Far-ultraviolet altitude profiles and molecular oxygen densities in an aurora

Abstract

The altitude profiles and absolute intensities of several far-ultraviolet emission features in a steady IBC I+ aurora were measured with a filterwheel photometer carried in an Aerobee rocket over Churchill, Manitoba, at 2241 CST on February 8, 1968. The volume emission rate of the Lyman-Birge-Hopfield (LBH) system of N2 was a nearly constant factor of 2.5 times the volume emission rate of 3914 A above 110 km; however, a falloff in this ratio, which possibly was the result of an abrupt change in the secondary electron energy spectrum at the lower border of the aurora, occurred below this altitude. The maximum intensity of the 1304 A resonance triplet of O was twice the total intensity of the 3914 A band of N2+ measured below the aurora. The brightness of the Lyman α line was 2.5 kR, and, although no auroral structure in the altitude profile of this feature was observed, there was a sudden 500 R increase in brightness that may have been caused by proton precipitation. The relative brightnesses of the far-ultraviolet features were about three times as large as previously reported. Molecular oxygen densities in the 110- to 120-km region were determined from the relative attenuation of LBH bands; the density at 115 km was 1.8×1011 per cm³, within a factor of 1.5. This value is somewhat higher than a previous absorption measurement and two recent mass spectrometer measurements that were also made in the winter over Churchill. Taken together, the measurements indicate that winter O2 densities near 115 km at high latitudes are at least a factor of 2 higher than those at temperate latitudes.