O I (7990 Å) Emission and radiative entrapment of auroral EUV

Auroral observations of the O I (7990 Å) multiplet intensity distribution have been performed using a 2‐m grille spectrometer and a Fabry‐Perot interferometer in Alaska during 1977 and 1982, respectively. In addition, an extensive set of 1‐m and ½‐m Ebert‐Fastie spectrometer data obtained at Poker Flat, Alaska, and Longyearbyen, Svalbard, from 1977 to 1980 has been analyzed. The grille spectrometer results for the singlet/doublet ratio of the O I (7990 Å) multiplet indicate a nonstatistical distribution in the excited state, consistent with preferential scattering of the singlet component of the O I (989 Å) EUV emission and in accord with recent radiative transfer calculations (Meier, 1982). The striking departure of the Fabry‐Perot results from the model may be due to blending of the O 2 (4, 4) band with the O I (7990 Å). Complete absorption of the EUV doublet by N 2 is precluded by the clear presence of the O I (7990 Å) doublet component in the auroral spectrum. A comparison of O I (7990 Å) intensities measured on the nightside with those from dayside magnetospheric cleft aurora shows a cascade contribution to the O I (8446 Å) line and ultimately the O I (1304 Å) transition that depends on the altitude of the aurora. This is attributed to the greater radiative entrapment of O I (989 Å) at low altitude and corresponding enhanced probability for branching from the 3 s ′ to the 3 p level in atomic oxygen.