HISTORICAL REVIEW OF GREAT AURORAS

Occurrences of great auroras of the past are discussed together with accounts Of visual and instrumental observations. Earlier observations revealed the extraordinarily great heights and unusual spectral characteristics of such displays as well as documenting their penetration to low latitudes. Theoretical studies have led to an understanding of the observed unusual spectroscopic characteristics including their bright red colour and the abnormal vibrational and rotational structure in the molecular bands observed in high-altitude forms. As a result of the increasing sophistication of observing techniques, the phenomena associated with great auroras have passed beyond the characterization of optical and magnetic effects, to include information on associated changes in the composition of the neutral upper atmosphere and effects in the ionosphere. In more recent times, studies have led to increased understanding of the magnetospheric processes that produce these auroras.