One aspect of the generation of type III solar radio bursts by stabilized streams of fast particles

Abstract

It is shown that when a stabilized stream (a proton stream, in particular) having fairly small velocity dispersion is injected instantaneously into the corona there may be two excitation modes for type III bursts. If the characteristic time scale of the stream is small as compared with the time for damping of plasma waves owing to electron-ion collisions in the corona, then at a fixed frequency the burst profile will have a smoothly growing front with subsequent smooth damping. From the qualitative viewpoint this case resembles the generation of a type III burst by an unstabilized stream. If the stream time scale is large as compared with the collision time, however, the type III burst profile will be quasioscillatory in nature (i. e., it will differ substantially from the time profile for the case of burst generation by an unstabilized stream). Such type III bursts having a time profile of quasioscillatory structure indicate that stabilization effects may, albeit quite rarely, play a decisive role in the expansion of streams of fast particles in the corona. i. Introduction Ginzburg and Zheleznyakov [1] were the first to attempt a detailed investigation of the mechanism for generation of type III bursts, which they assumed to be associated with streams of fast electrons penetrating the solar corona and exciting plasma waves in it. They also assumed that Rayteigh and Raman scattering of plasma waves by thermal fluctuations of the main plasma lead to the appearance of electromagnetic radiation at frequencies close to C~Le and 2COLe. A more effective model for formation of the second harmonic was later proposed within the framework of this same mechanism: Raman scattering of plasma waves by fluctuations of the plasma-wave type having a level substantially higher than that of the thermal fluctuations. This excess was caused either by induced scattering [2] or spontaneous scattering [3l of excited plasma waves into plasma waves. A significant aspect in the further development of type III bursts was the problem of the interaction of a stream of fast particles within the plasma waves excited by them. Owing to strong quasilinear relaxation, the excitation of plasma waves by an electron stream would appear at first glance to lead to retardation of the beam at absurdly small distances (the order of 108 cm) and to elimination of the instability. Thus it was difficult to understand how a stream of fast electrons could maintain its ability to generate plasma waves over the distance between the bottom layers of the corona and the earth's orbit.