Ground-based observation of high-altitude, high-temperature emission in the O2 atmospheric band nightglow

Abstract

O2(b1Σg+ - X 3Σg-) atmospheric band emission, when viewed from the ground outside the auroral zone, is considered to originate in the 90-100 km altitude region, from O-atom recombination. However, the v = 0,1 levels of the b1Σg+ state can also be generated through an energy transfer process, when O(1D) collides with O2. In the absence of specific altitude information, as in ground-based measurements, the two sources can still be distinguished because of different characteristic temperatures. We have analyzed sky spectra from the Keck I and II telescopes on Mauna Kea, contrasting solar minimum and maximum conditions and variations with time of night. Utilizing the O2(b-X) 1-1 band, we find that in averaged solar minimum conditions, ∼75% of the emission comes from an altitude corresponding to 200 K, while ∼25% has a temperature near 700 K and therefore originates at high altitude. The fraction of high-temperature emission is often considerably greater early in the evening. At solar maximum, the high altitude component can be dramatically enhanced in the early evening; emission in the 1-1 band reaches intensitities of 100-150 R, with a rotational temperature near 1000 K. We also observe high-J b-X 0-0 band lines penetrating the Fraunhofer A-band region, the normal absorption becoming emission. Emission intensites deduced from such observations indicate that the ionospheric 0-0 band is at least as intense as the 1-1 band, as expected. Copyright 2003 by the American Geophysical Union.