3RD-HARMONIC PLASMA EMISSION IN SOLAR-TYPE-II RADIO-BURSTS

We discuss consequences of the recently reported experimental evidence for third harmonic plasma emission during shock-excited solar radio b ursts (type II bursts). Spectrographic and partly imaging observations of three type II bursts displaying three drifting bands with frequenc ies related as 1 : 2 : 3 have been studied. The radio data of these ev ents were simultaneously recorded by the digital radiospectrograph of the Observatory of Solar Radioastronomy in Potsdam-Tremsdorf and the m ultifrequency radioheliograph of the Paris-Meudon Observatory in Nanca y. The data allow for determining the brightness temperature of radio emission in the three frequency bands. There are one to three orders o f magnitude difference between the brightness temperature of the secon d and the third harmonic plasma emission in our burst sample. Two non- linear processes - the coalescence of three plasma waves, and the coal escence of a plasma wave and an electromagnetic one at twice the plasm a frequency - are considered to explain the occurrence of a third harm onic. The analysis shows that both processes can fit the observed brig htness temperatures. The first process acts preferably at low phase ve locities of plasma waves and sharp electron density gradients in the s ource, the second in the case of high plasma wave phase velocities. Th is means regarding both processes, the occurrence of the third harmoni c in type II burst emission due to non-linear coronal plasma processes demands for some additional specific conditions in the shock or fores hock region. Finally, we propose a method to distinguish between the t wo invoked non-linear processes by a statistical investigation of a la rger type II burst sample.