se:uu:diva-308971OAI: oai:DiVA.org:uu-308971DiVA, id: diva2:1051186
Presentation
2016-12-20, Polhelmsalen, Regementsvägen 1, 752 37, Uppsala, 10:41 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, 171/12Available from: 2017-01-16 Created: 2016-12-01 Last updated: 2017-01-17Bibliographically approved
List of papers
1. Plasma regions, charged dust and field-aligned currents near Enceladus
Open this publication in new window or tab >>Plasma regions, charged dust and field-aligned currents near Enceladus
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2015 (English)In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 117, p. 453-469Article in journal (Refereed) Published
Abstract [en]

We use data from several instruments on board Cassini to determine the characteristics of the plasma and dust regions around Saturn's moon Enceladus. For this we utilize the Langmuir probe and the electric antenna connected to the wideband receiver of the radio and plasma wave science (RPWS) instrument package as well as the magnetometer (MAG). We show that there are several distinct plasma and dust regions around Enceladus. Specifically they are the plume filled with neutral gas, plasma, and charged dust, with a distinct edge boundary region. Here we present observations of a new distinct plasma region, being a dust trail on the downstream side. This is seen both as a difference in ion and electron densities, indicating the presence of charged dust, and directly from the signals created on RPWS antennas by the dust impacts on the spacecraft. Furthermore, we show a very good scaling of these two independent dust density measurement methods over four orders of magnitude in dust density, thereby for the first time cross-validating them. To establish equilibrium with the surrounding plasma the dust becomes negatively charged by attracting free electrons. The dust distribution follows a simple power law and the smallest dust particles in the dust trail region are found to be 10 nm in size as well as in the edge region around the plume. Inside the plume the presence of even smaller particles of about 1 nm is inferred. From the magnetic field measurements we infer strong field-aligned currents at the geometrical edge of Enceladus.

Keywords
Enceladus, Langmuir probe, Plasma, Charged dust, MAG, RPWS
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-268421 (URN)10.1016/j.pss.2015.09.010 (DOI)000364257400039 ()
Funder
Swedish National Space Board, 171/12Swedish National Space Board, 162/14
Available from: 2015-12-04 Created: 2015-12-04 Last updated: 2018-04-18Bibliographically approved

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