Parametric decay of Alfvenic wave packets in nonperiodic low-beta plasmas*

Parametric decay instability of Alfven waves is an important magnetohydrodynamic process that is relevant to the development of turbulence and energization of charged particles in laboratory, space, and astrophysical plasmas. While the instability has received extensive theoretical and numerical studies primarily in a periodic system, actual Alfvenic fluctuations are often finite in time and associated interactions nonperiodic. Here, we study the parametric decay of finite Alfvenic waves in nonperiodic low-beta plasmas using one-dimensional hybrid simulations. Compared with the periodic system, a wave packet under the absorption boundary condition shows very different decay dynamics, including reduced energy transfer, and localized density cavitation and ion heating. The resulting Alfvenic propagation is influenced by the growth rate, central frequency, and unstable mode bandwidth of the instability. A stable final state of the wave can be achieved provided the instability is insufficiently developed within the packet envelope. Under proper conditions, much enhanced backscatter cascades can also be excited with an amplitude even higher than that of the pump wave. These results may help interpret laboratory and spacecraft observation of Alfvenic dynamics, and refine our understanding of associated energy transport and ion heating.