题目内容
The splendid aurora light displays that appear in Earth’s atmosphere around the north and south magnetic(磁的) poles were once mysterious phenomena. Now, scientists have data from satellites and ground-based observations from which we know that the aurora brilliance is a massive electrical discharge similar to that occurring in a neon sign.
To understand the cause of auroras, first picture the Earth enclosed by its magnetosphere(磁层), a huge region created by the Earth’s magnetic field. Outside the magnetosphere, rushing toward the earth is the solar wind, a speedily moving body of ionized(离子化的) gases with its own magnetic field. Charged particles(粒子) in this solar wind speed earthward along the solar wind’s magnetic lines of force. The Earth’s magnetosphere is a barrier to the solar winds, and forces the charged particles of the solar wind to flow around the magnetosphere itself. But in the polar regions, the magnetic lines of force of the Earth and of the solar wind gather together. Here many of the solar wind’s charged particles break through the magnetosphere and enter Earth’s magnetic field. They then rush back and forth between the Earth’s magnetic poles very rapidly and ionize and excite the atoms of the upper atmosphere of the Earth, causing them to produce aurora radiations of visible light.
The colors of an aurora depend on the atoms producing them. The leading greenish-white light comes from low energy excitation of oxygen atoms. During huge magnetic storms oxygen atoms also undergo high energy excitation and produce pink light. Excited nitrogen(氮) atoms contribute bands of color varying from blue to violet. Viewed from outer space, auroras can be seen as dimly shining belts wrapped around each of the Earth’s magnetic poles. Each aurora hangs like a curtain of light stretching over the polar regions and into the higher latitudes. When the solar flares(闪光) that result in magnetic storms and aurora activity are very strong, aurora displays may extend as far as the southern regions of the United States.
1.What is it that directly gives off aurora light?
A. The Earth’s magnetic field.
B. The solar wind’s magnetic field.
C. The Earth’s ionized particles.
D. The solar wind’s charged particles.
2.What does the third paragraph mainly discuss?
A. The appearance of auroras around the Earth’s poles.
B. The periodic change in the display of auroras.
C. The factors that cause the variety of colors in auroras.
D. The covering area of auroras based on their colors.
3.What can we infer from the passage?
A. The magnetosphere increases the speed of particles from the solar wind.
B. The color of greenish-white appears least frequently in an aurora display.
C. Earth’s magnetic field contributes to the variety and difference of aurora’s colors.
D. The strength of the solar flares has a positive effect on the extending distance of aurora.
