Aurora Borealis

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Where is the best place to see the aurora borealis?What are the best places to see the Northern Lights? Rovaniemi, Finnish Lapland Ilulissat, Greenland. Tromso, Norway. Based in the heart of the  aurora  zone in the Norwegian Arctic, the city is widely regarded as one of the world’s best places to see the Northern Lights …Swedish Lapland.Reykjavik, Iceland. …Yukon, Canada. …

An aurora , sometimes referred to as polar lights (aurora polaris), northern lights (aurora borealis), or southern lights (aurora australis), is a natural light display in the Earth’s sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras are the result of disturbances in the magnetosphere caused by solar wind. These disturbances alter the trajectories of charged particles in the magnetospheric plasma. These particles, mainly electrons and protons, precipitate into the upper atmosphere (thermosphere/exosphere). The resulting ionization and excitation of atmospheric constituents emit light of varying colour and complexity. The form of the aurora, occurring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating particles. Most of the planets in the Solar System, some natural satellites, brown dwarfs, and even comets also host auroras.; The word « aurora » is derived from the name of the Roman goddess of the dawn, Aurora, who travelled from east to west announcing the coming of the sun. Ancient Greek poets used the name metaphorically to refer to dawn, often mentioning its play of colours across the otherwise dark sky A region that currently displays an aurora is called the « auroral oval », a band displaced by the solar wind towards the night side of Earth.[5] Early evidence for a geomagnetic connection comes from the statistics of auroral observations. In northern latitudes, the effect is known as the aurora borealis or the northern lights. The former term was coined by Galileo in 1619, from the Roman goddess of the dawn and the Greek name for the north wind. Most auroras occur in a band known as the « auroral zone », which is typically 3° to 6° wide in latitude and between 10° and 20° from the geomagnetic poles at all local times (or longitudes), most clearly seen at night against a dark sky. Elias Loomis (1860), and later Hermann Fritz (1881) and Sophus Tromholt (1881) in more detail, established that the aurora appeared mainly in the auroral zone. Auroras seen within the auroral oval may be directly overhead, but from farther away, they illuminate the poleward horizon as a greenish glow, or sometimes a faint red, as if the Sun were rising from an unusual direction. Auroras also occur poleward of the auroral zone as either diffuse patches or arcs, which can be subvisual. A geomagnetic storm causes the auroral ovals (north and south) to expand, bringing the aurora to lower latitudes. The instantaneous distribution of auroras (« auroral oval ») is slightly different, being centered about 3–5° nightward of the magnetic pole, so that auroral arcs reach furthest toward the equator when the magnetic pole in question is in between the observer and the Sun. The aurora can be seen best at this time, which is called magnetic midnight. The southern counterpart, the aurora australis or the southern lights, has features almost identical to the aurora borealis and changes simultaneously with changes in the northern auroral zone. The aurora australis is visible from high southern latitudes in Antarctica, Chile, Argentina, New Zealand, and Australia. The aurora borealis is visible from being close to the center of the Arctic Circle such as Alaska, Canada, Iceland, Greenland, Norway, Sweden and Finland.

Abstract Catalogues and other records of aurora-borealis events were used
to study the long-term spatial and temporal variation of these phenomena in
the period from 1700 to 1905 in the Northern Hemisphere. For this purpose,
geographic and geomagnetic coordinates were assigned to approximately 27 000
auroral events with more than 80 000 observations. They were analysed separately in three large-scale areas: i) Europe and North Africa, ii) North America,
and iii) Asia. There was a clear need to fill some gaps existing in the records so
as to have a reliable proxy of solar activity, especially during the 18th century.
In order to enhance the long-term variability, an 11-year smoothing window was
applied to the data. Variations in the cumulative numbers of auroral events
with latitude (in both geographic and geomagnetic coordinates) were used to
discriminate between the two main solar sources: coronal mass ejections and
high-speed streams from coronal holes. The characteristics of the associated
aurorae correlate differently with the solar-activity cycle.
Keywords: Solar Activity, Geomagnetic Storm, Aurora Borealis

Introduction
Aurorae are the most easily seen manifestations of the interaction (compression
and magnetic reconnection) between the solar wind and the Earth’s magnetosphere. Just naked-eye observations are sufficient, without any need for specific
training. They are therefore a useful tool with which to study the variability of
this process in past centuries because several catalogues, books, and reports of
various kinds are available. Here we shall concentrate on two recent centuries.

Instituto de Astrofısica de Canarias, 38200 La Laguna,
Spain. email: mva@iac.es
Departamento de Astrofısica, Universidad de La Laguna,
38205 La Laguna, Spain.
Departamento de F´ısica, Universidad de Extremadura,
Avda. Santa Teresa de Jornet 38, 06800, Merida, Spain.
email: jvaquero@unex.es
Departamento de Fısica, Universidad de Extremadura,
06071 Badajoz, Spain. email: maricruz@unex.es
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Vazquez, Vaquero, and Gallego
For longer periods of time (thousands of years), the use of cosmogenic isotopes,
such as 14C and 10Be, or of the nitrate content in polar ice is recommended
(Usoskin and Kovaltsov, 2012; Traversi et al., 2012).
The visibility of aurorae is limited to ring-shaped regions around the geomagnetic poles – the auroral ovals, centred at around 65 degrees magnetic latitude in
each hemisphere. Under the impact of a solar storm, the auroral ovals undergo
broadening, particularly on the night side. Low-latitude aurorae are very rare,
and are clearly associated with strong geomagnetic storms produced by solar
coronal mass ejections. They are generally red and diffuse, resulting primarily
from an enhancement of the 630.0 nm [OI] emission due to bombardment by soft
electrons (<100 eV) precipitating from the plasmasphere (Tinsley et al., 1986).
The typical altitude for a low-latitude aurora is 250 – 400 km (Roach et al., 1960).
They can be confused with distant fires or twilight due to their colour. The limits
of such aurorae are usually 15 and 45 degrees geomagnetic latitude. A similar
phenomenon, which shares the same energy source, is that of stable auroral red
arcs. They are observed mainly during the recovery phase of geomagnetic storms
(Rassoul et al., 1993; Nakazawa, Okada and Shiokawa, 2004).
The visual sensitivity thresholds of the green and red radiation of the aurorae
are between one and ten kilorayleighs, which means that only those aurorae
coming from moderate and strong geomagnetic storms will be visible and therefore part of our sample (Schroder, Shefov, and Treder, 2004). It is clear that the
meteorological conditions also play a role, although a cloud-free sky is not necessary. According to Livesey (1991), the zone with the greatest aurora-borealis
probability passes across northern Norway, over to Iceland, south of Greenland,
and over to the South of Hudson Bay in North America.
Fritz (1873) produced the first graph showing the geographical distribution
of auroral frequencies, measured in nights per year. The values ranged from a
minimum in the Mediterranean area to a maximum at a magnetic latitude of 67
degrees.
In previous articles (Vazquez et al., 2006; Vazquez and Vaquero, 2010), we
have studied low-latitude auroral events. We established a correlation with several indices of solar activity and drifts of magnetic latitude at two different sites,
verifying the adequacy of such terrestrial events to track the long-term variation
of solar activity in the past. For a monograph on historical aspects of solar
activity, see Vaquero and Vazquez (2009).
In the present work, we considered the period 1700 – 1905 for which various
documental sources of auroral observations are available, indicating the date and
place of the observations. This allowed geographic coordinates to be ascribed to
each observation, with a maximum of six sites being taken per day and per
catalogue. If necessary, a selection was made from the existing sample to give
the broadest coverage in both longitude and latitude. For a previous work on
auroral activity in this period, see Feyman and Silverman (1980), although their
study is restricted spatially to Scandinavian and New England sources.
Table 1 lists the documentary sources used in the present work. The number
of days in the period studied (1700 – 1905) was approximately 73 000 (200 years).
The available data for the Northern Hemisphere were divided into three continental areas corresponding to Europe (and North Africa), North America, and Asia.
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Aurora Borealis
Figure 1. Yearly number of aurorae in the following catalogues: (dotted line) Krivsk´y and
Pejml (1988) updated by Krivsk´y (1996), (thin solid line) Angot (1897), and (thick solid line)
Fritz (1873). The first two catalogues are limited to zones further south than 55 degrees of
geographic latitude.
The main sources of data were the catalogue of Fritz (1873) and the archives
of S. Silverman1
. For only a few places (less than ten) was it impossible to
unequivocally determine the site’s coordinates. More frequently, only the names
of countries or US states were described, and in these cases average coordinates
were assigned.
Fritz’s North American data are mainly based on the earlier catalogues of
Lovering (1866) and Loomis (1860, 1861). They are complemented for the later
phase of the 19th century with data from Greeley (1881) and different archives
collected by S. Silverman.
For Europe, the Fritz data were complemented with additional values for high
latitudes from Rubenson (1882) and Tromholt2
(1898). Particularly relevant are
the Greenland observations, contained in the Fritz catalogue and the Silverman
archives (see Stauning, 2011). For the rare low latitude observations, we included
data for the Iberian Peninsula3 and the Canary Islands (V´azquez and Vaquero
2010).
1These include: (a) Catalog of Ancient Auroral Observations, 666 BCE to 1951; (b) The
Auroral Notations from the Canadian Monthly Weather Review; (c) The New England Auroral Observations (1720 – 1998); and (d) Daily Auroral Reports, Southeastern Canada and
Northeastern US (1848 – 1853).
2For a biography of S. Tromholt see Moss and Stauning (2012).
3They include published data from Vaquero, Gallego, and Garcia (2003), Vaquero and Trigo
(2005), Aragones Valls and Orgaz Gargallo (2010), and Vaquero et al. (2010).
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Vazquez, Vaquero, and Gallego

Table 1. List of catalogues used for this study. The S. Silverman archives are available from
the National Space Data Center (http://nssdcftp.gsfc.nasa.gov/miscellaneous/aurora/).
Observing Area Catalogue Number of Number of
auroral days locations
Europe and N. Africa TOTAL 25 306 43 460
Fritz (1873) 9761 15 874
S. Silverman Archives 6089 11 810
Angot (1897) 439 753
Rubenson (1892) 5924 9809
Tromholt (1902) 2311 4328
Sunderland Archive 453 454
Hungary (Rethly and Berkes, 1963) 97 138
Visser (1942) 93 93
Harrison (2005) 8 8
W. Schroder (1966) 3 5
Iberian Peninsula 76 109
Vazquez and Vaquero (2010) 9 12
Different Journals 8 12
North America TOTAL 15 617 38 275
Fritz (1873) 5351 11 160
S. Silverman Archives 8605 23 674
Greeley (1881) 1379 2919
Lueders (1984) 121 121
Broughton (2002) 14 14
Mendillo and Keady (1976) 5 5
Eather (1980) 4 4
Asia TOTAL 324 359
Fritz (1873) 245 270
Yau et al. (1995) 27 27
Lee et al. (2004) 6 8
Willis, Stephenson and Fang (2007) 33 36
S. Silverman Archives 11 14
Basurah (2004) 2 2
Harrison (2005) 1 1
The data sample is clearly inhomogeneous in both space and time. This
reflects not only meteorological variations but also the difficulties of access of
some regions. For North America, there are many temporal gaps until 1746,
and, after that, no data are available for the years 1754, 1755, 1756, 1766, 1799,
1810, or 1812. There is a remarkable contribution for the entire period studied
from numerous forts (more than 60) located along the borders of the expanding
settlement of the western and northern territories.
After the Maunder Minimum, the works of Halley (1716) and Mairan (1733)
marked the beginning of modern studies of aurorae. Krivsky and Pejml (1988)
and Krivsky (1996) compiled a catalogue (archived at the National Geophysical
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Aurora Borealis
Table 2. Sites where aurorae were most frequently observed in Europe.
The geographical coordinates are expressed in degrees. (∗) Referred in the
catalogues only to the country, without assigning any defined site.
Site G. Latitude G. Longitude Auroral events
Godhaab (Nuuk) 64.175 -51.739 2164
Uppsala 59.85 17.63 1973
Stockholm 59.35 18.08 1593
Christiania (Oslo) 59.92 10.76 1536
Jakobshavn (Ilullissat) 69.17 -49.83 1487
Ofver-Tornea 66.38 23.65 1016
Saint Petersburg 59.95 30.32 968
Scotland (∗) 56.00 -4.00 951
Utrecht 52.09 5.12 818
Harnosand 62.66 17.94 813
Berlin 52.52 13.41 623
Paris 48.86 2.35 464
Lund 55.70 13.20 427
Data Centre) recording the aurorae visible in Europe in the period 1715 – 1850 at
latitudes lower than 55 degrees. They assumed that nearly all of the aurorae were
recorded after roughly 1720, and that therefore no normalization factor needed
to be applied (Figure 1). These records are compared with the catalogue of Angot
(1897) for the period 1700 – 1890, also covering observations at latitudes lower
than 55 degrees, and Fritz (1873) with no restrictions on latitude. Unfortunately,
no such global catalogues exist for the 20th century.
The reduction in the number of aurorae observed during the Dalton Minimum
is confirmed (Silverman, 1992; Broughton, 2002; Vaquero et al., 2003). Two
strong episodes of auroral activity are clearly seen around 1775 and 1850 (Figure
1). Around the turn of the 19th to the 20th century, the level of solar activity
decreases, as also does the visual-aurora monitoring. There is no recovery from
this decrease until the start of the space age in 1957 (Siscoe, 1980; Legrand and
Simon, 1987; Silverman, 1992).
It is our purpose now to proceed to a careful statistical study of these classical
catalogues, expanding them with other available sources. Our main emphasis is
on providing a 3D map (geographic coordinates and time) of the auroral activity
in the past, when auroral observations constitute the only information available
about heliosphere activity.
Figure 2 shows the locations of auroral observations on a Mercator projection
of the Northern Hemisphere. The accompanying Tables 2 and 3 indicate the
sites where most auroral events were reported for Europe and North America
respectively. The numbers are clearly low for Eastern Europe and Asia.
Figure 3 shows the histograms of the latitude distributions for Europe and
North America. They both suggest a bimodal distribution, with the maxima
at middle and high latitudes for North America and Europe, respectively. The
expected trend would be an increase of auroral events at high latitudes, but
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Vazquez, Vaquero, and Gallego
Table 3. Sites where aurorae were most frequently observed in North America.
The geographical coordinates are expressed in degrees.
Site G. Latitude G. Longitude Auroral events
Toronto 43.72 -79.34 1800
Cambridge, MA 42.37 -71.11 921
New Haven 41.31 -72.92 908
Gardiner, ME 44.21 -69.79 798
New York City and State 43.00 -75.00 734
Winnipeg 49.90 -97.14 727
York Factory 57.01 -92.31 518
Quebec 46.82 -71.22 494
Eastport 44.91 -67.00 486
Burlington 44.47 -73.15 340
Moose Factory 51.26 -80.59 287
Montreal 45.51 -73.55 259
Point Barrow 71.39 -156.48 251
in the case of North America the histograms reflect the slow settlement of the
northern areas. Moreover, access to southern data was delayed until these regions
(Louisiana, Florida, Texas, New Mexico) were annexed by the USA. A search of
18th century sources in Spanish would be worthwhile for this region.
We checked the relationship between the auroral observations in these catalogues and moonlight, since an aurora should be easier to observe when there is
little moonlight during the night. In particular, the light of the full Moon would
impede the observation of weak aurorae. Therefore, we expected relatively more
(fewer) reported aurorae when there was a new (full) Moon. Indeed, we found
that there was a clear decrease of observed events with increasing brightness of
the moon, as was expected.

  • Temporal Variation
    The number of aurorae is clearly related to the level of solar activity. The records
    reflect the well-known Sporer, Maunder, Dalton, and 1901 – 1913 minima, and
    include a previously unrecognized long-term minimum around 1765 (Silverman,
    1992, 1995).
    A double peak in the geomagnetic records has been observed, with the two
    maxima occurring i) shortly before the sunspot maximum and produced by
    transient events (Gonzalez et al., 2002), and ii) two years after the maximum
    and mainly produced by recurrent events (Tsuratani et al., 2006). Geomagnetic
    activity is greater in the second half of even-numbered cycles and in the first half
    of the odd-numbered cycles, giving rise to a 22-year variability (Chernosky, 1966:
    Vennerstrøn and Friis-Christensen, 1996). For a discussion of the geoeffectiveness
    of the different solar sources of geomagnetic disturbances, see Georgieva et al.
    (2006).
    SOLA: VVG2013spvdef1.tex; 9 September 2013; 0:24; p. 6
    Aurora Borealis
    The variation in time and latitude of the auroral observations recorded in our
    sample is shown in Figure 4. The European data reflect the Dalton Minimum
    very clearly. The lack of high-latitude records in the Fritz catalogue was remedied by including the Scandinavian observations of Tromholt and Rubenson.
    The aurorae observed in Barcelona in 1811 and 1812 are remarkable, and call
    for detailed confirmation. The existence of the secondary Silverman Minimum
    (around 1760) seems also to be confirmed. The middle panel of the figure reflects
    the paucity of North American data during the 18th century, especially at high
    latitudes. The settlement of the western regions produces a clear increase in the
    number of reports. The records in New England are, however, relatively frequent
    over the whole period. Finally, the bottom panel reflects the scarcity of the data
    that we were able to collect for auroral observations in Asia. In light of this,
    in the analyses below we shall concentrate mainly on European and American
    sources. The combination of the different auroral catalogues shows a quasi-80-
    year periodicity (the Gleissberg cycle), confirming previous findings (e.g. Siscoe,
    1980).
    The primary aim of this article was to study the long-term auroral variability
    gathering together different sources of auroral information. In the following we
    will calculate some correlations between sunspot numbers and different auroral
    parameters. In order to enhance this long-term variability an 11-year smoothing
    window will be applied to the data sets. Clearly, this procedure will increase
    the strength of the correlation, because we are entering a frequency common to
    the two sets, and will decrease the degrees of freedom of the series (number of
    independent points). To evaluate the statistical significance of the correlation we
    have applied the t-student test, calculating the significance given by the p-value.
    To take into account the effect of the smoothing in the calculation of the t- and
    p-parameters, we have reduced the number of independendent points by a factor
    of 11, the length of our smoothing window. We have verified that the effect of the
    smoothing on the correlations is very small when the length of the smoothing
    window is smaller than the length of the data set. For our sample this criteria is
    fullfilled when the number of available data is larger than ≈ 140 years. Finally,
    we can point out that the correction to the number of statistically independent
    points resulting from autocorrelation is negligible.
  • Latitudinal Variation
    3.1. Solar Source of the Aurorae
    One can divide the effects of solar activity on our planet via the solar wind as
    being due to two agents, depending on the topology of the magnetic field of the
    solar atmosphere (see Legrand and Simon, 1981).
    i) Closed Regions: The magnetic flux of the active regions is characterized
    by magnetic configurations with closed field lines dominating the variations in
    the total irradiance and emission in the high energy range of the solar spectrum
    (ultraviolet and X-rays). For historical reasons, the sunspot number is usually
    employed as a measure of this type of activity. Coronal mass ejections (CMEs)
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    Vazquez, Vaquero, and Gallego
    are transient phenomena linked to large-scale reorganizations of the magnetic
    field. They consist of huge emissions of solar particles that in some cases impact
    the Earth’s environment. The CME rate closely follows the solar cycle (Webb
    and Howard, 1994; Robbrecht, Berghmans, and Van der Linden, 2009) and the
    Gnevyshev gap (Gnevyshev, 1967), but the main physical parameters of CMEs
    lag the sunspot number by 19 months, at least for solar cycle 23 (Du, 2012).
    ii) Open regions: Large-scale magnetic regions have field lines open towards
    the interplanetary medium. They are the main source of a continuous outward
    flow of charged particles (protons, electrons, and helium nuclei) known as the
    solar wind. Long-lived coronal holes are the sources of high-speed solar wind,
    and are related to recurrent geomagnetic activity. They occur more frequently
    in the declining phase of the sunspot cycle (Verbanac et al., 2011).
    The solar magnetic field of the open regions, the open magnetic field (OMF),
    is frozen into this wind, thereby configuring the interplanetary magnetic field
    (IMF) which produces the heliosphere that fills practically the whole Solar
    System. Lockwood et al. (1999) and Lockwood (2003) derived the intensity of
    the interplanetary magnetic field from the aa geomagnetic intensity index, a
    record that extends back to 1868. They found that the average strength of the
    solar magnetic field has doubled in the last 100 years. Usoskin et al. (2002)
    carried out simulations of the variations of the OMF with a model based on
    the emergence and decay rates of active regions (Solanki, Schussler, and Fligge,
    2002). That model fits reasonably well other proxies of solar activity, such as
    the 10Be records in ice cores. Since solar cycle 14 (1902 – 1913), the geomagnetic
    activity lagged behind the sunspot number, but before that date the lag seems
    to have been less notable (Love, 2011).
    Siscoe (1980) discriminated between aurorae that are visible North and South
    of 54 degrees using Scandinavian records, noting that the southern data tracked
    the 11-year solar cycle more clearly. Bravo and Otaola (1990) studied the location
    of solar coronal holes and their influence on the auroral records. They found
    that the number of aurorae is positively correlated with polar coronal holes that
    reach solar latitudes below 60 degrees. Verbanac et al. (2011) found that highspeed streams originating in equatorial coronal holes are the main driver of solar
    activity in the declining phase.
    In order to differentiate between the distinct solar sources, we plotted the
    cumulative values, NLatj
    , of the number of aurorae visible for each 1.5 degree
    wide latitude band (see the definition below).
    NLatj = Σi=j
    0 NLati (1)
    The growth found is almost exponential. We took the resulting curve to consist
    of three segments, to each of which we made straight-line fits. The intersection of
    the three segments could represent the boundaries between different solar sources
    of the auroral event. The two plots of Figure 5 show the results for Europe and
    North America, respectively.
    The low-latitude segment would represent the aurorae produced by strong
    solar storms, with the northern limit at 49 degrees in Europe and 39 degrees in
    North America defining the so-called low-latitude aurorae. The middle segment
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    Aurora Borealis
    shows auroral events produced by CMEs of medium strength. In the upper
    latitude segment, the predominant role is played by the fast streams from coronal
    holes. In this case, the variation of the geographic latitude plays only a minor
    role. The limits are approximately 66 degrees for Europe and 44 degrees for
    North America.
    3.2. Width of the Auroral Oval
    Viewed from space, aurora are diffuse ovals of light around the geomagnetic poles.
    They can be regarded as the regions of the upper atmosphere with permanent
    luminescence (Feldstein, 1986). All of the great auroral expansions have been
    associated with intense values of the interplanetary magnetic field (Sheeley and
    Howard, 1980). The variation in the radius of the auroral oval in response to
    solar wind changes has been studied by Milan et al. (2009).
    Assuming a spherical Earth with a radius R = 6371 km, one calculates the
    area (A) of a longitude–latitude rectangle (area between two lines of latitude
    lat1 and lat2) (Moritz, 1980), as
    A = 2πR
    2
    (sin lat1 − sin lat2). (2)
    Figure 6 shows the variation of the annual maximum of the oval extension
    with the annual sunspot number. One must bear in mind that the auroral oval
    is really centred on the geomagnetic pole, so that the above expression is just
    a first-rough approximation. For the European data (sample 1720 – 1905) the
    level of correlation is not very strong but significant (r=0.58, p-value < 0.02).
    However, for the North American data (sample 1836 – 1905) the results are much
    noisier, probably reflecting the shortness of the sample with adequate data and
    the lack of observations of auroral events at extreme latitudes for many years.
    3.3. Low-latitude Aurorae
    A solar storm is accompanied by an expansion of the auroral oval towards the
    Equator on the night side of the Earth. Figure 7 shows the temporal variation of
    the yearly minimum latitude and its correlation with the sunspot number. For
    the North American sample, we restricted our observing period to the data after
    1803 due to the low latitudinal and temporal coverage at earlier dates. When
    the data were smoothed with an 11-year window, the correlations for both data
    samples were good (r = -0.74 and -0.71 for the European and the American
    data, reflecting the 80-year period of variability of solar activity. However, the
    level of significance was much better for the European (p-value < 0.001) than
    for the North American data (p-value < 0.05), due to the longer data set.
    The lowest latitudes were 24.09 degrees for Europe, 24.08 degrees for North
    America, and 18.97 degrees for Asia. All of these values correspond to the aurora
    of 4 February 1872 during Solar Cycle 11 (see Silverman, 2008).
    In this context, we should remark that low-latitude aurorae have also been
    observed during periods of weak to moderate geomagnetic activity. Silverman
    (2003) has shown from US auroral data from 1880 to 1940 that some of the
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    Vazquez, Vaquero, and Gallego

    auroral phenomena occurred under conditions of quiet or moderate magnetic
    activity and at low latitudes. He used the term “sporadic aurora” for this type
    of auroral phenomenon. Vaquero, Trigo and Gallego (2007) studied one such
    event observed in 1845 in the Iberian Peninsula. Willis, Stephenson, and Fang
    (2007) compiled 42 Chinese and Japanese auroral observations during the period
    1840 – 1911 and found that at least 29 of the 42 observations (i.e. 69 %) occurred
    at times of weak to moderate geomagnetic activity.
    3.4. High-latitude Aurorae
    The Greenland observations confirmed that, at high latitudes, the aurorae maximum coincides with the sunspot cycle minimum (Anonymous, 1883). In order
    to check the reliability of our method in discriminating between different solar
    sources, we looked for a critical latitude at which this anti-correlation is largest.
    For this purpose, we normalized the data above a critical latitude to the total
    number of aurora observations, and plotted the resulting temporal variation.
    The best correlation was obtained at 61.4 degrees geographic latitude for
    Europe (r = -0.70; p < 0.005) and 44.8 degrees for North America (r = -0.84; p
    < 0.01), both in phase with the sunspot number (Figure 8).The Geomagnetic Latitude
    Since the frequency of the aurorae is related to distance from the magnetic pole,
    it is more appropriate to plot the observations versus magnetic latitude.
    We computed the temporal evolution of the geomagnetic latitude for every
    observing location during the entire 1700 – 1905 period. It is common to define
    within the main geomagnetic field the geomagnetic latitude φ according to the
    expression tan φ = (tan I)/2 (see Stacey, 1992; Buforn, Pro, and Ud´ıas, 2012).
    We obtained the magnetic inclination (I) from the global geomagnetic model
    gufm1 (Jackson, Jonkers, and Walker, 2000), which can be applied in the period
    covered by this study. Therefore, the geomagnetic latitude has been calculated
    for each auroral record from the value of the magnetic inclination.
    We applied this procedure to all of the observing sites. Figure 9 shows the
    results for Europe, North America, and Asia. One observes the Dalton Minimum between the two high-activity episodes. However, the few more southern
    magnetic latitudes during the 18th century are mainly covered by Asian data.
    Two episodes with high-latitude aurorae occur in 1820 and 1840 close to the
    minimum phase of the corresponding solar cycles. An unusual aurora was visible
    close to the magnetic North Pole (Fort Conger) on 17 November 1882. These
    observations was made by the expedition commanded by A.W. Greeley from
    July 1882 to August 1883, in the framework of the activities of the First Polar
    Year (Taylor, 1981). Recently, Singh et al. (2012) have studied the characteristics
    of high-latitude storms above the classical auroral oval.
    Figure 10 shows the histograms of the geomagnetic latitudes for the three
    continental masses. The major contribution of the otherwise sparse Asian data
    to the low geomagnetic latitudes stands out. As with the geographic latitudes,
    SOLA: VVG2013spvdef1.tex; 9 September 2013; 0:24; p. 10
    Aurora Borealis
    the geomagnetic latitudes also have their absolute minima for the auroral event
    of 1872.
    Figure 11 shows the temporal variation of the yearly minimum geomagnetic
    latitude with the annual averaged sunspot number. The anti-correlation is similar
    for Europe and North America (-0.36 vs –0.32), but the level of significance is
    very low, especially for the American data. We must notice that the minimum
    geomagnetic latitude shows a decrease during the first half of the 19th century,
    most clearly visible for the American data.
    For the yearly number of aurorae above a critical magnetic latitude (Figures 12
    and 13), the best anti-correlation is found for a magnetic latitude of around 61.4
    and 63.4 for the European and American samples, respectively (see the insets in
    the figures for the variation of the correlation coefficients with the geomagnetic
    latitude). As expected from previous results, the level of significance for this
    correlation was better for the Europe (p < 0.005) than for the North America
    sample (p < 0.05).
    We repeated the calculation of the cumulative number of auroral events, but
    now for the geomagnetic latitudes. The results (Figure 14) show the behaviour
    to be similar for the two continents, although there is a lack of high geomagnetic
    latitudes for the North American sample (Canada and Alaska). Liritzis and
    Petropoulos (1987) already noted a marked change at a geomagnetic latitude of
    57 – 58 degrees.

    Conclusions
    We have studied the broad statistical properties (spatial and temporal variation)
    of several catalogues and reports of auroral observations during the 18th and
    19th centuries. To this end, we assigned geographic and geomagnetic latitudes to
    more than 80 000 auroral observations. For the analysis, we divided the Northern
    Hemisphere into three large continental masses corresponding to Europe, North
    America, and Asia.
    The sample presented numerous spatial and temporal gaps, mainly evident in
    the North American data due to the progressive European occupation of large
    areas outside the original New England states. It was clear that further investigation into low-latitude auroral events is important. The historical auroral records
    investigated can serve as a proxy for reconstructing the long-term variability of
    the heliosphere.
    To gain insight into the broad pattern of the spatial and temporal variations of auroral observations in the Northern Hemisphere, an 11-year smoothing
    procedure was applied to the data to enhance this long-term component.
    We suggested and applied a method to separate the contribution of the different solar sources of auroral events, which was based on the different slopes
    of the cumulative numbers of auroral events per latitude bin. The low-latitude
    segment mainly corresponded to strong CMEs taking place around the solar
    cycle maxima. The middle segment corresponded to standard CMEs, and the
    high-latitude segment to high-speed streams of solar wind originating from coronal holes. The limit between the last two sources is in a narrow belt between
    SOLA: VVG2013spvdef1.tex; 9 September 2013; 0:24; p. 11
    V´azquez, Vaquero, and Gallego
    61 and 64 degrees of geomagnetic latitude. One must bear in mind that the
    geoeffectiveness of such transitory solar events depends critically on the level of
    the IMF and its orientation with respect to the Earth’s magnetosphere.
    The spatial distribution of the available data is clearly determined by the
    populations living at high-latitude sites which were clearly greater in Europe
    than in North America. The same may apply to the low-latitude sites in the two
    continents. This masks the expected better correlation of the auroral parameters
    with the geomagnetic than with the geographic latitude.
    The existence of several minima was clearly seen. In addition to the Silverman
    and Dalton episodes, the discontinuity at the beginning of the 20th century merits
    special attention. This topic will be investigated in detail in a forthcoming article.
    Acknowledgements The authors wish to express their gratitude to Sam Silverman who has
    undertaken the immense task of collecting thousands of auroral reports around the world and
    making them available to the scientific community. Some well-known auroral catalogues (Fritz,
    Angot) used for this study are from Jack Eddy’s Compilation of Auroral Catalogues and were
    obtained from the Research Data Archive (RDA), which is maintained by the Computational
    and Information Systems Laboratory (CISL) at the National Center for Atmospheric Research
    (NCAR). NCAR is sponsored by the National Science Foundation (NSF). The original data are
    available from the RDA (dss.ucar.edu) with dataset number ds836.0. Support from the Junta
    de Extremadura (Research Group Grant No. GR10131) and the Ministerio de Econom´ıa y
    Competitividad of the Spanish Government (AYA2011-25945) is also gratefully acknowledged.
    Finally, we thank an anonymous referee for useful suggestions.
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