Abstract. Using high-time resolution measurements from the FAST and CLUSTER spacecraft we analyze different types of fine frequency structures of Auroral Kilometric Radiation (AKR). Based on the results from previous numerical simulations, we emphasize that the recorded frequency structures are the ones which are expected to occur on the low- and high-potential sides of an accelerating double layer. Electron holes producing steep perpendicular velocity gradients on the horseshoe velocity distribution function appear to be responsible for generating AKR fine-frequency structures. However, both ion and electron hole dynamics seem to play a major role in the generation of the subtle, fine structures moving across the radiation spectrum.