Third harmonic plasma emission in solar type II radio bursts
E.Ya. Zlotnik 1,
A. Klassen 2,
K.-L. Klein 3,
H. Aurass 2 and
G. Mann 2
Received 29 September 1997 / Accepted 18 November 1997
We discuss consequences of the recently reported experimental evidence for third harmonic plasma emission during shock-excited solar radio bursts (type II bursts). Spectrographic and partly imaging observations of three type II bursts displaying three drifting bands with frequencies related as 1 : 2 : 3 have been studied. The radio data of these events were simultaneously recorded by the digital radiospectrograph of the Observatory of Solar Radioastronomy in Potsdam-Tremsdorf and the multifrequency radioheliograph of the Paris-Meudon Observatory in Nançay. The data allow for determining the brightness temperature of radio emission in the three frequency bands. There are one to three orders of magnitude difference between the brightness temperature of the second and the third harmonic plasma emission in our burst sample.
Two non-linear processes - the coalescence of three plasma waves, and the coalescence of a plasma wave and an electromagnetic one at twice the plasma frequency - are considered to explain the occurrence of a third harmonic. The analysis shows that both processes can fit the observed brightness temperatures. The first process acts preferably at low phase velocities of plasma waves and sharp electron density gradients in the source, the second in the case of high plasma wave phase velocities. This means regarding both processes, the occurrence of the third harmonic in type II burst emission due to non-linear coronal plasma processes demands for some additional specific conditions in the shock or foreshock region. Finally, we propose a method to distinguish between the two invoked non-linear processes by a statistical investigation of a larger type II burst sample.
Key words: Sun: corona Sun: radio radiation shock waves
Send offprint requests to: E.Ya. Zlotnik
© European Southern Observatory (ESO) 1998
Online publication: March 3, 1998