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Plasma and dust interaction in the magnetosphere of Saturn
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Cassini spacecraft orbits Saturn since 2004, carrying a multitude of instruments for studies of the plasma environment around the planet as well as the constituents of the ring system. Of particular interest to the present thesis is the large E ring, which consists mainly of water ice grains, smaller than a few micrometres, referred to as dust. The first part of the work presented here is concerned with the interaction between, on the one hand, the plasma and, on the other hand, the dust, the spacecraft and the Langmuir probe carried by the spacecraft. In Paper I, dust densities along the trajectory of Cassini, as it passes through the ring, are inferred from measured electron and ion densities. In Paper II, the situation where a Langmuir probe is located in the potential well of a spacecraft is considered. The importance of knowing the potential structure around the spacecraft and probe is emphasised and its effect on the probe's current-voltage characteristic is illustrated with a simple analytical model. In Paper III, particle-in-cell simulations are employed to study the potential and density profiles around the Cassini as it travels through the plasma at the orbit of the moon Enceladus. The latter part of the work concerns large-scale currents and convection patterns. In Paper IV, the effects of charged E-ring dust moving across the magnetic field is studied, for example in terms of what field-aligned currents it sets up, which compared to corresponding plasma currents. In Paper V, a model for the convection of the magnetospheric plasma is proposed that recreates the co-rotating density asymmetry of the plasma.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xiii, 40 p.
Series
Trita-EE, ISSN 1653-5146 ; 2012:018
Keyword [en]
saturn, dusty plasma
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:kth:diva-93983ISBN: 978-91-7501-343-5 (print)OAI: oai:DiVA.org:kth-93983DiVA: diva2:524800
Public defence
2012-05-28, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20120507Available from: 2012-05-07 Created: 2012-05-03 Last updated: 2012-05-07Bibliographically approved
List of papers
1. Characteristics of charged dust inferred from the Cassini RPWS measurements in the vicinity of Enceladus
Open this publication in new window or tab >>Characteristics of charged dust inferred from the Cassini RPWS measurements in the vicinity of Enceladus
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2009 (English)In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 57, no 14-15, 1807-1812 p.Article in journal (Refereed) Published
Abstract [en]

The data obtained by the Cassini Radio and Plasma Wave Science (RPWS) instrument during the shallow (17.02.2005) and the steep (14.07.2005) crossings of the E-ring revealed a considerable electron depletion in proximity to Enceladus's orbit (the difference between the ion and electron densities can reach similar to 70 cm(-3)). Assuming that this depletion is a signature of the presence of charged dust particles, the main characteristics of dust down to submicron sized particles are derived. The differential size distribution is found to be well described by a power law with an index mu similar to 5.5-6 for the lower size limit a(min) = 0.03 mu m and mu similar to 7.3-8 for a(min) = 0.1 mu m. The calculated average integral dust number density is weakly affected by values of mu and a(min). For a greater than or similar to 0.1 mu m, both flybys gave the maximum dust density about 0.1-0.3 cm(-3) in the vicinity of Enceladus. Our results imply that the dust structure near Enceladus is characterized by approximately the same vertical length scale of 8000 km and reaches a maximum at the same radial distance (displaced outward of the orbit of Enceladus) as found by Kempf et al. [2008. The E-ring in the vicinity of Enceladus. Spatial distribution and properties of the ring particles. Icarus 193, 420-437], from the dust impact data.

Keyword
Saturn moons, E-ring, Cassini, Dust size distribution, Dusty plasma, e-ring, solar-system, plasma, saturn, particles, august
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-12938 (URN)10.1016/j.pss.2009.03.002 (DOI)000273099100017 ()2-s2.0-70450222133 (Scopus ID)
Note
QC 20100519Available from: 2010-05-19 Created: 2010-05-19 Last updated: 2017-12-12Bibliographically approved
2. On the interpretation of Langmuir probe data inside a spacecraft sheath
Open this publication in new window or tab >>On the interpretation of Langmuir probe data inside a spacecraft sheath
2010 (English)In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 81, no 10, 105106-1-105106-8 p.Article in journal (Refereed) Published
Abstract [en]

If a Langmuir probe is located inside the sheath of a negatively charged spacecraft, there is a risk that the probe characteristic is modified compared to that of a free probe in the ambient plasma. We have studied this probe-in-spacecraft-sheath problem in the parameter range of a small Langmuir probe (with radius r(LP)<<lambda(D)) using a modified version of the orbit motion limited (OML) probe theory. We find that the ambient electron contribution I-e(U-LP) to the probe characteristic is suitably analyzed in terms of three regions of applied probe potential U-LP. In region I, where the probe is negatively charged (i.e., U-LP<U-1, where U-1 is the potential in the sheath at the probe position), the probe characteristic I-e(U-LP) is close to that of OML theory for a free probe in the ambient plasma. In the probe potential range U-LP>U-1, there is first a transition region II in applied potential, U-1<U-LP<U-2, in which the key factor to determine the shape of I-e(U-LP) is a potential minimum U-M between the probe and the ambient plasma. This minimum gives the depth U-pl-U-M of a potential barrier that prevents the lowest energy ambient electrons from reaching the probe. For a high enough positive probe potential, in region III, the barrier becomes small. Here, I-e(U-LP) again approaches OML theory for a free probe. The boundary U-2 between regions II and III is somewhat arbitrary; we propose a condition on the barrier, U-pl-U-M << k(B)T(e)/e, as the definition of region III. The main findings in this work are qualitative rather than quantitative. The existence of the transition region points to that special care must be taken to extract plasma parameters from measured I(U-LP) as the probe characteristic is likely to depart from usual OML in crucial respects: (1) the ambient plasma potential U-pl falls into the transition region, but there is no obvious knee or other feature to identify it, (2) there is in this region no exponential part of I-e(U-LP) that can be used to obtain T-e, instead, (3) the probe size is important in determining the curve shape. We have tentatively applied our simplified probe-in-sheath model to real probe data from the Cassini spacecraft, taken in the dense plasma of Saturn's magnetosphere. We propose that our model gives a better description than OML of measured Langmuir probe sweeps in space plasmas where the Langmuir probe is situated within the spacecraft sheath, i.e., for long Debye lengths. The understanding of these probe sweep effects in such regions may improve by self-consistent particle simulations of the spacecraft environment.

Keyword
Ambient plasmas, Applied potentials, Cassini spacecraft, Curve shape, Debye length, Dense plasma, Key factors, OML theory, Parameter range, Particle simulations, Plasma parameter, Potential barriers, Potential minima, Potential range, Probe characteristics, Probe position, Probe size, Sheath problem, Space plasmas, Transition regions, Langmuir probes, Magnetosphere, Plasmas, Spacecraft
National Category
Astronomy, Astrophysics and Cosmology Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-12939 (URN)10.1063/1.3482155 (DOI)000283753400050 ()2-s2.0-78149444107 (Scopus ID)
Note
QC 20100519. Uppdaterad från submitted till published (20101213).Available from: 2010-05-19 Created: 2010-05-19 Last updated: 2017-12-12Bibliographically approved
3. Potential structure around th Cassini spacecraft near the orbit of Enceladus
Open this publication in new window or tab >>Potential structure around th Cassini spacecraft near the orbit of Enceladus
2010 (English)In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 17, no 10Article in journal (Refereed) Published
Abstract [en]

We present the results of numerical simulations of the potential structure around an object in a streaming plasma with parameters relevant for the Cassini spacecraft passing through Saturn's plasma disk near the orbit of Enceladus. Two-and three-dimensional particle-in-cell codes have been used allowing the potential of the simulated spacecraft body to develop self-consistently through the collection of charge by its surface. The dependence of the density and potential profiles on ambient plasma density, electron temperature, and ion drift speed is discussed. The spacecraft floating potential values, found in the simulations, are compared to those deduced from the analysis of Cassini Langmuir probe characteristics.

Keyword
plasma interaction, dust particles, simulation
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-12940 (URN)10.1063/1.3486523 (DOI)000283772200052 ()2-s2.0-78149249277 (Scopus ID)
Note
QC 20100519Available from: 2010-05-19 Created: 2010-05-19 Last updated: 2017-12-12Bibliographically approved
4. Dust-driven and plasma-driven currents in the inner magnetosphere of Saturn
Open this publication in new window or tab >>Dust-driven and plasma-driven currents in the inner magnetosphere of Saturn
2012 (English)In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 19, no 4, 042903- p.Article in journal (Refereed) Published
Abstract [en]

General equations for dust-driven currents and current systems J(D) in magnetized plasmas are derived and, as a concrete example, applied to the E ring of Saturn at radial distances 3R(S) < R < 5R(S). An azimuthal ring current J(D,phi) acts as a current generator and is coupled to two secondary dust-driven current systems down to the ionosphere of Saturn, both rotating with the magnetospheric plasma. One of these closes across the polar cap, and the other over a limited range in latitude. These dust-driven current systems are embedded in three systems of plasma-driven currents J(p): a ring current, a cross-polar-cap current system, and an ion pickup current system. Both the J(D) and the J(p) current systems have been quantitatively assessed from a data set for the E ring of Saturn in which the unknown distribution of small dust is treated by a power law extrapolation from the known distribution of larger dust. From data on the magnetic perturbations during a crossing of the equatorial plane, an approximate constraint on the fraction of the electrons that can be trapped on the dust is derived. For this amount of electron capture, it is demonstrated that all three types of dust-driven currents are, within somewhat more than an order of magnitude, of the same strength as the corresponding types of plasma-driven currents. Considering also that both plasma and dust densities vary with the geyser activity at the south pole of Enceladus, it is concluded that both the dust-driven and the plasma-driven contributions to the current system associated with the E ring need to be retained for a complete description.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2012
Keyword
saturn, magnetosphere, current system, field-aligned current, dusty plasma
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-93980 (URN)10.1063/1.3701995 (DOI)000309592100035 ()2-s2.0-84860477047 (Scopus ID)
Note

QC 20120504

Available from: 2012-05-03 Created: 2012-05-03 Last updated: 2017-12-07Bibliographically approved
5. The magnetospheric clock of Saturn: a self-organized plasma dynamo
Open this publication in new window or tab >>The magnetospheric clock of Saturn: a self-organized plasma dynamo
(English)In: Nature, ISSN 0028-0836, E-ISSN 1476-4687Article in journal, Letter (Other academic) Submitted
Place, publisher, year, edition, pages
Nature Publishing Group
Keyword
saturn, magnetosphere, corotation
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-93982 (URN)
Note
QS 2012Available from: 2012-05-03 Created: 2012-05-03 Last updated: 2017-12-07Bibliographically approved

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