单选题
The sun is stormy and has its own kind of weather. It is so hot and active that even the Sun"s gravity cannot hold its atmosphere in check! Energy flows away from the Sun toward the Earth in a stream of electrified particles that move at speeds around a million miles per hour. These particles are called plasma, and the stream of plasma coming from the Sun is called the solar wind. The more active the Sun, the stronger the solar wind.
The solar wind constantly streams toward the Earth, but don"t worry because a protective magnetic field surrounds our planet. The same magnetic field that makes your compass point north also steers the particles from the Sun to the north and south poles. The charged particles become trapped in magnetic belts around the Earth. When a large blast of solar wind crashes into the Earth"s magnetic field, the magnetic field first gets squeezed and then the magnetic field lines break and reconnect.
The breaking and reconnecting of the magnetic field lines can cause atomic particles called electrons trapped in the belts to fall into the Earth"s atmosphere at the poles. As the electrons fall into the Earth, they collide with gas molecules in the atmosphere, creating flashes of light in the sky. Each atmospheric gas glows a different color. Oxygen and nitrogen glows red and green and nitrogen glows violet-purple. As these various colors glow and dance in the night sky, they create the Northern Lights and the Southern Lights.
Watching auroras is fun and exciting, but normally you can only see them in places far north like Alaska and Canada. The movement of the aurora across the sky is usually slow enough to easily follow with your eyes but they can also pulsate, flicker, or even move like waves. During solar maximum, auroras are seen as far south as Florida, even Mexico!
Auroras often seem to be very close to the ground, but the lowest aurora is still about 100 kilometers above the ground, a distance much higher than clouds are formed or airplanes can fly. A typical aurora band can be thousands of kilometers long, a few hundred kilometers high, but only a few hundred meters thick.
We hope you are able to travel to far-north places like the Arctic Circle and see the Northern Lights at least once during your lifetime. We know you will never forget it!

单选题 The Sun"s gravity is too weak to keep its plasma from flowing to the Earth.

单选题 The Earth is quite safe with a magnetic field surrounding it to protect it from the attack by the solar wind.

单选题 Some scientists are worrying about the possible disappearance of the Earth"s protective magnetic field in the future.

单选题 The auroras are formed when the electrons falling into the Earth"s atmosphere at the poles and colliding with gas molecules in the atmosphere.

单选题 You cannot see the Northern Lights unless you are in Alaska or Canada.

单选题 Tens of thousands of tourists take special trips to Norway and Sweden every year to watch the Northern Lights.

单选题 An aurora is generally close to the ground and is very long and thick.