Evidence of Langmuir/$\mathcal{Z}$-mode Wave Decay into $\mathcal{Z}$-mode Electromagnetic Radiation in the Solar Wind

TL;DR

This paper reports the first direct observation of Langmuir/$\mathcal{Z}$-mode wave decay into electromagnetic $\mathcal{Z}$-mode radiation in the solar wind, confirmed by high-resolution measurements and particle-in-cell simulations.

Contribution

It provides the first observational evidence of nonlinear wave decay in the solar wind, supported by simulations that explain the trapping and decay process of wave packets.

Findings

First direct detection of wave decay in solar wind

Observation matches theoretical resonance and coherence conditions

Simulations reproduce observed decay features

Abstract

The nonlinear decay of Langmuir/$\mathcal{Z}$-mode waves into electromagnetic $\mathcal{Z}$-mode wave radiation at the plasma frequency is observed for the first time in the solar wind. This finding was enabled by the unprecedented high-resolution electric and magnetic field measurements provided by the Radio Plasma Waves (RPW) instrument aboard the Solar Orbiter spacecraft, which encountered an electron beam associated with a Type III radio burst. The decay process is definitively identified through multiple lines of evidence: satisfaction of frequency and wavevector resonance conditions, strong phase coherence and temporal coincidence between the interacting waves, exclusion of competing mechanisms, and full agreement with theoretical predictions. Particle-in-cell simulations, conducted under close beam-plasma conditions, successfully reproduce the key features of the observations. Notably, they suggest that the wave packet observed by Solar Orbiter may be trapped within an extended, nearly flat-bottomed density well, where the decay process is not overcome by wave scattering on random density fluctuations and subsequent mode conversion effects.