Third plasma harmonic radiation in type II bursts

Abstract

We present observational evidence for simultaneous fundamental, second and third harmonic radio emission during an excessively strong type II burst on February 16, 1984. This burst was emitted from an active region behind the limb allowing for fair resolution of the wave bands. If interpreted as a triple harmonic system, three different, nearly equally probable mechanisms for higher harmonic emission are qualitatively discussed. These are a four-wave process which involves very strong Langmuir waves, a decay process first proposed by Cairns (1987) for higher harmonic emission near the Earth's bow shock, and time evolution of the emitted frequency during Langmuir wave collapse. In sufficiently strong coronal shock waves, both of the former mechanisms may be more efficient than under solar wind conditions. In the third mechanism, Langmuir wave collapse may be driven by strong electron beams as are expected to exist in quasiparallel shocks where electron reflection may be strongest. We discuss the differences between the signatures of these mechanisms.