Aurora Borealis: Wikis

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The Aurora Borealis shines above Bear Lake
Red and green Aurora in Fairbanks, Alaska
Aurora australis in Antarctica
Aurora Timelapse.ogv
Aurora timelapse video

Auroras, sometimes called the northern and southern (polar) lights or aurorae (singular: aurora), are natural light displays in the sky, usually observed at night, particularly in the polar regions. They typically occur in the ionosphere. They are also referred to as polar auroras.

In northern latitudes, the effect is known as the aurora borealis, named after the Roman goddess of dawn, Aurora, and the Greek name for north wind, Boreas, by Pierre Gassendi in 1621.[1] The aurora borealis is also called the northern polar lights, as it is only visible in the sky from the Northern Hemisphere, with the chance of visibility increasing with proximity to the North Magnetic Pole. (Earth's is currently in the arctic islands of northern Canada.) 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. The northern lights have had a number of names throughout history. The Cree call this phenomenon the "Dance of the Spirits." In the Middle Ages the auroras have been called a sign from God (see Wilfried Schröder, Das Phänomen des Polarlichts, Darmstadt 1984).

Its southern counterpart, the aurora australis or the southern polar lights, has similar properties, but is only visible from high southern latitudes in Antarctica, South America, or Australasia. Australis is the Latin word for "of the South."

Auroras can be spotted throughout the world and on other planets. It is most visible closer to the poles due to the longer periods of darkness and the magnetic field.

Contents

Auroral mechanism

Auroras are the result of the emissions of photons in the Earth's upper atmosphere, above 80 km (50 miles), from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state. They are ionized or excited by the collision of solar wind particles being funneled 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:

oxygen emissions
Green or brownish-red, depending on the amount of energy absorbed.
nitrogen 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 state.

Oxygen is 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 do not have time to happen, and eventually even green light emissions are prevented.

This is why there is a colour differential with altitude; at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything.

A predominantly red aurora australis

Auroras are associated with the solar wind, a flow of ions continuously flowing outward from the sun. The Earth's magnetic field traps these particles, many of which travel toward the poles where they are accelerated toward earth. Collisions between these ions and atmospheric atoms and molecules causes energy releases in the form of auroras appearing in large circles around the poles. Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind.[2] Seen from space, these fiery curtains form a thin ring in the shape of a monk's tonsure.

Forms and magnetism

Northern lights over Calgary

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 perspective.

Although it was first mentioned by Ancient Greek explorer/geographer 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. Kristian Birkeland (1908)[3] 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).

On 26 February 2008, THEMIS probes were able to determine, for the first time, the triggering event for the onset of magnetospheric substorms.[4] 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.[5] Dr. Vassilis Angelopoulos of the University of California, Los Angeles, the principal investigator for the THEMIS mission, claimed, "Our data show clearly and for the first time that magnetic reconnection is the trigger." [6]

Still more evidence for a magnetic connection are the statistics of auroral observations. Elias Loomis (1860) and later in more detail Hermann Fritz (1881)[7] 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[8]) 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

Schematic of Earth's 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 corona. The solar wind usually reaches Earth with a velocity around 400 km/s, density around 5 ions/cm3 and magnetic field intensity around 2–5 nT (nanoteslas; 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.[9]

Earth's magnetosphere is formed by the impact of the solar wind on the Earth's magnetic field. It forms an obstacle to the solar wind, diverting it, at a distance of about 70,000 km, forming a bow shock 12,000 km to 15,000 km further upstream. The width of the magnetosphere abreast of Earth, is typically 190,000 km, and on the night side a long "magnetotail" of stretched field lines extends to great distances.

The magnetosphere is full of ions trapped as the solar wind passes the earth. Perturbations in the solar wind increase this flow of ions. The excess moving along field lines and eventually accelerated toward the poles are responsible for changes in the aurora.

Frequency of occurrence

Aurora australis 1994 from Bluff, New Zealand

Auroras are common near the Poles. They are occasionally seen in temperate latitudes, when a 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.[citation needed] However, within the auroral zone the likelihood of an aurora occurring depends mostly on the slant of IMF lines (the slant is known as Bz), 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 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 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 (IMF) comes from the Sun and is carried outward with 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 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. 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 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 variations.

Auroral events of historical significance

The auroras which occurred as a result of the "great geomagnetic storm" on both August 28 and September 2, 1859 are thought to be perhaps the most spectacular ever witnessed throughout recent recorded history. Balfour Stewart, in a paper [10][11] to the Royal Society on November 21, 1861, described both auroral events as documented by a self-recording magnetograph at the Kew Observatory 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 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. It was reported by the New York Times [12][13][14] that in Boston on Friday September 2, 1859 the Aurora was "so brilliant that at about one o'clock ordinary print could be read by the light"[13][15][16]. One o’clock Boston time on Friday September 2, would have been 6:00 GMT and the self-recording magnetograph at the Kew Observatory was recording the geomagnetic storm, which was then one hour old, at its full intensity; this is amazingly accurate news reporting. Between 1859 and 1862 Elias Loomis published a series of nine papers on the Great Auroral Exhibition of 1859 in the American Journal of Science 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 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 (201,000 km) 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 occurred between two operators of the American Telegraph Line between Boston and Portland, Maine, on the night of September 2, 1859 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.[15] Such events led to the general conclusion that

The effect of the Aurora on the electric telegraph is generally to increase or diminish the electric current generated in working the wires. Sometimes it entirely neutralizes them, so that, in effect, no fluid is discoverable in them . The aurora borealis seems to be composed of a mass of electric matter, resembling in every respect, that generated by the electric galvanic battery. The currents from it change coming on the wires, and then disappear: the mass of the aurora rolls from the horizon to the zenith.[17]

Origin

Aurora australis (September 11, 2005) as captured by NASA's IMAGE satellite, digitally overlaid onto the The Blue Marble composite image. An animation created using the same satellite data is also available.

The ultimate energy source of the aurora is the solar wind flowing past the Earth. The magnetosphere and solar wind consist of plasma (ionized gas), which conducts electricity. It is well known (since Michael Faraday'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. 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.

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 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 Earth), and similarly near the south magnetic pole. 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 magnetotail, and some exist in the upward extension of the ionosphere, which may extend (with diminishing density) some 25,000 km around Earth.

Bright auroras are generally associated with Birkeland currents (Schield et al., 1969;[18] Zmuda and Armstrong, 1973[19]) 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 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[20]), 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).

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. 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.

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 Alfvén (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

Aurora Borealis from Expedition 6.ogg
Aurora Borealis from the International Space Station

Again, our understanding is very incomplete. A rough guess may point out three main sources:

  1. 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.
  2. Dynamo action involving plasma squeezed towards Earth 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 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.
  3. 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.
Diffuse aurora observed by the DE-1 satellite

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 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.[citation needed] "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

Jupiter aurora. The bright spot at far left is the end of field line to Io; spots at bottom lead to Ganymede and Europa.
An aurora high above the northern part of Saturn. Image taken by the Cassini spacecraft spacecraft. A movie showing images from 81 hours of observations of Saturn's aurora is also available.

Both Jupiter and Saturn 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 Telescope. Uranus and Neptune have also been observed to have auroras.[21]

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. 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 Io, Europa, and Ganymede themselves, e.g., using the Hubble Space Telescope. 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 Express. The aurora was located at Terra Cimmeria, 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 Surveyor, 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.[21][22]

History of aurora theories

In the past theories have been proposed to explain the phenomenon. These theories are now obsolete.

  • Benjamin Franklin 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.[23]
  • 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 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 energy.
  • The aurora is produced by solar wind 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

25 Second exposure of the Aurora Australis from Amundsen-Scott S.P.S.

Images of aurora are significantly more common today due to the rise of use of digital cameras that have high enough sensitivities.[24] 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 Malin pioneered multiple exposure using multiple filters for astronomical photography, recombining the images in the laboratory to recreate the visual display more accurately. [1] 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. [2] Terrestrial features often find their way into aurora images, making them more accessible and more likely to be published by the major websites.[25] It is possible to take excellent images with standard film (using ISO ratings between 100 and 400) and a single-lens reflex camera with full aperture, a fast lens (f1.4 50 mm, for example), and exposures between 10 and 30 seconds, depending on the aurora's display strength.[26]

Early work on the imaging of the Aurora was done in 1949 by the University of Saskatchewan using the SCR-270 radar.

In folklore

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".[27]

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 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 lack of references.[28]

The first Old Norse account of norðrljós is found in the Norwegian chronicle Konungs Skuggsjá from AD 1230. 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.[29]

In ancient Roman mythology, Aurora is the goddess of the dawn, 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.[30]

See also


References

  • "Secrets of the Polar Aurora"
  • "Exploration of the Earth's magnetosphere" - overview of the magnetosphere, including auroras; and including extensive bibliographies of scientific articles
  • Eather, Robert H. (1980). Majestic Lights: The Aurora in Science, History, and The Arts. Washington, DC: American Geophysical Union. ISBN 0-87590-215-4.  (323 pages)
  • Syun-Ichi Akasofu (April 2002). "Secrets of the Aurora Borealis". Alaska Geographic Series (Graphic Arts Center Publishing Company) 29 (1). 
  • Savage, Candace Sherk (1994 / 2001). Aurora: The Mysterious Northern Lights. San Francisco: Sierra Club Books / Firefly Books. ISBN 0-87156-419-X.  (144 pages)
  • Hultqvist, Bengt (2007). "The Aurora". in Kamide, Y.; Chian, A. Handbook of the Solar-Terrestrial Environment. Berlin Heidelberg: Springer-Verlag. pp. 331–354. doi:10.1007/978-3-540-46315-3_13. ISBN 978-3-540-46314-6. 
  • Sandholt, Even; Carlson, Herbert C.; and Egeland, Alv (2002). "Optical Aurora". Dayside and Polar Cap Aurora. Netherlands: Springer Netherlands. pp. 33–51. doi:10.1007/0-306-47969-9_3. ISBN 978-0-306-47969-4. 
  • Phillips, Tony (October 21, 2001). "'tis the Season for Auroras". NASA. http://science.nasa.gov/headlines/y2001/ast26oct_1.htm. Retrieved 2006-05-15. 
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Footnotes

  1. ^ Books.Google.com, Bibliographical History of Electricity and Magnetism by Paul Fleury Mottelay. Published by Read Books, 2007, ISBN 1406754765. p 114.
  2. ^ NASA.gov
  3. ^ Birkeland, Kristian (1908). "The Norwegian Aurora Polaris Expedition 1902-3."
  4. ^ NASA.gov
  5. ^ Sciencemag.org
  6. ^ Space.com
  7. ^ Fritz, Hermann (1881). "Das Polarlicht."
  8. ^ Feldstein, Y. (1963). "Some problems concerning the morphology of auroras and magnetic disturbances at high latitudes", Geomagnetism and Aeronomy, 3, 183-192.
  9. ^ Alaska.edu, Solar wind forecast from a University of Alaska website
  10. ^ On the Great Magnetic Disturbance of August 28 to September 7, 1859, as Recorded by Photography at the Kew Observatory. (Abstract) by Balfour Stewart, Proceedings of the Royal Society of London, Vol. 11, (1860 - 1862), pp. 407-410
  11. ^ JSTOR.org, On the Great Magnetic Disturbance Which Extended from August 28 to September 7, 1859, as Recorded by Photography at the Kew Observatory by Balfour Stewart, Philosophical Transactions of the Royal Society of London, Vol. 151, (1861), pp. 423-430
  12. ^ NYtimes.com, The Aurora Borealis; The Brilliant Display on Sunday Night. Phenomena Connected with the Event. Mr. Meriam's Observations on the Aurora--E. M. Picks Up a Piece of the Auroral Light. The Aurora as Seen Elsewhere--Remarkable Electrical Effects.; New York Time, August 30, 1859, Tuesday; Page 1, 3087 words
  13. ^ a b NYtimes.com, Auroa Australis; Magnificent Display on Friday Morning Mr. Merlam's Opinions on the Bareul Light--One of his Friends Finds a Place of the Aurora on his Lion-corp. The Aurural Display in Boston.; New York Times, September 3, 1859, Saturday; Page 4, 1150 words
  14. ^ NYtimes.com, Auroral Phenomena; Remarkable Effect of the Aurora Upon the Telegraph Wires; New York Times, September 5, 1859, Monday; Page 2, 1683 words
  15. ^ a b Eyewitness reports of the great auroral storm of 1859 Advances in Space Research, Volume 38, Issue 2, 2006, Pages 145-154, Green, et al.
  16. ^ Books.Google.com, The Journal of Education for Upper Canada; The Late Aurora Borealis and the Telegraph 1859, Page 132, Ryerson, et al.
  17. ^ The British Colonist, Vol. 2 No. 56, October 19, 1859, page 1, accessed online at BritishColonist.ca, on February 19, 2009.
  18. ^ Schield, M. A.; Freeman, J. W.; & Dessler, A. J. (1969) "A Source for Field-Aligned Currents at Auroral Latitudes", Journal of Geophysical Research, 74, 247-256.
  19. ^ Armstrong J. C., & Zmuda, A. J. (1973). "Triaxial magnetic measurements of field-aligned currents at 800 kilometers in the auroral region: Initial results", Journal of Geophysical Research, 78, 6802-6807.
  20. ^ Persson, Hans (1963). "Electric field along a magnetic line of force in a low-density plasma", Physics of Fluids, 6, 1756-1759.
  21. ^ a b ESA Portal - Mars Express discovers aurorae on Mars
  22. ^ Universe Today » Mars Express Finds Auroras on Mars
  23. ^ LOC.gov
  24. ^ SpaceWeather.com - News and information about meteor showers, solar flares, auroras, and near-Earth asteroids
  25. ^ SpaceWeather.com
  26. ^ SpaceWeather.com 2001 image
  27. ^ Bullfinch's Mythology
  28. ^ Viking Answer Lady Webpage - The Aurora Borealis and the Vikings
  29. ^ IRF.se
  30. ^ University of Minnesota Style Manual

External links


Travel guide

Up to date as of January 14, 2010
(Redirected to Northern Lights article)

From Wikitravel

This article is a travel topic.

long-exposure photo of the Aurora Borealis in Alaska
long-exposure photo of the Aurora Borealis in Alaska

The Northern Lights or Aurora Borealis is a natural phenomenon that can paint the night sky with unearthly, surreal color. To observers at far-northern latitudes, they're a frequent occurrence, but many who live in more temperate climates have never seen them, even though they're sometimes seen as far south as 35 degrees north latitude. This article will help you improve your chances of seeing the Lights if you journey north.

Understanding the lights better

The Northern Lights are similar to a sunset in the sky at night, but appear occasionally in arcs or spirals usually following the earth's magnetic field. They are most often light green in color but sometimes a variety of other colors. The Aurora Borealis is caused by charged particles ejected from the sun. When these particles reach the earth, they collide with gas atoms in the earth's atmosphere causing them to energise which results in a spectacular multi-coloured light show.

Northern Lights in Finnish Lapland
Northern Lights in Finnish Lapland

Contrary to intuition, seeing the Northern Lights isn't just a matter of heading "north." The Lights usually circle the globe in a circular or elliptical band centered on the earth's North Magnetic Pole, which is not at the same location as the North Geographic Pole, but rather is offset in the direction of northern Canada. Furthermore, auroral displays aren't strongest at the North Magnetic Pole; the band of greatest auroral activity is usually offset from the Magnetic Pole by 20 degrees or so.

This quirk is actually fairly convenient for would-be aurora watchers. It means that locations in the north-central United States, for example Minnesota and North Dakota, and also southern Canada see Northern Lights much more frequently than they would if the Lights were centered on the North Geographic Pole. Alaska and Lapland (the northern part of Finland, Norway and Sweden) also fall in the region of greatest probability, while the far-north territory of Siberia that misses out on some of the Lights (because the Magnetic Pole is displaced away from that region) tends to be more inaccessible to the traveler.

A curiosity is that the exact location of the North Magnetic Pole varies from year to year, sometimes by tens of miles. The Pole has been moving north for a few years now as of 2006; it's now near Ellesmere Island in the nearly uninhabited far north of Canada. As a consequence, the advantages of being on the "right side" of the earth are not as pronounced as they were some years ago. Still, there's a slight North American bias even today in your chance of seeing the Lights.

This said, the actual latitudes of the Lights vary considerably. In times of high solar activity (more on that later), the Lights may be seen in North America at latitudes as low as 35 degrees north, meaning that all but the southernmost parts of the United States may get a display. The offset of the Pole keeps solar storms from benefiting Europe quite as strongly, but most of the countries of northern Europe will get displays during periods of solar storms.

When

There are both seasonal and long-term variations in the likelihood of a Northern Lights display. On a yearly basis, the Lights are at their peak in September and March, but also in October and April. The reasons for this trend aren't fully known, but it's definitely real, not just an artifact of the weather or other viewing conditions. The Lights can best be seen at around midnight.

In the longer term, auroral displays are correlated with an 11-year cycle in sunspot activity and other perturbations of the sun; the more restless the sun, the more aurorae. Unfortunately, 2006-7 corresponds to a minimum in solar activity, and therefore the number of Northern Lights-lit nights. However, at the most favorable latitudes, the Lights are still likely to be seen even at solar minimum; it's mainly at lower latitudes that they get scarce during the inactive times. The next maximum in solar activity will be in about 2012, with frequent Northern Lights displays likely for another two or three years after that, so you have plenty of time to plan a trip.

In addition to these more or less regular variations in frequency of the aurora, there are also less predictable, erratic displays resulting from solar storms. Some of these, particularly near solar-activity maximum, can lead to visible Northern Lights remarkably far south, if you're in an area with clear, transparent night skies. The "Alerts" section below will help you stay on top of solar activity and prepare for some viewing when a solar storm does occur.

Last but not least, don't forget the weather forecast — aurora occur very high up in the atmosphere, and if there are clouds in the way you will not see anything.

Prepare

Alerts

If you have the luxury of being able to travel into aurora-viewing territory on short notice, you can improve your chances of seeing something by being aware of "space weather," the things going on beyond the earth's atmosphere as a result of solar activity. A good site for this information is http://www.sec.noaa.gov/ operated by the (US) National Oceanic and Atmospheric Administration, precisely for the purpose of keeping up on space weather. (The commercial site http://www.spaceweather.com/ presents much of the same information in digested, more accessible form.)

If a major solar storm develops that is forecast to have a good chance of producing Northern (and Southern) Lights, your time to respond will be measured in hours to a few days, rather than either minutes or weeks. The forecasts will usually include some indication of how far from the magnetic poles the activity is expected to extend. For purposes of travel planning, it's a good idea to plan conservatively and go to a locale somewhat closer to the pole than the maximum extent of the aurora; things don't always work out as forecast, and the Lights may be relatively weak and/or confined to the northern horizon if you're at the southern edge of the activity, either limitation possibly creating difficulties for you in viewing owing to light pollution.

Photography

Taking good pictures of the Northern Lights is very difficult, since they're fast-moving, often faint and against a pitch-dark background, all of which befuddles consumer point-and-shoots. Here's what you need for a sporting chance:

  • A camera that supports manual exposure (5 to 40 seconds).
  • A fast lens (aperture f/2.8 or better)
  • Fast film (800 ASA or better), or equivalent ISO setting on a digital camera
  • A tripod to hold the long exposure
  • Cable release or self-timer to trigger shots without stirring the camera
  • Kangerlussuaq, Greenland: 99% Chance of visual between November to March. If Husky and snowmobile wides are intended, mid-late winter is recommended.
  • Jukkasjärvi, Northern Sweden

Stay safe

Okay, so how about the Southern Lights?

Aurorae happen in a circle/ellipse about the South Magnetic Pole just as they do about the north one, and the South Magnetic Pole is similarly offset from the geographic South Pole. Would-be observers of the Southern Lights or Aurora Australis benefit from the happy accident that the offset of the South Magnetic Pole is generally in the direction of Australia, although the Pole itself is still in Antarctica like the geographic one. Tasmania is therefore relatively favored for the Aurora Australis, and southern Australia gets more than its share of Lights relative to latitude. All of the considerations about maximizing your chances of seeing the Northern Lights apply equally to seeing the southern ones.

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Gaming

Up to date as of February 01, 2010

From Wikia Gaming, your source for walkthroughs, games, guides, and more!

Aurora Borealis


Aurora and Psyche "get back together" in City of Heroes, V2 #1

Statistics
Real name Aurora Scott
Status Active
Affiliations Vindicators
Previous affiliations Freedom Phalanx
Notable powers Energy (light ?) blasts, Telepathy
Notes Independent at last.


Aurora Borealis is one of the signature heroes of City of Heroes.

History

Aurora had a very unique introduction to the "big time" of the hero world. When Sister Psyche burned out during the Rikti War, Aurora was the one her comatose mind choose to 'mind-ride'. for the next three years both women shared the same body, and generally went by the name of Sister Psyche. In the end it was her husband, Calvin Scott, who had enough of it. Though not an experienced contact, he was able to direct heroes though task forces that eventually led to Psyche being freed back into her own body, although her mental control of Malaise was lost in the process. Aurora then took on his role as Psyche's sidekick, although it's amazing the two of them can still stand each other after so long of forced cohabitation.

In-Game

Aurora is the trainer for Independence Port, near the hospital where Sister Psyche once rested.


This article uses material from the "Aurora Borealis" article on the Gaming wiki at Wikia and is licensed under the Creative Commons Attribution-Share Alike License.

Simple English

Redirecting to Aurora


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