Parametric Instability: An Evolutive Mechanism for the Alfvénic Turbulence in the Solar Wind

Fluctuations in fast streams or in slow Alfvénic streams of the solar wind, and in the high-latitude wind, are characterized by high cross-helicity and a low level of compressions. Such properties, which are typical of Alfvénic fluctuations, tend to decline with increasing heliocentric distance. Parametric decay, where the energy of an initial Alfvén wave is progressively transferred to both backward-propagating Alfvén and compressive modes, has been proposed as a mechanism responsible for such a behavior. Over the years, the parametric process has been studied, both analyt- ically and numerically, in many configurations, from monochromatic waves to increasingly complex situations which include broad-band turbulent configurations with one- and two-dimensional spectra. In this paper, we give a brief review of this theoretical development, discussing its relevance in the context the evolution of Alfvénic turbulence in the solar wind.