Polarisation Analysis of in-situ observations of Langmuir/Z-Mode Waves by Solar Orbiter

Type III solar radio emissions originate from a mode conversion of electrostatic Langmuir waves generated by an energetic electron beam. This electron beam creates a bump-on-tail instability in the electron velocity distribution function, generating Langmuir waves as the electrons propagate along the magnetic field lines in the solar wind. The Solar Orbiter spacecraft enables us to study Type III radio emissions and the Langmuir waves locally generated in the solar wind with a very high temporal resolution. Using electron velocity distribution measurements obtained by the Solar Wind Analyser (SWA) and Energetic Particle Detector (EPD) instruments, we model the plasma environment and numerically solve the dispersion relation. After deriving the dispersion relation for the generalized Langmuir/Z-mode, we successfully match the waveform data observed by the Radio and Plasma Waves (RPW) instrument with the theoretical prediction. Furthermore, we analyse the wave polarization properties, including a coherent high frequency magnetic component. Combining the solution of the dispersion relation and observations, we constrain the wave vector and provide valuable insights into the wave polarisation properties.