Statistical analysis of the spatial relationship between the radio and optical aurora: Further evidence for refraction

Recent studies have found strong evidence that ionospheric refraction can explain some of the large aspect angle VHF radio aurora observations. The spatial relationship between such observations made with the Bistatic Auroral Radar System (BARS) and simultaneous 557.7-nm emissions detected by an All-Sky Imager (ASI) has been found consistent with the above idea (Hall et al., 1990). In this paper, further results of a cross-correlation analysis of the BARS and ASI intensities are presented. Two-dimensional as well as one-dimensional (along radar beam direction) cross-correlation functions are computed for a large statistical sample of 4 months of data. It is found that for all levels of ASI intensity and/or BARS power, stronger radar echoes predominantly occur on the remote side of the optical aurorae. The results are in agreement with the model calculation of refraction by Watermann (1990). In the premidnight sector, some of the brightest aurorae are found coincident with the strongest radar echoes, and a significant fraction of weaker radar echoes occur equatorward of fainter auroral forms.