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- Alfvén, Hannes, et al.
(författare)
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Voyager saturnian ring measurements and the early history of the solar-system
- 1986
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Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 34:2, s. 145-154
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Tidskriftsartikel (refereegranskat)abstract
- The mass distribution in the Saturnian ring system is investigated and compared with predictions from the plasma cosmogony. According to this theory, the matter in the rings has once been in the form of a magnetized plasma, in which the gravitation is balanced partly by the centrifugal force and partly by the electromagnetic forces. As the plasma is neutralized, the electromagnetic forces disappear and the matter can be shown to fall in to of the original saturnocentric distance. This causes the so called “cosmogonic shadow effect”, which has been demonstrated earlier for the asteroidal belt and in the large scale structure of the Saturnian ring system.The relevance of the cosmogonic shadow effect is investigated for parts of the fine structures of the Saturnian ring system. It is shown that many structures of the present ring system can be understood as shadows and antishadows of cosmogonic origin. These appear in the form of double rings centered around a position a factor 0.64 (slightly less than) closer to Saturn than the causing feature. Voyager data agree with an accuracy better than 1%.
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| 7. |
- Brenning, Nils, et al.
(författare)
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Radiation from an electron beam in magnetized plasma : excitation of a whistler mode wave packet by interacting, higher-frequency, electrostatic-wave eigenmodes
- 2017
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Ingår i: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 59:12
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Tidskriftsartikel (refereegranskat)abstract
- Infrequent, bursty, electromagnetic, whistler-mode wave packets, excited spontaneously in the laboratory by an electron beam from a hot cathode, appear transiently, each with a time duration tau around similar to 1 mu s. The wave packets have a center frequency f(W) that is broadly distributed in the range 7 MHz < f(W) < 40 MHz. They are excited in a region with separate electrostatic (es) plasma oscillations at values of f(hf), 200 MHz < f(hf) < 500 MHz, that are hypothesized to match eigenmode frequencies of an axially localized hf es field in a well-defined region attached to the cathode. Features of these es-eigenmodes that are studied include: the mode competition at times of transitions from one dominating es-eigenmode to another, the amplitude and spectral distribution of simultaneously occurring es-eigenmodes that do not lead to a transition, and the correlation of these features with the excitation of whistler mode waves. It is concluded that transient coupling of es-eigenmode pairs at f(hf) such that vertical bar f(1, hf) - f(2, hf)vertical bar = f(W) < f(ge) can explain both the transient lifetime and the frequency spectra of the whistler-mode wave packets (f(W)) as observed in lab. The generalization of the results to bursty whistler-mode excitation in space from electron beams, created on the high potential side of double layers, is discussed.
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- Böhlmark, Johan, et al.
(författare)
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Measurement of the magnetic field change in a pulsed high current magnetron discharge
- 2004
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Ingår i: Plasma Sources Science and Technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 13:4, s. 654-661
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we present a study of how the magnetic field of a circular planar magnetron is affected when it is exposed to a pulsed high current discharge. Spatially resolved magnetic field measurements are presented and the magnetic disturbance is quantified for different process parameters. The magnetic field is severely deformed by the discharge and we record changes of several millitesla, depending on the spatial location of the measurement. The shape of the deformation reveals the presence of azimuthally drifting electrons close to the target surface. Time resolved measurements show a transition between two types of magnetic perturbations. There is an early stage that is in phase with the axial discharge current and a late stage that is not in phase with the discharge current. The later part of the magnetic field deformation is seen as a travelling magnetic wave. We explain the magnetic perturbations by a combination of E × B drifting electrons and currents driven by plasma pressure gradients and the shape of the magnetic field. A plasma pressure wave is also recorded by a single tip Langmuir probe and the velocity (~103 m s−1) of the expanding plasma agrees well with the observed velocity of the magnetic wave. We note that the axial (discharge) current density is much too high compared to the azimuthal current density to be explained by classical collision terms, and an anomalous charge transport mechanism is required.
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- Karlsson, Tomas, et al.
(författare)
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On enhanced aurora and low-altitude parallel electric fields
- 2005
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Ingår i: Physica Scripta. - 0031-8949 .- 1402-4896. ; 72:5, s. 419-422
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Tidskriftsartikel (refereegranskat)abstract
- Enhanced auroras are bands of enhanced intensity, sometimes observed along the bottom edges of auroral arcs. It is here proposed that they sometimes are due to local electron energization by low-altitude. downward-directed DC electric fields. which arise as a consequence of the ionospheric response to small-scale auroral current systems. Simulations of such structures are presented. and the electron energization mechanism is discussed. The enhanced auroras would, in this model, appear parallel and close to (perhaps only a few kin front) an ordinary auroral arc. Detailcd 3-D observations of auroral arc structures could resolve if enhanced auroras sometimes take this form.
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