The polarisation of second harmonic coronal type III bursts

The process of second harmonic plasma emission is considered, where two simplifying approximations made in previous treatments are relaxed. The revised theory can account for strong polarisation observed in some harmonic coronal type III bursts, and predicts that a correlation between these bursts and the fastest electron beam speeds associated with type III emission should be apparent. These high observed degrees of polarisation could not be explained in the earlier theory. In most cases, polarisation in the sense of the magnetoionic o-mode is predicted. The predicted degree of polarisation is shown to be strongly dependent on the form of the Langmuir wave number spectrum, and in particular on the magnitude of the wave number offset between beam-driven and backscattered Langmuir waves.