The Direct Amplification of Electromagnetic Waves by Electron Beams: An Alternate Explanation for Solar Type III Bursts

Abstract

The direct amplification of electromagnetic (EM) waves by the beam-plasma instability is proposed as an alternative for solar type III bursts in corona and planetary space, the main properties of type III bursts at meter bands are compared to this model, including the cutoff frequency, the timescale, the relative bandwidth, the polarization sense and degree, and the harmonic emission. This mechanism may explain the reason why the cutoff frequency or the low-frequency limit of type III bursts observed by the Ulysses spacecraft is much larger than the local electron plasma frequency, which may have resulted from the variation of the heliocentric distance of the spacecraft as well as the ratio between the local electron plasma and cyclotron frequencies.