Plasma instability in front of ejected energetic electrons and type III solar radio bursts

Type III radio bursts are signatures of the fluxes of near-relativistic electrons ejected during solar flares. These bursts are frequently observed by spacecraft such as the Parker Solar Probe. It has been traditionally believed that these electron beams generate Langmuir waves through the two-stream instability, which are then converted into electromagnetic waves. In this study, we revise that model, by examining how the electron distribution becomes truncated due to the “time-of-flight” effect, as the beam travels through a randomly inhomogeneous and gently varying solar wind plasma. Rather than the two-stream instability, this truncation destabilizes the distribution and leads to the generation of Langmuir waves via a linear instability; we confine our analysis to this linear regime and do not take into account the backreaction of the generated Langmuir waves on the electron distribution, which is nonlinear. The instability grows until slower electrons arrive and dampen the waves. Our qualitative analysis shows that the resulting wave intensity growth and decay closely match the intensity–time profile of observed type III radio bursts at the fundamental frequency, supporting this modified theory.