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On the structure and dynamics of Saturn's inner plasma disk
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. (Astronomy and Space Physics)
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This licentiate thesis presents our investigation of Saturn's inner plasma disk. The thesis gives an overview of the Cassini-Huygens project, what a plasma is and how we use the Langmuir probe to investigate it, various difficulties related to the measurements, the structure of the magnetosphere of Saturn, with special focus on the inner magnetosphere and the region around the Saturnian moon Enceladus. For our investigation we use the Cassini Langmuir probe to derive ion density and ion velocity in the region from 2.5 to 12 Saturn radii. We show that the dominant part of the plasma torus, ion density above ~15 particle/cm3, is located in between 2.5 and 8 Saturn radii (1 RS = 60,268 km) from the planet, with a north-southward extension of 2 RS. The plume of the moon Enceladus is clearly visible as an ion density maximum of 105 cm-3, only present at the south side of the ring plane, as expected since the Enceladus plumes are located in the south polar region. Also the azimuthal ion velocity vi,Θ is estimated, showing a clear general trend in the region between 3 and 7 RS, described by vi,Θ =1.5R2-8.7R+39. The average vi,Θ starts to deviate from corotation speed at around 3 RS and reaches down to ~68 % of corotation close to 5 RS. The Langmuir probe data show a clear day/night side asymmetry in both ion density and ion velocity, most prominent in the radial region 4-6 RS from the center of Saturn. The ion densities ni varies from an average of ~35 cm-3 for the lowest dayside values close to noon up to ~70 cm-3 for the highest nightside values around midnight. The azimuthal ion velocities vi,Θ varies from ~28-32 km/s at the lowest dayside values around noon to ~36-40 km/s at the highest nightside values around midnight. This gives an azimuthal ion velocity difference between noon and midnight of Δvi,Θ ~5-10 km/s. The day/night asymmetry is suggested to be due to dust-plasma interaction.

Place, publisher, year, edition, pages
Uppsala: Uppsala universitet , 2013. , 57 p.
Keyword [en]
Saturn, E-ring, plasma disk
National Category
Engineering and Technology
Research subject
Space and Plasma Physics
Identifiers
URN: urn:nbn:se:uu:diva-218772OAI: oai:DiVA.org:uu-218772DiVA: diva2:697244
Presentation
2013-10-07, Polhemsalen, Lägerhyddsvägen 1, Uppsala, 15:00 (English)
Opponent
Supervisors
Available from: 2014-06-24 Created: 2014-02-17 Last updated: 2014-06-24Bibliographically approved
List of papers
1. Ion densities and velocities in the inner plasma torus of Saturn
Open this publication in new window or tab >>Ion densities and velocities in the inner plasma torus of Saturn
2012 (English)In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 73, no 1, 151-160 p.Article in journal (Refereed) Published
Abstract [en]

We present plasma data from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe (LP), mapping the ion density and velocity of Saturn's inner plasma torus. Data from 129 orbits, recorded during the period from the 1st of February 2005 to the 27th of June 2010, are used to map the extension of the inner plasma torus. The dominant part of the plasma torus is shown to be located in between 2.5 and 8 Saturn radii (1 RS=60,268 km) from the planet, with a north-southward extension of ±2RS. The plasma disk ion density shows a broad maximum in between the orbits of Enceladus and Tethys. Ion density values vary between 20 and 125 cm-3 at the location of the density maximum, indicating considerable dynamics of the plasma disk. The equatorial density structure, |z|<0.5RS, shows a slower decrease away from the planet than towards. The outward decrease, from 5 R S, is well described by the relation neq=2.2×10 4(1/R)3.63. The plume of the moon Enceladus is clearly visible as an ion density maximum of 105 cm-3, only present at the south side of the ring plane. A less prominent density peak, of 115 cm-3, is also detected at the orbit of Tethys, at ∼4.9 RS. No density peaks are recorded at the orbits of the moons Mimas, Dione, and Rhea. The presented ion velocity vi,θ shows a clear general trend in the region between 3 and 7 RS, described by vi, θ=1.5R2-8.7R+39. The average vi,θ starts to deviate from corotation at around 3 RS, reaching ∼68% of corotation close to 5 RS.

Place, publisher, year, edition, pages
Elsevier, 2012
Keyword
Cassini, E-ring, Ion density, Ion velocity, Plasma disk, Saturn magnetosphere, Alpha particles, Magnetosphere, Orbits, Plasma waves, Plasmas, Velocity, Ions
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-192893 (URN)10.1016/j.pss.2012.09.016 (DOI)000314007400024 ()
Available from: 2013-01-28 Created: 2013-01-25 Last updated: 2017-12-06Bibliographically approved
2. Dayside/nightside asymmetry of ion densities and velocities in Saturn's inner magnetosphere
Open this publication in new window or tab >>Dayside/nightside asymmetry of ion densities and velocities in Saturn's inner magnetosphere
2014 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 41, no 11, 3717-3723 p.Article in journal, Letter (Refereed) Published
Abstract [en]

We present Radio and Plasma Wave Science Langmuir probe measurements from 129 Cassini orbits, which show a day/night asymmetry in both ion density and ion velocity in the radial region 4–6 RS (1 RS = 60,268 km) from the center of Saturn. The ion densities ni vary from an average of ∼35 cm−3 around noon up to ∼70 cm−3 around midnight. The ion velocities vi,θ vary from ∼28–32 km/s at the lowest dayside values to ∼36–40 km/s at the highest nightside values. The day/night asymmetry is suggested to be due to the radiation pressure force acting on negatively charged nanometer-sized dust of the E ring. This force will introduce an extra grain and ion drift component equivalent to the force of an additional electric field of 0.1–2 mV/m for 10–50 nm sized grains.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-227095 (URN)10.1002/2014GL060229 (DOI)000339280200005 ()
Available from: 2014-06-24 Created: 2014-06-24 Last updated: 2017-12-05Bibliographically approved

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