F. J. Polanco-Rodríguez, C. Krafft, P. Savoini
Abstract The nonlinear decay of Langmuir/ Z -mode waves into electromagnetic Z -mode wave radiation near the plasma frequency is observed for the first time in the solar wind, during the encounter of the Solar Orbiter satellite with an electron beam associated with a type III radio burst. This result was achieved through the high-resolution electric and magnetic field measurements provided by the Radio Plasma Waves instrument on board the spacecraft. The decay process is identified through multiple lines of evidence: satisfaction of Doppler-shifted frequency resonance conditions, strong phase coherence and temporal coincidence between the interacting waves, dominance over competing mechanisms, and full agreement with theoretical predictions. Two-dimensional 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.