Structured type III radio bursts observed in interplanetary space

Context: The last few decades has seen numerous studies dedicated to fine structures of type III radio bursts observed in the metric to decametric wavelengths. Majority of explanations of the structured radio emission involve the propagation of electron beam through the strongly inhomogeneous plasma in the low corona. Until now only few studies of single type III bursts with fine structures, observed in the hecto-kilometric wavelengths, were reported. Aims: Herein we report about existence of numerous structured type III radio bursts observed during the STEREO era by all three WAVES instruments on board STEREO A, B, and Wind. The aim of the study is to report, classify structured type III bursts, and present the characteristics of their fine structures. The final goal is to try to understand the physical mechanism responsible for the generation of structured radio emission. Methods: In this study we used data from all available spacecraft, specifically the STEREO and the Wind spacecraft. We employ 1D density models to obtain the speed of the source of type III radio emission, the electron beam. We also perform spectral analysis of the fine structures in order to compare their characteristics with the metric-decametric fine structures. Results: The presented similarities of the type III fine structures in the metric to decametric and interplanetary wavelengths indicate that the physical processes responsible for the generation of structured type III radio bursts could be the same, at the heights, all the way from the low corona to the interplanetary range. We show that the observed structuring and intermittent nature of the type III bursts can be explained by the variation in the level of density fluctuations, at different distances from the Sun.