The 300‐ to 900‐Å spectrum of a nightside aurora

Results of a satellite observation of the nighttime auroral emission spectrum between 300 and 900 Å at 8‐Å resolution are presented. The spectrum was taken from a 600‐km altitude looking down on a bright, well‐defined auroral arc located near the midnight meridian in the north polar region on March 10, 1979, when AE = −500γ (Kp = 4). The most prominent features in this wavelength range are identified as the O II 538‐, 617‐, 718‐, and 834‐Å emission lines, which have intensities exceeding 50 R. A number of weaker O II (and possibly N II and O III) lines are found widely scattered from 430 to 800 Å at intensity levels of ≃ 10–40 R. As in the dayglow, the measured intensity ratio of the 540 to 585 Å features r ⩾ 4.1 is discrepant with laboratory measurements of the O II 538/581‐Å line ratio. Since He I, 537‐Å contamination is probably negligible, this result indicates that at least 70% of the observed 540‐Å feature is due to the O II 2p³ 4S0‐3s 4P transition multiplets at 539 Å.