What Is the Lowest Latitude of Discrete Aurorae During Superstorms?

Abstract From a survey of published accounts of visual sightings of aurorae, a compilation is presented of the lowest identified geomagnetic latitude at which discrete aurorae were seen at local zenith during magnetic storms having intensities with maximum nT. The compilation includes data for the superstorms of 2 September 1859, 4 February 1872, and 15 May 1921. A statistical model is developed representing the equatorward boundary of discrete aurorae versus storm intensity. The model indicates that a once‐per‐century storm would likely induce discrete aurorae at zenith down to a geomagnetic latitude of . Insofar as aurorae can be taken as a proxy for electrojet currents, such a storm would expose many nighttime electric‐power systems in the contiguous United States or Europe to high levels of geomagnetic disturbance. A Carrington‐class storm would induce discrete aurorae down to . These exposures are much greater than those indicated in recent numerical simulations of extreme magnetic storms. Using the model to infer storm intensity from reports of low‐latitude aurorae, a storm on 28 August 1859 likely had maximum nT. That this storm occurred just a few days before the Carrington storm of 2 September (maximum nT) deserves attention. A storm that occurred on 17 September 1770 is estimated to have had maximum nT. A vision described by Ezekiel could have been inspired by aurorae from a storm with maximum nT.