Day-night ratio of auroral absorption for breakup events

In a recent paper Brown and Barcus [1963] reported the results of a study of the day-night ratio for auroral absorption events. This showed that the ratio for slowly varying events is essentially unity and indicates that the absorption region is located at heights where the negative ion-electron ratio λ is small compared to unity. Current estimates of ionospheric parameters [Webber, 1962] would place this in the vicinity of the 90-km level of the atmosphere. The previous study showed considerable variability in the absorption accompanying magnetic disturbances of a given intensity. Since all-sky camera records showed widespread, diffuse aurora during these events, this suggests that the variability arises from changes in the electron energy spectrum, from event to event, rather than displacements of the ionization relative to the riometer antenna pattern. In view of this, it is of interest to examine other phases of auroral absorption to see if the electron energy spectrum ever changes to the extent that the absorbing region drops to heights where the negative ion-electron ratio λ is comparable to unity. This might be found, for example, during intense disturbances such as magnetic storms or, possibly, the breakup phase of aurora, with its rapid growth in luminosity and drastic changes in activity of auroral forms.