The Aurora

1. The Aurora By Alexandria C. Kakela

2. General • Auroras are associated with the solar wind, a flow of electrically charged 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 cause energy releases in the form of auroras appearing in large circles around the poles. Seen from space, these fiery curtains form a thin ring. • There are different colors at different altitudes. At high altitude oxygen red dominates, then oxygen green, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. • Green is the most common of all auroras. Behind it is pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and green), and lastly pure blue.

3. Where and when • An auroral display in the Northern Hemisphere is called the aurora borealis, or the northern lights. A similar phenomenon in the Southern Hemisphere is called the aurora australis. • They appear chiefly as arcs, clouds, and streaks - some move, brighten, or flicker suddenly. The most common color in an aurora is green, but displays that occur extremely high in the sky may be red or purple due to differences in the composition of the air. Most auroras occur about 60 to 620 miles above the earth. Some extend lengthwise across the sky for thousands of miles. • The auroras follow the sunspot cycle. Solar activity runs on a 22-year cycle — 11 positive years and 11 negative years. Auroras are more frequent and brighter during the positive phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind.

4. Old aurora theories • 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. • 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 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 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.

5. Mythology and popular culture • 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. • In Bulfinch's Mythology 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 armor sheds a strange flickering light, which flashes up over the northern skies, making what men call the "aurora borealis", or "Northern Lights". • The first Old Norse account is found in the Norwegian chronicle Konungs Skuggsjá from AD 1230. The chronicler has heard about this phenomenon from friends returning from Greenland, and he gives three possible explanations: that the ocean was surrounded by huge 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. • English author Philip Pullman's trilogy of novels, His Dark Materials, explores the nature of the aurora from a science-fiction standpoint. They are said to be areas where the boundaries between parallel universes are especially weak. In the world of Lyra Belacqua, when one peers into the aurora borealis, he or she can faintly see the skyline of a city in another universe. • An image of an aurora was used as part of the emblem for the 1994 Winter Olympics in Lillehammer.

6. On other planets - 1 • Both Jupiter and Saturn have magnetic fields much stronger than Earth's, and both have large 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 are powerful sources of auroras on Jupiter. These arise from electric currents along field lines 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. • Auroras have also been observed on Io, Europa, and Ganymede themselves, using the Hubble Space Telescope. These are generated when Jupiter's magnetospheric plasma impacts their very thin atmospheres.

7. On Other Planets - 2 • 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. The total size of the emission region was about 18 miles across, and possibly about 5 miles high. Scientists observed that the region of the emissions corresponded to an area where the strongest magnetic field is localized. This indicates that the origin of the light emission actually was a flux of electrons moving along the crust and exciting the upper atmosphere of Mars.

8. Bibliography • www.wikipedia.org • http://fairbanks-alaska.com/northern-lights-alaska.htm • http://www.grantchronicles.com/auroraborealismk.htm