Critical comparison of OII(732–733 nm), OI(630 nm), and N2(1PG) emissions in auroral rays

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Abstract
All‐sky spectral imagery has been used to study the altitude distribution of OII(732–733 nm), OI(630 nm), and N2(1PG) emissions in tall auroral rays appearing at the poleward edge of the auroral oval. The OII(732–733 nm) emission layer was consistently higher than the OI(630 nm) emission layer in these structures; the peak emission altitudes were estimated to be ∼380 km and ∼240 km, respectively. Neither the high absolute altitude of the OII(732–733 nm) peak nor its >100 km offset from the OI(630 nm) peak are consistent with published model results, which consider electron impact on atomic oxygen as the sole source of auroral O+(2P). The present results suggest that O+(2P) may also be excited directly from ambient ionospheric O+(4S) atoms. Other contributing factors may include a significant underestimation of the rate coefficients for O+(2P) quenching, or field‐aligned up‐flows of ionospheric O+.

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