Sammendrag
The generation mechanisms for so called naturally enhanced ion-acoustic echoes, observed at mid- and high-latitude Incoherent Scatter observatories, are still under discussion. Important questions in this respect are whether or not simultaneously observed up- and down-shifted enhancements really occur simultaneously, or if they are the result of temporal and/or spatial averaging, and also how these enhancements are related to auroral activity. By using the EISCAT Svalbard Radar as an interferometer, we have established that when up- and down-shifted enhanced shoulders are 1) observed simultaneously and 2) both are caused by structures sufficiently narrow to be detected by the interferometer technique, they originate predominantly from the same volume. Furthermore, the corresponding implied increase in the scattering cross-section, up to 50 dB above incoherent scattering, eliminates theoretical explanations based on marginal stability. To explore the relation of the enchanced echoes with auroral activity, an image intensified video camera was used in coordination with the radar. Our results throw new light on previous low-resolution observations, which associate the enhanced echoes with stable red arcs. The video data has helped establish a clear correlation between the enhanced echoes and dynamic auroral activity on sub-second time scales, showing a threshold connection between the auroral intensity and the triggering of the radar enhancements. It appears that the up- and down-shifted enhanced echoes correlate with fine auroral structures in slightly different ways.
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