Laboratoire de Mécanique des Fluides et d'Acoustique - UMR 5509

LMFA - UMR 5509
Laboratoire de Mécanique des Fluides et d’Acoustique
Lyon
France


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Accueil > Activités de Recherche > Turbulence et Instabilités > Séminaires informels > Séminaires passés

Fabio Feraco

Alfvén waves and parametric instability in the solar wind plasma in MHD approximation

Mardi 16 mai 2017 13h, INSA, salle de conférences LMFA bât. Jacquard

Alfvén waves and parametric instability in the solar wind plasma in MHD approximation

Alfvén waves are exact solutions of the linearized Magnetohydrodynamics (MHD) equations for a homogeneous, incompressible and ideal plasma, even in the large-amplitude, circularly polarized, case. They play a major role in several plasma settings such as the plasmas of solar origin, and in particular in the Solar Wind (SW). The latter is a flow of charged particles originating in the solar corona which expands into the outer-space developing turbulence and it can be considered as a natural wind tunnel.
However, as a consequence of this, a large amplitude Alfvénic fluctuation, travelling in a homogeneous and incompressible plasma, should propagate indefinitely without producing any turbulent effect since the non-linear terms in the MHD equations vanish for purely outward propagating fluctuations. In the low-latitude SW, near to the ecliptic plane, where the magnetic field is less homogeneous and the plasma is more compressible, Alfvén waves spontaneously decay in backward propagating waves, thus producing turbulence. In the polar wind, where the background magnetic field is more homogeneous and density fluctuations have smaller amplitudes, instead, the mechanism through which the turbulence is produced is less clear.
In this framework, I will characterize a special type of plasma instability, called “parametric”, and show how this provides a mechanism to produce correlation between kinetic and magnetic fields and thus sustain the non-linear coupling between Alfvén fluctuations with opposite polarization.
This will be achieved by means of pseudo-spectral simulations over different values of the β parameter.

PNG - 113.3 ko
Numerical results regarding the evolution of the mean density for β=0.5 in the one-dimensional case (red) and the two-dimensional-case (green).
PNG - 111.1 ko
Numerical results regarding the evolution of the mean density for β=0.9 in the one-dimensional case (red) and the two-dimensional-case (green).