Reading Comprehension
Directions: Read the passage and choose the one best answer, 1, 2, 3, or 4, to each question. Then mark the correct choice on your answer sheet.
1. For hundreds of years, people living in the northern regions of the world have observed the sky come alive with magnificent waves of green, pink, and purple light. This stunning natural display is called the Aurora Borealis, commonly known as the Northern Lights. For many years, no one understood what caused it, which led to the creation of numerous myths and traditional stories. Today, however, scientific research provides a clear explanation for this remarkable phenomenon.
2. The process begins with the sun. The sun continuously releases a flow of electrically charged particles into space, known as the solar wind. After traveling millions of kilometers, some of these particles eventually arrive near Earth. Fortunately, our planet is surrounded by a strong magnetic field that shields us from most harmful solar radiation. This magnetic field is weaker near the North and South Poles, where it acts like a channel, guiding the solar particles into the upper atmosphere around these regions.
3. The real "magic" occurs when these energetic solar particles strike atoms and gas molecules high in the atmosphere, generally between 100 and 400 kilometers above the Earth's surface. During these collisions, energy is transferred to the gas particles, causing them to become excited. However, they can only remain in this condition briefly. As they return to their normal state, they release the extra energy in the form of small flashes of light. When countless numbers of these collisions take place simultaneously they create the spectacular light display visible from Earth.
4. The colors of the aurora depend on both the type of gas being affected and the altitude at which the interaction occurs. The most frequently seen color, bright green appears when solar particles collide with oxygen atoms at heights between approximately 100 and 300 kilometers. Red lights, which occur less often, are also produced by oxygen but at elevations higher than 300 kilometers. Nitrogen molecules can produce pink and dark red shades.