Fascinating Polar Lights
Aurora Borealis - also known as the Northern Lights - is a vivid demonstration of the Earth's magnetic field interacting with charged particles from the Sun. It's beautiful too, and worth braving a cold night when visiting the high latitudes of the northern (or southern).
Aurora borealis are centered on the Earth's magnetic poles, visible in a roughly circular region around them. Because the magnetic and geographic poles are not identical, sometimes the northern lights are visible further south than one might expect, while in other places it is further north. [Aurora photos: Northern lights dazzles in night sky pictures]
In the Northern Hemisphere, the Northern Lights Zone runs along the northern coast of Siberia, Scandinavia, Iceland, the southern tip of Greenland, and northern Canada and Alaska. Northern lights are visible south of the zone, but they are less likely to occur the further you go. The Southern Hemisphere auroral zone is mostly over Antarctica or in the Southern Ocean. The southern lights (or Aurora Australis) are a must to Tasmania, and there are occasional sightings in southern Argentina or the Falkland Islands - but these are rare. Here are some dazzling facts about these light shows.
Different ions make different colors
Aurora displays are created when protons and electrons stream from the sun's surface and slam into the earth's magnetic field. Since the particles are charged, they move in spirals along the magnetic field lines, the protons in one direction and the electrons in the other. These particles in turn hit the atmosphere. As they follow the magnetic field lines, most of them enter the atmospheric gases in a ring around the magnetic poles where the magnetic field lines come together.
The air consists mainly of nitrogen and oxygen atoms with oxygen becoming a larger component on the high altitudes’ auroras pass - starting about 60 miles and going up to 600 miles. When the charged particles hit them, they gain energy. Eventually they relax, give up the energy and let go of photons of certain wavelengths. Oxygen atoms emit green and sometimes red light, while nitrogen is orange or red.
They are visible from spaceSatellites can bring images of aurora out of orbit - and the images they get are pretty eye-catching. In fact, the northern lights are bright enough that they appear strongly on Earth's nightside, even if you were looking at them from another planet. The International Space Station's orbit is inclined enough that it even plows through the heavenly lights. Most of the time no one notices how the charged particle density is so low. Rodney Viereck, director of the Space Weather Prediction Test Bench at the National Oceanic and Atmospheric Administration (NOAA), said the only time it matters is during particularly intense solar storms when radiation is high. At this point, the astronauts only need to move to a more protected area of the station. (Ironically, strong solar storms can actually reduce the amount of radiation around the space station, because of the interactions of charged particles with the Earth's magnetic field). Meanwhile, the ISS astronauts can align beautiful northern lights panoramas.
You have other planetsVoyager 1 and 2 became the first probes to retrieve images of the northern lights on Jupiter and Saturn, and later on Uranus and Neptune. Since then, the Hubble Space Telescope has taken pictures of them as well. Aurora borealis on either Jupiter or Saturn are much larger and stronger than on Earth because these planets magnetic fields are orders of magnitude more intense. On Uranus, northern lights get stranger because the planet's magnetic field is roughly vertical, but the planet rotates on its side. That means instead of bright rings on other worlds, Uranus' aurora borealis look more individual bright spots, at least when indicated by the Hubble Space Telescope spied in 2011. But it's not clear that's always the case because no spaceship has seen the planet up close since 1986.
The lights can move southOccasionally the northern lights are visible further from the poles than usual. During periods of high solar activity, the southern limit for northern lights can be seen as far south as Oklahoma and Atlanta - as it was in October 2011. A likely record at the Battle of Fredericksburg, Virginia in 1862, during the Civil War when the northern lights appeared. It notes many soldiers in their diaries. Viereck said it is actually more difficult now than it was a century ago to tell when the northern lights are very bright because so many Americans live in cities and wash out the lights of the aurora. "You could have had a big auroral storm in New York City and if you looked you didn't notice," he said.
Divine signs?Speaking of this Civil War Aurora, a few observers took the swirling light show as a bad omen (notably Elizabeth Lyle Saxon, who wrote about the phenomenon in her 1905 book, "A Southern Woman's War Time Memories"), although most people only used it as one looked unusual and impressive display. In areas where the lights are infrequent, they were often considered bad omen, like the ancient Greeks. The Inuit, who see the northern lights more often, thought the lights were ghosts playing in the sky, and some groups would tell children not to play outside at night to make the aurora disappear and take them away. Laplanders thought the lights were the spirits of the dead. In the southern hemisphere the Maori and Aboriginal people of Australia have associated fires in the spirit world of the southern lights. Oddly enough, the Norse and Icelandic literature seems to mention a lot of Northern Lights. The Vikings thought the displays could be fires that surrounded the edge of the world, an emanation of flame from the northern ice, or reflections of the sun as it happened on the other side of the earth. All three ideas were considered to be rational, not supernatural explanations in the medieval period.
The northern lights look like fire, but they wouldn't feel like one. Although the temperature of the upper atmosphere can reach thousands of degrees Celsius, the heat is based on the average speed of the molecules. After all, that's what temperature is. But feeling warmth is a different matter - the density of the air is so low at 60 miles (96 km) that a thermometer would register temperatures well below zero where aurora shows to occur.
Cameras see better
Aurora borealis are relatively weak, and the redder light is often at the limit of what human retinas can pick up. Cameras, however, are often delicate, and with a long exposure setting and a clear dark sky you can pick up some spectacular shots.
You can't predict a show
One of the most difficult problems in solar physics is the shape of a magnetic field in a coronal mass ejection (CME), know that's basically a huge blob of charged particles ejected from the sun. Such CMEs have their own magnetic fields. The problem is that it is next to impossible to tell which direction the CME area is hitting until it is pointing. A hit creates a spectacular magnetic storm and dazzling aurora with it or a hiss. There is currently no way to know in advance. NOAA has an online map that can tell you what auroral activity looks like on a given day, show the extent of the "auroral oval" and where you are more likely to catch the light.
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