Non-Thermal Sources and Amplified Emission in Solar Radio Bursts

Abstract

The possibility of amplifying radio waves in a plasma is explored along the lines pioneered by Twiss (1958) ; amplification depends on the macroscopic absorption coefficient assuming a negative value. The conditions for negative absorption are specified in terms of the electron-energy distribution, the statistical weight of the energy levels of the radiating electrons and their mean emissivity. The latter is characteristic of particular microscopic radiation mechanisms.

One necessary condition for negative absorption is a positive gradient in the electron-energy distribution ; this condition is probably met by the streams of fast electrons which have been identified as the sources of solar radio bursts.

A second condition a negative gradient in the product of statistical weight and electron emissivity-has to be investigated for the various microscopic radiation mechanisms.

We find that bremsstrahlung and synchrotron radiation from highly relativistic electrons cannot lead to amplification; gyro-radiation from mildly relativistic electrons and Čerenkov plasma waves can. In the latter case Bohm and Gross (1949) demonstrated that amplification was due to coherent emission from bunched electrons in a stream.

These conclusions on amplified and temperature-limited radiations from non-thermal electron streams are applied to different types of solar radio bursts. It is suggested that the drifting bursts of Types II and III originate in radiation from amplified longitudinal plasma waves. Microwave bursts and several phases of Type-IV emission are probably non-thermal, but temperature limited, gyro-synchrotron radiation. It is tempting to invoke amplified gyro radiation to explain Type-1 and other bursts superimposed on long continuum radiation.