Nitrogen airglow sources: Comparison of Triton, Titan, and Earth

Abstract

Detailed calculations of the emission rates of nitrogen airglow in the upper atmospheres of Triton and Titan by direct solar excitation, photoelectrons, and magnetospheric electrons are compared with the Voyager Ultraviolet Spectrometer (UVS) observations. The principal spectral features of Triton's airglow are shown to be consistent with precipitation of magnetospheric electrons with power dissipation − 5 × 108 W. Solar excitation rates of the dominant N2 and N+ emission features are factors of 2–7 weaker than magnetospheric electron excitation. On Titan the calculated disk center and bright limb N+(1085 Å) intensities by solar excitation agree with observed values, whereas if the 970Å feature is mostly N2 c4 band emission, the calculated photoelectron excitation intensity is only 40% of the observed value. The difference could be due to magnetospheric electron precipitation. But the calculated LBH intensity by photoelectrons is in agreement with observations and suggests that magnetospheric electrons play a minor role in Titan's UV airglow. On the Earth solar/photoelectron excitation explains the observed N+(1085 Å) and LBH intensities, accounts for only 40% of the N+(916 Å) intensity, and overpredicts substantially the N2 c4 (0–0) band intensity.