Aurora

Aurora  is a natural display of light in the sky that can be seen with the unaided eye only at night. 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. Auroras are the most visible effect of the sun's activity on the earth's atmosphere.

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.

Auroral displays are associated with the solar wind, a continuous flow of electrically charged particles from the sun. When these particles reach the earth's magnetic field, some get trapped. Many of these particles travel toward the earth's magnetic poles. When the charged particles strike atoms and molecules in the atmosphere, energy is released. Some of this energy appears in the form of auroras.

Auroras occur most frequently during the most intense phase of the 11-year sunspot cycle. During this phase, dark patches on the sun's surface, called sunspots, increase in number. Violent eruptions on the sun's surface, known as solar flares, are associated with sunspots. Electrons and protons released by solar flares add to the number of solar particles that interact with the earth's atmosphere. This increased interaction produces extremely bright auroras. It also results in sharp variations in the earth's magnetic field called magnetic storms. During these storms, auroras may shift from the polar regions toward the equator.

From : http://www.grantchronicles.com/auroraborealismk.htm

 

The Advantage and Disadvantage of Nuclear Power

Nuclear power is generated by using uranium which is a metal mined in various part of the world. The first large scale of nuclear power station was opened at Calder Hall in Cumbria, England in 1956.

Some military ships and submarines have nuclear power plant for engine. Nuclear power produces around 11% of the world's energy needed, and produces huge amounts of energy. It cause no pollution as we would get when burning fossil fuels. The advantages of nuclear plant are as follow:

• It costs about the same coal, so it is not expansive to make.
• It does not produce smoke or carbon dioxide, so it does not contribute to the greenhouse effect.
• It produces huge amounts of energy from small amount of uranium.
• It produces small amount of waste.
• It is reliable. 

On the other hand, nuclear power is very, very dangerous. It must be sealed up and buried for many years to allow the radioactivity to die away. Furthermore, although it is reliable, a lot of money has to be spent on safety because if it does go wrong, a nuclear accident ca be a major accident.

People are increasingly concerned about this matter. In the 1990's nuclear power was the fastest growing source of power in many parts of the world.

From : http://bos-sulap.blogspot.com/2010/02/contoh-example-of-discussion-text-on.html