A possible case of sporadic aurora observed at Rio de Janeiro
Denny M. Oliveira, Hisashi Hayakawa, Ankush Bhaskar, Eftyhia Zesta, Geeta Vichare
AA possible case of sporadic aurora observed at Rio de Janeiro ∗ Denny M. Oliveira † , Hisashi Hayakawa , Ankush Bhaskar , Eftyhia Zesta , andGeeta Vichare Goddard Planetary Heliophysics Institute, University of Maryland, Baltimore County, Baltimore, MD,United States NASA Goddard Space Flight Center, Greenbelt, MD, United States Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Japan Institute for Advanced Researches, Nagoya University, Nagoya, 4648601, Japan Rutherford Appleton Laboratory, Chilton, United Kingdom Catholic University of America, Washington D.C., United States Indian Institute of Geomagnetism, Plot 5, Sector 18, New Panvel (West), Navi Mumbai, India
Abstract
Being footprints of major magnetic storms and hence major solar eruptions, mid-to low-latitude aurorae have been one of the pathways to understand solar-terrestrialenvironments. However, it has been reported that aurorae are also occasionally ob-served at low latitudes under low or even quiet magnetic conditions. Such phenomenaare known as “sporadic aurorae”. We report on a historical event observed by a scien-tist of the Brazilian Empire in Rio de Janeiro on 15 February 1875. We analyze thisevent on the basis of its spectroscopic observations, along with its visual structure andcoloration, to suggest this event was a possible case of sporadic aurorae. Given theabsence of worldwide aurora observations on that day as a consequence of low mag-netic activity recorded on the days preceding the observation, in addition to a detaileddescription, the event observed can most likely be classified as a sporadic aurora. Wediscuss the geographic and magnetic conditions of that event. Thus, we add a possiblecase of sporadic aurora in the South American sector.
Extreme space weather events, such as the events of August-September 1859 (Carring-ton, 1859; Farrona, Gallego, Vaquero, & Dom´ınguez-Castro, 2011; Gonz´alez-Esparza &Cuevas-Cardona, 2018; Green & Boardsen, 2006; Hayakawa, Ebihara, Hand, et al., 2018;Hayakawa, Ebihara, Willis, et al., 2019; Hayakawa et al., 2016; Kimball, 1960), February1872 (Hayakawa, Ebihara, Willis, et al., 2018; Meldrun, 1872; Silverman, 2008), and May1921 (Love, Hayakawa, & Cliver, 2019; Silverman & Cliver, 2001), cause extremely intensemagnetic storms. Among other effects, one of the most interesting visual phenomena isthe occurrence of very intense and bright aurorae. Aurorae during extreme events are not ∗ Paper published in
Earth, Planets and Space , Special issue
Solar-Terrestrial Environment Predic-tion: Toward the Synergy of Science and Forecasting Operation of Space Weather and Space Climate , doi: https://doi.org/10.1186/s40623-020-01208-z † Electronic address: [email protected]; [email protected] a r X i v : . [ phy s i c s . s p ace - ph ] J un nly observed at high latitudes, but at low latitudes (22-23 ◦ ) as well (C´ardenas, S´anchez,& Dom´ınguez, 2016; Gonz´alez-Esparza & Cuevas-Cardona, 2018; Green & Boardsen, 2006;Hayakawa, Ebihara, Hand, et al., 2018; Hayakawa, Ebihara, Willis, et al., 2019; Hayakawaet al., 2016; Humble, 2006; Kimball, 1960; Silverman, 1995, 2008).As such, mid- and low-latitude aurorae have formed footprints of major magnetic stormsand hence major geo-effective solar eruptions (Schlegel & Schlegel, 2011; Shiokawa, Ogawa,& Kamide, 2005; Silverman, 2006; Vallance Jones, 1992; Willis, Henwood, & Stephenson,2006; Willis, Vaquero, & Stephenson, 2009). Therefore, such auroral reports have been oneof the key pathways to understand solar-terrestrial interactions in the past in terms of theirlong-term variability and cyclicity (Dom´ınguez-Castro et al., 2016; Lockwood & Barnard,2015; Lockwood et al., 2016; Silverman, 1992; Usoskin et al., 2015, 2013; V´azquez, Vaquero,Gallego, Roca Cort´es, & Pall´e, 2016).However, auroral phenomena have been rarely seen at low latitudes during moderateand even quiet magnetic conditions. Such events are known as sporadic aurorae (Silverman,2003). Botley (1963) introduced this term to the scientific community, citing previousdescriptive usages of the same word by Abbe (1895). She introduced nine cases of lowlatitude aurorae in Europe and in the Middle East observed in the 12th and 19th centuries.Botley (1963) was the first to clearly define sporadic aurorae as comprise such instances asa single ray in a sky otherwise seemingly clear of auroral light, or isolated patches well tothe equatorial side of a great display”. Botley (1963) also noted references to reports oftwo low-latitude aurora occurrences without the occurrence of high-latitude aurorae (Eddie,1894; Fritz, 1881).It took another four decades for the next paper on sporadic aurora to be published. Sil-verman (2003) provided a survey of considerable sporadic aurora observations in low-latituderegions of the United States during a time span of over half a century, and highlighted theoccurrence of sporadic aurorae in the context of mid- to low-latitude aurorae during moder-ate to low magnetic activity. That paper was later followed by other papers with reports onsporadic aurora sightings from Iberia and the Canary Islands (Vaquero, Trigo, & Gallego,2007; V´azquez & Vaquero, 2010), East Asia (Willis, Stephenson, & Fang, 2007), Mexico(Vaquero, Gallego, & Dom´ınguez-Castro, 2013), and the Philippines (Hayakawa, Vaquero,& Ebihara, 2018). Interestingly, Shiokawa et al. (2005) reported three cases of instrumentalobservations of mid-latitude aurora in Hokkaido (Japan) under fairly moderate magneticactivity as well.Silverman (2003) speculated that sporadic aurorae may be caused by localized andephemeral magnetospheric energy input into the low-latitude ionosphere, but he does notclearly suggest any physical mechanisms that may explain this phenomenon. In fact, con-sidering the known correlation between intensity of magnetic disturbance and equatorwardboundary of auroral ovals (Yokoyama, Kamide, & Miyaoka, 1998), Silverman (2003)’s com-prehensive survey was striking and casted an open question on its physical mechanism.Hayakawa, Vaquero, and Ebihara (2018) suggested that at least part of sporadic auroraemight have been caused by the impact of inclined interplanetary shocks (see also Oliveira etal., 2018; Oliveira & Samsonov, 2018) that strike the magnetosphere in the pre-dusk sector.However, despite all these efforts, a comprehensive understanding of the causes of sporadicaurorae still remains an open question in space weather research.The main goal of this article is to show an aurora observation report published in a Riode Janeiro’s newspaper on 17 February 1875, hitherto unknown to the scientific community.Based on the event descriptions, the expertise of the observer, and the sporadic auroracharacteristics presented in this introduction, as well as the magnetic latitude location ofRio de Janeiro and the low magnetic activity on the days before the observation, we will show2hat the event was most likely a sporadic aurora. The paper is structured as follows. Section2 brings brief descriptions of the observational site and the observer. Section 3 introducesthe report along with its interpretation based on current aurora knowledge. Finally, thepaper is concluded in section 4 along with a final remark. Brazil was a Portuguese colony during the period 1500 to 1822. Due to military and com-mercial sanctions imposed by Napoleon to Lisbon in the beginning of the 19th century, thethrone of the Portuguese Empire exiled from Lisbon to Rio de Janeiro in 1808 (Fausto, 1994).Later, John VI of Portugal returned back to Lisbon and left his son Peter I as the ruler of theKingdom of Brazil. Then, on 7 September 1822, Peter I proclaimed Brazil’s independence ofPortugal, and became the first emperor of Brazil (Fausto, 1994). In 1827, seven years beforehis death, Peter I founded the Imperial Observatory (Morize, 1987), today known as theNational Observatory (
Observat´orio Nacional ), still located in Rio de Janeiro. After PeterI’s death, his son Peter II became the second and last emperor of Brazil, when it became aRepublic on 15 November 1889 (Fausto, 1994). Peter II was a monarch very interested inscience who supported many contemporary scientists, and used the auspices of the ImperialObservatory for astronomical observations and scientific discussions (Benevides, 1979).
The likely sporadic aurora reported here was observed from the facilities of the ImperialObservatory by the Frenchman Emanuel Liais (1828-1900) on 15 February 1875. Liais wasthe director of the Imperial Observatory in 1875, having been directly appointed by Peter II,after leaving the position as the director-adjunct of the Paris Observatory in France (Morize,1987). The observer was a professional 19th century scientist. While at the Imperial Ob-servatory, Liais conducted research on astronomy with emphasis on planetary motion andcomets, discovering one himself in 1860 (Liais, 1860). He also published a popular bookon astronomy (Liais, 1865). Liais had considerable experience and expertise with opticalphysics and instrumentation. He published on the 1858 total solar eclipse observation fromBrazil, being among the first to photograph the solar corona (Aubin, 2016; Liais, 1861), andapparently had a good understanding of atmospheric effects with respect to their altitudeoccurrences (Liais, 1859; Muniz Barreto, 1997). Liais also published on aurora observationsfrom his home town Cherburg, France, on the Halloween day of 1853 (Liais, 1853). More sur-prisingly, Liais even suggested methods to measure auroral altitudes, showing that auroraeoccur far higher than meteorological phenomena (Liais, 1851), as is well known today (e.g.,Roach, Moore, Bruner Jr., Cronin, & Silverman, 1960). According to Muniz Barreto (1997),these findings would have contributed to classify auroral phenomena as magnetic phenomenaas opposed to meteorological phenomena if they had been published in a scientific journalwith higher audience. 3 xygenNitrogenSulfur
400 450 500 550 600 650 700
Wavelength (nm)
Sunlight
Figure 1:
From top to bottom, line spectra of oxygen, nitrogen, sulfur, and solar (reflected) light, withwavelengths in the horizontal axes. Whereas the auroral lights by oxygen and nitrogen show bright emissionlines, the sunlight and its reflection (solar reflected light) show dark absorption lines.
Emmanuel Liais observed the aurora event on 15 February 1875 from Rio de Janeiro, Brazil.At that time, the Imperial Observatory was hosted by the
Morro do Castelo (Castle Hill), anold church whose geographic coordinates are 22.75 ◦ S, 43.10 ◦ W. Liais took notes of his obser-vations and wrote a report to the local Jornal do Commercio (1875) (Commerce Newspaper).This report was found in the data base of the National Digital Library of the National Li-brary of Brazil ( http://bndigital.bn.gov.br ), hereafter BNDigital). We transcribed thefull text of early modern Portuguese with its original spelling and grammar style in AppendixA.1 and translated it into English in Appendix A.2.The observer noted the occurrence of the aurora by 19:45 local mean time (LMT), or ∼ There are only a few magnetic field observations that were regularly recorded around theworld during the 19th century. The only magnetic indices that can be used for that periodare the ak index (Nevanlinna, 2004) and the aa index (Mayaud, 1972). Unfortunately, there5
600 1650 1700 1750 1800 1850 1900 1950 2000 year − − − − − − − − M L A T [ d e g ] Time evolution of Rio de Janeiro’s magnetic latitude
RJ MLATCarringtonevent (1859)This report1781-1788observations
Figure 2:
Time evolution of Rio de Janeiro’s magnetic latitude computed with the geomagnetic fieldmodel GUFM1 (Jackson et al., 2000). The light purple bar shows the period corresponding to SanchesDorta’s magnetic and aurora observations in Rio de Janeiro during the period 1781-1788 (Vaquero & Trigo,2005, 2006). The green and brown vertical lines mark the Carrington event (e.g., Green & Boardsen, 2006;Hayakawa, Ebihara, Hand, et al., 2018; Hayakawa, Ebihara, Willis, et al., 2019) and the eventual sporadicaurora observation here reported. is no ak index for that date, but there is aa index for that date. The aa index is a 3-hour time resolution magnetic index derived from two magnetic observatories in Englandand Australia that are nearly antipodal to each other (Mayaud, 1972). The Aa index isthen derived from the aa index by taking its daily averages. More detail of these indicescan be found in the literature (Mayaud, 1980; Rostoker, 1972). The aa and Aa indices areprovided by the British Geological Survey website. Magnetic latitudes are computed by thegeomagnetic field GUFM1 model (Jackson, Jonkers, & Walker, 2000) from 1600 to 1990.This model is complimentary to the International Geomagnetic Reference Field (IGRF)model (Th´ebault et al., 2015). IGRF can compute magnetic fields from 1900 onwards, butGUFM1 can compute magnetic fields as far back as 1590 due to the compilation of a massivedata base obtained from observational logs compiled on ships at sea and ports around theworld (Jackson et al., 2000; Jonkers, Jackson, & Murray, 2003).The solid orange line in Figure 2 shows the time evolution of Rio de Janeiro’s magneticlatitude (MLAT) from 1600 to 1990. The model shows that MLAT increased from –18.4 ◦ in1600 to its maximum value (the closest value to the magnetic equator) slightly above –12 ◦ around 1816 when it started to decrease again. The highlighted light purple area (discussedlater) corresponds to the 1781-1788 interval between aurorae observed from Rio de Janeiro.The dashed green vertical line marks the Carrington event occurrence (1859), while thedashed brown vertical line indicates the event reported in this letter (1875).Figure 3 shows the aa index for the interval 1-24 February 1875. The solid orange lineindicates 3-hour aa index (in nT), while the shaded green line indicates the daily-averagedAa index. The dashed blue line corresponds to the beginning of Liais’ observations (19:45LMT or 1645 UT) reported to the Jornal do Commercio (1875). The plot documents thatthe A(a)a indices showed some weak/mild activity 2-4 days prior to the aurora observation,6 Days of February 1875 (Greenwich Mean Time) g e o m a g n e t i c i nd e x [ n T ] British Geological Survey data
Aaaaobservationonset16:45 UT
Figure 3:
British Geological Survey 3-hour aa index (solid orange line) and the 24-hour Aa index (solidgreen line/shaded green area) for the interval 1-24 February 1875. The dashed blue line indicates the onsetof the aurora observation (15 February 1875 at 19:45 LMT/16:45 GMT) reported by Emmanuel Liais to theJornal do Commercio (1875). with maximum Aa around 27 nT. This magnetic activity is consistent with sunspot numberobservations recorded a few days before, with very low values and one day with the obser-vation number of 60 (Clette & Lef`evre, 2016; Clette, Svalgaard, Vaquero, & Cliver, 2014).The low magnetic activity conditions during that sporadic aurora event is consistent withthe description suggested by Silverman (2003).Additionally, the results of this study may also explain the reason why great auroradisplays observed from Brazil have not been found/reported in the contemporary recordsfor the Carrington event yet. As seen in Figure 3, Rio de Janeiro’s MLAT by 1859 wasvery low, around –12.2 ◦ . While we surveyed auroral reports in Brazilian newspapers duringthe Carrington event in the BNDigital database, we found only references to great auroradisplays and even telegraph system failures in North America and Europe, with nothingbeing reported as having been observed from Brazil.Since the equatorward boundary of the auroral oval is reconstructed ∼ ◦ MLAT(Hayakawa, Ebihara, Hand, et al., 2018) assuming aurora height ∼
400 km (Ebihara et al.,2017; Roach et al., 1960), the expected elevation of auroral visibility would be at best 3 ◦ above the horizon and hence it was quite difficult to observe auroral displays in the sky of Riode Janeiro (and other Brazilian locations) during the Carrington event. However, it wouldbe worth surveying potential auroral reports in Argentine, Chile, and Uruguay, countriesthat are located in regions of higher MLATs in South America. Another significant event,the magnetic storm of 4 February 1872, also triggered great aurora displays at low latitudes(Hayakawa, Ebihara, Willis, et al., 2018; Silverman, 2008). Silverman (2008) reported onpossible aurora sightings in latitudes as low as 10o or even 3 ◦ , while he casted a caveat on7heir reliability. The author reported aurora sightings on the French Reunion Island, in theIndian Ocean (21.12 ◦ S, 55.54 ◦ E). We found mention to these aurora sightings during thatstorm in Brazilian newspapers while searching the BNDigital database, but none occurringfrom Rio de Janeiro or anywhere else in Brazil.Another possibility is to interpret Liais optical observations as equatorial plasma bubbles(EPBs), which are structures with depleted plasma density usually formed after sunset inthe bottomside ionosphere and move from west to east (Kelley, 2009; Liu, Pedatella, &Hocke, 2017; Mendillo & Tyler, 1983). Since plasma bubbles are faint structures that canbe hardly seen by the naked eye (e.g., Wiens, Ledvina, Kintner, Afewerki, & Mulugheta,2006), Liais event most likely cannot be classified as an EPB event.It should be mentioned that this is not the first report of an aurora observation performedfrom Rio de Janeiro, despite its proximity to the magnetic equator. In fact, Vaquero andTrigo (2005) and Vaquero and Trigo (2006) and Carrasco, Trigo, M., and Vaquero (2017)presented a series of magnetic observations and aurora sightings conducted by the Portugueseastronomer Sanches Dorta in the 18th century. According to the authors, the observationsconducted by Sanches Dorta, during the period 1781 to 1788, must very likely have occurredduring times of elevated magnetic activity. However, according to Figure 2, the MLATs ofRio de Janeiro during these events were around –12.5 ◦ (highlighted purple area). If theevents reported by Vaquero and Trigo (2005), Vaquero and Trigo (2006) and Carrasco etal. (2017) were in fact caused by great magnetic storms, their visibility would have reachedMLATs closer to the magnetic equator in comparison to the low-latitude Carrington auroraepreviously reported (Green & Boardsen, 2006; Hayakawa, Ebihara, Cliver, et al., 2019;Hayakawa, Ebihara, Hand, et al., 2018; Hayakawa, Ebihara, Willis, et al., 2019). Sporadic aurorae occur at low-latitude areas (Abbe, 1895; Botley, 1963) during moderate-to-low magnetic or even quiet conditions (Silverman, 2003). However, despite being impressive,this space weather phenomenon is not very well known by the community. In addition, thisphenomenon does not happen very often, and there are only a few publications reportingon sporadic aurora sightings (Abbe, 1895; Botley, 1963; Boyer, 1898; Hayakawa, Vaquero,& Ebihara, 2018; Silverman, 2003; Vaquero et al., 2013, 2007; V´azquez & Vaquero, 2010;Willis et al., 2007).In this letter, we presented for the first time a report on a possible sporadic auroraobservation performed from Rio de Janeiro, Brazil, on 15 February 1875. This is the firstsporadic aurora report in South America, and the second one in the southern hemisphere(the first observation was reported by Eddie (1894). Additionally, this is the second sporadicaurora observed near the magnetic equator. The original report was authored by EmmanuelLiais, then director of the Imperial Observatory of Rio de Janeiro, and published in theJornal do Commercio (1875) of the same city. Given the scientific expertise, the contents ofscientific descriptions and the experience of the observer, particularly with respect to the useof a spectroscope, Liais’ report may be considered credible and possible misinterpretationof the observed phenomenon, such as caused by atmospheric optics (Hayakawa, Vaquero, &Ebihara, 2018; Usoskin et al., 2017), may be discarded. The aurora description presentedby Liais is consistent with sporadic aurorae (Abbe, 1895; Botley, 1963; Silverman, 2003).In addition, the very low magnetic latitude of Rio de Janeiro and the weak/mild magneticactivity during the observations are consistent with a previous sporadic aurora observationnear the magnetic equator (Hayakawa, Vaquero, & Ebihara, 2018).8urthermore, in addition to the sporadic aurora causes presented in the introductorysection, we speculate that sporadic aurorae may also be caused by the flow of solar windphase fronts with some inclination in the equatorial plane toward the dusk flank. As sug-gested by Cameron, Jackel, and Oliveira (2019), such flows of solar wind phase fronts duringtimes of low magnetic activity or quiet conditions would increase magnetic activity over timedue to shear and viscosity effects, and the sudden release of this energy may cause sporadicaurorae. More observations and possibly numerical simulations are needed in order to testthese hypotheses and advance the knowledge of sporadic aurora triggering.
A The report
A.1 Original transcript written in old Portuguese
Rio de Janeiro, 17 de fevereiro de 1875Jornal do Commercio
Aurora austral : O Sr. Emmanuel Liais, director do observatorio astronomico do Riode Janeiro, enviou-nos ontem as sequintes observaes que fez sobre a aurora austral, de quej´a dmos resumida not´ıcia:A’s 7 3/4 da noite foi a minha ateno despertada por uma especie de vo espalhado sobretodo o co formando uma serie de listras esbranquiadas, que comeavam ao sul sobre umarco de circulo, cujo centro achava-se abaixo do horisonte, na direo da agulha magnetica deinclinao. As listras ou raios eram de tal extenso que atravessavam o co do sul ao norte, ondeconvergiam para o ponto diametralmente oposto.Esta disposio, reproduzindo a forma das auroras boreaes, fez-me desde logo suppr quebem podia ser uma aurora austral o phenomeno que presenciava; infelizmente no podiaaffirmal-o por causa da presena da lua momentos depois, porm, fiquei inteiramente conven-cido disso, graas a outras circumstancias que o acompanhram.Com effeito, depois de cinco minutos de observao, passaram de oste para lste, e duasvezes manifestaram-se variaes successivas de intensidade nos raios, como se d frequentementenas auroras boreaes e austraes. Alm disso, passados mais alguns minutos os raios, cujasintensidades haviam augmentado, tomaram na parte inferior uma tenue cr avermelhada ena superior verde desmaiada, que no podia ser effeito da luz reflectida da lua.Observei ento com o espectroscopio, onde appareciam linhas brilhantes, indicio certo deluzes proprias. Todas elas pertenciam ao enxofre, substancia que, como sabido, encontra-seem quantidade apreciavel na atmosphera.Em seguida olhei para o norte, onde vi dous relampagos, e notei que se formavam pe-quenas nuvens de uma frma variavel pelo effeito de condensao e da dissoluo de vapores.Muitas nuvens caminhavam na direo de lste, um pouco ao sul, passando abaixo dos raios daaurora; ao mesmo tempo estes diminuiam de intensidade e as cres da parte inferior tinhamdesapparecido.Observei mais um halo fraco em volta da lua, dentro de um tenue vo de vapor, e como jo assignalou por Bravais, este halo era um tanto mais forte nas intercepes com os raios daaurora. Pouco depois estes raios comearam a encurtar e a retirar-se para o Sul.Foi ento que deixando a observao s 8 horas e 20 minutos, mandei para o Jornal a noticiadeste phenomeno. Quando tornei a subir ao terrao os numerosos raios existiam ainda, poremmais curtos e mais fracos. Pelas 8 horas e 40 minutos comearam a desapparecer, e s 9 horas9 se vio pequenos vestigios delles junto ao horisonte, de lste a oeste particularmente.A’s 10 horas parecia quererem formar-se outra vez dous ou tres raios, mas desapparece-ram pouco depois, e pequenas nuvens condensaram-se sobre diversos pontos do co. Nadamais aconteceu at s 3 horas da madrugada, occasio em que fui chamado para vr dous raiosbrilhantes que tinham reapparecido a leste, e que por causa da ausencia da luz lunar, cham-mavam mais a atteno.Depois de diminuirem, reapparecram quatro raios na mesma regio, mais fracos, porm,do que os primeiros, e duraram at que a luz do dia nascente veio fazer cessar de todo ophenomeno, e ao amanhecer o co mostrou-se coberto de tenues cirrus.So estes os pormenores da observao, cujas deduces faro o objeto de uma memoria especial.
A.2 Translation of the original transcript into modern English
Rio de Janeiro, February 17, 1875Commerce Newspaper
Aurora australis
Mr. Emmanuel Liais, director of the astronomical observatory ofRio de Janeiro, sent us yesterday the following observations that he made on the auroraaustralis, on which we already reported:At 7:45pm my attention was caught by a kind of bridal veil spread all over the skyforming a series of white stripes, that started in the south on a circular arc whose centerwas below the horizon, in the direction of the magnetic needle inclination. The stripes orrays were of such an extension that they passed through the sky from south to north, wherethey converged at the diametrically opposite point.Such disposition, producing the form of aurora borealis, made me suppose at once thatit could be an aurora australis the phenomenon I witnessed; unfortunately I could not affirmthat because of the Moon’s presence: moments later, however, I was entirely convinced thatwas the case, thanks to other circumstances that accompanied it.In fact, after five minutes of observation, they passed from west to east, and twicesuccessive appearances with a variety of ray intensities occurred, as often happens withaurora borealis and australis. In addition, after a few more minutes the rays, whose intensityhad augmented, took in their inferior part a reddish color and in their superior part a faintgreenish color that could not result from the effect of the light reflected by the Moon.I observed then with a spectroscope, where bright lights appeared, the certain evidenceof proper lights. All lights belonged to sulfur, a substance that, as is well known, is foundin large amounts in the atmosphere.Then I looked towards the north, where I saw two light bolts, and noticed that smallclouds formed in a variable form by effects of condensation and dissolution of vapors. Suchclouds moved towards the east direction, slightly to the south, passing below the aurorarays; at the same time the rays decreased in intensity and the colors of the inferior part haddisappeared.I observed one more weak halo around the Moon inside a thin veil of vapor, and aspointed out by Bravais, such halo was somewhat stronger in the interceptions with theaurora rays. Later such rays started to get shorter and move southward.Then when I left the observation at 8:20pm I sent to the Newspaper the news of thisphenomenon. When I went back up to the terrace the numerous rays still existed, howeverthey were shorter and weaker. Around 8:40pm they began to disappear, and at 9:00pm itwas possible to see only their vestiges together in the horizon, particularly from east to west.10t 10:00pm two or three rays appeared to be formed once more, but they disappearedlater, and small clouds condensed over several points in the sky. Nothing more took placeuntil 3:00am, occasion on which I was called to see two very bright rays that had reappearedtowards the east, which caught more attention because of the absence of lunar light.After diminishing, four rays appeared in the same region, however, weaker in comparisonto the first ones, and lasted until the light of the breaking day brought the phenomenon toan end, and at dawn it was shown that the sky was covered by a faint cirrus.These are the details of the observations, whose deductions will make an object of aspecial memory.
Declarations
List of abbreviations
MLAT: magnetic latitude; LMT: local mean time; GMT: Greenwich mean time; BNDig-ital: Digital Library of the National Library of Brazil; IGRF: International GeomagneticReference Field; EPB: equatorial plasma bubble.
Ethics approval and consent to participate
Not applicable.
Authors contributions
DMO, who is proficient in Portuguese, surveyed the BNDigital database to search for his-torical accounts of auroral observations from Brazil. He also used GUFM1 to computemagnetic fields and coordinates. HH provided background of space weather events in his-tory particularly with respect to sporadic aurorae. EZ contributed with the interpretation ofthe historical data and observations in the light of current auroral scientific understanding.AB contributed with interpreting spectroscopic observations. GV provided fundamentalinformation on the ionospheric and magnetic field variations at low latitudes. All authorsread and approved the final manuscript.
Consent for publication
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Availability of data and materials
The aa and Aa indices are available for download at the British Geological Survey website . Fig-ure 1 and its data can be retrieved from .11 unding
DMO thanks the financial support of the NASA grants 13-SRITM132-0011 and HSRMAG142-0062, under contract with UMBC. HH acknowledges the JSPS Grand-in-Aid grant JP17J06954,JP15H05816, JP15H05812, and JP15K21709. AB acknowledges the support by the NASALiving With a Star Jack Eddy Postdoctoral Fellowship Program, administered by theCatholic University of America.
Acknowledgments
The authors thank the National Library of Brazil (Biblioteca Nacional do Brasil) for provid-ing and keeping a public newspaper archive ( ). The authors also acknowledge theBritish Geological Survey for providing the magnetic index data used in this investigationand Yusuke Ebihara for his helpful comments. Finally, we thank Martin Rehfeld for provid-ing and making a FORTRAN code version of the GUFM1 model public at his web-servicehosting GitHub website ( https://github.com/martinrehfeld ). References
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