Auroras, sometimes called the
northern and southern (polar) lights or
aurorae (
singular: aurora), are natural light displays in the
skyThe sky is the part of the atmosphere or of outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a deep blue surface because of the air's scattering of sunlight. The sky is...
, usually observed at
nightNight sky is a commonly used term most often employed to refer to the sky as it is seen at night. The term is usually associated with astronomy, with reference to views of heavenly bodies such as stars, the Moon and planets that become visible on a clear night after the Sun has set.The night sky...
, particularly in the
polar regionsA geographical pole is either of the two points—the north pole and the south pole—on the surface of a rotating planet where the axis of rotation meets the surface of the body...
. They typically occur in the
ionosphereThe ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
. They are also referred to as
polar auroras. In northern
latitudeLatitude, usually denoted by the Greek letter phi gives the location of a place on Earth north or south of the equator. Lines of Latitude are the imaginary horizontal lines shown running east-to-west on maps that run either north or south of the equator...
s, the effect is known as the
aurora borealis, named after the
RomanAncient Rome was a civilization that grew out of a small agricultural community founded on the Italian Peninsula as early as the 10th century BC. Located along the Mediterranean Sea, it became one of the largest empires in the ancient world....
goddessRoman mythology, or Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its main city, Ancient Rome. It can be considered as having two parts; One part, largely later and literary, consists of borrowings from Greek mythology...
of dawn,
AuroraAurora is the Latin word for dawn
, the goddess of dawn in Roman mythology and Latin poetry. Aurora is comparable to the Greek goddess Eos, though Aurora did not bring with her any resonance of a greater archaic goddess.-Roman mythology:...
, and the
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
name for north wind,
BoreasIn Greek mythology, the Anemoi were wind gods who were each ascribed a cardinal direction, from which their respective winds came, and were each associated with various seasons and weather conditions...
, by
Pierre GassendiPierre Gassendi was a French philosopher, priest, scientist, astronomer, and mathematician. With a church position in south-east France, he also spent much time in Paris, where he was a leader of a group of free-thinking intellectuals. He was also an active observational scientist, publishing the...
in 1621. The aurora borealis is also called the
northern polar lights, as it is only visible in the sky from the
Northern HemisphereThe Northern Hemisphere is the half of a planet that is north of the equator—the word hemisphere literally means 'half sphere'. It is also that half of the celestial sphere north of the celestial equator...
, the chance of visibility increasing with proximity to the
North Magnetic PoleThe Earth's North Magnetic Pole is the point on the Earth's surface at which the Earth's magnetic field points vertically downwards . This point moves gradually with time. As described later in this article, the North Magnetic Pole is physically a magnetic field south pole...
, which is currently in the arctic islands of northern
CanadaCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
. Auroras seen near the magnetic pole may be high overhead, but from further away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the sun were rising from an unusual direction. The aurora borealis most often occurs near the equinoxes; from September to October and from March to April. The northern lights have had a number of names throughout history. The
CreeCree is one of the largest group of First Nations/Aboriginals in North America, located mainly across Canada and historically in the United States from Minnesota westward but are found today in Montana....
people call this phenomenon the "
Dance of the Spirits." In the middle age the auroras has been called by sign of God (see Wilfried Schröder, Das Phänomen des Polarlichts, Darmstadt 1984).
Auroras can be spotted throughout the world. It is most visible closer to the poles due to the longer periods of darkness and the magnetic field.
Its southern counterpart, the
aurora australis or the
southern polar lights, has similar properties, but is only visible from high southern latitudes in
Antarctica| style="border-top:solid 1px #ccd2d9; padding:0.4em 1em 0.4em 0; vertical-align:top;" | 14,000,000 km
2 280,000 km
2 13,720,000 km
2 |-! style="border-top: solid 1px #ccd2d9; padding: 0.4em 1em 0.4em 0; vertical-align: top;...
,
South AmericaSouth America is the southern continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere...
, or
AustralasiaAustralasia is a region of Oceania: Australia, New Zealand, the island of New Guinea, and neighbouring islands in the Pacific Ocean. The term was coined by Charles de Brosses in Histoire des navigations aux terres australes . He derived it from the Latin for "south of Asia" and differentiated the...
.
Australis is the
LatinLatin is an Italic language originally spoken in Latium and Ancient Rome. Through the Roman conquest, Latin spread throughout the Mediterranean and a large part of Europe...
word for "of the South."
Auroral mechanism
Auroras are the result of the emissions of photons in the Earth's upper
atmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
, above 80 km (50 miles), from ionized
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
atoms regaining an electron, and
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
and
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
atoms returning from an
excited stateExcitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
to ground state. They are ionized or
excitedExcitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
by the collision of
solar windThe solar wind is a stream of charged particles ejected from the upper atmosphere of the sun. It consists mostly of electrons and protons with energies of about 1 keV. The stream of particles varies in temperature and speed with the passage of time...
particles being funnelled down, and accelerated along, the Earth's magnetic field lines; excitation energy is lost by the emission of a photon of light, or by collision with another atom or molecule
https://weboodi.oulu.fi/oodi/opintjakstied.jsp?OpinKohd=112250&MD5avain=&OnkoIlmKelp=0&takaisin=ilmsuor.jsp&OnkoIlmKelp=1&takaisin=ilmsuor.jsp&haettuOrg=-1&sortJarj=2&haettuOpas=-1&haettuOppAin=-1&haettuLk=-1&haettuOpetKiel=-1&NimiTunniste=761011Y&AlkPvm=&PaatPvm=&Selite=&Sivu=0&haeOpintJaks=haeopintojaksot&haeVainIlmKelp=0&haeMyosAlemOrg=1&eHOPSopinkohtlaj=&eHOPSpaluusivu=&eHOPSilmsuor=1&Kieli=6.
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
emissions: Green or brownish-red, depending on the amount of energy absorbed.
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
emissions: Blue or red. Blue if the atom regains an electron after it has been ionized. Red if returning to ground state from an
excited stateExcitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
.
Oxygen is a little unusual in terms of its return to ground state, it can take three quarters of a second to emit green light, and up to two minutes to emit red. Collisions with other atoms or molecules will absorb the excitation energy and prevent emission. The very top of the atmosphere is both a higher percentage of oxygen, and so thin that such collisions are rare enough to allow time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions don't have time to happen, and eventually even green light emissions are prevented.
This is why there is a colour differential with altitude, high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything.
Auroras are mostly only visible when a
coronal mass ejectionA coronal mass ejection is an ejection of material from the solar corona, usually observed with a white-light coronagraph.The ejected material is a plasma consisting primarily of electrons and protons , plus the entraining coronal magnetic field.-Impact of a CME:When the ejection reaches the...
, or similar events, fires
plasmaPlasma may refer to:* Blood plasma, the yellow-colored liquid component of blood, in which blood cells are suspended* Plasma , an ionized gas, the fourth state of matter...
, and also
magnetic fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
, from the surface of the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
toward the Earth
http://www.nasa.gov/worldbook/aurora_worldbook.html. The relatively high density of material means a higher intensity of Aurora, and the snapping of some field lines of the Earth's own magnetic field, and their subsequent reconnect, funnels and accelerates the charged particles
https://weboodi.oulu.fi/oodi/opintjakstied.jsp?OpinKohd=112250&MD5avain=&OnkoIlmKelp=0&takaisin=ilmsuor.jsp&OnkoIlmKelp=1&takaisin=ilmsuor.jsp&haettuOrg=-1&sortJarj=2&haettuOpas=-1&haettuOppAin=-1&haettuLk=-1&haettuOpetKiel=-1&NimiTunniste=761011Y&AlkPvm=&PaatPvm=&Selite=&Sivu=0&haeOpintJaks=haeopintojaksot&haeVainIlmKelp=0&haeMyosAlemOrg=1&eHOPSopinkohtlaj=&eHOPSpaluusivu=&eHOPSilmsuor=1&Kieli=6 down in a large circle around the Earth's poles. Seen from space, these fiery curtains form a thin ring in the shape of a
monks tonsureTonsure is the practice of some Christian churches, mystics, Buddhist novices and monks, and some Hindu temples of cutting the hair from the scalp of clerics, devotees, or holy people as a symbol of their renunciation of worldly fashion and esteem.-History:...
, or man's bald spot.
Forms and magnetism
Typically the aurora appears either as a diffuse glow or as "curtains" that approximately extend in the east-west direction. At some times, they form "quiet arcs"; at others ("active aurora"), they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that aurora is shaped by Earth's magnetic field. Indeed, satellites show electrons to be guided by magnetic field lines, spiraling around them while moving towards Earth.
The similarity to curtains is often enhanced by folds called "striations". When the field line guiding a bright auroral patch leads to a point directly above the observer, the aurora may appear as a "corona" of diverging rays, an effect of
perspectivePerspective, in context of vision and visual perception, is the way in which objects appear to the eye based on their spatial attributes; or their dimensions and the position of the eye relative to the objects...
.
Although it was first mentioned by
Ancient GreekAncient Greek is the historical stage in the development of the Greek language spanning across the Archaic , Classical , and Hellenistic periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek...
explorer/
geographerA geographer is a scientist whose area of study is geography, the study of Earth's physical environment and human habitat.Though geographers are historically known as people who make maps, map making is actually the field of study of cartography, a subset of geography...
PytheasPytheas of Massilia , 4th century BC, was a Greek geographer and explorer from the Greek colony, Massilia . He made a voyage of exploration to northwestern Europe at about 325 BC. He travelled around and visited a considerable part of Great Britain. Some of his observations may refer to...
, Hiorter and
CelsiusAnders Celsius was a Swedish astronomer. He was professor of astronomy at Uppsala University from 1730 to 1744, but traveled from 1732 to 1735 visiting notable observatories in Germany, Italy and France. He found the Uppsala Astronomical Observatory in 1741, and in 1742 he proposed the Celsius...
first described in 1741 evidence for magnetic control, namely, large magnetic fluctuations occurred whenever the aurora was observed overhead. This indicates (it was later realized) that large
electric currentElectric current can mean, depending on the context, a flow of electric charge or the rate of flow of electric charge ....
s were associated with the aurora, flowing in the region where auroral light originated.
Kristian BirkelandKristian Olaf Birkeland was a Norwegian scientist. He is best remembered as the person who first elucidated the nature of the Aurora borealis. In order to fund his research on the aurorae, he invented the electromagnetic cannon and the Birkeland-Eyde process of fixing nitrogen from the air...
(1908) deduced that the currents flowed in the east-west directions along the auroral arc, and such currents, flowing from the dayside towards (approximately) midnight were later named "auroral electrojets" (see also
Birkeland currentA Birkeland current is a specific magnetic field aligned current in the Earth’s magnetosphere which flows from the magnetotail towards the Earth on the dawn side and in the other direction on the dusk side of the magnetosphere. Lately, the term Birkeland currents has been expanded by some authors...
s).
On 26 February 2008,
THEMISThemis is an ancient Greek goddess. She is described as "of good counsel", and is the embodiment of divine order, law, and custom. Themis means "law of nature" rather than human ordinance, literally "that which is put in place", from the verb τίθημι, títhēmi, "to put"...
probes were able to determine, for the first time, the triggering event for the onset of magnetospheric substorms . Two of the five probes, positioned approximately one third the distance to the moon, measured events suggesting a magnetic reconnection event 96 seconds prior to auroral intensification . Dr. Vassilis Angelopoulos of the
University of California, Los AngelesThe University of California, Los Angeles is a research university located in the Westwood neighborhood of Los Angeles, California, in the United States. It was founded in 1919 and is the second-oldest general-purpose campus in the University of California system...
, the principal investigator for the THEMIS mission, claimed, "Our data show clearly and for the first time that magnetic reconnection is the trigger." .
Still more evidence for a magnetic connection are the statistics of auroral observations.
Elias LoomisElias Loomis was an American mathematician.- Life and work :Loomis was born in Willington, Connecticut in 1811. He graduated at Yale College in 1830, was a tutor there for three years, 1833-36 and then spent the next year in scientific investigation in Paris...
(1860) and later in more detail Hermann Fritz (1881) established that the aurora appeared mainly in the "auroral zone", a ring-shaped region with a radius of approximately 2500 km around Earth's magnetic pole. It was hardly ever seen near the geographic pole, which is about 2000 km away from the magnetic pole. The instantaneous distribution of auroras ("auroral oval", Yasha/Jakob Feldstein 1963) is slightly different, centered about 3-5 degrees nightward of the magnetic pole, so that auroral arcs reach furthest towards the equator around midnight. The aurora can be seen best at this time.
Solar wind and the magnetosphere
The Earth is constantly immersed in the
solar windThe solar wind is a stream of charged particles ejected from the upper atmosphere of the sun. It consists mostly of electrons and protons with energies of about 1 keV. The stream of particles varies in temperature and speed with the passage of time...
, a rarefied flow of hot plasma (gas of free electrons and positive ions) emitted by the Sun in all directions, a result of the million-degree heat of the Sun's outermost layer, the
coronaA corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometres into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
. The solar wind usually reaches Earth with a velocity around 400 km/s, density around 5 ions/cm
3 and magnetic field intensity around 2–5 nT (
nanoteslasThe tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honor of the Serbian-American inventor, physicist, and electrical engineer Nikola Tesla...
; Earth's surface field is typically 30,000–50,000 nT). These are typical values. During
magnetic stormsA geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in space weather. Associated with solar coronal mass ejections , coronal holes, or solar flares, a geomagnetic storm is caused by a solar wind shock wave which typically strikes the Earth's magnetic...
, in particular, flows can be several times faster; the
interplanetary magnetic fieldThe Interplanetary Magnetic Field is the term for the Sun’s magnetic field carried by the solar wind among the planets of the Solar System....
(IMF) may also be much stronger.
The IMF originates on the Sun, related to the field of
sunspotA sunspot is an area on the Sun's surface that is marked by intense magnetic activity, which inhibits convection, forming areas of reduced surface temperature. They can be visible from Earth without the aid of a telescope...
s, and its
field lines (lines of force)In physics, the term magnetism is used to describe how materials respond on the microscopic level to an applied magnetic field; to categorize the magnetic phase of a material. For example, the most well known form of magnetism is ferromagnetism such that some ferromagnetic materials produce their...
are dragged out by the solar wind. That alone would tend to line them up in the Sun-Earth direction, but the rotation of the Sun skews them (at Earth) by about 45 degrees, so that field lines passing Earth may actually start near the western edge ("limb") of the visible sun.
Earth's
magnetosphereA magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
is the space region dominated by its magnetic field. It forms an obstacle in the path of the solar wind, causing it to be diverted around it, at a distance of about 70,000 km (before it reaches that boundary, typically 12,000–15,000 km upstream, a
bow shockA bow shock is a boundary between a magnetosphere and an ambient medium. For stars, this is typically the boundary between their stellar wind and the interstellar medium....
forms). The width of the magnetospheric obstacle, abreast of Earth, is typically 190,000 km, and on the night side a long "magnetotail" of stretched field lines extends to great distances.
When the solar wind is perturbed, it easily transfers energy and material into the magnetosphere. The electrons and ions in the magnetosphere that are thus energized move along the magnetic field lines to the polar regions of the atmosphere.
Frequency of occurrence
The aurora is a common occurrence in the Poles. It is occasionally seen in temperate latitudes, when a strong magnetic storm temporarily expands the auroral oval. Large magnetic storms are most common during the peak of the eleven-year
sunspot cycleA sunspot is an area on the Sun's surface that is marked by intense magnetic activity, which inhibits convection, forming areas of reduced surface temperature. They can be visible from Earth without the aid of a telescope...
or during the three years after that peak. However, within the auroral zone the likelihood of an aurora occurring depends mostly on the slant of IMF lines (the slant is known as B
z), being greater with southward slants.
Geomagnetic stormA geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in space weather. Associated with solar coronal mass ejections , coronal holes, or solar flares, a geomagnetic storm is caused by a solar wind shock wave which typically strikes the Earth's magnetic...
s that ignite auroras actually happen more often during the months around the
equinoxAn equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the Sun being vertically above a point on the Equator...
es. It is not well understood why geomagnetic storms are tied to Earth's seasons while polar activity is not. But it is known that during spring and autumn, the interplanetary magnetic field and that of Earth link up. At the
magnetopauseThe magnetopause is the abrupt boundary between a magnetic field, and surrounding plasma. The magnetopause ripples, flaps, and moves inward and outward in response to varying solar wind conditions....
, Earth's magnetic field points north. When B
z becomes large and negative (i.e., the IMF tilts south), it can partially cancel Earth's magnetic field at the point of contact. South-pointing B
z's open a door through which energy from the solar wind can reach Earth's inner magnetosphere.
The peaking of B
z during this time is a result of geometry. The interplanetary magnetic field (IMF) comes from the Sun and is carried outward with the solar wind. Because the Sun rotates the IMF has a
spiral shapeThe Parker spiral is the shape of the Sun's magnetic field as it extends through the solar system. Unlike the familiar shape of the field from a bar magnet, the Sun's extended field is twisted into an arithmetic spiral by the magnetohydrodynamic influence of the solar wind...
. Earth's magnetic dipole axis is most closely aligned with the Parker spiral in April and October. As a result, southward (and northward) excursions of B
z are greatest then.
However, B
z is not the only influence on geomagnetic activity. The Sun's rotation axis is tilted 8 degrees with respect to the plane of Earth's orbit. Because the solar wind blows more rapidly from the Sun's poles than from its equator, the average speed of particles buffeting Earth's magnetosphere waxes and wanes every six months. The solar wind speed is greatest — by about 50 km/s, on average — around 5 September and 5 March when Earth lies at its highest heliographic latitude.
Still, neither B
z nor the solar wind can fully explain the seasonal behavior of geomagnetic storms. Those factors together contribute only about one-third of the observed semiannual variations.
Auroral events of historical significance
The auroras which occurred as a result of the "
great geomagnetic stormThe solar storm of 1859, also known as the Solar Superstorm, or the Carrington Event, was the most powerful solar storm in recorded history.-Sunspots:...
" on both August 28 and September 2, 1859 are thought to be perhaps the most spectacular ever witnessed throughout recent recorded history.
Balfour StewartBalfour Stewart was a Scottish physicist.Stewart was born in Edinburgh, and was educated at Dundee, the University of St Andrews, and the University of Edinburgh. The son of a tea merchant, he was for some time engaged in business in Leith and in Australia, but, returning to his studies of physics...
, in a paper to the
Royal SocietyThe Royal Society of London for the Improvement of Natural Knowledge, known simply as the Royal Society, or even the Royal, is a learned society for science that was founded in 1660 and is considered by most to be the oldest such society still in existence...
on November 21, 1861, described both auroral events as documented by a self-recording
magnetographA magnetograph is one of two types of scientific instrument:* A special type of magnetometer that records a time plot of the local magnetic field near the instrument; or...
at the
Kew ObservatoryKew Observatory was an astronomical and terrestrial magnetic observatory located in Kew, London, England.In the past, the Kew Observatory participated in assessing and rating Swiss timepiece movements for accuracy. As marine navigation adopted the usage of mechanical timepieces for navigational...
and established the connection between the September 2, 1859 auroral storm and the Carrington-Hodgson flare event when he observed that “it is not impossible to suppose that in this case our luminary was taken
in the act.” The second auroral event, which occurred on September 2, 1859 as a result of the exceptionally intense Carrington-Hodgson white light
solar flareA solar flare is a large explosion in the Sun's atmosphere that can release as much as 6 × 10
25 joules of energy. The term is also used to refer to similar phenomena in other stars, where the more accurate term stellar flare applies....
on September 1, 1859 produced aurora so widespread and extraordinarily brilliant that they were seen and reported in published scientific measurements, ship's logs and newspapers throughout the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
, Europe,
Japanis an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, People's Republic of China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
and
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
. It was reported by the
New York Times
that in
BostonBoston is the capital and largest city of the Commonwealth of Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is considered the economic and cultural center of the region and is sometimes regarded as the unofficial "Capital of New England"...
on Friday September 2, 1859 the Aurora was "so brilliant that at about one o'clock ordinary print could be read by the
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
". One o’clock Boston time on Friday September 2, would have been 6:00 GMT and the self-recording
magnetographA magnetograph is one of two types of scientific instrument:* A special type of magnetometer that records a time plot of the local magnetic field near the instrument; or...
at the
Kew ObservatoryKew Observatory was an astronomical and terrestrial magnetic observatory located in Kew, London, England.In the past, the Kew Observatory participated in assessing and rating Swiss timepiece movements for accuracy. As marine navigation adopted the usage of mechanical timepieces for navigational...
was recording the
geomagnetic stormA geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in space weather. Associated with solar coronal mass ejections , coronal holes, or solar flares, a geomagnetic storm is caused by a solar wind shock wave which typically strikes the Earth's magnetic...
, which was then one hour old, at its full intensity; this is amazingly accurate news reporting. Between 1859 and 1862
Elias LoomisElias Loomis was an American mathematician.- Life and work :Loomis was born in Willington, Connecticut in 1811. He graduated at Yale College in 1830, was a tutor there for three years, 1833-36 and then spent the next year in scientific investigation in Paris...
published a series of nine papers on the Great Auroral Exhibition of 1859 in the
American Journal of ScienceThe American Journal of Science is America's longest-running scientific journal, having been published continuously since its conception in 1818 by Professor Benjamin Silliman, who edited and financed it himself...
where he collected world wide reports of the auroral event. The aurora is thought to have been produced by one of the most intense
coronal mass ejectionA coronal mass ejection is an ejection of material from the solar corona, usually observed with a white-light coronagraph.The ejected material is a plasma consisting primarily of electrons and protons , plus the entraining coronal magnetic field.-Impact of a CME:When the ejection reaches the...
s in history, very near the maximum intensity that the Sun is thought to be capable of producing. It is also notable for the fact that it is the first time where the phenomena of auroral activity and electricity were unambiguously linked. This insight was made possible not only due to scientific
magnetometerA magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument. Magnetism varies from place to place and differences in Earth's magnetic field can be caused by the differing nature of rocks and the interaction...
measurements of the era but also as a result of a significant portion of the of telegraph lines then in service being significantly disrupted for many hours throughout the storm. Some telegraph lines however, seem to have been of the appropriate length and orientation which allowed a current (
geomagnetically induced currentGeomagnetically induced currents , affecting the normal operation of long technological conductor systems, are a manifestation at ground level of space weather. During space weather events electric current systems in the magnetosphere and ionosphere experience large spatiotemporal variations...
) to be induced in them (due to Earth's severely fluctuating
magnetosphereA magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
) and actually used for communication. The following conversation occurred between two operators of the American Telegraph Line between
BostonBoston is the capital and largest city of the Commonwealth of Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is considered the economic and cultural center of the region and is sometimes regarded as the unofficial "Capital of New England"...
and
Portland, MainePortland is the largest city in the U.S. state of Maine and the county seat of Cumberland County. The 2007 estimated city population was 62,875. Portland is Maine's cultural, social and economic capital. It is also the principal city of the Portland-South Portland-Biddeford metropolitan area, with...
, on the night of September 2, 1859 and reported in the
Boston Traveler:
The conversation was carried on for around two hours using no battery power at all and working solely with the current induced by the aurora, and it was said that this was the first time on record that more than a word or two was transmitted in such manner. Such events led to the general conclusion that
Origin
The ultimate energy source of the aurora is the solar wind flowing past the Earth. The magnetosphere and solar wind consist of
plasmaIn physics and chemistry, plasma is a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it...
(ionized gas), which conducts electricity. It is well known (since
Michael FaradayMichael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....
's [1791 - 1867] work around 1830) that when an electrical conductor is placed within a magnetic field while relative motion occurs in a direction that the conductor cuts
across (or is cut
by), rather than
along, the lines of the magnetic field, an electrical current is said to be induced into that conductor and electrons will flow within it. The amount of current flow is dependent upon a) the rate of relative motion and b) the strength of the magnetic field, c) the number of conductors ganged together and d) the distance between the conductor and the magnetic field, while the
direction of flow is dependent upon the direction of relative motion.
Dynamo-In Engineering:* Dynamo, a magnetic device originally used as an electric generator* Dynamo theory, a theory relating to magnetic fields of celestial bodies* Solar dynamo, the physical process that generates the Sun's magnetic field-Europe:...
s make use of this basic process ("the
dynamo effectThe dynamo theory proposes a mechanism by which a celestial body such as the Earth generates a magnetic field.-History of theory:In 1905, shortly after composing his special relativity paper, Albert Einstein described the origin of the Earth's magnetic field as being one of the great unsolved...
"), any and all conductors, solid or otherwise are so affected including plasmas or other fluids.
In particular the solar wind and the magnetosphere are two electrically conducting fluids with such relative motion and should be able (in principle) to generate electric currents by "dynamo action", in the process also extracting energy from the flow of the solar wind. The process is hampered by the fact that plasmas conduct easily along magnetic field lines, but not so easily perpendicular to them. So it is important that a temporary magnetic connection be established between the field lines of the solar wind and those of the magnetosphere, by a process known as
magnetic reconnectionMagnetic reconnection is the process whereby magnetic field lines from different magnetic domains are spliced to one another, changing their patterns of connectivity with respect to the sources. It is a violation of an approximate conservation law in plasma physics, and can concentrate mechanical...
. It happens most easily with a southward slant of interplanetary field lines, because then field lines north of Earth approximately match the direction of field lines near the
north magnetic poleThe Earth's North Magnetic Pole is the point on the Earth's surface at which the Earth's magnetic field points vertically downwards . This point moves gradually with time. As described later in this article, the North Magnetic Pole is physically a magnetic field south pole...
(namely,
into Earth), and similarly near the
south magnetic poleThe Earth's South Magnetic Pole is the wandering point on the Earth's surface where the geomagnetic field lines are directed vertically upwards...
. Indeed, active auroras (and related "substorms") are much more likely at such times. Electric currents originating in such way apparently give auroral electrons their energy. The magnetospheric plasma has an abundance of electrons: some are magnetically trapped, some reside in the
magnetotailA magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
, and some exist in the upward extension of the
ionosphereThe ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
, which may extend (with diminishing density) some 25,000 km around Earth.
Bright auroras are generally associated with
Birkeland currentA Birkeland current is a specific magnetic field aligned current in the Earth’s magnetosphere which flows from the magnetotail towards the Earth on the dawn side and in the other direction on the dusk side of the magnetosphere. Lately, the term Birkeland currents has been expanded by some authors...
s (Schield et al., 1969; Zmuda and Armstrong, 1973) which flow down into the ionosphere on one side of the pole and out on the other. In between, some of the current connects directly through the ionospheric E layer (125 km); the rest ("region 2") detours, leaving again through field lines closer to the equator and closing through the "partial ring current" carried by magnetically trapped plasma. The ionosphere is an
ohmic conductorIn electrical circuits, Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them....
, so such currents require a driving voltage, which some dynamo mechanism can supply. Electric field probes in orbit above the polar cap suggest voltages of the order of 40,000 volts, rising up to more than 200,000 volts during intense magnetic storms.
Ionospheric resistance has a complex nature, and leads to a secondary Hall current flow. By a strange twist of physics, the magnetic disturbance on the ground due to the main current almost cancels out, so most of the observed effect of auroras is due to a secondary current, the auroral electrojet. An auroral electrojet index (measured in nanotesla) is regularly derived from ground data and serves as a general measure of auroral activity.
However, ohmic resistance is not the only obstacle to current flow in this circuit. The convergence of magnetic field lines near Earth creates a "mirror effect" which turns back most of the down-flowing electrons (where currents flow upwards), inhibiting current-carrying capacity. To overcome this, part of the available voltage appears along the field line ("parallel to the field"), helping electrons overcome that obstacle by widening the bundle of trajectories reaching Earth; a similar "parallel potential" is used in "tandem mirror" plasma containment devices. A feature of such voltage is that it is concentrated near Earth (potential proportional to field intensity; Persson, 1963), and indeed, as deduced by Evans (1974) and confirmed by satellites, most auroral acceleration occurs below 10,000 km. Another indicator of parallel electric fields along field lines are beams of upwards flowing O+ ions observed on auroral field lines.
While this mechanism is
probably the main source of the familiar auroral arcs, formations conspicuous from the ground, more energy might go to other, less prominent types of aurora, e.g. the diffuse aurora (below) and the low-energy electrons precipitated in magnetic storms (also below).
Some O+ ions ("conics") also seem accelerated in different ways by plasma processes associated with the aurora. These ions are accelerated by plasma waves, in directions mainly perpendicular to the field lines. They therefore start at their own "mirror points" and can travel only upwards. As they do so, the "mirror effect" transforms their directions of motion, from perpendicular to the line to lying on a cone around it, which gradually narrows down.
In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as
auroral kilometric radiationAuroral kilometric radiation is the intense radio radiation emitted in the acceleration zone of the polar lights. The radiation mainly comes from cyclotron radiation from electrons orbiting around the magnetic field lines of the Earth...
(AKR, discovered in 1972). Ionospheric absorption makes AKR observable from space only.
These "parallel potentials" accelerate electrons to auroral energies and seem to be a major source of aurora. Other mechanisms have also been proposed, in particular, Alfvén waves, wave modes involving the magnetic field first noted by
Hannes AlfvénHannes Olof Gösta Alfvén was a Swedish plasma physicist and Nobel laureate for his work on the theory of magnetohydrodynamics. He was originally trained as an electrical power engineer and later moved to research and teaching in the fields of plasma physics...
(1942), which have been observed in the lab and in space. The question is however whether these waves might just be a different way of looking at the above process, because this approach does not point out a different energy source, and many plasma bulk phenomena can also be described in terms of Alfvén waves.
Other processes are also involved in the aurora, and much remains to be learned. Auroral electrons created by large geomagnetic storms often seem to have energies below 1 keV, and are stopped higher up, near 200 km. Such low energies excite mainly the red line of oxygen, so that often such auroras are red. On the other hand, positive ions also reach the ionosphere at such time, with energies of 20-30 keV, suggesting they might be an "overflow" along magnetic field lines of the copious "ring current" ions accelerated at such times, by processes different from the ones described above.
Sources and types
Again, our understanding is very incomplete. A rough guess may point out three main sources:
- Dynamo action
The dynamo theory proposes a mechanism by which a celestial body such as the Earth generates a magnetic field.-History of theory:In 1905, shortly after composing his special relativity paper, Albert Einstein described the origin of the Earth's magnetic field as being one of the great unsolved...
with the solar wind flowing past Earth, possibly producing quiet auroral arcs ("directly driven" process). The circuit of the accelerating currents and their connection to the solar wind are uncertain.
- Dynamo action involving plasma squeezed towards Earth by sudden convulsions of the magnetotail
A magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
("magnetic substorms"). Substorms tend to occur after prolonged spells (hours) during which the interplanetary magnetic field has an appreciable southward component, leading to a high rate of interconnection between its field lines and those of Earth. As a result the solar wind moves magnetic fluxMagnetic flux, represented by the Greek letter Φ , is a measure of quantity of magnetism, taking into account the strength and the extent of a magnetic field...
(tubes of magnetic field lines, moving together with their resident plasma) from the day side of Earth to the magnetotail, widening the obstacle it presents to the solar wind flow and causing it to be squeezed harder. Ultimately the tail plasma is torn ("magnetic reconnectionMagnetic reconnection is the process whereby magnetic field lines from different magnetic domains are spliced to one another, changing their patterns of connectivity with respect to the sources. It is a violation of an approximate conservation law in plasma physics, and can concentrate mechanical...
"); some blobs ("plasmoidA plasmoid is a coherent structure of plasma and magnetic fields. Plasmoids have been proposed to explain natural phenomena such as ball lightning, magnetic bubbles in the magnetosphere, and objects in cometary tails, in the solar wind, in the solar atmosphere, and in the heliospheric current sheet...
s") are squeezed tailwards and are carried away with the solar wind; others are squeezed towards Earth where their motion feeds large outbursts of aurora, mainly around midnight ("unloading process"). Geomagnetic storms have similar effects, but with greater vigor. The big difference is the addition of many particles to the plasma trapped around Earth, enhancing the "ring current" which it carries. The resulting modification of Earth's field allows aurora to be visible at middle latitudes, on field lines much closer to the equator.
- Satellite images of the aurora from above show a "ring of fire" along the auroral oval (see above), often widest at midnight. That is the "diffuse aurora", not distinct enough to be seen by the eye. It does not seem to be associated with acceleration by electric currents (although currents and their arcs may be embedded in it) but to be due to electrons leaking out of the magnetotail.
Any magnetic trapping is leaky—there always exists a bundle of directions ("loss cone") around the guiding magnetic field lines where particles are not trapped but escape. In the
radiation beltsThe Van Allen radiation belt is a torus of energetic charged particles around Earth, which is held in place by Earth's magnetic field. This field is not uniformly distributed around the Earth. On the sunward side, it is compressed because of the solar wind, while on the other side it is elongated...
of Earth, once particles on such trajectories are gone, new ones only replace them very slowly, leaving such directions nearly "empty". In the magnetotail, however, particle trajectories seem to be constantly reshuffled, probably when the particles cross the very weak field near the equator. As a result, the flow of electrons in all directions is nearly the same ("isotropic"), and that assures a steady supply of leaking electrons.
The energization of such electrons comes from magnetotail processes. The leakage of negative electrons does not leave the tail positively charged, because each leaked electron lost to the atmosphere is quickly replaced by a low energy electron drawn upwards from the ionosphere. Such replacement of "hot" electrons by "cold" ones is in complete accord with the 2nd law of thermodynamics.
Other types of aurora have been observed from space, e.g. "poleward arcs" stretching sunward across the polar cap, the related "theta aurora", and "dayside arcs" near noon. These are relatively infrequent and poorly understood. There are other interesting effects such as flickering aurora, "black aurora" and subvisual red arcs. In addition to all these, a weak glow (often deep red) has been observed around the two polar cusps, the "funnels" of field lines separating the ones that close on the day side of Earth from lines swept into the tail. The cusps allow a small amount of solar wind to reach the top of the atmosphere, producing an auroral glow.
On other planets
Both
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
and
SaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
have magnetic fields much stronger than Earth's (Jupiter's equatorial field strength is 4.3 gauss, compared to 0.3 gauss for Earth), and both have large radiation belts. Aurora has been observed on both, most clearly with the
Hubble Space TelescopeThe Hubble Space Telescope is a space telescope that was carried into orbit by the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, the Hubble is one of the largest and most versatile, and is well-known as both a vital...
. Uranus and Neptune have also been observed to have auroras.
The auroras on the gas giants seem, like Earth's, to be powered by the solar wind. In addition, however, Jupiter's moons, especially
IoIo is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of 3,642 kilometres, the fourth-largest moon in the Solar System. It was named after Io, a priestess of Hera who became one of the lovers of Zeus.With over 400 active volcanoes, Io is the most...
, are powerful sources of auroras on Jupiter. These arise from electric currents along field lines ("field aligned currents"), generated by a dynamo mechanism due to the relative motion between the rotating planet and the moving moon. Io, which has active volcanism and an ionosphere, is a particularly strong source, and its currents also generate radio emissions, studied since 1955. Auroras have also been observed on
IoIo is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of 3,642 kilometres, the fourth-largest moon in the Solar System. It was named after Io, a priestess of Hera who became one of the lovers of Zeus.With over 400 active volcanoes, Io is the most...
, Europa, and Ganymede themselves, e.g., using the
Hubble Space TelescopeThe Hubble Space Telescope is a space telescope that was carried into orbit by the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, the Hubble is one of the largest and most versatile, and is well-known as both a vital...
. These are generated when Jupiter's magnetospheric plasma impact their very thin atmospheres.
Auroras have also been observed on Venus and Mars. Because Venus has no intrinsic (planetary) magnetic field, Venusian auroras appear as bright and diffuse patches of varying shape and intensity, sometimes distributed across the full planetary disc. Venusian auroras are produced by the impact of electrons originating from the solar wind and precipitating in the night-side atmosphere. An aurora was also detected on Mars, on August 14, 2004, by the SPICAM instrument aboard
Mars ExpressMars Express is a space exploration mission being conducted by the European Space Agency . The Mars Express mission is exploring the planet Mars, and is the first planetary mission attempted by the agency. "Express" originally referred to the speed and efficiency with which the spacecraft was...
. The aurora was located at
Terra CimmeriaTerra Cimmeria is a large Martian region, centered at and covering 5400 km at its broadest extent. Terra Cimmeria is one part of the heavily cratered, southern highland region of the planet....
, in the region of 177° East, 52° South. The total size of the emission region was about 30 km across, and possibly about 8 km high. By analyzing a map of crustal magnetic anomalies compiled with data from
Mars Global SurveyorThe Mars Global Surveyor was a US spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. It began the United States's return to Mars after a 10-year absence. It completed its primary mission in January 2001 and was in its third extended mission phase when, on , the...
, scientists observed that the region of the emissions corresponded to an area where the strongest magnetic field is localized. This correlation indicates that the origin of the light emission actually was a flux of electrons moving along the crust magnetic lines and exciting the upper atmosphere of Mars.
History of aurora theories
In the past theories have been proposed to explain the phenomenon. These theories are now obsolete.
- Benjamin Franklin
Benjamin Franklin was one of the Founding Fathers of the United States of America. A noted polymath, Franklin was a leading author and printer, satirist, political theorist, politician, scientist, inventor, civic activist, statesman, soldier, and diplomat...
theorized that the "mystery of the Northern Lights" was caused by a concentration of electrical charges in the polar regions intensified by the snow and other moisture.
- Auroral electrons come from beams emitted by the Sun. This was claimed around 1900 by Kristian Birkeland
Kristian Olaf Birkeland was a Norwegian scientist. He is best remembered as the person who first elucidated the nature of the Aurora borealis. In order to fund his research on the aurorae, he invented the electromagnetic cannon and the Birkeland-Eyde process of fixing nitrogen from the air...
, whose experiments in a vacuum chamber with electron beams and magnetized spheres (miniature models of Earth or "terrellas") showed that such electrons would be guided towards the polar regions. Problems with this model included absence of aurora at the poles themselves, self-dispersal of such beams by their negative charge, and more recently, lack of any observational evidence in space.
- The aurora is the overflow of the radiation belt
The Van Allen radiation belt is a torus of energetic charged particles around Earth, which is held in place by Earth's magnetic field. This field is not uniformly distributed around the Earth. On the sunward side, it is compressed because of the solar wind, while on the other side it is elongated...
("leaky bucket theory"). This was first disproved around 1962 by James Van AllenJames Alfred Van Allen was an American space scientist at the University of Iowa. The Van Allen radiation belts were named after him, following the 1958 satellite missions in which Van Allen had argued that a Geiger counter should be used to detect charged particles.-Honors:* TIME magazine Man of...
and co-workers, who showed that the high rate at which energy was dissipated by the aurora would quickly drain all that was available in the radiation belt. Soon afterwards it became clear that most of the energy in trapped particles resided in positive ions, while auroral particles were almost always electrons, of relatively low energy.
- The aurora is produced by solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the sun. It consists mostly of electrons and protons with energies of about 1 keV. The stream of particles varies in temperature and speed with the passage of time...
particles guided by Earth's field lines to the top of the atmosphere. This holds true for the cusp aurora, but outside the cusp, the solar wind has no direct access. In addition, the main energy in the solar wind resides in positive ions; electrons only have about 0.5 eV (electron volt), and while in the cusp this may be raised to 50–100 eV, that still falls short of auroral energies.
Images
Images of aurora are significantly more common today due to the rise of use of
digital cameraA digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor....
s that have high enough sensitivities. Film and digital exposure to auroral displays is fraught with difficulties, particularly if faithfulness of reproduction is an objective. Due to the different spectral energy present, and changing dynamically throughout the exposure, the results are somewhat unpredictable. Different layers of the film emulsion respond differently to lower light levels, and choice of film can be very important. Longer exposures aggregate the rapidly changing energy and often blanket the dynamic attribute of a display. Higher sensitivity creates issues with graininess.
David MalinDavid Malin is a British-Australian astronomer and photographer.Malin trained as a chemist and originally worked in England as microscopist...
pioneered multiple exposure using multiple filters for astronomical photography, recombining the images in the laboratory to recreate the visual display more accurately.
http://www.davidmalin.com/index.html For scientific research, proxies are often used, such as ultra-violet, and re-coloured to simulate the appearance to humans. Predictive techniques are also used, to indicate the extent of the display, a highly useful tool for aurora hunters.
http://www.sec.noaa.gov/pmap/index.html Terrestrial features often find their way into aurora images, making them more accessible and more likely to be published by the major websites. It is possible to take excellent images with standard film (using
ISO ratingsFilm speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. Relatively insensitive film, with a correspondingly lower speed index requires more exposure to light to produce the...
between 100 and 400) and a
single-lens reflex cameraA single-lens reflex camera is camera that uses a semi-automatic moving mirror system which permits the photographer to sometimes see exactly what will be captured by the film or digital imaging system, as opposed to pre-SLR cameras where the view through the viewfinder could be significantly...
with full
apertureIn optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
, a fast lens (f1.4 50 mm, for example), and exposures between 10 and 30 seconds, depending on the aurora's display strength.
Early work on the imaging of the Aurora was done in 1949 by the
University of SaskatchewanThe University of Saskatchewan is a coeducational public research university located on the east side of the South Saskatchewan River in Saskatoon, Saskatchewan, Canada, founded over 100 years ago in 1907. The University of Saskatchewan Act was passed by the Legislative Assembly of Saskatchewan in...
using the SCR-270 radar.
In folklore
In
Bulfinch's MythologyBulfinch's Mythology is a collection of the works of Thomas Bulfinch, renamed after him and published after his death. It is a classic work of mythology and is still being printed, even 150 years after the first work, Age of Fable, was published in 1855....
from 1855 by
Thomas BulfinchThomas Bulfinch was an American writer, born in Newton, Massachusetts. Bulfinch belonged to a well educated Bostonian merchant family of modest means. His father was Charles Bulfinch, the architect of the Massachusetts State House in Boston and parts of the U.S. Capitol in Washington, D.C....
there is the claim that in
Norse mythologyNorse, North Germanic, or Scandinavian mythology comprises the myths of North Germanic pre-Christian religion.Most of the written sources for Norse mythology were assembled in medieval Iceland in Old Norse, notably as the Edda....
:
- The Valkyrior
In Norse mythology, a valkyrie is one of a host of female figures who decide who will die in battle. The valkyries bring their chosen to the afterlife hall of the slain, Valhalla, ruled over by the god Odin, where the deceased warriors become einherjar...
are warlike virgins, mounted upon horses and armed with helmets and spears. /.../ When they ride forth on their errand, their armour sheds a strange flickering light, which flashes up over the northern skies, making what men call the "aurora borealis", or "Northern Lights".
While a striking notion, there is not a vast body of evidence in the Old Norse literature supporting this assertion. Although auroral activity is common over
ScandinaviaScandinavia is a geographical region in northern Europe that includes, and is named after, the Scanian Province. It consists of the kingdoms of Norway, Sweden, and Denmark...
and
IcelandThe Republic of Iceland is a European island country located in the North Atlantic Ocean. It has a population of about 320,000 and a total area of 103,000 km². Its capital and largest city is Reykjavík, whose surrounding area is home to approximately two thirds of the national population...
today, it is possible that the Magnetic North Pole was considerably further away from this region during the centuries before the documentation of Norse mythology, thus explaining the lack of references.
The first Old Norse account of norðurljós
is found in the Norwegian chronicle Konungs SkuggsjáKonungs skuggsjá is a Norwegian educational text from around 1250, an example of speculum literature that deals with politics and morality...
from AD 1230. The chronicler has heard about this phenomenon from compatriots returning from
GreenlandGreenland is an autonomous constituent country within the Kingdom of Denmark located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago...
, and he gives three possible explanations: that the ocean was surrounded by vast fires, that the sun flares could reach around the world to its night side, or that
glacierA glacier is a perennial mass of ice which moves over land. A glacier forms in locations where the mass accumulation of snow and ice exceeds ablation over many years...
s could store energy so that they eventually became
fluorescentFluorescence is the emission of visible light by a substance that has absorbed light of a differing, usually invisible, wavelength. Absorption of a photon triggers the emission of a photon with a longer wavelength. A shorter wavelength emission is sometimes observed from multiple photon absorption...
.
In ancient Roman mythology, Aurora is the
goddess of the dawnAurora is the Latin word for dawn
, the goddess of dawn in Roman mythology and Latin poetry. Aurora is comparable to the Greek goddess Eos, though Aurora did not bring with her any resonance of a greater archaic goddess.-Roman mythology:...
, renewing herself every morning to fly across the sky, announcing the arrival of the sun. The persona of Aurora the goddess has been incorporated in the writings of Shakespeare, Lord Tennyson and Thoreau.
Modern style guides recommend that the names of meteorological phenomena, such as aurora borealis, be uncapitalized.
See also
External links