The Great Aurora of January 1770 observed in Spain

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).

Island. For this reason, Spanish auroral reports can be a good detector of large geomagnetic storm. For example, during the great Carrington storm, auroral displays were also seen in Spain (Vaquero et al., 2008). In today's world, geomagnetic storms can cause major problems due to our dependence on technology which is vulnerable to electromagnetic perturbations. Some potential consequences of these events are disturbances in communication systems, power blackouts, and permanent damage to transformers (Pulkkinen, 2007). It is therefore essential to understand and predict the behaviour of these phenomena.
In the past, several aurorae borealis have been observed in Spain and Portugal. The first Spanish catalogue was made in the 19th century (Rico Sinobas, 1855;Vaquero et al., 2003). Vaquero & Trigo (2005) and Vaquero et al. (2010) report systematic aurora observations in Lisbon compiled by Jacob Praetorius and Henrique Schulze during the 18th century, and in Barcelona by Francisco Salvá during the period 1780-1825. Other, non-systematic, auroral observations from Spain and Portugal have been published by Vaquero et al. (2003) and Carrasco et al. (2017). Aragonès & Ordaz (2010) compiled 80 auroral display observed in Spain and Portugal during the 18th century. This catalogue includes original descriptions of the great aurora that occurred in January 1770. Schröder (2010) discussed the development of this aurora, showing that it was observed at middle and low latitudes based on Fritz (1873) and Angot (1896). It was also used in Vázquez et al. (2006) to show that the open magnetic field better describes middle and low latitude auroral occurrences.
The objective of the present work is to analyse several records made by Spanish observers concerning an aurora that occurred on 18 January 1770, records contained in the catalogue published by Aragonès & Ordaz (2010). In Sec. 2, we provide the locations where this auroral display was observed. Section 3 summarizes the original aurora records given in Aragonès & Ordaz (2010). Section 4 analyses and discusses the results, and Sec. 5 presents the main conclusions.

Geographic Distribution of the Observations
Aragonès & Ordaz (2010) compiled a catalogue with 80 auroral display observed in the Iberian Peninsula and the Balearic and Canary Islands during the 18th century. In particular, this catalogue contains information about a notable aurora that occurred on 18 January 1770, including descriptions and the observational sites. This aurora was also sighted at other, more northern, European locations such as London (51° 30' N, 0° 7' W) and Berlin (52° 31' N, 13° 23' E), and at others of similar latitude to those in Spain such as Rome (41° 53' N, 12° 30' E) and Naples (40° 50' N, 14° 15' E) (Fritz, 1873;Angot, 1896;Schröder, 2010).
Several observers located at different places in Spain registered this exceptional aurora (Fig. 1)  The aurora observed in Córdoba was sighted on 18 January 1770 from 17:30 (local time) to midnight, but some of the countryfolk ("hombres de campo", i.e., men living and working outside the city itself) affirmed that the aurora remained until sunrise on the following day. According to the Professor of Arts of the Colegio de la Asunción, the aurora developed in the northern sky, and was changing in shape, brightness, and colour (Nipho, 1770). Figure 2 is the header of the article in which the description of the observation of the aurora in Córdoba on 18 January 1770 was published. Original text -"[…] dia 18.
de enero, como a las cinco y media de la tarde […] Comenzose á dexar ver por el Nordeste, y poco despues se fue manifestando por el Norte haciendo columnas, ó rafagas de bastante diametro, y que se dividian unas de otras […] Es cosa digna de notar que esta Aurora era en su mayor parte materia inflamada, y que solo por abaxo blanqueaba un poco […] á las once y media yá nada se descubria quedó la noche tan obscura, como qualesquiera otra sin Luna […] aseguran algunos hombres del campo haberse conservado esta aurora yá mas, yá menos viva hasta el amanecer"; translation -"On 18 January 1770 around 17:30 […] It started to be seen in the northeast, and after it manifested in the north as columns or bursts with great diameter and divided from each other […] It is noteworthy that this Aurora was for the most part inflamed matter, and it only blanched a little at the bottom […] At 23:30, nothing was discoverable, the night was as dark as any other without a moon, […] some countryfolk [men from outside the city] declared that the aurora was preserved more or less alive until sunrise." In Madrid, the aurora was observed at nightfall to the north. It presented several shapes, a red colour, and a brightness that rose and fell over time. The sky was clear and the aurora was visible all night (Subirás y Barra, 1770). Original text -"Jueves último dia 18 al anochecer se dexó ver una fuerte Aurora Boreal […] la Aurora Boreal consistía entonces en una faxa como de 20° de ancho de un color rojo muy debil, y casi uniforme en toda su longitud, de figura de una faxa esferica ó semicorona que salia se de hacia al este, y pasando por el cenit de Madrid se terminaba hacia el Nor-nordoeste […] el color rojo se iba aumentando, y disminuyendo por poco tiempo á trechos […] A las 6 de la noche refieren era semejante a lo que expreso de las 10 ½"; translation -"Thursday 18 at dusk, a strong Aurora Borealis was seen, […] the Aurora Borealis The description provided by an anonymous observer in Castellon indicated that the aurora was sighted in the northern sector of the sky, that it was red in colour, and that it remained in the same state from 19:30 (local time), when it started to be seen, until midnight (Anonymous, 1770a). This observer reported that the aurora was also seen the next day (19 January) at around 20:00 (local time). Original text -"A las 7 ½ me avisaron avia una[s] nubes coloradas subí a la torre de casa y vi unas Auroras Boreales una al Nordoestoest […] y la otra menos grande a Nord de un color rojo obscuro melancolico […] A las onse de la noche volví a verla y encontre de 4 a 5 auroras boreales de NE a NO formando nubes mas altas que anchas […] me han asegurado que a las 12 permanesian en el mismo estado"; translation -"At 19:30, I was alerted that there were reddish clouds, I went up to the tower of my house and I saw some aurorae borealis, one to the west-northwest […] and the other, less large, to the north with a dark melancholic red colour […] At 23:00, I went back to see it and found from 4 to 5 aurorae borealis from northeast to northwest forming clouds higher than wide […] other people have assured me that they remained in the same state at 24:00." Another anonymous observer at Gerri de la Sal noted that "a tower of fire" appeared in the north of the sky at 18:30 (local time), and it later extended from east to west showing different shapes and white and ash colours. It disappeared at midnight (Anonymous, 1770b). Original text -"[…] a cosa de las sis, y mitja aparegue en est emisferi en la part de tramontana una com torre de foch, que feia unas vias de color de llet y a cosa de mitja hora se dividí, y extengué ab una gran nuvolada,  , 1951). This observer indicated that the red glow of the aurora spread over all the northern part with a very bright light from one hour after twilight to midnight. Original text -"Algo más de una hora después del crepúsculo, se extendió por la ciudad el rumor de que los montes Taganana quizás estuvieran ardiendo […] Salí a observar el fuego, pero para mi sorpresa me encontré ante una verdadera aurora boreal […] y la aparición en forma de llama de color rojo sangre se extendía por todo el norte desde el este hasta unos pocos grados más allá del oeste, con una luz muy brillante"; translation -"Something more than an hour after twilight, there spread throughout the city a rumour that the Taganana hills might be burning […] I went out to observe the fire, but, to my surprise, I found myself before a true aurora borealis […] and the appearance [of the aurora] in the shape of a flame of a blood-red colour extended over all the north from east to a few degrees beyond west, with a very bright light." A complete English translation of that record can be consulted in Vázquez et al. (2006). For the case of Barcelona, Aragonès & Ordaz (2010) found that an aurora was observed from 19:00 to midnight as a cloud of fire in the sky split into four parts with a great radiance (Sagarriga, 1907, p. 57). Aragonès & Ordaz (2010) presented the description of the aurora that was made by Richard (1771), noting that the same variations of the phenomenon observed in Genoa were also seen in Cádiz. Moreover, Aragonès & Ordaz (2010) reported that this aurora was also recorded in Valencia as "one of the most complete aurorae observed" (Pastor Fuster, 1830, p. 471), and by the priest Leonardo Hernández Tolosa in Badajoz where several people observed how the northern sector appeared reddened from sunset until two hours after midnight (Hernández Tolosa, 1992, p. 198;Vaquero, 2001).

Analysis and Discussion of Results
The aurora that occurred on 18 January 1770 was observed over a wide zone of latitudes that were abnormally low given the typical behaviour of this kind of phenomenon. In Spain, the aurora was recorded from San Cristóbal de la Laguna (28.5° N) to Gerri de la Sal (42.3° N). It was also observed over all of the rest of Europe (Richard, 1771;Fritz, 1873;Angot, 1896).
The likelihood of seeing an aurora is generally greater at higher geomagnetic latitudes. To investigate this condition, we computed the geomagnetic latitudes of the locations considered in the present work at which aurorae were registered on 18  (Vaquero et al., 2016): Lalande, Horrebow, and Staudach. The solar activity declined from the beginning of December to January. It then rose to reach 11 group counts on 19 January 1770, the maximum value for that 3-month period which it also reached on 7 February 1770. It is evident therefore that the solar activity level was high when the great aurora was observed on 18 January 1770. According to Horrebow's records, the group count on that day as well as the day before was 10, whereas Staudach registered 6 sunspot groups on 18 January 1770. This significant difference in the sunspot counts between Horrebow and Staudach could be due to the use of different telescopes and methodologies to observe sunspots. We would note that, approximately nine solar rotations later, around 18 September 1770, another large