Detection of Filamentary Structures in the Polar Ionosphere Using the EISCAT Svalbard Radar as an Interferometer

Where currents are carried by a plasma, these currents tend to be bursty and to focus into discrete channels, often with a filamented structure. This situation is very common in nature and in the laboratory. Here we present evidence of bursty and filamentary structures in the terrestrial aurora, observed through the enhancements of radar echoes produced by plasma instability in the filaments. By recording the raw time series from the radar directly, we have resolved the time development of the observed phenomena and we find significant variations on sub-second time scales. Using an interferometric technique to attain sub-beam spatial resolution, we have estimated the horizontal scale size of the structures, and find it to be comparable to the smallest known scale size of optical aurora, which despite extensive theoretical attempts remain little understood (e.g., Borovsky, JGR 98, 6101-6138, 1993). Enhanced radar echoes of this type have been observed previously at several Incoherent Scattering Radar observatories. One of the suggested explanations would enhance only one shoulder. Hence, temporal or spatial averaging of short-lived or spatially small structure was hypothesized to explain simultaneous enhancement of both shoulders. Our time/space resolved observations let us test this hypothesis with the result that time averaging is ruled out, whereas the enhancement of both shoulders appears to occur in the same volume.