Observations of the helium II 304-A and helium I 584-A atmospheric dayglow radiation

Two photometers with bandpasses of 170–500 and 170–800 A were employed to observe dayglow emissions in the extreme ultraviolet (EUV) over an altitude range of 90–186 km. We have identified the emissions observed with these photometers as resonantly scattered He I 584-A and He II 304-A radiations. At 186 km, 209 ± 70 rayleighs of 584-A and 9.3 ± 3.1 rayleighs of 304-A radiation were measured. We have compared these observations to theoretical calculations of resonance scattering of solar emissions from geocoronal He and He+. Using the Jacchia (1971) atmospheric model for He, we find that the observed brightness of the 584-A emission requires that the solar 584-A line width be 0.014 ± 0.004 A. In this model the maximum overhead brightness of 584-A dayglow would occur at 900 km, and its magnitude would be 1.9 ± 0.6 kR. Our observation of 304-A brightness requires that the overhead column density of the He+ ions be 4.2 × 1011 ions/cm² column. This value is consistent with a constant-density plasmasphere model with a He+ ion density of 320 ions/cm³ in the plasmasphere. There is evidence of additional EUV dayglow emission in the 500- to 800-A band near 140 km that is still unidentified.