Electromagnetic radiation by turbulent, magnetized and randomly inhomogeneous solar radio sources generated by electron beams

Abstract

During Type III solar radio bursts, electromagnetic waves are radiated at plasma frequency $\omega_p$ and its harmonics by electrostatic wave turbulence generated by electron beams ejected by Sun in randomly inhomogeneous solar wind and coronal plasmas. These emissions, detected since decades by spacecraft and radiotelescopes, are split by the plasma magnetic field into three modes $\mathcal{X}$, $\mathcal{O}$ and $\mathcal{Z}$ of different dispersion, polarization and radiation properties. This work demonstrates, using three independent and converging approaches, that only a small fraction of electromagnetic energy radiated at $\omega_p$ ($\lesssim10\%$) is escaping from beam-generated radio sources, mainly as $\mathcal{O}$-mode waves and, depending on plasma conditions, as $\mathcal{X}$-mode waves. Most energy is radiated in $\mathcal{Z}$-mode and can therefore be only observed close to sources. Results have major implications for solar radio emission and provide strong support for interpretation of observations performed up to close distances to Sun by Parker Solar Probe and Solar Orbiter spacecraft.