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Sökning: swepub > Umeå universitet > Stenflo Lennart

  • Resultat 1-10 av 172
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1.
  • Stenflo, Lennart, et al. (författare)
  • Nonlinear beam generated plasma waves as a source for enhanced plasma and ion acoustic lines
  • 2011
  • Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 18:15, s. 052107-1-052107-14
  • 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.
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2.
  • Brodin, Gert, et al. (författare)
  • Three-wave coupling coefficients for a magnetized plasma
  • 2012
  • Ingår i: Physica Scripta. - Bristol : Institute of Physics. - 0031-8949 .- 1402-4896. ; 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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3.
  • Brodin, Gert, et al. (författare)
  • A new decay channel for upper-hybrid waves
  • 2016
  • Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 91:10
  • Tidskriftsartikel (refereegranskat)abstract
    • We look here at a three-wave interaction process involving only electrostatic waves in an electron plasma with stationary ions. Special attention is given to the case with an upper-hybrid wave as a pump wave, where a new decay channel is pointed out. The corresponding growth rate is calculated.
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4.
  • Brodin, Gert, et al. (författare)
  • A simple electron plasma wave
  • 2017
  • Ingår i: Physics Letters A. - : Elsevier BV. - 0375-9601 .- 1873-2429. ; 381:11, s. 1033-1035
  • Tidskriftsartikel (refereegranskat)abstract
    • Considering a class of solutions where the density perturbations are functions of time, but not of space, we derive a new exact large amplitude wave solution for a cold uniform electron plasma. This result illustrates that most simple analytical solutions can appear even if the density perturbations are large.
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5.
  • Brodin, Gert, et al. (författare)
  • Alfven wave interactions within the Hall-MHD description
  • 2013
  • Ingår i: Journal of Plasma Physics. - : Cambridge University Press (CUP). - 0022-3778 .- 1469-7807. ; 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.
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6.
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7.
  • Brodin, Gert, et al. (författare)
  • Large amplitude electron plasma oscillations
  • 2014
  • Ingår i: Physics Letters A. - : Elsevier. - 0375-9601 .- 1873-2429. ; 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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8.
  • Brodin, Gert, et al. (författare)
  • Nonlinear dynamics of a cold collisional electron plasma
  • 2017
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 24:12
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the influence of collisions on the dynamics of a cold non-relativistic plasma. It is shown that even a comparatively small collision frequency can significantly change the large amplitude wave solution.
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9.
  • Brodin, Gert, et al. (författare)
  • Nonlinear dynamics of large amplitude modes in a magnetized plasma
  • 2014
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 21:12
  • Tidskriftsartikel (refereegranskat)abstract
    • We derive two equations describing the coupling between electromagnetic and electrostatic oscillations in one-dimensional geometry in a magnetized cold and non-relativistic plasma. The nonlinear interaction between the wave modes is studied numerically. The effects of the external magnetic field strength and the initial electromagnetic polarization are of particular interest here. New results can, thus, be identified. 
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10.
  • Brodin, Gert, et al. (författare)
  • Nonlinear wave interactions of kinetic sound waves
  • 2015
  • Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 33:8, s. 1007-1010
  • Tidskriftsartikel (refereegranskat)abstract
    • We reconsider the nonlinear resonant interaction between three electrostatic waves in a magnetized plasma. The general coupling coefficients derived from kinetic theory are reduced here to the low-frequency limit. The main contribution to the coupling coefficient we find in this way agrees with the coefficient recently presented in Annales Geophysicae. But we also deduce another contribution which sometimes can be important, and which qualitatively agrees with that of an even more recent paper. We have thus demonstrated how results derived from fluid theory can be improved and generalized by means of kinetic theory. Possible extensions of our results are outlined.
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