The Great Aurora of January 1770 observed in Spain

TL;DR

This paper analyzes historical records of a notable aurora observed across Spain on 18 January 1770, revealing its characteristics, widespread observation, and connection to high solar activity, providing insights into auroral phenomena at low geomagnetic latitudes.

Contribution

It presents a detailed analysis of 1770 aurora observations in Spain, linking the event to high solar activity and low-latitude auroral occurrence, based on historical records and geomagnetic calculations.

Findings

Aurora observed across multiple Spanish locations from sunset to midnight.

Descriptions include red, white, and ash colors with changing brightness and shape.

Event coincided with high solar activity, including 10 sunspot groups.

Abstract

An analysis is made of the records made by Spanish observers of a notable aurora on 18 January 1770 in order to study the characteristics of this event. The records indicate that the phenomenon was observed in both continental and insular territories of Spain, in particular at San Crist\'obal de la Laguna, C\'adiz, C\'ordoba, Badajoz, Valencia, Castell\'on, Madrid, Barcelona, and Gerri de la Sal. The most equatorward observational site was San Crist\'obal de la Laguna (28.48{\deg} N, 16.32{\deg} W) in the Canary Islands. In general, the descriptions put its duration from sunset to midnight, but the observers from C\'ordoba and Madrid report the aurora as being visible during the last hours of the night, and it was even observed the following day at Castell\'on. All the observers described the aurora as red in colour, while white and ash colours were also reported at C\'ordoba and Gerri de la Sal. The brightness and shape of auroral display changed over time. Calculations of the geomagnetic latitudes of the observation locations gave San Crist\'obal de la Laguna as the southernmost (26{\deg} N) to Gerri de la Sal the northernmost (35{\deg} N), and indicate this aurora was observed over a wide range of abnormally low latitudes for such a phenomenon. Solar activity around the event was high, with the astronomer Horrebow registering 10 sunspot groups on that date (18 January 1770).