Nonlinear reflection process of linearly-polarized, broadband Alfv\'en waves in the fast solar wind

TL;DR

This study uses numerical simulations to explore how linearly polarized broadband Alfvén waves undergo nonlinear reflection in the solar wind, revealing a new energy transfer mechanism that explains observed fluctuation patterns.

Contribution

It demonstrates nonlinear wave-wave interactions causing Alfvén wave reflection for linearly polarized waves, a process not previously detailed in this context.

Findings

Nonlinear reflection involves wave steepening and energy transfer to backscattered waves.

The process explains the increasing sunward fluctuation energy with distance in the solar wind.

Backscattering occurs via decay instability, influenced by wave polarization and broadband spectrum.

Abstract

Using one-dimensional numerical simulations, we study the elementary process of Alfv\'{e}n wave reflection in a uniform medium, including nonlinear effects. In the linear regime, Alfv\'{e}n wave reflection is triggered only by the inhomogeneity of the medium, whereas in the nonlinear regime, it can occur via nonlinear wave-wave interactions. Such nonlinear reflection (backscattering) is typified by decay instability. In most studies of decay instabilities, the initial condition has been a circularly polarized Alfv\'{e}n wave. In this study we consider a linearly polarized Alfv\'en wave, which drives density fluctuations by its magnetic pressure force. For generality, we also assume a broadband wave with a red-noise spectrum. In the data analysis, we decompose the fluctuations into characteristic variables using local eigenvectors, thus revealing the behaviors of the individual modes. Different from circular-polarization case, we find that the wave steepening produces a new energy channel from the parent Alfv\'en wave to the backscattered one. Such nonlinear reflection explains the observed increasing energy ratio of the sunward to the anti-sunward Alfv\'{e}nic fluctuations in the solar wind with distance against the dynamical alignment effect.