aurora borealis. click for movie the aurora borealis (northern lights), and the aurora australis...
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• The aurora borealis (northern lights), and the aurora australis (southern lights) are beautiful, dynamic, luminous displays seen in the night sky in the northern and southern latitudes, near the poles.
• The aurora is a very large-scale phenomenon encircling the entire polar regions, but when one views a particular display from the ground only a very small portion is visible. Most aurora have basically a curtain like or ribbon like form. As the auroral activity increases, folds develop, the complexity and extent of the folds depending on the degree of activity. Astronauts looking down on the polar region are in a much better position than those of us who are earthbound to observe the aurora.
• Though the aurora appear to come near to the ground, the light originates high in the atmosphere. The lowest aurora are about 100 kilometers or 62 miles above the ground, with the highest extending to 4 times that distance. This is much higher than clouds or the highest flying aircraft (besides the Space Shuttle).
• Looking toward the south, the crew of the Space Shuttle Endeavor made this stunning time exposure of the aurora australis (southern lights) in April of 1994.
A Side View of the Aurora
Sailing upside down, 115 nautical miles above Earth, the crew of the Space Shuttle Endeavor made this spectacular time exposure of the southern aurora (aurora australis) in October of 1994. The dark object at lower left is the Earth. The aurora is in the atmosphere above Earth.
The aurora is a permanent feature of the earth's upper atmosphere. It is actually an oval centered on the magnetic north and south poles. The magnetic poles are some distance, 700 meters, away from the geographic poles. The size and shape of the auroral ovals change depending on how hard and how fast the solar wind is blowing. When the sun is quiet and the solar wind is calm, the aurora oval is small and thin. When the sun becomes more active and the solar wind hits the earth's magnetic field with strong gusts, the aurora oval becomes wider and stretches south.
As a result of the oval's position, prime viewing is in northern and interior Alaska, and northern Canada. In Fairbanks and Nome, Alaska, the northern lights can be seen almost 200 days a year. In Europe, only the northernmost part of Scandinavia falls in the prime viewing region. In the southern hemisphere, the oval falls mostly over Antarctica, so the southern aurora (aurora australis) is photographed infrequently.
The aurora is always present, but for most people in the United States and Europe, it rarely stretches far enough south for them to see, especially with light pollution in the major population centers. It is not bright enough to see in daylight, so you can see it for more hours in the winter than in summer. The aurora is most often seen around midnight, though bright displays can occur at any time. If a bright display occurs early in
the evening, there is a good chance that another display will follow a couple of hours later.
Red aurora over Alaska
HOW DOES THE AURORA HAPPEN?
The Sun is a stormy place and has its own weather. It is so hot and dynamic that it cannot keep its atmosphere contained by its gravity. Instead, energy flows out from the Sun toward the Earth in a stream of electrified particles. Moving at a million miles per hour, this hot, ionized gas - called plasma - carries particles and magnetic fields from the Sun outward past the planets. This stream of charged particles is called the solar wind. The solar wind is constantly streaming toward Earth from the Sun. When the Sun is more active, observed as sunspots and other changes,
the solar wind blows harder.
PLASMA APPROACHES THE MAGNETOSPHERE
The Earth is shielded from the full blast of these particles by its magnetosphere, a large bubble of magnetic field that deflects the solar wind. Here is an artist's depiction of the blast of the solar wind in white, the Earth and its magnetosphere, depicted as curved lines coming from the Earth, and the satellites NASA has launched to study the solar wind.
PLASMA STRIKES THE MAGNETOSPHERE
This drawing shows the blast of solar wind as it reaches Earth's
CAUSE OF THE NORTHERN LIGHTS
Now we have the idea that the Sun is spewing out charged particles that stream toward Earth and are the cause of the aurora, the northern lights. But, how does this make the sky glow?
Some of the particles in the solar wind are captured by the Earth's magnetic field and speed up as they travel down magnetic field lines toward the auroral ovals. The particles gain energy so that when they blast into the atmosphere they cause the air to glow. When these high energy particles collide with oxygen and nitrogen in Earth's atmosphere, some of the energy is turned into light, the aurora.
EACH COLOR COMES FROM ELECTRONS IN A DIFFERENT MOLECULE
In the upper atmosphere accelerated electrons blast into oxygen and nitrogen atoms and molecules and give them energy. This energy is absorbed causing a "quantum leap" in the electrons of the atom or molecule. This "excited" atom cannot keep this energy for long and releases the energy as a small burst of light, called a photon, of a particular wavelength. The wavelength determines the color or the light. Now the atom or molecule is back to its unexcited state, having given off it's energy in the form of light. Billions of atoms and molecules undergoing these electron excitations are what produce the light in the aurora. The color of the light is determined by the particular "quantum" of energy absorbed and released by the atom or molecule.
ELECTRONS POP BACK INTO ORBITS TO PRODUCE LIGHT
ELECTRONS POP BACK INTO ORBITS TO PRODUCE LIGHT
• Neon lights produce light in the same way. The neon atoms in the gas discharge tubes get energized by the electricity passing through the tube. The atoms become excited and their electrons make "quantum leaps" to outer regions in the atom. When the electrons lose this energy they give off a small burst of light, called a photon, and return to a lower energy level within the atom. Turning off the electricity causes this light to disappear since the electrons have no way to become sufficiently excited without the high voltage of the power supply. Similarly, the electrons from the solar wind have to have sufficient energy to knock an electron into a higher energy state in the atoms and molecules in the upper atmosphere.
Auroral light is similar to light from a color television set. In the picture tube, a beam of electrons strikes the screen, making it glow in different colors, depending on the type of chemicals that coat the picture tube. The chemicals are called phosphors and glow red, green or blue.
Auroral light is from the air glowing as high-energy electrons stream down Earth's magnetic field lines and collide with molecules in the atmosphere. Each gas in the atmosphere glows with a particular color, depending on whether it is neutral or charged, and on the energy of the particle that hits it. Atomic oxygen, about 60 miles up, is the source of the greenish-white light common in aurora displays. High altitude atomic oxygen, about 200 miles up, can also emit a dark red light under some circumstances resulting in the "bloody red" auroras produced during great magnetic storms.
Nitrogen molecules, lower in the atmosphere, produce a red light when they are struck by electrons. This is the faint red that is often seen along the bottom edge of a aurora curtain.
High in the atmosphere nitrogen molecules can become ionized and emit blues and violets.
HOW IS THE AURORA PRODUCED
HOW DOES THE SOLAR WIND ORIGINATE?
The sun's surface temperature is approximately 6,000 Kelvin, much cooler than the interior, which is several million Kelvin. In the Sun's atmosphere, or corona, the temperature rises again to several million Kelvin. At these temperatures collisions between gas particles are so violent that hydrogen atoms disintegrate into charged particles, electrons and protons. This ionized material is called a plasma. This plasma flows outward in all directions and becomes known as the solar wind.
The solar wind is mostly hydrogen ions (protons) and electrons. It carries the magnetic field from the Sun into interplanetary space. The solar wind varies tremendously in speed and density. Its speed and density tend to go up when it comes from active regions on the Sun, like sunspots, solar flares, and coronal holes.
The solar wind streams outward through the solar system. At the distance of the Earth from the Sun, one astronomical unit or 93 million miles, it has an average density of 8 particles per cubic centimenter and an average speed of 400 km/sec (1,440,000 km/hr, 893,000 mi/hr). On average, solar wind originating from around the equator on the Sun takes approximately four days to reach Earth.
WHAT IS THE SOLAR WIND COMPOSED OF?
Observations of comet tails gave scientists the first hint that the Sun produced a solar wind composed of particles. The tail of a comet always points away from the Sun because of the force of the solar wind.
The tail of comet Hyakutake, visible in this recent (March 26, 1996) color image, is composed of dust and gas driven off the icy comet nucleus by the Sun's heat and blown away by the solar wind. Bathed in sunlight, the gas molecules break down and are excited, producing a characteristic glow. This glow is responsible for visible light from the tail.
The image, taken March 7, 1996, by the Solar and Heliospheric
Observatory (SOHO), is an ultraviolet image of the 1 million degree
plumes from the sun's surface near the south pole.
An "eruptive prominence" or blob of 60,000-degree gas, over 80,000 miles long, was ejected at a speed of at least 15,000 miles per hour. The gaseous blob is shown to the left in each image. These eruptions occur when a significant amount of cool dense plasma or ionized gas escapes from the normally closed, confining, low-level magnetic fields of the Sun's atmosphere to streak out into the interplanetary medium, or heliosphere. Eruptions of this sort can produce major disruptions in the near Earth environment, affecting communications, navigation systems and even power grids.
Here is an image captured by NASA's Skylab during an active phase in 1973. It shows a very large solar prominence, from a very active Sun.
• On September 1, 1859, Richard Carrington, an English astronomer and solar physicist, was sketching sunspots and noticed a very bright spot appearing among the spots. He made the first notation of what we now call a solar flare, an intense explosion on the Sun. About one day later, a large part of Europe was covered by active aurora. In fact, there is also some record to indicate that the aurora was seen as far south as Honolulu on that day.
• The graph below shows the solar cycle, the regularly repeating active and quiet phases of the Sun. Below, in red, are years with major auroral activity. There is an obvious correlation between periods when the Sun is very active and the appearance of major aurora.