Sammendrag
The phenomenon of naturally occuring and strongly enhanced ion-acoustic scattering was first reported by Foster et al., (Geophys. Res. Lett. 15(2), 160, 1988) and data with these unusual spectra has for a long time been discarded together with data contaminated by satellite echoes. These observations have been discussed under many names: Coherent echoes; anomalous echoes; naturally enhanced ion-acoustic line (NEIAL); and Anomalous Ion Spectra (AIS). Several theories have been developed to explain these observations, but none of them are entirely satisfactory, either from not being able to explain all features of the observations, or from requiring physical conditions not observed elsewhere (i.e. very filamented high current densities). A review of the observations and the suggested theories was made by Sedgemore-Schulthess and St.-Maurice (Surv. Geophys. 22(1), 55, 2001). In an attempt to provide observational evidence to help decide between these theories, in part by seeing whether narrow auroral structures appear as localised scattering structures within the radar beam, Grydeland et al. (Geophys. Res. Lett. 30(6), 1338, doi:10.1029/2002GL016362, 2003) for the first time used the two antennas of the EISCAT Svalbard Radar (ESR) as an interferometer, which has given us the possibility of detecting scattering structures localised along the baseline between the two antennas within the radar beam. In addition to the ordinary ESR receiver system, we used data acquisition equipment based on the MIDAS-W Software Radar prototype. This system was used in a raw data collection mode, where a 500 kHz wide band of the 7.5 MHz intermediate frequency of the receiver for each antenna was sampled, and the time series stored directly to disk. Further processing was done off-line. From the time series, autocorrelation functions for both signals were computed, as well as the cross correlation function. Normalising the cross correlation by the geometric mean of the autocorrelation functions gives us a coherence spectrum, which provides information on the size of a discrete scattering structure in the direction along the baseline, which is perpendicular to the magnetic field. We present examples of our observations -- spectra, cross-correlations and coherence functions from the Software Radar receiver. We discuss the interferometric technique in some detail, and touch upon some of the ways in which the processing changes for cross-correlation estimation. We also discuss the effect of applying a windowing function to the auto- or cross- correlation estimate in order to produce a spectrum.
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