SPECTRA OF TYPE-B RED LOWER BORDERS

A low-light-level television camera and a television spectrograph were used to study the variation of color with elevation angle in auroras having type-B red lower borders. It was found that much of what is com monly described as type-B aurora results from a simple separation betw een the prompt red and blue molecular nitrogen emissions on the one ha nd and the persistent green oxygen line on the other hand. The red and blue emissions combine to produce magenta which appears on the leadin g edge of rapidly moving auroral features. True type-B aurora, not dep endent on motion, was observed to result from a suppression of the gre en line at low altitudes, presumably due to quenching. While the quenc hing of the green is the predominant effect, there are, in addition, c hanges in the shade of magenta resulting from an enhanced red/blue rat io associated with the phenomenon of auroral hems (bands of enhanced l uminosity along the lower borders of auroral curtains). In one case th ere was also evidence for a redistribution of the intensities of the n itrogen first positive bands, possibly by intercollisional transfer of energy from the W states to the B states as proposed by Benesch [1981 ]. In sum, evidence was found to support each of the prominent theorie s for type-B auroras and that the various mechanisms can coexist.