Spatial relationship between large aspect angle VHF radio aurora and 557.7‐nm emissions: Evidence for refraction

A recently proposed explanation for large aspect angle VHF backscatter observations, based on refraction through auroral ionization structures in the E region (Moorcroft, 1989), has been confirmed by an experimental comparison of observations of auroral E region coherent backscatter at 48.5 MHz (made with the Bistatic Auroral Radar System or BARS) with simultaneous 557.7‐nm emissions detected with a CCD‐based all‐sky imager. The apparently large aspect angle radio auroral echoes are found to occur predominantly on the side of the visual aurora away from the radar for both the premidnight and the postmidnight sectors. For the Red Lake BARS radar this means that the strongest echoes occur to the north of the visual form, in sharp contrast with previous comparisons, particularly for the premidnight sector, which found that radar echoes occur predominantly southward of the visual aurora. These results are consistent with the proposed explanation based on refraction in the auroral E region.