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Sökning: swepub > Umeå universitet > Stenflo Lennart > Engelska

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1.
  • Brodin, G, et al. (författare)
  • Alfven wave interactions within the Hall-MHD description
  • 2013
  • Ingår i: Journal of Plasma Physics. - Cambridge University Press (CUP). - 0022-3778. ; 79, s. 909-911
  • Tidskriftsartikel (refereegranskat)abstract
    • We show that comparatively simple expressions for the Alfven wave coupling coefficients can be deduced from the well-known Hall-magnetohydrodynamics (MHD) model equations.
2.
  • Brodin, G., et al. (författare)
  • Large amplitude electron plasma oscillations
  • 2014
  • Ingår i: Physics Letters A. - Elsevier. - 0375-9601. ; 378:22-23, s. 1632-1635
  • Tidskriftsartikel (refereegranskat)abstract
    • We consider a cold plasma in order to find new large-amplitude wave solutions in the long-wavelength limit. Accordingly we derive two generic coupled equations which describe the energy exchange between the electrostatic and electromagnetic waves. A new kind of quasi-periodic behavior is found. Our derivations may be considered as a prerequisite to extended studies of stimulated Raman scattering for cases where the wave amplitudes are so large that standard perturbation techniques are not applicable.
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3.
  • Brodin, G, et al. (författare)
  • On the parametric decay of waves in magnetized plasmas
  • 2009
  • Ingår i: Journal of Plasma Physics. - Cambridge UK : Cambridge University Press. - 0022-3778. ; 75, s. 9-13
  • Tidskriftsartikel (övrigt vetenskapligt)abstract
    • We reconsider the theory for three-wave interactions in cold plasmas. In particular, we demonstrate that previously overlooked formulations of the general theory are highly useful when deriving concrete expressions for specific cases. We also pointout that many previous results deduced directly from the basic plasma equations contain inappropriate approximations leading to unphysical results. Finally, generalizations to more elaborate plasma models containing, for example, kinetic effects are given.
4.
  • Brodin, Gert, et al. (författare)
  • Stimulated Brillouin scattering in magnetized plasmas
  • 2013
  • Ingår i: Journal of Plasma Physics. - Cambridge University Press. - 0022-3778. ; 79:6, s. 983-986
  • Tidskriftsartikel (refereegranskat)abstract
    • Previous theory for stimulated Brillouin scattering is reconsidered and generalized. We introduce an effective ion sound velocity that turns out to be useful in describing scattering instabilities.
5.
  • Brodin, Gert, 1963-, et al. (författare)
  • Three-wave coupling coefficients for a magnetized plasma
  • 2012
  • Konferensbidrag (övrigt vetenskapligt)abstract
    • The resonant interaction between three waves in a uniform magnetized plasma is reconsidered. Starting from previous kinetic expressions, that contain a general but too little used result, we are able to improve the formulas. This leads to an explicit expression for the three wave coupling coefficient which applies for arbitrary wave propagation in a magnetized Vlasov plasma.
6.
  • Brodin, G, et al. (författare)
  • Three-wave coupling coefficients for a magnetized plasma
  • 2012
  • Ingår i: Physica Scripta. - Institute of Physics. - 0031-8949. ; 85:3, s. 035504
  • Tidskriftsartikel (refereegranskat)abstract
    • The resonant interaction between three waves in a uniform magnetized plasma is reconsidered. Starting from previous kinetic expressions, which contain a general but too little used result, we are able to improve the formulae. This leads to an explicit expression for the three-wave coupling coefficient which applies for arbitrary wave propagation in a magnetized Vlasov plasma.
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7.
  • Daldorff, LKS, et al. (författare)
  • Nonlinear beam generated plasma waves as a source for enhanced plasma and ion acoustic lines
  • 2011
  • Ingår i: Physics of Plasmas. - 1070-664X. ; 18:15, s. 052107-052114
  • Tidskriftsartikel (refereegranskat)abstract
    • Observations by, for instance, the EISCAT Svalbard Radar (ESR) demonstrate that the symmetry of the naturally occurring ion line in the polar ionosphere can be broken by an enhanced, nonthermal, level of fluctuations (naturally enhanced ion-acoustic lines, NEIALs). It was in many cases found that the entire ion spectrum can be distorted, also with the appearance of a third line, corresponding to a propagation velocity significantly slower than the ion acoustic sound speed. It has been argued that selective decay of beam excited primary Langmuir waves can explain some phenomena similar to those observed. We consider a related model, suggesting that a primary nonlinear process can be an oscillating two-stream instability, generating a forced low frequency mode that does not obey any ion sound dispersion relation. At later times, the decay of Langmuir waves can give rise also to enhanced asymmetric ion lines. The analysis is based on numerical results, where the initial Langmuir waves are excited by a cold dilute electron beam. By this numerical approach, we can detect fine details of the physical processes, in particular, demonstrate a strong space-time intermittency of the electron waves in agreement with observations. Our code solves the full Vlasov equation for electrons and ions, with the dynamics coupled through the electrostatic field derived from Poisson's equation. The analysis distinguishes the dynamics of the background and beam electrons. This distinction simplifies the analysis for the formulation of the weakly nonlinear analytical model for the oscillating two-stream instability. The results have general applications beyond their relevance for the ionospheric observations.
8.
  • Eliasson, Bengt, et al. (författare)
  • Full-scale simulation study of stimulated electromagnetic emissions:
  • 2010
  • Ingår i: JOURNAL OF PLASMA PHYSICS. - Cambridge University Press. - 0022-3778. ; 76, s. 369-375
  • Tidskriftsartikel (refereegranskat)abstract
    • A full-scale numerical study is performed of the nonlinear interaction between a large-amplitude electromagnetic wave and the Earths ionosphere; and of the stimulated electromagnetic emission emerging from the turbulent layer, during the first 10 milliseconds after switch-on of the radio transmitter. The frequency spectra are downshifted in frequency and appear to emerge from a region somewhat below the cutoff of the O mode, which is characterized by Langmuir wave turbulence and localized Langmuir envelopes trapped in ion density cavities. The spectral features of escaping O-mode waves are very similar to those observed in experiments. The frequency components of Z-mode waves, trapped in the region between the O- and Z-mode cutoffs show strongly asymmetric and downshifted spectra.
9.
  • Eliasson, Bengt, et al. (författare)
  • Full-scale simulation study of the initial stage of ionospheric turbulence
  • 2008
  • Ingår i: Journal of Geophysical Research. - 0148-0227. ; 113:0148-0227, s. A02305-1-A02305-9
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a full-scale simulation of the nonlinear interaction between an intense electromagnetic wave and the Earth's ionosphere, by means of a generalized Zakharov model. The radio wave propagates from the neutral atmosphere into the ionospheric plasma layer and reaches the turning points of the ordinary and extraordinary wave modes. At the turning point of the ordinary mode, a parametric instability takes place in which the electromagnetic wave decays into an electron plasma wave and an ion acoustic wave with a typical wavelength of one meter. This is followed by collapse and caviton formation and trapping of the intense electron plasma wave. The cavitons lead to an efficient excitation of slow X (or Z) waves that propagate further into the denser ionospheric layer at higher altitudes. We use a realistic ion (oxygen) mass, length scales, and other plasma parameters. This numerical study should be useful for understanding the nonlinear interaction between intense radio waves and the ionosphere.   
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10.
  • Gradov, Oleg, et al. (författare)
  • Non-linear effects in a cold electron plasma with non-uniform density profile
  • 2008
  • Ingår i: Physica Scripta. - 0031-8949. ; 77, s. 055503-1-055503-3
  • Tidskriftsartikel (refereegranskat)abstract
    •  A new step forward on the theory for two-dimensional wave propagation is outlined for a non-uniform plasma with a smooth density profile. A way to excite envelope solitary waves with certain shapes is described. The corresponding wave space structure is calculated, and the restrictions on the wave profile along the direction of wave propagation are noticed.
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