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　　the northern lights

　　the sun is stormy(狂暴的) 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(控制，支配)! energy flows away from the sun toward the earth in a stream of electrified particles(电粒子) that move at speeds around a million miles per hour. these particles are called plasma(等离子体), 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(磁场) surrounds our planet.the same magnetic field that makes your compass point north also steer(引向) 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(断开又闭合).

　　the breaking and reconnecting of the magnetic field lines can cause atomic particles(原子粒子) called electrons(电子) trapped in the belts to fall into the earth’s atmosphere at the poles. as the electrons fall to the earth, they collide with gas molecules(分子)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(跳动), 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!

　　1.the sun’s gravity is too weak to keep its plasma from flowing to the earth.

　　a. right b. wrong c. not mentioned

　　解析：

　　题干的意思是：太阳的重力太弱，等离子逃离太阳，流向地球。这一说法与文章第一段第一句和第二句表达的意思不符。句子表达的重点不是太阳的重力太弱，而是太阳太热，其活动有太剧烈，造成等离子逃离太阳，流向地球。

　　2.the earth is quite safe with a magnetic field surrounding it to protect it from the attack by the solar wind.

　　a. right  b. wrong c. not mentioned

　　解析：

　　题干认为地球安全因为有磁场在高空包围着地球，将太阳风挡在地球大气层外。这与文章第二段第一句的意思是一致的。

　　3. some scientists are worrying about the possible disappearance of the earth’s protective magnetic field in the future.

　　a. right b. wrong c. not mentioned

　　解析：

　　题干认为有些科学家担心包围地球起防护作用的磁场有一天会消失。这在文章中没有提到。

　　4.the auroras are formed when the electrons falling into the earth’s atmosphere at the poles and colliding with gas molecules in the atmosphere.

　　a. right b. wrong c. not mentioned

　　解析：

　　文章第二段和第三段解释极光的成因。当强大的太阳风侵入地球磁场，磁场的磁力线会断开和闭合，等离子流的电子进入大气层，流向极地上空，与大气层的气体分子发生碰撞，发出极光。这个意思与文章相符。

　　5.you cannot see the northern lights unless you are in alaska or canada.

　　a. rightb. wrong c. not mentioned

　　解析：

　　根据文章表述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(跳动), flicker, or even move like waves. during solar maximum, auroras are seen as far south as florida, even mexico!可见题干是错误的表述。

　　6. tens of thousands of tourists take special trips to norway and sweden every year to watch the northern lights.

　　a. right b. wrong c. not mentioned

　　解析：

　　题干的意思是：每年有几万游客专程前往挪威和瑞典观看北极光。文章中找不到这一内容。

　　7. an aurora is generally close to the ground and is very long and thick.

　　a. right b. wrong c. not mentioned

　　解析：

　　题干说极光通常很接近地面，又长又厚。这一说法有两点与文章第五段的介绍不符。第一点，极光不是接近地面，而是“似乎很接近地面”，事实上，最靠近地面的极光离地面也有100公里。第二点，极光不厚，only a few hundred meters thick.

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