Parametric decay of Alfv\'enic wave packets in nonperiodic low-beta plasmas

TL;DR

This study uses hybrid simulations to explore how finite-size Alfvén wave packets decay in nonperiodic low-beta plasmas, revealing unique dynamics influenced by system size, instability factors, and conditions for secondary decay.

Contribution

It provides new insights into the decay behavior of finite Alfvén wave packets in nonperiodic plasmas, highlighting the effects of system size, boundary conditions, and instability parameters.

Findings

Decay dynamics differ from periodic systems, showing reduced energy transfer.

Localized density cavitation and ion heating occur during decay.

Secondary decay can be triggered under certain conditions.

Abstract

The parametric decay of finite-size Alfv\'en waves in nonperiodic low-beta plasmas is investigated using one-dimensional hybrid simulations. Compared with the usual small periodic system, a wave packet in a large system under the absorption boundary condition shows different decay dynamics, including reduced energy transfer and localized density cavitation and ion heating. The resulting Alfv\'en wave dynamics are influenced by several factors of the instability including the growth rate, central wave frequency, and unstable bandwidth. A final steady state of the wave packet may be achieved when the instability does not have enough time to develop within the residual packet, and the packet size shows well-defined scaling dependencies on the growth rate, wave amplitude, and plasma beta. Under the proper conditions enhanced secondary decay can also be excited in the form of a narrow, amplified wavepacket. These results may help interpret laboratory and spacecraft observations of Alfv\'en waves, and refine our understanding of associated energy transport and ion heating.