Parametric Decay of Obliquely Propagating Alfvén Waves: Transverse Coupling and Proton Parallel Acceleration

We present two-dimensional hybrid simulations of the evolution of large amplitude shear Alfvén waves in oblique propagation with respect to the ambient magnetic field. As in the case of parallel propagation, Alfvén waves are unstable to parametric decay resulting in the excitation of ion-acustic modes and of backward propagating Alfvén waves. Ion-acoustic modes, due to non-linear trapping, support the acceleration of protons with formation of a beam along the mean magnetic field. In contrast to the parallel propagating case, oblique shear Alfvén modes allow for the generation of a broad band spectrum of coupled Alfvénic and compressive modes (also in oblique propagation), selected by the resonance condition for the three-wave coupling. This leads to a direct coupling of the parent waves to small scales, supporting a strong transverse magnetic field modulation, once saturation is attained.