An aurora (plural: aurorae or auroras) is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere). The charged particles originate in the magnetosphere and solar wind and, on Earth, are directed by the Earth's magnetic field into the atmosphere.
Aurora is classified as diffuse or discrete aurora. Most aurorae occur in a band known as the auroral zone which is typically 3° to 6° in latitudinal extent and at all local times or longitudes. The auroral zone is typically 10° to 20° from the magnetic pole defined by the axis of the Earth's magnetic dipole. During a geomagnetic storm, the auroral zone will expand to lower latitudes.
The diffuse aurora is a featureless glow in the sky which may not be visible to the naked eye even on a dark night and defines the extent of the auroral zone. The discrete aurora are sharply defined features within the diffuse aurora which vary in brightness from just barely visible to the naked eye to bright enough to read a newspaper at night. Discrete aurorae are usually observed only in the night sky because they are not as bright as the sunlit sky. Aurorae occur occasionally poleward of the auroral zone as diffuse patches or arcs (polar cap arcs) which are generally invisible to the naked eye.
In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas, by Pierre Gassendi in 1621. Auroras seen near the magnetic pole may be high overhead, but from farther 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.
Discrete aurorae often display magnetic field lines or curtain-like structures, and can change within seconds or glow unchanging for hours, most often in fluorescent green. The aurora borealis most often occurs near the equinoctes. The northern lights have had a number of names throughout history.
Its southern counterpart, the aurora australis (or the southern lights), has almost identical features to the aurora borealis and changes simultaneously with changes in the northern auroral zone and is visible from high southern latitudes in Antarctica, South America, New Zealand and Australia.
There is the claim from 1855 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 armor 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 not a vast body of evidence in the Old Norse literature giving this interpretation, or even much reference to auroras. Although auroral activity is common over Scandinavia and Iceland today, it is possible that the Magnetic North Pole was considerably farther away from this region during the relevant period of Norse mythology. Today, the Northern Lights are visible in Iceland from September to April.
The first Old Norse account of Noroljos is found in the Norwegian chronicle Konungs Skuggsja from AD 1230, (long after the Viking age). 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.
Magnetic control of the aurora was mentioned by Ancient Greek explorer/geographer Pytheas, Hiorter, and Celsius described in 1741 evidence that large magnetic fluctuations occurred whenever the aurora was observed overhead. It was also later realized that large electric currents were associated with the aurora, flowing in the region where auroral light originated. Multiple superstitions and obsolete theories explaining the aurora have emerged over the centuries.
Seneca speaks diffusely on auroras in the first book of his Naturales Quaestiones, drawing mainly from Aristotle; he classifies them "putei" or wells when they are circular and "rim a large hole in the sky", "pithaei" when they look like casks, "chasmata" from the same root of the English chasm, "pogoniae" when they are bearded, "cyparissae" when they look like cypresses), describes their manifold colors and asks himself whether they are above or below the clouds. He recalls that under Tiberius, an aurora formed above Ostia, so intense and so red that a cohort of the army, stationed nearby for fireman duty, galloped to the city.
In 1619 A.D., Galileo Galilei coined the term "aurora borealis" after Aurora, the Roman goddess of morning renewing herself every morning to fly across the sky, announcing the arrival of the sun. He had the misconception that the auroras he saw were due to sunlight reflecting from the atmosphere. The persona of Aurora the goddess has been incorporated in the writings of Shakespeare, Lord Tennyson, and Thoreau. The name Aurora, however, simply comes from the Latin word for the dawn. The goddess was not associated with polar light phenomena, in Roman myth.
In the traditions of Aboriginal Australians, the Aurora Australis is commonly associated with fire. For example, the Gunditjmara people of western Victoria called auroras "Puae buae", meaning "ashes", while the Gunai people of eastern Victoria perceived auroras as bushfires in the spirit world. When the Dieri people of South Australia said that an auroral display was "Kootchee", an evil spirit creating a large fire. Similarly, the Ngarrindjeri people of South Australia referred to auroras seen over Kangaroo Island as the campfires of spirits in the 'Land of the Dead'. Aboriginal people in southwest Queensland believed the auroras to be the fires of the "Oola Pikka", ghostly spirits who spoke to the people through auroras. Sacred law forbade anyone except male elders from watching or interpreting the messages of ancestors they believed were transmitted through auroras.
American Civil War
After the Battle of Fredericksburg, the lights could be seen from the battlefield that night. The Confederate Army took it as a sign that God was on their side during the battle as it was very rare that one could see the lights in Virginia. The painting Aurora Borealis (1865) by American landscape painter Frederic Edwin Church is widely interpreted to represent the conflict of the American Civil War.
Native American Mythology
A variety of Native American myths surround the spectacle. Early European explorer Samuel Hearne traveled with Chipewyan Dene in 1771 and recorded their views on the aurora borealis, or the "ed-thin", as they called it, meaning caribou. Dene experience was that stroking caribou fur created sparks much like the aurora. They also believed that the lights were the spirits of their departed friends dancing in the sky, and when the lights shined the brightest it meant that their deceased friends were very happy. The Cree called the phenomenon the "Dance of the Spirits". In Medieval Europe, the auroras were commonly believed to be a sign from God.
Walter William Bryant wrote in his book Kepler (1920) that Tycho Brahe "seems to have been something of a homeopathist, for he recommends sulfur to cure infectious diseases "brought on by the sulphurous vapors of the Aurora Borealis."
Europe Middle Ages ....
In Europe, in the Middle Ages, the auroras were commonly believed a sign from God.
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.
Auroras occur on other planets. Similar to the Earth's aurora, they are visible close to the planet's magnetic poles.
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 extensive radiation belts. Auroras have been observed on both, most clearly with the Hubble Space Telescope. Uranus and Neptune have also been observed to have auroras.
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 also have been observed on the surfaces of Io, Europa, and Ganymede, using the Hubble Space Telescope.
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 was a flux of electrons moving along the crust magnetic lines and exciting the upper atmosphere of Mars.
The brown dwarf star LSR J1835+3259 was discovered to have auroras in July 2015, the first extra-solar auroras discovered. The aurora is a million times brighter than the Northern Lights, mainly red in color, because the charged particles are interacting with hydrogen in its atmosphere. It is not known what the cause is. Some have speculated that material maybe being stripped off the surface of the brown dwarf via stellar winds to produce its own electrons. Another possible explanation is an as-yet-undetected planet or moon around the dwarf, which is throwing off material to light it up, as is the case with Jupiter and its moon Io. More about Auroras
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