Low-Latitude Auroras: The Magnetic Storm of 14-15 May 1921

We review solar geophysical data relating to the great magnetic storm of 14-15 May 1921, with emphasis on observations of the low-latitude visual aurora. From the reports we have gathered for this event the lowest geomagnetic latitude of definite overhead aurora (coronal form) was 40 deg and the lowest geomagnetic latitude from which auroras were observed on the poleward horizon in the northern hemisphere was 30 deg. For comparison, corresponding overhead/low-latitude values of 48 deg/32 deg and 41 deg/20 deg were reported for the great auroras on 28-29 August and 1-2 September 1859, respectively. However for the 1921 event, there is a report of aurora from Apia, Samoa, in the southern hemisphere, within 13 deg of the geomagnetic equator. This report by professional observers appears to be credible, based on the aurora description and timing, but is puzzling because of the discrepancy with the lowest latitude of observation in the northern hemisphere and the great implied aurora height (approximately 2000 km, assuming overhead aurora at Auckland, New Zealand). We discuss various possibilities that might account for this observation.