On celestial events, auroral activity, and the solar cycle in classical antiquity

Auroral activity is known to be connected to the ∼11-yr solar cycle. Ancient records of celestial events, if they can be related to auroral activity, can provide information about solar activity in the past. A record of such events, extracted from Livy's history of Rome, was previously analyzed and an ∼11-yr periodicity was identified, suggesting that the events were auroral in nature. The goal of the present analysis is to estimate the form of the ∼11-yr cycle in the rate of events. The estimated rate is bimodal. This is similar to modern variations in geomagnetic activity and lends independent support to the earlier conclusion that at least some of these ancient events represent aurorae.