Most auroras occur in far northern and southern regions. 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. Most auroras occur about 60 to 620 miles (97 to 1,000 kilometers) above the earth. Some extend lengthwise across the sky for thousands of miles or kilometers.

Our sun provides the "fuel" needed to produce the Aurora. The Sun continuously emits charged particles, which are the by-products of thermonuclear reactions occurring inside the Sun. These charged particles, such as protons, electrons, and ions, form the Solar Wind, which travels through space away from the sun at speeds of about 400 km/s (about a million miles per hour), and collides with planets, moons, comets, etc. Results of these collisions can sometimes be quite dramatic. For example, the Solar Wind causes comets' tails to point away from the Sun, and the collision of the Solar Wind with the Earth's atmosphere causes the Aurora.

Before entering the atmosphere, the Solar Wind is being captured by the Earth's magnetosphere. This is the region surrounding the Earth formed by the Earth's magnetic field. To imagine the magnetic field of Earth, think of Earth as a giant bar magnet with its poles more or less aligned with the geographic North and South poles of Earth. The magnetic field lines of this magnet form giant arches, stretching well outward into space, and connecting the poles. As the charged particles of the Solar Wind approach the magnetic field, they are forced to change their course, and begin a spiral motion along the lines of the magnetic field. On the side of the Earth facing the sun, the Earth's magnetosphere is "squashed" by the incoming Solar Wind. On the side of the Earth facing away from the Sun the magnetosphere becomes elongated. This happens when the Earth's magnetic field tries to hold the magnetosphere in place, while the Solar Wind tries to stretch it out. At the Earth's magnetic poles the Earth's magnetic field lines converge to form a "funnel" into which the trapped Solar Wind can be channeled. The magnetic field lines "dressed" with the charged particles of the captured Solar Wind are called the Van Allen Belts. Charged particles of the Solar Wind trapped in the Van Allen Belts, will be sooner or later drawn up to the Earth's magnetic poles via the Earth's magnetic field lines.

Figure 1

The Aurora has a variety of shapes, colors, and structures, and also changes in time. As a result no two Aurorae are the same. There are a series of terms used in describing the various forms of Aurora. One of these terms is the arc, which describes the Aurora as a simple slightly curving arc of light across the night sky. Another term is band, which refers to Aurora which has an irregular shape with kinks or folds. There is also a patch, in which case the Aurora looks like a speck of light resembling a cloud. The term veil describes Aurora as a large area of uniform light that covers most of the sky. Ray, refers to an Aurora that appears to be made up of straight vertical shafts, that are aligned in the direction of the earth's magnetic field lines. Sometimes the Aurora can occur in the shape of a fluttering curtain or drapery, and sometimes looks like a light shower (corona). Aurorae can also be classified according to their behavior and movement. Quiet Aurorae usually have a uniform intensity over a longer period of time (minutes). There are also pulsating Aurorae whose brightness changes in a periodic manner. More extreme cases of this Aurora can occur. One of them is the flickering Aurora, whose brightness changes about 5-10 times per second. Another is the flaming Aurora, where bursts of light appear at the base of the Auroral form and then rapidly move up and disappear at the top. Auroral brightness is rated on a scale of 0 to 4, 0 being a barely visible Aurora, and 4 being a very bright Aurora. Auroral colors can cover the entire light spectrum. On rare occasions sounds such as hissing, swishing, rustling or crackling were reported, to accompany an Aurora.

Where and When does the Aurora Occur ?

Where:

The Aurora occurs most often in regions known as the "Auroral Ovals". These are rings of a radius roughly 1500mi (2400 km) around the Earth's magnetic Poles. The Aurora occurs at a height of about 60 miles (96 km) above the Earth's surface. The word Aurora Australis refers to the Aurora that occurs in the southern hemisphere (southern lights), whereas Aurora Borealis refers to the Aurora that occurs in the northern hemisphere (northern lights). On rare occasions, Aurora can occur at lower latitudes, and even at the equator. For example, a majestic Aurora was seen above Paris, France, on April 15, 1869. For two days, on March 13-14, 1989, the Aurora was seen almost over the entire United States. At lower latitudes the Aurora is less pronounced, and often obscured by intense air and light pollution of more inhabited areas of Earth today. In the past, it was much easier to observe this phenomenon at lower latitudes. The Aurora also occurs on other planets. For example, recent pictures taken by the "Voyager" space probe revealed extensive Auroral activity on Jupiter.

When:

Auroral activity correlates with the activity of the Sun. The more active is the Sun, the greater the number of Aurorae. Whereas the less active is the Sun the smaller the number of Aurorae. The activity of the Sun changes periodically. It takes 11-years, for the sun to change from its period of low activity (Solar Minima) to its period of high activity (Solar Maxima), and then back to its Solar Minima. This so called 11-year solar cycle results in a greater or less number of Auroral occurrences.