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Wednesday, November 22 through Sunday, November 26.For other uses, see Aurora Borealis (disambiguation).For other uses, see Aurora (disambiguation).Some scientists therefore call them "polar auroras" (or "aurorae polaris").In northern latitudes, it is known as the aurora borealis, named after the Roman goddess of the dawn, Aurora, and the Greek name for north wind, Boreas.It often appears as a greenish glow (or sometimes a faint red), as if the sun were rising from an unusual direction.The aurora borealis is also called the northern polar lights, as 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.Auroral mechanism
2 Auroral forms and magnetism
3 The solar wind and magnetosphere
4 Frequency of occurrence
4.The origin of the aurora
6 Sources and types of aurora
6.Auroras on other planets
8 History of Aurora theories
9 Auroral images
10 Aurora in folklore
11 References
11.Auroral mechanism
Auroras are produced by the collision of charged particles, mostly electrons but also protons and heavier particles, from the magnetosphere, with atoms and molecules of the Earth's upper atmosphere (at altitudes above 80 km).Further magnetic reconnection accelerates the particles towards earth.The collisions in the atmosphere electronically excite atoms and molecules in the upper atmosphere.The excitation energy can be lost by light emission or collisions.Most aurorae are green and red emission from atomic oxygen.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 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.Pytheas, Hiorter and 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 currents were associated with the aurora, flowing in the region where auroral light originated.Still more evidence for a magnetic connection are the statistics of auroral observations.The aurora can be seen best at this time.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.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 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.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.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.As a result, southward (and northward) excursions of Bz are greatest then.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.September 5 and March 5 when Earth lies at its highest heliographic latitude.Auroral events of historical significance
The auroras which occurred as a result of the "great geomagnetic storm" on both August 28, 1859 and September 2, 1859 are thought to be perhaps the most spectacular ever witnessed throughout recent recorded history.Hodgson white light solar flare 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 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 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.Portland operator: "Will do so.Boston: "Mine is disconnected, and we are working with the auroral current.How do you receive my writing?"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.Shall I go ahead with business?"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 the Blue Marble composite image.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 conducts electricity.Dynamos make use of this basic process ("the dynamo effect"), any and all conductors, solid or otherwise are so affected including plasmas or other fluids.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.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.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 storms.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.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.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.The Aurora Borealis as viewed from the ISS Expedition 6 team.Lake Manicouagan is visible to the bottom left.Some O+ ions ("conics") also seem accelerated in different ways by plasma processes associated with the aurora.Hz known as auroral kilometric radiation (AKR, discovered in 1972).Ionospheric absorption makes AKR observable from space only.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.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.V, 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.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.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.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").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 magnetotail.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.Other types of aurora have been observed from space, e.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.The cusps allow a small amount of solar wind to reach the top of the atmosphere, producing an auroral glow.The bright spot at far left is the end of field line to Io; spots at bottom lead to Ganymede and Europa.Rayleigh), and both have large radiation belts.Aurora has been observed on both, most clearly with the Hubble Space Telescope.Uranus and Neptune have also been observed to have aurorae.The auroras on the gas giants seem, like Earth's, to be powered by the solar wind.In addition, however, Jupiter's moons, especially Io, 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.Aurorae have also been observed on Io, Europa, and Ganymede themselves, e.Auroras have also been observed on Venus and Mars.Because Venus has no intrinsic (planetary) magnetic field, Venusian aurorae appear as bright and diffuse patches of varying shape and intensity, sometimes distributed across the full planetary disc.An aurora was also detected on Mars, on August 14, 2004, by the SPICAM instrument aboard Mars Express.History of Aurora theories
In the past theories have been proposed to explain the phenomenon.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.The aurora is the overflow of the radiation belt ("leaky bucket theory").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 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.V, that still falls short of auroral energies.Auroral images
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.David Malin pioneered multiple exposure using multiple filters for astronomical photography, recombining the images in the laboratory to recreate the visual display more accurately.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.Please help improve this article by adding reliable references.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.While a striking notion, there is nothing in the Old Norse literature supporting this assertion.Although auroral 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.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 conclusion.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.The Algonquin believed the lights to be their ancestors dancing around a ceremonial fire.In Russian folklore aurora borealis was associated with dragon ("Ognenniy Zmey"), who came to women and seduced them in the absence of their husbands.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.Service's most popular poem, The Cremation of Sam McGee, begins, "The Northern Lights have seen queer sights, But the queerest they ever did see, Was the night on the marge of Lake Lebarge, I cremated Sam McGee."In "The Ballad of the Northern Lights", Service refers to the theory that the source of the glow is an enormous deposit of radium in the polar region.Hatton of the Astronomy Department, University of Chicago
Eather, Robert H.Majestic Lights: The Aurora in Science, History, and The Arts."Secrets of the Aurora Borealis".Graphic Arts Center Publishing Company.Aurora: The Mysterious Northern Lights.Phillips, Tony (October 21, 2001).Eyewitness reports of the great auroral storm of 1859.The external links in this article may not comply with Wikipedia's content policies or guidelines.This page was last modified on 22 February 2008, at 20:03.All text is available under the terms of the GNU Free Documentation License.Look up Aurora, aurora in Wiktionary, the free dictionary.Aircraft and aerospace vehicles
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