Coordinated measurements of auroral light intensities and Riometric radio-wave absorption

Digitized allsky images of auroral optical emissions, recorded by a low light level TV system at Sanae (70.3°S, 2.4°W, L=4.0), have been mapped onto the angular sensitivity functions of both a broad, single‐beam riometer and narrow beams of an imaging riometer. During the rising phase of a substorm before local magnetic midnight, on 11 May, 1992, the integrated optical intensities in the inner four beam directions of the 16‐beam imaging riometer followed each other fairly closely, while the absorptions showed marked spatially inhomogeneous distributions. Regions of ionospheric absorption appeared not to coincide spatially with regions of discrete optical structures, but were adjacent to these structures. A small step of ∼0.09 dB in absorption was observed shortly after onset of the substorm on 27 July, 1979, and again on 11 May, 1992, but before the sharp increase in absorption. This step in absorption may be due to highly enhanced E‐region electron temperatures arising from strong dc electric fields connected to auroral arcs. During the event of 9 June, 1991, which started with an SSC at 00:40 UT, enhancements in absorptions followed enhancements in optical emissions repeatedly after 52 s.