The application of spectroscopic studies of the aurora to thermospheric neutral composition

The relationship between selected auroral emission features and the relative atomic oxygen abundance in the auroral thermosphere is investigated using observations acquired at Poker Flat, Alaska, and model results from an electron transport code. The emission rate ratio I(7320)/I(4278) due to transitions in O ⁺ and N ⁺₂ ions, respectively, is computed for a range of characteristic energies of auroral electron precipitation using the transport model. Simultaneous measurements of I(7320)/I(4278) and I(6300) during several nights of observations yield an empirical functional relationship between the characteristic energy and the "red to blue" ratio, I(6300)/I(4278), independently of any assumed excitation mechanisms. The I(8446)/I(4278) ratio (where the 8446 Å line results from an allowed transition in atomic oxygen) predicted by the model is compared with a large set of observed ratios to establish the magnitude of the scaling factor required to change the relative abundance of atomic oxygen given by the MS1S-86 empirical model. Using the ratio method we show that on 25 September 1987, a day with strong auroral activity, half of the original atomic oxygen in the gas column whence the auroral emissions originated disappeared during the course of the night.