Anomalous UV auroral spectra during a large magnetic disturbance

Abstract

Ultraviolet and far ultraviolet auroral spectra (1100–2900 Å) were taken during a very disturbed period (Kp = 7+) on June 2, 1978, by the S3‐4 polar‐orbiting satellite over the nightside, winter southern hemisphere polar region. The spectra from the equatorward section (−53° to −60° geomagnetic latitude) of the auroral precipitation showed many striking differences from those of the diffuse aurora in other orbits or the poleward section of the diffuse auroral region (−60° to −65° geomagnetic latitude) on the same orbit. The differences are as follows: (1) intensity ratios of the nitrogen atomic lines (1744 Å and 2143 Å) to the Lyman‐Birge‐Hopfield (LBH) (3–10) band were 2 instead of 1; (2) intensity ratio of the oxygen line (1356 Å) to the LBH (3–10) band was 15 instead of 4; (3) rotational temperature of the Vegard‐Kaplan band system was 1000 K instead of 400 K; and (4) effective vibrational temperature of the LBH band system was above 3000 K instead of below 1000 K. These and other characteristics are consistent with the assumption that observed spectra may originate from two different altitude regimes. Emissions from low altitudes (approximately 120 km) were produced by typical keV diffuse auroral‐electron precipitation, and those from high altitudes (approximately 200 km) were produced by keV heavy particle precipitation. The lack of significant enhancement of Lyman α emission indicates a very weak proton precipitation. We believe that these low‐latitude anomalous ultraviolet spectral features are likely due to the keV ion‐atom oxygen precipitation, previously observed by a mass spectrometer at 800 km.