The aurora (plural aurorae/auroras)
is a bright glow observed in the night sky, usually in the polar zone. For this
reason some scientists call it a "polar aurora" (or "aurora polaris"). In
northern latitudes, it is known as the aurora borealis, which is named after the
Roman goddess of the dawn, Aurora, and the Greek name for north wind, Boreas.
Especially in Europe, it often appears as a reddish glow on the northern
horizon, as if the sun were rising from an unusual direction. The aurora
borealis is also called the northern lights since it is only visible in the
North sky from the Northern Hemisphere. The aurora borealis most often occurs
from September to October and from March to April. Its southern counterpart,
aurora australis, has similar properties. Australis is the Latin word for "of
Auroras are now known to be caused by the collision of charged particles
(e.g. electrons), found in the magnetosphere, with atoms in the Earth's upper
atmosphere (at altitudes above 80 km). These charged particles are typically
energized to levels between 1 thousand and 15 thousand electronvolts and, as
they collide with atoms of gases in the atmosphere, the atoms become energized.
Shortly afterwards, the atoms emit their gained energy as light. Light emitted
by the Aurora tends to be dominated by emissions from atomic oxygen, resulting
in a greenish glow (at a wavelength of 557.7 nm) and - especially at lower
energy levels and at higher altitudes - the dark-red glow (at 630.0 nm of
wavelength). Both of these represent forbidden transitions of electrons of
atomic oxygen that, in absence of newer collisions, persist for a long time and
account for the slow brightening and fading (0.5-1 s) of auroral rays. Many
other colors - especially those emitted by atomic and molecular nitrogen (blue
and purple, respectively) - can also be observed. These, however, vary much
faster and reveal the true dynamic nature of auroras.
movie of an aurora display on Sep/24/2006 in British Columbia, Canada.
As well as visible light, auroras emit infrared (NIR and IR) and ultraviolet
(UV) rays as well as X-rays (e.g. as observed by the Polar spacecraft). While
the visible light emissions of auroras can easily be seen on Earth, the UV and
X-ray emissions are best seen from space, as the Earth's atmosphere tends to
absorb and attenuate these emissions.
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 the earth's
magnetic field. Indeed, satellites show auroral electrons to be guided by
magnetic field lines, spiraling around them while moving earthwards.
The curtains often show folds called "striations", which are curtain-like.
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 perspective.
In 1741, Hiorter and Celsius first noticed other evidence for magnetic
control, namely, large magnetic fluctuations occurred whenever the aurora was
observed overhead. This indicates (it was later realized) that large electric
currents were associated with the aurora, flowing in the region where auroral
light originated. Kristian Birkeland (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"
Still more evidence for a magnetic connection are the statistics of auroral
observations. Elias Loomis (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 the magnetic pole of the
earth, not its geographic one. It was hardly ever seen near that pole itself.
The instantaneous distribution of auroras ("auroral oval", Yasha [or Yakov]
Felds[h]tein 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 earth is constantly immersed in the solar wind, 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 solar corona. The solar wind usually reaches Earth with a velocity around
400 km/s, density around 5 ions/cc and magnetic field intensity around 2–5 nT (nanoteslas;
the earth's surface field is typically 30,000–50,000 nT). These are typical
values. During magnetic storms, in particular, flows can be several times
faster; the interplanetary magnetic field (IMF) may also be much stronger.
The IMF originates on the sun, related to the field of sunspots, and its
field lines (lines of force) 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 (a numerical
simulation of the solar wind illustrates that: Solar wind forecast).
The earth's magnetosphere 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 shock 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
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 causing the aurora.
compilation of Aurora Borealis pictures and videos.
Frequency of occurrence
The aurora is a common occurrence in the ring-shaped zone. 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 cycle 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 (known as Bz, pronounced "bee-sub-zed" or
"bee-sub-zee"), being greater with southward slants.
Geomagnetic storms that ignite auroras actually happen more often during the
months around the equinoxes. It is not well understood why geomagnetic storms
are tied to the earth's seasons when polar activity is not. It is known,
however, that during spring and autumn, the earth's and the interplanetary
magnetic field link up. At the magnetopause, Earth's magnetic field points
north. When Bz 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 Bz's open a door through which energy from the solar
wind can reach Earth's inner magnetosphere.
The peaking of Bz during this time is a result of geometry. The
interplanetary magnetic field comes from the sun and is carried outward the
solar wind. Because the sun rotates the IMF has a spiral shape. 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 Bz are greatest
However, Bz 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 September 5 and March 5 when Earth lies at its
highest heliographic latitude.
Still, neither Bz 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 variation.
The aurora which occurred as a result of the "great geomagnetic storm" on
both 28 August and 2 September 1859, are thought to be perhaps the most
spectacular ever witnessed throughout recent recorded history. The latter, which
occurred on September 2 as a result of the exceptionally intense
Carrington-Hodgson white light solar flare on September 1, 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 States, Europe, Japan and Australia. It was said in the New York Times
that "ordinary print could be read by the light [of the aurora]". The aurora is
thought to have been produced by one of the most intense coronal mass ejections
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 magnetometer
measurements of the era but also as a result of a significant portion of the
125,000 miles 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 current) to be induced in them (due to Earth's severely fluctuating
magnetosphere) and actually used for communication. The following conversation
was had between two operators of the American Telegraph Line between Boston and
Portland on the night of the 2nd and reported in the Boston Traveler:
Boston operator (to Portland operator): "Please cut off your battery
[power source] entirely for fifteen minutes." Portland operator: "Will do so. It is now disconnected." Boston: "Mine is disconnected, and we are working with the auroral
current. How do you receive my writing?" Portland: "Better than with our batteries on. - Current comes and goes
gradually." Boston: "My current is very strong at times, and we can work better
without the batteries, as the aurora seems to neutralize and augment our
batteries alternately, making current too strong at times for our relay magnets.
Suppose we work without batteries while we are affected by this trouble." Portland: "Very well. Shall I go ahead with business?" Boston: "Yes. Go ahead."
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.
Aurora australis (September 11, 2005) as captured by NASA's
IMAGE satellite, digitally overlaid onto planet earth
The origin of the aurora
The ultimate energy source of the aurora is the solar wind flowing past the
Both the magnetosphere and the solar wind consist of plasma (ionized gas),
which can conduct electricity. It is well known (since Michael Faraday's work
around 1830) that if two electric conductors are immersed in a magnetic field
and one moves relative to the other, while a closed electric circuit exists
which threads both conductors, then an electric current will arise in that
circuit. Electric generators or dynamos make use of this process ("the dynamo
effect"), but the conductors can also be 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. It is therefore important that a temporary magnetic
interconnection be established between the field lines of the solar wind and
those of the magnetosphere, by a process known as magnetic reconnection. 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 pole (namely, into the earth), and
similarly near the southern pole. Indeed, active auroras (and related "substorms")
are much more likely at such times.
Electric currents originating in such fashion apparently give auroral
electrons their energy. The magnetospheric plasma has an abundance of electrons:
some are magnetically trapped, some reside in the magnetotail, and some exist in
the upward extension of the ionosphere, which may extend (with diminishing
density) some 25,000 km around the earth.
Bright auroras are generally associated with Birkeland currents (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 conductor, 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
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.
"stunningly beautiful to watch and very
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 voltage" 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 radiation (AKR, discovered
in 1972). Ionospheric absorption makes AKR observable from space only.
Aurora Borealis over Canada
photographed by Expedition Six. (NASA)
These "parallel voltages" 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 Alfvén (1942), which have been observed in the lab and in space. The
question is however whether this 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
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 of aurora
Again, our understanding is very incomplete. A rough guess may point out
three main sources:
Dynamo action 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 earthward by sudden convulsions of
the magnetotail ("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 flux (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 reconnection"); some blobs ("plasmoids") are squeezed
tailwards and are carried away with the solar wind; others are squeezed
earthwards 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 the 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
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 belts 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
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
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.
Space does not allow discussion of other 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.
Jupiter aurora. The bright spot
at far left is the end of field line to Io; spots at bottom lead to Ganymede and
Auroras on other planets
Both Jupiter and Saturn have magnetic fields much stronger than Earth's
(Uranus, Neptune and Mercury are also magnetic), and both have large radiation
belts. Aurora has been observed on both, most clearly with the Hubble Space
These auroras seem, like Earth's, to be powered by the solar wind. In
addition, however, Jupiter's moons, especially Io, are also powerful sources of
auroras. These arise from electric currents along field lines ("field aligned
currents"), generated by a dynamo mechanism due to 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.
An aurora has recently been detected on Mars, even though it was thought that
the lack of a strong magnetic field would not make one possible. 
History of Aurora theories
In the past theories have been proposed to explain the phenomenon. These
theories are now obsolete.
Auroral electrons come from beams emitted by the sun. This was claimed
around 1900 by Kristian Birkeland, whose experiments in a vacuum chamber with
electron beams and magnetized spheres (miniature models of the 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 ("leaky bucket theory").
This was first disproved around 1962 by James Van Allen 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
The aurora is produced by solar wind particles guided by the 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 of aurora are significantly more common today due to the rise in
digital camera use with high enough sensitivities.  Film and digital exposure
to auroral displays is fraught with many difficulties, particularly if
faithfulness of reproduction is an important 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 Malin pioneered multiple exposure using multiple filters
for astronomical photography, recombining the images in the laboratory to
recreate the visual display more accurately.  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.  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 (employing ISO ratings between 100 and
400) and an SLR with full aperture, a fast lens (f1.4 50mm, for example), and
exposures between 10 and 30 seconds, depending on the aurora's display strength.
In Bulfinch's Mythology from 1855 by Thomas Bulfinch there is the
claim that in Norse mythology:
The Valkyrior 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 nothing in the Old Norse literature
supporting this assertion. Although aurora activity is common over Scandinavia
and Iceland 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 absent references. 
The first Old Norse account of norđurljós is instead found in the
Norwegian chronicle Konungs Skuggsjá from AD 1250. The chronicler has
heard about this phenomenon from compatriots returning from Greenland, 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
glaciers could store energy so that they eventually became fluorescent. 
An old Scandinavian name for northern lights translates as "herring flash".
It was believed that northern lights were the reflections cast by large swarms
of herring onto the sky.
Another Scandinavian source refers to "the fires that surround the North and
South edges of the world". This has been put forward as evidence that the Norse
ventured as far as Antarctica, although this is insufficient to form a solid
The Finnish name for northern lights is revontulet, fox fires.
According to legend, foxes made of fire lived in Lapland, and revontulet
were the sparks they whisked up into the atmosphere with their tails.
In Estonian they are called virmalised, spirit beings of higher
realms. In some legends they are given negative characters, in some positive
The Sami people believed that one should be particularly careful and quiet
when observed by the northern lights (called guovssahasat in Northern
Sami). Mocking the northern lights or singing about them was believed to be
particularly dangerous and could cause the lights to descend on the mocker and
The Algonquin believed the lights to be their ancestors dancing around a
In Inuit folklore, northern lights were the spirits of the dead playing
football with human skulls over the sky. The Inuit also used the aurora to get
their children home after dark by claiming that if you whistled in their
presence they would come down and split their heads from their body to play
football with it.
In Latvian folklore northern lights, especially if red and observed in
winter, are believed to be fighting souls of dead warriors, an omen foretelling
disaster (especially war or famine).
In Scotland, the northern lights were known as "the merry dancers" or na
fir-chlis. There are many old sayings about them, including the Scottish
Gaelic proverb "When the merry dancers play, they are like to slay." The
playfulness of the merry dancers was supposed to end occasionally in quite a
serious fight, and next morning when children saw patches of red lichen on the
stones, they say amongst themselves that "the merry dancers bled each other last
night". The appearance of these lights in the sky was considered a sign of the
approach of unsettled weather.
Many prospectors during the Klondike Gold Rush believed that the Northern
Lights were the reflection of the mother lode of all gold.
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