A dynamic theory of type III solar radio bursts

Abstract

Numerous experimental data indicate that type III solar radio bursts are generated by the streams of fast electrons in the corona. The process of the electron acceleration in the flare region is, in general, of the character of a short time local explosion. As a result, a spatially limited stream with inhomogeneous front and back is formed in the corona. The present paper shows that such a spatial structure of fast electrons radically changes the dynamics of the stream instability development. In particular, for example, despite strong quasilinear relaxation, in this case the electron stream can generate plasma waves in the corona for a long time due to faster electrons escaping out of the front of the stream. The extension of the stream in the outer corona where the collisions are negligible is of similar character. The maximum of the energy density in the packet of excited plasma waves travels in the corona with constant mean velocity which is defined by the fast-electron energy at the moment of their formation in the flare region. Therefore, in spite of the considerable influence of quasilinear effects on the stream motion, the velocity of type III sources found in terms of the drift velocity remains unchanged. This creates the illusion of stream stabilization. The energy dissipation of plasma waves for the low frequency type III bursts is fully determined by Landau damping in the tail of the stream. Because of this, the temperature estimates of the outer corona from time profiles of type III bursts are incorrect. The theoretical curves of the time variation of radiation at different frequencies agree well with the experimental data in the hectometer wave range under the assumption that the electromagnetic wave generation takes place at the second harmonic of the plasma frequency. In this case it is necessary to decrease the electron density in the solar corona at distances of about 5 to 30 R⊙ 4 times as compared with the previous densities previously derived from type III data.