Spectral characteristics of two types of low latitude aurorae

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Abstract
Auroral emissions at low latitudes appear to be due to energy loss from trapped ring current ions or electrons of energy of order of 1eV rather than keV electrons accelerated along field lines as in the ordinary polar aurora. Two types of low latitude aurorae have been observed in Texas in our measurements during the last few years. The features observed to predominate in the first type are N2+1N bands showing high vibrational/rotational development, OI 7774 Å emissions, and sometimes Hydrogen Balmer β emissions. These spectral characteristics indicate precipitation of the ring current particles themselves, directly as ions, and/or indirectly as energetic neutrals following charge transfer with exospheric H or O neutrals. The second type consists predominantly of [OI]6300Å and [NI] 5200 Å emission and other features of excitation potential only a few eV, with spectral characteristics indicating excitation by electrons from a population with kT ∼ 1eV. The second type can give the brighter displays more easily visible to the naked eye, although the first appears to be more frequent. The time variations of the emissions have been found to be closely related to fluctuations of periods less than an hour in the magnetograms from nearby observatories.