The ultraviolet spectrum of a dayside aurora: 530–1500 Å

Abstract

Rocket-borne spectrometers have observed the high-latitude dayside aurora above Cape Parry, N.W.T. Ultraviolet spectra of optical emissions produced by ambient precipitating particles have been obtained in the wavelength region between 530 and 1500 Å, over a range of spectrometer line-of-sight orientations, from 100 km to the rocket apogee of 452 km. The spectrum below 1500 Å is dominated by transitions from neutral and singly ionized atomic oxygen. NI, NII, and molecular nitrogen emissions, which are prominent in day airglow and nighttime auroral spectra measured by the same instrumentation, are very weak, indicating energy from the dayside auroral particles is transferred to the atmosphere above most of the N2. Relative line strengths of OI and OII transitions in the high-latitude dayside aurora differ in comparison with either airglow or nighttime auroral observations. The differences arise from differing excitation mechanisms and radiative-transfer characteristics and indicate the far-UV region of the spectrum can serve as a useful probe for remotely sensing various auroral phenomena. Weak emission near 1170 Å is identified as the OI 3s′3D–2p42D intercombination transition at 1172 Å. Observed OI 1172 Å intensities are consistent with the branching-ratio measurements of Morrison