The O+ 834‐Å dayglow: Satellite observations and interpretation with a radiation transfer model

The O+ 834‐Å dayglow observations made with the EUV spectrometer on the Air Force satellite STP‐78‐1 are interpreted with the use of a radiation transfer model. At 600‐km altitude the dayglow intensity was observed to vary with magnetic latitude from 30 R to 300 R in the near zenith direction (θ = 40°) and from 400 R to 500 R in the near nadir direction (θ = 140°); the correspondingly large near zenith to near nadir intensity ratio of 0.3–0.65 over much of the dayside indicates that the O+ ions in the topside ionosphere constitute an optically thick medium for resonance scattering of 834‐Å airglow emission even at this high altitude. Simultaneous measurements of the O+ density from the AE‐E spacecraft at 460‐km altitude near the equator are used to normalize the STP 78‐1 airglow data. A g value of 1.1×10−8 s−1 at zero optical depth is required for the ionization excitation of atomic oxygen leading to the production of O+ atoms in 4P state. The latitudinal distribution of O+ density derived from the 834‐Å airglow data shows a double peak (at ±13° latitude) surrounding an equatorial trough, characteristic of the equatorial anomaly, at altitudes below 1000 km and a single peak at the magnetic equator above 1000 km. At high latitudes (>20°) the O+ densities in the summer hemisphere are substantially lower than those in the winter hemisphere.