A possible case of Sporadic Aurora in 1843 from Mexico

José Manuel Vaquero^1*, María Cruz Gallego² and Fernando Domínguez-Castro³

¹ Departamento de Física, Centro Universitario de Mérida, Universidad de Extremadura, Av. Santa Teresa de Jornet, 38 06800 Mérida, España. *Corresponding author: jvaquero@unex.es

² Departamento de Física, Universidad de Extremadura, Badajoz, España.

³ Departamento de Física Universidad de Extremadura Badajoz, España.

Received: September 09, 2012;
accepted: October 22, 2012;
published on line: December 14, 2012

Resumen

Recientemente, algunos autores han mostrado que algunas auroras pueden ser observadas a latitudes relativamente bajas cuando la actividad geomagnética es baja o moderada. Este tipo tan especial de aurora recibe el nombre de "aurora esporádica". Presentamos y analizamos en este trabajo un posible caso de "aurora esporádica" observada desde México el 19 de abril de 1843. Además, estudiamos la actividad solar y auroral alrededor de este evento.

Palabras clave: Aurora esporádica, actividad geomagnética, actividad solar, México, historia de la Geofísica.

Abstract

Key words: Sporadic aurora, geomagnetic activity, solar activity, Mexico, history of Geophysics.

Introduction

In general, low-latitude auroras are related to great events of space weather, usually associated with huge Coronal Mass Ejection (CME). Vazquez et al. (2006) show that low-latitude aurorae occur mainly in the decaying phase of the solar cycle, as defined by the sunspot number, coinciding with the maximum of the Open Magnetic Field (OMF) that is a better descriptor of the state of the heliosphere. Nowadays, the studies of these episodes are important because they provide additional information on potentially damaging space weather events (Lanzerotti, 2007). In fact, there are some studies of important space weather events in the past. Some of them can be highlighted such as the cases that have occurred on 2 September 1859 (Cliver, 2006; Tsurutani et al, 2003; Ribeiro et al, 2011), 24-25 October 1870 (Vaquero et al, 2008), 25 September 1909 (Silverman, 1995), 14-15 May 1921 (Silverman and Cliver, 2001), and 25 January 1938 (Barlow, 1938; Botley, 1938; Bernhard, 1938; Hess et al, 1938).

However, Silverman (2003) provided sufficient data to demonstrate that an auroral phenomenon can be observed at low-latitude under conditions of quiet to moderate magnetic activity. This infrequent phenomenon was called "sporadic aurora" by Silverman (2003) and constitutes a puzzle in magnetospheric physics. Later, Willis et al. (2007) demonstrated that at least 29 out of the 42 Chinese and Japanese auroral observations (from 1840 to 1911) occurred at times of weak or moderate geomagnetic activity (aa or Ak index < 50). Moreover, Vaquero et al. (2007) presented a case of "sporadic aurora" observed from Spain on the 29 December 1845.

Therefore, we have some reports of "sporadic aurora" in USA (Silverman, 2003), Asia (Willis et al., 2007), and Spain (Vaquero et al., 2007) but records of this phenomenon from other places of the world are not available. The aim of this paper is to present a possible case of "sporadic aurora" observed from Mexico on the 19 April 1843.

Historical record

Mexico is a country with an interesting historical and scientific heritage, despite its location far from main centers of scientific research in the 18^th and 19^th centuries (located generally in European countries as England, France or Germany). Some examples can be cited in the branch of solar physics as the historical sunspot observations preserved in its archives (Vaquero, 2004; Vaquero and Moreno-Corral, 2008) or the records of the great aurora observed in 1789 (Vazquez et al., 2006).

According to this report, Mr. Soyano, professor of philosophy of the Seminary- College, observed a light in the sky to the north on the night of 19 Apri. Despite the weather and clouds, the light was visible. This light was observed from 21:00 to 23:00 approximately (Local Time). Although the clouds did not allow to see its exact form, the observer indicates that it was a semicircle whose center was in the direction NNO1/4. The reporter indicated that it was not the zodiacal light. The phenomenon was not caused by the light of the moon, nor by flashes of lightning. This was clearly an aurora borealis.

Discussion

This record can be associated to a sporadic aurora establishing that the phenomenon was observed under conditions of quiet to moderate magnetic activity and from a low-latitude place.

We would like to compare the date of this aurora observed in Mexico with geomagnetic activity to establish clearly that it is a sporadic aurora. However, note that the more usual magnetic indexes are not availables for early dates. For example, Ak index is only available since 1844 (Nevanlinna and Kataja, 1993; Nevanlinna, 2004) and aa index is only available from 1868 (Mayaud, 1980). Therefore, we can compare the observation date with the geomagnetic IDV index (Svalgaard and Cliver, 2005, 2010), available from 1835, and with some proxies of geomagnetic activity, namely sunspot number and the number of auroras observed at middle latitudes.

Figure 1 shows the Inter-Diurnal Variability (IDV) index from 1835 until 1855 introduced by Svalgaard and Cliver (2005) and improved by Svalgaard and Cliver (2010). IDV is computed as the average difference from one day to the next between hourly mean values of the horizontal component, H (measured one hour after midnight), without regard to sign. The most interesting characteristic of IDV index is that correlate highly with the near heliospheric magnetic field (HMF) strength B. Data provided by Svalgaard and Cliver (2010) shown low values of the IDV index (and, therefore, low values of the HMF strength B) around the date of observation of the event considered here.

Figure 2 shows the yearly number of auroras observed in middle latitudes according to Krivsky and Pejml (1988). Auroral activity during the period 1830-1870 was relatively high with peak values in the years 1830, 1841, 1850, 1859 and 1870 (Figure 2, upper panel). Auroral activity was relatively low in early and late 19th century, corresponding with secular solar activity minima. In the year 1843, auroral activity was low with only 39 auroras observed (Figure 2, lower panel).

As similar picture is obtained from the sunspot numbers (Figure 3). We use two different versions of the sunspot number: International Sunspot Number (Clette et al., 2007) and Group Sunspot Number (Hoyt and Schatten, 1998). Figure 3 (upper panel) shows these solar indexes from 1820 to 1860 corresponding to solar cycles 7, 8 and 9. Solar activity around 1843 was low or very low. In October and November 1842, sunspot number values were in the range 30-40. However, the values decrease to10 approximately after December 1842. The values corresponding to April 1843, when the aurora was observed from Mexico, are very low: 9.5 (ISN) and 7 (GSN). Obviously, the low solar activity corresponding to the year 1843 can also be checked in sunspot areas (Vaquero et al., 2004).

Long-term variation of the geomagnetic latitude of Mexico for the last two millennia was studied in Vazquez et al. (2006) varying between low values of 14° and 30° N. The geomagnetic latitude of Mexico in 1843 was 29.5° N approximately.

Conclusion

We have presented a historical record that clearly describes the aurora borealis seen from Mexico on 19 April 1843. However, geomagnetic, solar and auroral activity during this time was low according to the evolution of the IDV index, sunspot number and the number of auroras observed at middle latitudes respectively. Furthermore, the geomagnetic latitude of Mexico at that time was low (approximately 29.5° N). Therefore, we think that this is a case of "sporadic aurora".

Acknowledgements

Support from the Junta de Extremadura (Research Group Grant No. GR10131) and Ministerio de Economfa y Competitividad of the Spanish Government (AYA2011-25945) is gratefully acknowledged.

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