Parametric instability and turbulent cascades in space plasmas

Spacecraft observations show the presence, in the polar Solar Wind (SW) plasma, of outward propagating Alfvénic fluctuations with a very broadband spectrum. This is true only up to a certain frequency in the spectrum, after which a consistent amount of energy is present also in the spectra of inward propagating Alfvén and density fluctuations. A mechanism able to explain the production of inward propagating modes and density fluctuations is the parametric instability, in which Alfvén waves can decay and produce back-scattered waves and density perturbations. In this work, we show some recent results obtained with a (2+1/2)D-MHD pseudo-spectral numerical code in which an attempt is made to reproduce an initial condition similar to that observed in the real SW. The evolution of the instability generates a nonlinear cascade that redistributes the energy towards the small scales in all directions. A striking feature of this evolution is the presence of a few localized, energy containing, coherent pressure-balanced structures.