On Low-Frequency Type III Solar Radio Bursts Observed in Interplanetary Space

This article puts forth an alternative theory of interplanetary type III radio bursts, based on the cyclotron-maser instability. The model suggests that the radio emission is not generated in local interplanetary space but originates much closer to the corona. It postulates that the radiation remains trapped inside the density-depleted duct until it emerges out into free space at the end of the duct, which is located in the interplanetary space. With this model a number of outstanding problems associated with interplanetary type III bursts can be naturally resolved. Termination of type III bursts at low frequency can be explained by the fact that the ratio of plasma- to gyrofrequency, f_p/f_g, in the source region exceeds the maximum allowable value for the maser instability. The low starting frequency can be explained by the fact that the parameter f_p/f_g in the source region falls below the minimum allowable value for the maser instability to operate. The rapid increase of temporal width of the dynamic spectrum near the termination frequency may be due to the low group speed associated with waves trapped inside the duct.