Excitation of Alfven Wave Parametric Decay in 3D Open-Boundary Low-Beta Plasma*

We present the first 3D, open-boundary, hybrid kinetic-fluid simulations of Alfven wave parametric decay instability (PDI) in a low-beta plasma. The Alfven wave PDI—where a large forward pump Alfven wave decays into a backward child Alfven wave and a forward ion acoustic wave—is a significant process for understanding wave dissipation and plasma heating. However, details regarding how PDI is excited in realistic 3D open systems and how the finite perpendicular wave scale—as found in both laboratory and space plasmas—affects the excitation remain elusive. Our 3D simulations and theoretical analysis reveal that PDI excitation is strongly influenced by the wave damping present, including electron-ion collisional damping (represented by constant resistivity) and geometrical attenuation associated with the finite-scale Alfven wave, and ion Landau damping of the child acoustic wave. The perpendicular wave scale alone, however, plays no discernible role: waves of different perpendicular scales exhibit similar instability excitation as long as the magnitude of the parallel ponderomotive force remains unchanged. This new understanding of 3D Alfven wave PDI physics is essential for advancing laboratory studies of the fundamental plasma process and may provide insights into the role of PDI in low-beta space plasmas.