Sammendrag
A much debated topic within the Incoherent Scatter Radar (ISR) community over the last decade or so has been the phenomenon of naturally occuring and strongly enhanced ion-acoustic scattering. This phenomenon was first reported by Foster et al., (Geophys. Res. Lett. 15(2), 160-163, 1988) and data with such unusual spectra was for a long time discarded along with data contaminated by satellite echoes. Several theories have been developed to explain these observations, but none of them are entirely satisfactory. A review of the observations and the suggested theories was recently made by Sedgemore-Schulthess and St.-Maurice (Surv. Geophys. 22(1), 55-92, 2001).
In an attempt to provide observational evidence to help decide between these theories, to explore the possibility of any connection between this phenomenon and optically visible auroral forms, and to also to see if the enhanced scattering is due to localised scattering structures within the radar beam, we have used the two antennas of the EISCAT Svalbard Radar (ESR) as an interferometer, recording time series directly and separately from each antenna, while they were both pointing in the same direction, which is along the Earth's magnetic field. This gives us an opportunity for both high time-resolution observations, better than what is possible through the ordinary receiver system of the radar, and also offers 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, we compute powerspectra for each signal, as well as the cross-spectrum and coherence between the two signals.
The observations were made during a combined radar and optical campaign conducted on Svalbard during January of 2002. The University of Calgary Portable Auroral Imager (PAI) pointed a narrow field-of-view camera along the direction of the magnetic field, from an observation point about 7 km away from the ESR.
We present examples of our observations -- powerspectra, cross-spectra and coherences -- from the Software Radar receiver showing that the enhanced scattering is due to scattering from narrow filaments elongated along the magnetic field, and much narrower than the radar beam in the horizontal direction. We also show how the enhanced scattering varies with orders of magnitude within the 2-5 second timescales usually employed for pre-integration in the ordinary receivers at IS radars. During these observations the camera detected highly structured and dynamic aurora, which moved in and out of the radarbeam during the event. We present the connection between the visible auroral activity, as measured in and adjacent to the radarbeam, and the measured enhanced scattering from the radar, in the form of cross-correlation coefficients, and discuss its interpretation.
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