Spectral characteristics of low latitude aurorae on October 21,1989

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During the main phase of geomagnetic storm on October 21,1989,the airglow spectrograph with a Schmidt camera of effective aperture F/0.7 observed distinct auroral spectra at low latitudes as in Niigata, geomagnetic latitude 27.7°, Japan. Although the aurora was not seen with the naked eye, it was observed by the method of ordinary airglow measurements, which required much longer exposure time. From the observed spectra, we can get the auroral components by subtracting a background spectrum which contains airglow and scattering city lights. We used a reference spectrum. which was constructed without aurorae under the same observing conditions of aurora spectrum. By subtracting the reference spectrum from that of observed spectrum, the following features emerged for the low latitude aurora on October 21,1989 : 1. Great enhancement in [OI] 6300 and 6364 A doublet, whose intensities at the zenith were 6.7 and 2.0 kR respectively, 2. Enhanced [OI] 5577 A emission of 1.4 kR at the zenith, and 3. no enhancements of N_2^+ 1.N. of 3914 and 4278 A, [NI] 5200 A, and of H Balmer alpha 6563 A. These facts might suggest that precipitating particles responsible for this aurora are electrons with energy as low as ∿ 10 eV, and not positive and/or neutral particles.
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