A diffuse coloured light in the upper atmosphere (100 km/60 mi) over polar regions, visible at night. It is caused by charged particles from the Sun colliding with oxygen and nitrogen atoms in the atmosphere. It is seen most frequently in the auroral zones, which have a radius of c.22° around the geomagnetic poles. In the N it is known as the aurora borealis or northern lights; in the S as the aurora australis.
For other uses, see Aurora (disambiguation).The aurora 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 (IPA /ɔˈɹɔɹə bɔɹiˈælɪs/), which is named after the Roman goddess of the dawn, Aurora, and the Greek name for north wind, Boreas, since in Europe especially 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 from the Northern Hemisphere. The aurora borealis most often occurs from September to October and from March to April.
Auroral Mechanism
Auroras are now known to be caused by the collision of charged particles (e.g. 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). These, however, vary much faster and reveal the true dynamic nature of auroras.
As well as visible light, auroras emit ultraviolet (UV) rays as well as X-rays (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.
Auroral forms and magnetism
Typically the aurora appears either as a diffuse glow or as "curtains" that approximately extend in the east-west direction. 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. 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" (see also Birkeland currents).
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 of with a radius of approximately 2500 km around the magnetic pole of the earth, not its geographic one. 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.
Aurora australisThe solar wind and magnetosphere
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 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.
The earth's magnetosphere is the space region dominated by its magnetic field.
Frequency of occurrence
The aurora is a common occurrence in the ring-shaped zone. 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.
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.
However, Bz is not the only influence on geomagnetic activity.
Still, neither Bz nor the solar wind can fully explain the seasonal behavior of geomagnetic storms.
The sun gives off high-energy charged particles (also called ions) that travel out into space at speeds of 300 to 1200 kilometres per second. As the solar wind interacts with the edge of the earth's magnetic field, some of the particles are trapped by it, and they follow the lines of magnetic force down into the ionosphere, the section of the earth's atmosphere that extends from about 60 to 600 kilometres above the earth's surface. When the particles collide with the gases in the ionosphere they start to glow, producing the spectacle that we know as the auroras, northern and southern.
Auroral events of historical significance
The aurora which occurred as a result of the "great geomagnetic storm" on both the 28th of August and 2nd of September, 1859, are thought to be perhaps the most spectacular ever witnessed throughout recent recorded history. The latter, which occurred on September 2nd as a result of the exceptionally intense Carrington-Hodgson white light solar flare on September 1st, 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. The aurora is thought to have been produced by one of the most intense coronal mass ejections in history very near the maximum intensity it is thought that the sun is capable of producing. The great aurora of 1859 is also notable for the fact that it is the first time where the phenomena of auroral activity and electricity were unambiguously linked. 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. - 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. Go ahead."
The conversation was carried on for around two hours using no battery power at all and working soley 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.
The origin of the aurora
The ultimate energy source of the aurora is the solar wind flowing past the Earth.
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.
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. 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.
Bright auroras are generally associated with Birkeland currents (Schield et al., 1969;
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.
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; 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.
In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as auroral kilometric radiation (AKR, discovered in 1972).
These "parallel voltages" accelerate electrons to auroral energies and seem to be a major source of aurora.
Other processes are also involved in the aurora, and much remains to be learned. 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.
Aurora Borealis (file info) Aurora Borealis from ISS In case of problems, see media help.Sources and types of aurora
Again, our understanding is very incomplete. 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. others are squeezed earthwards where their motion feeds large outbursts of aurora, mainly around midnight ("unloading process"). 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.
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.
The energization of such electrons comes from magnetotail processes.
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. 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.
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 telescope.
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.
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. 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. 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.
Auroral images
Images of aurora are significantly more common today due to the rise in digital camera use with high enough sensitivities. 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. 2001 image
Guide to seeing and imaging aurora australis here: GuideAurora in folklore
In Bulfinch's Mythology from 1855 by Thomas Bulfinch there is the claim that in Norse mythology:
While a striking notion, there is nothing in the Old Norse literature supporting this assertion.
The first Old Norse account of norðurljós is instead found in the Norwegian chronicle Konungs Skuggsjá from AD 1250.
An old Scandinavian name for northern lights translates as "herring flash".
Another Scandinavian source refers to "the fires that surround the North and South edges of the world".
The Finnish name for northern lights is revontulet, fox fires.
The Sami people believed that one should be particularly careful and quiet when observed by the northern lights (called guovssahasat in Northern Sami).
The Algonquin believed the lights to be their ancestors dancing around a ceremonial fire.
In Inuit folklore, northern lights were the spirits of the dead playing football with a walrus skull 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 burn you up.
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.
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