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Auroral mechanism The aurora is now known to be caused by electrons of typical energy of 1-15 keV, i.e. the energy obtained by the electrons passing through a voltage difference of 1,000-15,000 volts. The light is produced when they collide with atoms of the upper atmosphere, typically at altitudes of 80-150 km. It tends to be dominated by emissions of atomic oxygen--the greenish line at 557.7 nm and (especially with electrons of lower energy and higher altitude) the dark-red line at 630.0 nm. Both these represent forbidden transitions of atomic oxygen from energy levels which (in absence of collisions) persist for a long time, accounting for the slow brightening and fading (0.5-1 sec) of auroral rays. Many other lines can also be observed, especially those of molecular nitrogen, and these vary much faster, revealing the true dynamic nature of the aurora. Auroras can also be observed in ultra-violet (UV) light, a very good way of observing it from space (but not from ground--the atmosphere absorbs UV). The Polar spacecraft even observed it in X-rays. The image is very rough, but precipitation of high-energy electrons can be identified. Auroral forms and magnetism
The 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 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. 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 distances. Frequency of occurrence
Auroral events of historical significance The aurora which occured 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 occured on September 2nd as a result of the exceptionally intense Carrington-Hodgson white light solar flare on September 1st, produced aurora so widespread and extraordinarilly 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 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. 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 majorly 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 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 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 undoubtedly 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. 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 plasmas conduct easily along magnetic field lines but not so 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 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 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. 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 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 of aurora
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. 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. * Obsolete theories Auroral sounds Throughout history people have written and spoken of sounds associated with auroral displays, often describing them as crackling, hissing, buzzing, or whistling. Danish explorer Knud Rasmussen mentioned them indirectly in 1932 while describing the folk traditions of Greenland Eskimos. The same sounds in the same context are mentioned in an account written by Canadian anthropologist Ernest Hawkes in 1916. Cornelius Tacitus (AD 56-120), an ancient Roman historian, wrote that people from the north of (modern) Germany claimed to hear them. Today, many people continue to report these sounds, but despite their many anecdotal reports, nobody has yet managed to record the sounds, and there are scientific problems with the idea of the sounds being true sound waves originating in the auroras. The energy of the auroras and other factors make it extremely improbable that any sounds directly produced by auroral discharges would reach the ground, and the coincidence of sounds with the visible changes in the auroras conflicts with the necessary propagation time for any sounds from the discharges themselves. Some people speculate that local electrostatic phenomena induced by the auroras might explain the sounds; theories associated with brush discharges seem to fit the reported observations best, although no theory thus far provides a completely satisfactory explanation. 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. 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. 2001 image Aurora 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". * 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. * The first Old Norse account of norðurljós 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 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. In Estonian they are called virmalised, spirit beings of higher realms. On some legends they are given negative characters, on some positive ones. 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 kill him. 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. 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. Aurora in popular culture | |||||||||||||||
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