To understand the cause of auroras, first picture the Earth enclosed by itsmagnetosphere, a huge region created by the Earth's magnetic field. Outside themagnetosphere, blasting toward the earth is the solar wind, a swiftly movingplasma of ionized gases with its own magnetic filed. Charged particles in thissolar wind speed earthward along the solar wind's magnetic lines of force with aspiraling motion. The Earth's magnetosphere is a barrier to the solar winds, andforces the charged particles of the solar wind to flow around the magnetosphereitself. But in the polar regions, the magnetic lines of force of the Earth andof the solar wind bunch together. Here many of the solar wind's chargedparticles break through the magnetosphere and enter Earth's magnetic field. Theythen spiral back and forth between the Earth's magnetic poles very rapidly. Inthe polar regions, electrons from the solar wind ionize and excite the atoms andmolecules of the upper atmosphere, causing them to emit aurora radiations ofvisible light.
The colors of an aurora depend on the atoms emitting them. The dominantgreenish white light comes from low energy excitation of oxygen atoms. Duringhuge magnetic storms oxygen atoms also undergo high energy excitation and emitcrimson light. Excited nitrogen atoms contribute bands of color varying fromblue to violet. Viewed from outer space, auroras can be seen as dimly glowingbelts wrapped around each of the Earth's magnetic poles. Each aurora hangs likea curtain of light stretching over the polar regions and into the higherlatitudes. When the solar flares that result in magnetic storms and auroraactivity are very intense, aurora displays may extend as far as the southernregions of the United States.
Studies of auroras have given physicists new information about the behaviorof plasmas, which has helped to explain the nature of outer space and is beingapplied in attempts to harness energy from the fusion of atoms.
1. What does the passage mainly discuss?
(A) The methods used to observe auroras from outer space
(B) The formation and appearance of auroras around the Earth's poles
(C) The factors that cause the variety of colors in auroras
(D) The periodic variation in the display of auroras
2. The word phenomena in line 2 is closest in meaning to
(A) ideas
(B) stars
(C) events
(D) colors
3. The word picture in
(A) frame
(B) imagine
(C) describe
(D) explain
4. The passage describes the magnetosphere as a barrier (line 10)because
(A) its position makes it difficult to be observed from Earth
(B) it prevents particles from the solar wind from easily entering Earth'satmosphere
(C) it increases the speed of particles from the solar wind
(D) it is strongest in the polar regions
5. The word them in line 16 refers to
(A) polar regions
(B) electrons
(C) atoms and molecules
(D) aurora radiations
6. According to the passage , which color appears most frequently in anaurora display?
(A) greenish-white
(B) crimson
(C) blue
(D) violet
7. The word emit in line 20 is closest in meaning to
(A) change from
(B) connect with
(C) add to
(D) give off
8. The word glowing in line 22 is closest in meaning to
(A) shining
(B) moving
(C) charging
(D) hanging
9. Auroras may be seen in the southern regions of the United Sates when
(A) magnetic storms do not affect Earth
(B) solar flares are very intense
(C) the speed of the solar wind is reduced
(D) the excitation of atoms is low
10. The passage supports which of the following statements aboutscientists'n derstanding of
auroras?
(A) Before advances in technology, including satellites, scientists knewlittle about auroras.
(B) New knowledge about the fusion of atoms allowed scientists to learnmore about auroras.
(C) Scientists cannot explain the cause of the different colors inauroras.
(D) Until scientists learn more about plasma physics, little knowledgeabout auroras will be available.
11. Which of the following terms is defined in the passage ?
(A) magnetosphere (line 6)
(B) electrons (line 15)
(C) ionize (line 15)
(D) fusion (line 29)
BCBBC ADABA A
line 5 is closest in meaning to

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