Hybrid simulations of large-amplitude Alfvénic fluctuations: the role of parametric instabilities in proton heating and acceleration

Abstract:

The solar wind is observed to display many non-thermal features such as a preferential perpendicular heating and a field-aligned proton beam population that coexist with a spectrum of large-amplitude Alfvénic fluctuations. While it has become clear that turbulent fluctuations can contribute to the thermodynamic evolution of the solar wind, it remains to understand what is the origin of the persistent field-aligned beam and what mechanisms allow for the observed non-adiabatic expansion of the solar wind. It is known that large-amplitude Alfvénic fluctuations tend to be unstable to parametric instabilities, which result in a decay process of the initial wave into different daughter waves depending upon the amplitude of the pump wave and the plasma beta. Here we revisit this problem by means of multidimensional hybrid simulations and investigate the stability of Alfvénic fluctuations, the saturation mechanisms of the decay process(es), and the final nonlinear state reached for different pump wave amplitudes and plasma beta values.

We find that the decay process in multi-dimensions persists at large values of the plasma beta via the filamentation/magnetosonic decay instabilities that lead to a nonlinear state characterized by a turbulent, heated plasma displaying a field-aligned beam at the Alfvén speed. By adopting a test-particle approach, we discuss the resulting anisotropic proton heating and particle acceleration with an emphasis on the importance of discontinuities produced by wave steepening in accelerating particles at the Alfvén speed.