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A study of the structure and dynamics of Saturn's inner plasma disk
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics. (Astronomy and Space Physics)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents a study of the inner plasma disk of Saturn. The results are derived from measurements by the instruments on board the Cassini spacecraft, mainly the Cassini Langmuir probe (LP), which has been in orbit around Saturn since 2004. One of the great discoveries of the Cassini spacecraft is that the Saturnian moon Enceladus, located at 3.95 Saturn radii (1 RS = 60,268 km), constantly expels water vapor and condensed water from ridges and troughs located in its south polar region. Impact ionization and photoionization of the water molecules, and subsequent transport, creates a plasma disk around the orbit of Enceladus. The plasma disk ion components are mainly hydrogen ions H+ and water group ions W+ (O+, OH+, H2O+, and H3O+). The Cassini LP is used to measure the properties of the plasma. A new method to derive ion density and ion velocity from Langmuir probe measurements has been developed. The estimated LP statistics are used to derive the extension of the plasma disk, which show plasma densities above ~20 cm-3 in between 2.7 and 8.8 RS. The densities also show a very variable plasma disk, varying with one order of magnitude at the inner part of the disk. We show that the density variation could partly be explained by a dayside/nightside asymmetry in both equatorial ion densities and azimuthal ion velocities. The asymmetry is suggested to be due to the particle orbits being shifted towards the Sun that in turn would cause the whole plasma disk to be shifted. We also investigate the ion loss processes of the inner plasma disk and conclude that loss by transport dominates loss by recombination in the entire region. However, loss by recombination is still important in the region closest to Enceladus (~±0.5 RS) where it differs with only a factor of two from ion transport loss. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 53 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1298
Keyword [en]
Planetary magnetospheres, Saturn, magnetospheric dynamics, Saturn's inner plasma disk, ring plasma, ion densities, ion velocities, dayside/nightside asymmetry, ion loss rates, Cassini, Langmuir probe, RPWS
National Category
Fusion, Plasma and Space Physics
Research subject
Space Physics
Identifiers
URN: urn:nbn:se:uu:diva-263278ISBN: 978-91-554-9353-0 (print)OAI: oai:DiVA.org:uu-263278DiVA: diva2:858073
Public defence
2015-11-19, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board
Available from: 2015-10-27 Created: 2015-09-29 Last updated: 2015-11-10
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
3. Transport and chemical loss rates in Saturn's inner plasma disk
Open this publication in new window or tab >>Transport and chemical loss rates in Saturn's inner plasma disk
Show others...
2016 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, no 3, 2321-2334 p.Article in journal (Refereed) Published
Abstract [en]

The Kronian moon Enceladus is constantly feeding its surrounding with new gas and dust, from cryovolcanoes located in its south polar region. Through photoionization and impact ionization of the neutrals a plasma disk is created, which mainly contains hydrogen ions H+ and water group ions W+. This paper investigates the importance of ion loss by outward radial transport and ion loss by dissociative recombination, which is the dominant chemical loss process in the inner plasma disk. We use plasma densities derived from several years of measurements by the Cassini Radio and Plasma Wave Science (RPWS) electric field spectrums and Langmuir probe (LP), to derive the total flux tube content NL2. Our calculation show that NL2 agrees well with earlier estimates within L shell 8. We also show that loss by transport dominates chemical loss in between L shell 2.5 and 10. The loss rate by transport is ∼5 times larger at 5 Saturn radii (1 RS = 60,268 km) and the difference is increasing as L7.7 for larger radial distances, for the total ion population. Chemical loss may still be important for the structure of the plasma disk in the region closest to Enceladus (∼±0.5 RS) at 3.95 RS, since the transport and chemical loss rates only differ by a factor of ∼2 in this region. We also derive the total plasma content of the plasma disk from L shell 4 to 10 to be 1.9×10^33 ions, and the total ion source rate for the same region to be 5.8×10^27 s^−1. The equatorial ion production rate P, ranges from 2.6×10^−5 cm^−3s^−1 (at L = 10) to 1.1×10^−4 cm^−3s^−1 (at L = 4.8). The net mass loading rate is derived to be 123 kg/s for L shell 4 to 10. 

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-263274 (URN)10.1002/2015JA021784 (DOI)000374730900032 ()
Funder
Swedish National Space Board, DNR 162/14 DNR 166/14Swedish Research Council, DNR 621-2014-450 5526
Available from: 2015-09-29 Created: 2015-09-29 Last updated: 2017-12-01Bibliographically approved
4. Density structures, ion drift speeds, and dynamics in Saturn's inner plasma disk
Open this publication in new window or tab >>Density structures, ion drift speeds, and dynamics in Saturn's inner plasma disk
(English)Manuscript (preprint) (Other academic)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-263277 (URN)
Available from: 2015-09-29 Created: 2015-09-29 Last updated: 2015-11-10

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