Most measurements of cosmic radio noise absorption in the polar regions have been made with riometers using broad-beam (∼60 deg) antennas. Only limited information about the spatial structure and dynamics of the aurora can be obtained by this means. Recent trends in riometry have emphasized the use of multiple narrow-beam antennas operated in a fixed beam or one-dimensional scanning mode to examine smaller ionospheric regions. A further step in this direction has been the development of the imaging riometer for ionospheric studies (IRIS). This instrument provides a two-dimensional image of regions of enhanced cosmic noise absorption at 38.2 MHz with a spatial resolution as small as 20 km and time resolution of 1 s. The IRIS antenna is a 64-element dipole array, phased to produce 49 independent beams viewing an ionospheric area about 200 km square at 90 km altitude. One IRIS instrument has been operating at the South Pole station since January, 1988; a second instrument was recently installed at Sondre Stromfjord, Greenland. This paper presents a technical description of the IRIS system. The response of the IRIS to a sun-aligned absorption arc is illustrated and compared with that of a broad-beam riometer. An intense, localized structure that developed within the arc was observed by IRIS to propagate in the magnetic poleward direction with a speed of 1 km/s.
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