Parametric decay of parallel and oblique Alfven waves in the expanding solar wind

TL;DR

This study uses 2D MHD simulations to explore how large-amplitude Alfven waves decay in the expanding solar wind, revealing new insights into wave stability, mode excitation, and the influence of expansion on instability growth.

Contribution

It demonstrates the first direct excitation of transverse daughter modes in an expanding environment and shows how expansion suppresses parametric decay, explaining wave persistence in the solar wind.

Findings

Transverse cascade of daughter modes occurs in oblique waves.

Expansion reduces and can suppress parametric decay instability.

Oblique waves exhibit direct excitation of transverse modes.

Abstract

The long-term evolution of large-amplitude Alfven waves propagating in the solar wind is investigated by performing two-dimensional MHD simulations within the expanding box model. The linear and nonlinear phases of the parametric decay instability are studied for both circularly polarized waves in parallel propagation and for arc-polarized waves in oblique propagation. The non-monochromatic case is also considered. In the oblique case, the direct excitation of daughter modes transverse to the local background field is found for the first time in an expanding environment, and this transverse cascade seems to be favored for monochromatic mother waves. The expansion effect reduces the instability growth rate, and it can even suppress its onset for the lowest frequency modes considered here, possibly explaining the persistence of these outgoing waves in the solar wind.