O-I (7990-A) EMISSION AND RADIATIVE ENTRAPMENT OF AURORAL EUV

Auroral observations of the O I (7990 A) multiplet intensity distribution have been performed using a 2-m grille spectrometer and a Fabry-Perot interferometer in Alaska during 1977 and 1982, respectively. The results indicate a nonstatistical distribution in the excited state, consistent with preferential scattering of the singlet component of the O I (989 A) EUV emission and in accord with recent radiative transfer calculations. Complete absorption of the EUV doublet by N//2 is precluded by the clear presence of the O I (7990 A) doublet component in the auroral spectrum. A comparison of O I (7990 A) intensities measured on the nightside with those from dayside magnetospheric cleft aurora shows a cascade contribution to the O I (8446 A) line and ultimately the O I (1304 A) transition that depends on the altitude of the aurora. This is attributed to the greater radiative entrapment of O I (989 A) at low altitude.