Comparison of the aurora of September 1/2, 1859 with other great auroras

The maximum equatorward boundary of a number of visual auroral events for the period subsequent to that of the great storms of August and September 1859 are examined. These boundaries, in terms of dipole magnetic latitude, are related to the half-daily aa index. A least squares regression line, magnetic latitude = 55.86  − 0.056aa, with a correlation coefficient of 0.84, adequately represents the data. The September 1859 event, in the context of these other storms, fits into the totality of the events examined as one of the two largest events of the sample, but is otherwise not unique. The discussion includes indications for further work in this area.