Upper atmospheric temperatures from Doppler line widths—IV: A detailed study using the OI 5577 å auroral and nightglow emission

Abstract

The measurement of Doppler temperatures in the upper atmosphere has been continued in Saskatoon, using a field-compensated Michelson interferometer. This device possesses certain advantages over the Fabry-Perot spectrometer, the most important being that its luminosity may be of the order of a thousand times greater at the same effective resolution. A brief description of the instrument and the techniques used in this particular application are given. A large number of temperature values, from both aurora and nightglow, have been obtained with this instrument, using the OI 5577 Å line. This made it possible to obtain some correlation with local time, sky position, magnetic field variations, and particularly with emission brightness. The detailed results for the nights of April 7/8, May 10/11 and May 14/15, 1964 are presented. The most striking result is that a systematic relationship between the auroral brightness and observed temperature is often obtained over periods up to a few hours. Brightness and temperature are usually inversely related, and changes in the relationship have been observed to occur at the same time as magnetic bays. This inverse relationship is crudely interpreted as following the atmospheric density curve; that is, that a uniform excitation per atom exists down to some cutoff height determined by the particle energy. This cutoff height thus determines both the auroral brightness and the observed temperature. A more quantitative interpretation has been given using calculated luminosity distributions. These suggest that while the inverse relation is in effect, auroral brightness (and temperature) changes result primarily from changes in the particle energy spectrum rather than changes in the flux. This systematic relationship also seems to preclude the existence of major temperature changes through auroral heating at these levels. The nightglow observations give an average temperature of about 180°R and brightness in the range of 200–300 R, in agreement with observations at lower latitudes. When compared with the auroral data, the indication is that aurora and nightglow are unrelated phenomena in the 5577 Å emission.