An aurora is an erratic, usually dim, atmospheric phenomenon typically observed in the night sky from locations at high latitudes. They are more commonly known as the "northern lights" and they may first appear as a faint, opaque lights low in the north. They are too dim for the human eye to detect any color but vivid enough to outline to clouds close to the horizon and they could develop into a stable greenish bow or form dazzling, twirling curtains of yellow-green light. The most striking exhibits that are observable are from areas at middle latitudes, such as central Europe and the United States. A crimson glow pervades a large amount of the heavens and it was this figure that provoked European scientists of the 1600s to call the occurrence aurora borealis, which literally "northern dawn." However, it also takes place at high southern latitudes, where it is properly called aurora australis, "southern dawn." Similar processes happen in both hemispheres, not just on Earth, but on other planets as well. And today, scientists simply refer to this phenomenon as an aurora. The ethereal body of an aurora is comprised of quiescent patches, veils, and arcs, and rapidly moving rays and curtains. (http://www.astronomy.com/asy/default.aspx?c=a&id=2088)

            Auroras are created by the Van Allen belt which contains mainly ions of elements from interstellar space. These particles are confined in earth’s magnetic field. Charged particles favorably move in the direction of earth magnetic field lines, and not across the field lies. These particles, often from solar magnetic storms or from earth’s upper atmosphere, are guided by the earth’s magnetic field towards the earth magnetic poles. When they interact with air molecules, they cause the atmosphere to glow, which we see as n and s lights—aurora borealis and australis. (The Cosmos, P. 106)

            The factors that make them up are a combination of our atmosphere, magnetic field, charged particles, and the sun’s atmosphere and magnetic field. The air we breathe in our atmosphere is really a mixture of several gases, mostly nitrogen and oxygen, with traces of hydrogen, helium and various compounds. The existence of a strong magnetic field on Earth is essential in that the field lines go into and out of the Earth around its magnetic poles, in which the lines are closest together the field is strongest. Also, there is invisible plasma around the Earth, made of lots of charged particles and there are always electrons and positive ions in the surrounding magnetic field. Charged particles in a magnetic field are guided by the field and they travel along magnetic field lines as if they were wires, circling around the lines in a long spiral as they go. Charged particles are the "ammunition" of an aurora and the energetic electrically charged particles (mostly electrons) accelerate along the magnetic field lines into the upper atmosphere, where they collide with gas atoms, causing the atoms to give off light.

            The Sun also has an atmosphere and a magnetic field that extend into space. The Sun's atmosphere is made of helium and hydrogen, which is itself made of subatomic particles: protons and electrons and these particles are constantly boiling off the Sun and streaming outward at very high speeds.  Together, the Sun's magnetic field and particles are called the "solar wind." Auroras are caused be the Sun's solar wind, a continuous stream of charged particles that heads out into the solar system. When these particles reach the Earth, they are deflected towards the North and South poles by its magnetic field, then the particles then hit the atmosphere, and cause the shimmering lights to appear. The strength of the solar wind varies over an 11 year cycle, connected to sunspot activity. Sometimes the sun violently throws off material in an event called a coronal mass ejection (CME). If these head towards the Earth as part of the solar wind, the auroras produced are particularly impressive! In fact they can even be seen as far south from the Arctic as England. 

            This wind is always pushing on the Earth's magnetic field, changing its shape and the compressed field around the earth the magnetosphere. The Earth's field is compressed on the day side, where the solar wind flows over it. It is also stretched into a long tail like the wake of a ship, which is called the magnetotail, and points away from the Sun. Squeezing the Earth's magnetic field takes energy, just the way it takes energy to compress a balloon with air in it. The whole process is still not fully understood, but energy from the solar wind is constantly building up in the magnetosphere, and this energy is what powers auroras.

            Solar particles are always entering the tail of the magnetosphere from the solar wind and moving toward the Sun. Now and then, when conditions are right, the build-up of pressure from the solar wind creates an electric voltage between the magnetotail and the poles and it can reach about 10,000 volts! The voltage pushes electrons (which are very light) toward the magnetic poles, accelerating them to high speeds and they zoom along the field lines towards the ground to the north and south, until huge numbers of electrons are pushed down into the upper layer of the atmosphere, called the ionosphere.

            In the ionosphere, the speeding electrons collide violently with gas atoms and  this gives the gas atoms energy, which causes them to release both light and more electrons. In this way, the gases of the ionosphere glow and conduct flowing electric currents into and out of the polar region and the electrons flowing back out don't have as much energy as the speedy incoming ones had - that energy went into creating the aurora. Solar wind is the power source for auroras. It has also been known for a long time that there is a connection between activity on the Sun and auroral activity on the earth. The Sun and its wind are constantly changing; thus, the flow of particles and the intensity of the solar wind's magnetic field increase when the Sun is more active. Scientists now know that certain kinds of high-energy solar events can result in very large and unusual auroras. 

            `The Earth’s magnetosphere is stretched from a simple, symmetrical dipole field into a windsock-shaped bubble, that is, it has a north and south magnetic pole (not to be confused with the geographic poles). If there were no solar wind, Earth's magnetosphere would make a near-perfect dipole, but when the magnetized solar wind blows past the Earth, it compresses the day (or sunlit) side of Earth's field and stretches out the night side into a long tail. 

            These types of solar activity include coronal mass ejections (CME's) and sudden solar flares. In these events, parts of the Sun's outer atmosphere practically explode, producing huge bursts of solar wind packed with as many sub-atomic particles as a mountain. It takes 2 to 4 days for solar wind and particles to reach Earth. When these events arrive, they strike the magnetosphere like a shock wave and inject huge amounts of energy into the magnetic field, often causing enormous and unusual auroras. We will also see that such intense "gusts" of solar wind can affect where auroras can be seen.

            The colors of auroras vary depending on the height at which most of the collisions occur. Street lamps and 'neon' signs emit different colors of light due to the types of gas inside them and the same applies to auroras.  If the predominant gas is oxygen, as it is above 300 km altitude, auroras will be red, but this is rare, and only occurs at times of maximum solar activity. The most common color is yellow or green, again caused by oxygen, but at lower altitudes and Nitrogen at about 100 km produces a red light often seen at the lower edges of auroras. 

            Very high in the ionosphere (above 300 km or 180 miles), oxygen is the most common atom, and collisions there can create a rare red aurora. The strong yellow-to-green light that is most common is produced by collisions with oxygen at lower altitudes, between 100 and 300 km. Around 100 km, nitrogen molecules produce a red light that often seems to form the lower fringes on aural curtains. Lighter gases high in the ionosphere, like hydrogen and helium, make colors like blue and purple, but our eyes cannot always see them in the night sky. Good photographic film can be more sensitive to some colors than our eyes. Eyes see best in the green-yellow-orange part of the spectrum, where the Sun emits most of its light.

            Looking at the auroras from space, they look like almost circular bands of light around the North and South Poles. At the North Pole, it's called aurora borealis, or northern lights, and at the South Pole it's called the aurora australis, or southern lights. From spacecraft observations made in October, 2002, scientists noticed that these circular bands of aurora shift in opposite directions to each other depending on the orientation of the sun's magnetic field, which travels toward the Earth with the solar wind flow. They also noted that the auroras shift in opposite directions to each other depending on how far the Earth's northern magnetic pole is leaning toward the sun.
            The magnetosphere is complex and vast and the structure is shaped by the Solar wind, and solar storms that can affect communications, power grids, and other important aspects of modern life on Earth.  What was most surprising was that both the northern and southern auroral ovals were leaning toward the dawn (morning) side of the Earth for this event. The scientists suspect the leaning may be related to "imperfections" of the Earth's magnetic field. The Aurora Australis, or Southern Lights shows a spiked band of red and green aurora above the Earth's Limb. Calculated to be at altitudes ranging from 80 - 120 km (approx. 50-80 miles), the auroral light shown is due to the "excitation" of atomic oxygen in the upper atmosphere by charged particles (electrons) streaming down from the magnetosphere. The Earth's magnetic field provides an obstacle in the flow of the solar wind, and it becomes compressed into what looks like an extended tear-drop shaped bubble known as the "magnetosphere," which protects the Earth by shielding it from the solar wind. However, under certain conditions charged particles from the solar wind are able to get through Earth's magnetic shield and get energized. When this happens, they crash into the Earth's upper atmosphere and create the light which we see as an "aurora."

            There are numerous historical accounts of the northern lights from places far south of its common position. The Roman philosopher Seneca wrote that an aurora in 37 a.d. hoaxed the emperor into sending troops to assist what he thought was the blazing seaport of Ostia, "when the glowing of the sky lasted through a great part of the night, shining dimly like a vast and smoking fire." An early Chinese record described it as a "red cloud spreading all over the sky." In 1583, similar "fires in the air" assembled thousands of French pilgrims, who prayed to avoid the fury of God. On September 15, 1839, an intense aurora dispatched firefighters all over London.

            Some current information include that scientists looking at the Earth’s northern and southern auroras were surprised to find they are not mirror images of each other, as was once thought. The main cause behind the differences appears to be the interaction between the Sun’s outer atmosphere and the Earth’s magnetic field. Analysis of the images from NASA’s Polar spacecraft and the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft showed how the auroras move and change, based on the “tilt” of the Earth’s magnetic field toward the Sun and conditions in the solar wind. By knowing how auroras react to the solar wind, scientists can better determine the impacts of space weather in the future. The new discovery by scientists from NASA, the University of Iowa, Iowa City, and the University of California at Berkeley, shows that auroras may be more complicated than previously thought.
            The aurora form near-circular bands around both the northern and southern poles of the Earth, known as the auroral ovals. These phenomena also are known as the aurora borealis, or northern lights, and the aurora australis, or southern lights, but it was expected that the auroral ovals would be mirror images of each other.  ”This is the first analysis to use simultaneous observations of the whole aurora in both the Northern and Southern Hemispheres to track their locations,” said lead author Timothy J. Stubbs of the Laboratory for Extraterrestrial Physics at NASA’s Goddard Space Flight Center, Greenbelt, Md.
            The Sun’s outer atmosphere is an extremely thin electrified gas, or “plasma,” better known as the “solar wind,” since it blows constantly out from the Sun at around 250 miles per second. The Earth’s magnetic field provides an obstacle in the solar wind flow and becomes compressed into an extended teardrop-shaped bubble known as the “magnetosphere.”  The magnetosphere protects the Earth by shielding it from the solar wind. However, under certain conditions charged particles from the solar wind are able to penetrate this magnetic shield and become energized. Collisions between these charged particles and the Earth’s upper atmosphere emit light which we observe as an “aurora.”
            Stubbs and his colleagues used data from the two spacecraft to study the auroras. By luck the orbits of Polar and IMAGE were aligned so the entire auroral ovals in both hemispheres could be observed simultaneously in detail. Stubbs and his colleagues noted four important items in their study of auroras observed in October 2002. As predicted, they observed the auroral ovals shift in opposite directions to each other depending on the orientation of the Interplanetary Magnetic Field (IMF). The IMF is the Sun’s magnetic field that travels out into space with the solar wind. They noted the auroral ovals also shift in opposite directions to each other depending on how far the Earth’s northern magnetic pole is leaning toward the Sun (known as the “dipole tilt angle”). Following a change in the orientation of the IMF, they observed the southern auroral oval shift toward the Sun while the northern auroral oval remained in about the same location. The scientists believe the southern aurora moved because the solar wind was able to penetrate into the magnetosphere in the southern hemisphere, but not in the northern hemisphere.
            What was most surprising was that both the northern and southern auroral ovals were leaning toward the dawn (morning) side of the Earth for this event. The scientists suspect the leaning may be related to “imperfections” of the Earth’s magnetic field. The Earth has a similar type of magnetic field to that which occurs around a simple bar magnet, which causes iron filings to arrange themselves in loops around it. ”Because Earth’s magnetic field is not a perfect dipole, we think this fact plays some role in causing the auroras to not be mirror images of each other,” Stubbs said. 

The Cosmos, P. 106
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http://www.geo.mtu.edu/weather/aurora/

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http://www.astronomy.com/asy/default.aspx?c=a&id=2088

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(http://www.exploratorium.edu/learning_studio/auroras/index.html)

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www.geo.mtu.edu/weather/aurora/