An observation of the helium I 584‐A dayglow radiation between 400 and 1000 km

Retarding potential analysis of photoelectrons created by the earth's EUV airglow has permitted us to measure the height profile of the He I 584-A and H I 1216-A dayglow lines from 400 to 1016 km. Maximum 584-A emission occurred at 500 km during rocket ascent and amounted to 0.7±0.3 kR. The large uncertainty reflects several experimental difficulties, one of which was the presence of a large and variable background signal. This backgorund is believed to have been caused by photoelectrons produced at the instrument baffles by direct sunlight. We have concluded that there is no important contribution to the signal from ionospheric electrons. The height profile of the 584-A line was consistent with that expected for resonance scattering of solar 584-A radiation by telluric helium in diffusive equilibrium at 927°K. The helium density required was 6.5±0.7×106 cm−3 at 176 km. However, the very large emission rates observed demand that the solar 584-A (full) line width be 0.023±0.007 A if the flux was 109 photons cm−2 sec−1. Lyman α airglow (14 kR at 1000 km) varied in a way consistent with expectations for resonance scattering in an atmosphere containing 3×107 H atoms cm−3 at 100 km. There is some evidence in the Lyman α data for a rather strong dependence of exospheric hydrogen abundance on the solar zenith angle.