'Plasma Emission' without Langmuir Waves

Abstract

Recent observations have confirmed that the level of Langmuir waves associated with type III streams of electrons in the interplanetary medium is usually too low to account for the observed radio emission by the accepted 'plasma emission' processes, and it has been suggested that emission mechanisms which do not require Langmuir waves should be explored. Four such mechanisms are discussed. One is a parametric instability leading directly to second-harmonic emission; it is found inapplicable under conditions of interest here. The other three processes all involve ion-sound turbulence. One which is known in a different context is turbulent bremsstrahlung. Turbulent bremsstrahlung of transverse waves is found to compare unfavourably with the other two processes, which are scattering of an ion-sound (s) wave into a transverse (t) wave and double emission of both waves simultaneously. These latter two processes are related by a crossing symmetry and are treated together with the following results: (i) The processes become greatly enhanced when the beat (w±w', k±k') between the t wave and the s wave nearly satisfies the dispersion relation for Langmuir (I) waves. (ii) A bump-in-the-tail instability (due to electrons with dF(v)/dv > 0) can cause the transverse waves to grow due to double emission; this growth has been likened to a freeelectron maser. (iii) The familiar bump-in-the-tail instability for resonant I waves can be suppressed by the ion-sound waves, and the double-emission instability then takes over with about the same growth rate as the original I-wave instability. (iv) The conditions for the double-emission instability to occur are probably satisfied at least some of the time for type III streams. It is concluded that although 'plasma emission' without Langmuir waves may be possible in principle, it is unlikely to play any role in type III bursts.