| 1. |
- Olson, Jonas, 1983-, et al.
(författare)
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Dust-driven and plasma-driven currents in the inner magnetosphere of Saturn
- 2012
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 19:4, s. 042903-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Olson, Jonas, et al.
(författare)
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On the interpretation of Langmuir probe data inside a spacecraft sheath
- 2010
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 81:10, s. 105106-1-105106-8
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Tidskriftsartikel (refereegranskat)abstract
- 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)<U-1, there is first a transition region II in applied potential, U-1
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| 3. |
- Olson, Jonas, et al.
(författare)
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The magnetospheric clock of Saturn-A self-organized plasma dynamo
- 2013
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 20:8, s. 082901-
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Tidskriftsartikel (refereegranskat)abstract
- The plasma in the inner magnetosphere of Saturn is characterized by large-amplitude azimuthal density variations in the equatorial plane, with approximately a sinusoidal dependence on the azimuthal angle [D. A. Gurnett et al., Science 316, 442 (2007)]. This structure rotates with close to the period of the planet itself and has been proposed to steer other nonaxisymmetric phenomena, e. g., the Saturn kilometric radiation SKR [W. S. Kurth et al., Geophys. Res. Lett. 34, L02201 (2007)], and inner-magnetosphere magnetic field perturbations [D. J. Southwood and M. G. Kivelson, J. Geophys. Res. 112(A12), A12222 (2007)]. There is today no consensus regarding the basic driving mechanism. We here propose it to be a plasma dynamo, located in the neutral gas torus of Enceladus but coupled both inwards, through electric currents along the magnetic field lines down to the planet, and outwards through the plasma flow pattern there. Such a dynamo mechanism is shown to self-regulate towards a state that, with realistic parameters, can reproduce the observed configuration of the magnetosphere. This state is characterized by three quantities: the Pedersen conductivity in the polar cap, the ionization time constant in the neutral gas torus, and a parameter characterizing the plasma flow pattern. A particularly interesting property of the dynamo is that regular (i.e., constant-amplitude, sinusoidal) variations in the last parameter can lead to complicated, non-periodic, oscillations around the steady-state configuration.
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| 4. |
- Yaroshenko, V.V., et al.
(författare)
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Characteristics of charged dust inferred from the Cassini RPWS measurements in the vicinity of Enceladus
- 2009
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Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 57:14-15, s. 1807-1812
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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